The volume of the earth is at all times dependent on two sets of
antagonistic forces, (1) the attractive or centripetal, consisting of
gravity and the molecular and sub-molecular attractions, and (2) the
resistant forces—which are not necessarily centrifugal—consisting of
heat and the resistant molecular and sub-molecular forces.
1. _The centripetal agencies._
=Gravity.=—The most obvious of the concentrating forces is gravity,
and in most questions relating to great segmental movements, it has
been thought sufficient to consider gravity alone, but it is by no
means certain that this does not lead to serious error. In studying
the causes and effects of earth movements, it is necessary to consider
both gravitational _energy_ and gravitational _force_. Gravitational
_energy_ is greatest when the mass is most widely dispersed, and least
when most concentrated. Gravitational _force_ is greatest when the mass
is most concentrated, and least when most dispersed. The gravitational
energy of the earth matter was at its maximum when it was most widely
diffused in the supposed nebulous condition. It will perhaps reach
its minimum at some future period when the shrinkage shall reach its
limit. In passing from an expanded condition to a more concentrated
condition, potential energy, or energy of position, is transformed
into other forms of energy, chiefly heat. The heat thus developed
is an important factor in the earth’s dynamics. The total amount of
gravitational energy involved in the earth’s evolution is unknown, for
neither the maximum dispersion of the earliest state, nor the ultimate
condensation, is known. It is not difficult, however, to compute the
amount of transformation of gravitational energy into heat, or other
forms of energy, during a given degree of condensation. If a mass
equal to that of the earth were originally infinitely scattered, the
gravitational energy given up by it in condensing into a homogeneous
sphere of the earth’s present size would, if all transformed into heat,
suffice to raise the temperature of an equal mass of water 8900° C.
(Hoskins), or an equal mass of rock (specific heat of .2), 44,500° C.
If the mass were more condensed toward the center, as is the actual
case, the heat would be considerably greater. If the condensation
toward the center followed the Laplacian law (p. 564), the heat would
be sufficient to raise the earth mass 48,900° C., assuming its specific
heat to be .2, which is about the average specific heat of rock at
the surface (Lunn). A further shrinkage of one mile would transform
an additional amount of gravitational energy into heat about equal in
amount to Tait’s estimate of the loss of heat from the surface of the
earth in 100,000,000 years (see p. 572). If the radial shrinkage has
been 32 miles, or even 16 miles, the amount of heat generated is very
much greater than the estimated loss from the surface.
How much gravitational energy can possibly be transformed into heat and
other forms of energy in the future, can only be computed by making
assumptions as to the possible extent of further contraction, and that
involves hypotheses as to the atomic and sub-atomic constitution of the
earth’s matter, and its behavior under the prodigious pressures of the
earth’s interior. All shrinkage develops added gravitational force and
further tendency to shrinkage, which follows when the heat generated by
the shrinkage is lost; and where the process may end, in a body of the
dimensions of the earth, is beyond present determination. If there were
no limit to the density that might be attained, it would be impossible
to assign any limit to the energy that might be transformed. It has
usually been assumed that contraction could not go on indefinitely
because the atoms would come into actual contact, and prevent further
increase of density. This conception rests on the recently prevalent
hypothesis of the atomic constitution of matter; but the more recent
hypotheses that substitute multitudes of revolving corpuscles or
electrons for irreducible atoms, do not carry the same presumption of a
rigorous limit to condensation. It is not therefore prudent to try to
set such a limit, or to make it a feature in the dynamical doctrines
of the earth. It is even less prudent to try to measure the limit of
future conversion of the gravitational energy of the sun into heat, and
so to set a limit to the habitability of the earth.
The _force_ of gravity may be defined as the effort of gravitational
energy to change into other forms of energy. It is most familiarly
expressed in terms of weight, which is the resultant of the
gravitational force of the whole earth upon a given portion. Weight is
determined by the distances and directions of the given portion from
all parts of the attracting mass, the amount of the attraction being
directly as the mass and inversely as the square of the distance,
modified by the direction. It is greatest about 610 miles below the
surface, where it is 1.0392 times that at the surface. Below this point
it declines, and at the center it is zero. The sum total of the earth’s
gravitative force at the present time is equivalent to about 6 × 10²¹
tons. This gives rise to a pressure of about 3,000,000 atmospheres at
the center of the earth.
Gravitational force is also expressed in terms of the earth’s ability
to accelerate the velocity of falling bodies at its surface, which is
now approximately 32 feet per second. For certain purposes, the force
of gravity may be better pictured by means of the velocity required
to overbalance it, which is 6.9 miles per second; e.g. a body shot
away from the surface at a speed exceeding 6.9 miles per second, would
escape from the control of the earth if the influence of the atmosphere
and other bodies is neglected; while a body shot away at less than this
speed would return to the earth.
=Molecular and sub-molecular attractions.=—In addition to gravity,
there are at least three additional classes of attractive agencies
whose laws appear to differ from those of gravity, viz. cohesion,
chemical affinity, and sub-atomic attraction, using these terms in
their comprehensive generic senses. The thought has been entertained
that these might be reducible to forms of gravity in ulterior analysis,
but it does not appear from existing evidence that the laws of their
attractions are conformable to the Newtonian law of the inverse
square of the distance, to which gravity conforms. Apparently the
forces of the molecular, atomic, and sub-atomic attractions increase
at higher rates, and have individual peculiarities of action quite
different from gravity. It would be of the utmost service to geological
philosophy if these laws of molecular and sub-molecular attractions
were firmly established, and could be applied to the conditions of
heat and pressure under which the matter of the interior of the earth
exists. In the absence of such determinations, we can do little more
than recognize that the matter of the interior of the earth tends to
condense itself by the aid of molecular and sub-molecular attractions,
supplemental to the attraction of gravity.
=Cohesion and crystallization.=—The force of gravity between small
bodies is exceedingly feeble, but it is cumulative, every particle in a
mass attracting every other particle, so that in great masses the force
becomes enormous. In cohesion, and probably in the other molecular and
sub-molecular attractions, the particles attract very strongly the
particles with which they are in close relations, but beyond minute
distances their effects are insensible. The force of crystallization
is felt for a very short distance from the crystal, and “mass action”
is probably dependent on a function of similar kind, acting at a very
small distance, but the range of these forces is very limited in
comparison with that of gravity.
Rock matter, as a rule, tends to become crystalline by the assembling
of like molecules in systematic order. The general effect is
condensation, though this is not universally the case, for in some
instances the crystalline arrangement results in expansion. The
crystallizing force may be regarded as a specialized variety of
cohesion which usually coöperates with gravity to produce increased
density. In cases of expansion it seems clear that the organizing force
does not act according to the law of gravity. The intensity of the
force exhibited in the formation of ice illustrates the superiority of
the molecular force over the gravitative force in small masses; but in
a planet of ice of very moderate dimensions, the internal pressure of
gravity would overcome the crystalline force, which illustrates the
superiority of gravity in large masses.
While the crystalline force may thus in exceptional cases operate
against gravity, it is known that in most cases it not only operates
with it, but is controlled by it, in this sense—that where a substance
has two forms of crystallization, it will take the denser one when the
pressure is great. The inference is that if the less dense form of
crystallization takes place under slight pressure, and subsequently
the pressure is greatly increased, the form of crystallization will
change from the less to the more dense.[247] It is probable that in
general those forms of molecular arrangement will be assumed in the
deep zones which give the greatest density, and this probably includes
concretionary, colloidal, and other forms of aggregation, as well as
crystallization.
=Diffusion.=—The same law probably holds relative to diffusion, though
in a molecular sense diffusion is the opposite of crystallization,
for in crystallization, like comes to like, while in diffusion the
molecules distribute themselves among those of unlike nature. Diffusive
action, quite familiar in gases and liquids, takes place to some extent
in solids. The molecules of plates of gold and lead brought into
intimate contact under pressure mutually diffuse among one another.
So gases seem to be very generally diffused or “occluded” in rocks,
though the nature of this relation is imperfectly determined. It
is known that pressure upon gases promotes their diffusion through
liquids and solids. It is inferred that pressure upon a solid tends
to the diffusion of the entrapped gases within it, but it is not to
be inferred from this that pressure upon rock promotes the absorption
of gases into it, but rather the opposite. It is probable that great
pressure with high heat promotes the diffusion of entrapped gases or
other diffusible substances through the rock-mass, and at the same time
tends to their extrusion along lines of least resistance; but this is
an inference rather than a demonstration.
=Chemical combination.=—The general effect of chemical combination
under pressure is greater density. In reversible reactions capable of
conditions of chemical or physical equilibrium, pressure invariably
favors the formation of the denser of any possible products.
=Sub-atomic forces.=—Recent investigation has made it probable
that atoms are composite, embracing many exceedingly minute
bodies—corpuscles or electrons—in a state of extremely high activity
and possessed of marvelous energy notwithstanding their minuteness.
This discovery possesses deep interest to the geologist because it
seems to reveal sources of energy of almost incalculable potency, some
portions of which at least are being constantly freed and added to
the previously recognized supplies of energy. Attempts have been made
during the past few decades to limit the habitable age of the earth,
both retrospectively and prospectively, by the smallness of the sum
total of energy derivable from gravity. In these estimates slight
recognition has been given to the resources of molecular and atomic
energy, and none at all to the possibilities of sub-atomic energy. It
would be going quite too far to assume that these sub-atomic energies
are all available for the perpetuation of habitable conditions on
the earth or in the solar system, but we are doubtless justified in
appealing to them as an offset to all dicta restricting the period
of the earth’s habitability by supposed insufficiencies of energy
deduced merely from the estimated resources of gravity. The banishment
of the idea of the atom as a minute, incompressible, undecomposable
sphere takes away the theoretical limit of compressibility, and by
so doing cuts away the groundwork for assigning definite limits even
to the resources of gravity, since, as already indicated, unlimited
condensation gives theoretically unlimited transformation of the
potential energy of gravity.
While we must await with such patience as we can command the
development of fuller knowledge concerning the nature and laws of the
molecular, atomic, and sub-atomic energies, and their applicability to
the activities resident in the interior of the earth, it is permissible
even now to assume that, besides the simple compressive action of
gravity, there are at work varied forms of molecular aggregation, of
atomic combination, and perhaps of sub-atomic change, tending toward
increased density, and that the ulterior limit of these processes is
quite undetermined. The condensational forces are now restrained at
certain temporary limits by the antagonistic resistant forces, some of
which, such as heat, are the products of the condensational forces, and
are gradually being dissipated, permitting further condensation. Where
the process may ultimately end, we dare not attempt to say. On the
other hand, we are not compelled to accept assigned limits that seem to
be inconsistent with the phenomena which the earth actually presents.
2. _The resisting agencies._
=Heat.=—The most familiar of the active agencies that resist
condensation is heat. Upon this the existing volume of the earth is
immediately dependent, in some large part at least. As this heat is
dissipated, the earth shrinks. This shrinkage increases the force of
gravity, and hence the internal pressure increases, and, if further
compression takes place as the result of this increased pressure,
additional heat is developed, which checks further condensation until
it is dissipated. It is this kind of creative and self-checking action
that determines the volume of great gaseous bodies like the sun. Though
their matter is far from its ultimate density, and their self-gravity
is enormous, they condense slowly, because, with every stage of
condensation, heat is generated which antagonizes gravity and checks
condensation, until at least a part of the heat is radiated away.
As the force of gravity increases with every stage of condensation,
the heat developed to hold it in check must increase, and hence the
famous law of Lane, that a gaseous body like the sun grows hotter as
it condenses. This law holds good while the body remains in a gaseous
state in which the maintenance of the volume is essentially dependent
on heat. When a body becomes liquid or solid, its volume is dependent
in part on forms of resistance other than heat, and the force of the
law is abated, though the principle still holds good. In small solids,
the principle has little application, since the force of self-gravity
is slight compared to the resisting forces, and very little new heat
is generated as the body loses that which it has; but in large bodies,
like the earth, where the condensational forces are enormous and the
internal temperature is very high, it is not improbable that the heat
generated at every stage of condensation is relatively large. It has
been inferred by some students of the phenomena that the conditions
in the interior of the earth are essentially those of gaseous matter,
so far as molecular relations are concerned, because the temperatures
are thought to be above the critical temperatures of the substances
composing it. If this be true, the new heat generated with each stage
of condensation is large. However this may be, it seems safe to infer
that in so far as the volume of the interior mass is dependent on heat
resistance, _the loss of existing heat leads to the generation of
new heat_. The amount of this new heat must be enough, together with
the residual heat and the other forces of resistance, to match the
new condensational forces. The molecular and sub-molecular forces of
resistance other than heat, are probably responsible for some large
part of the resistance to the increased condensational force, but how
much is not determined.
=All resistance perhaps due to motion.=—As now interpreted, the force
of resistance of heat is due to the impact of the flying particles
of the heated matter. The other forms of resistance to compression
have not usually been interpreted in this way, but the tendency of
recent investigation is to place them in the same dynamic class.
A cold solid body offers resistance to compression that is in no
obvious way dependent on heat motion. In small bodies this resistance
is immeasurably greater than the self-gravity of the body. It is so
great that it can only be partially overcome by any force which human
ingenuity can bring to bear upon it. This form of resistance has thus,
not unnaturally, come to be regarded as approximately immeasurable, and
perhaps as grading into actual immeasurability, and as resting back
upon the actual contact of irreducible atoms. But the recent researches
which have developed grounds for the conception that even the atoms
are composite, lead to the further conception that their resistance
to compression is dependent on the movement of their constituent
corpuscles or electrons. This encourages the broad conception that the
whole of the resistance to compression arises from molecular, atomic,
and sub-atomic _motions_, of which heat is merely one form.
While all this is yet on the frontier of physical progress, these
conceptions may well be recognized in framing interpretations of the
agencies which determine the volume of the earth, and which control
the changes that take place in it from age to age. The result of their
combined action at any stage is _a state of temporary equilibrium_
between gravity, aided by the molecular, atomic, and sub-atomic
attractions, on the one hand, and heat, aided by the molecular,
atomic, and sub-atomic resistances, on the other. The vital problem
is to ascertain _the original condition of balance_ between these
antagonistic forces, and the changes which have affected that balance
since. The original state of balance is necessarily a matter of
hypothesis, and the best that can be done at present is to picture
as clearly as possible the different hypotheses that have been
entertained, and the different consequences that logically flow from
them. The most important factor in the case is the original amount and
distribution of internal heat.
ALTERNATIVE VIEWS OF ORIGINAL HEAT DISTRIBUTION.
The hypothetical modes of origin of the earth will be treated in the
historical section. Suffice it here to say that one view is that the
earth was once gaseous, passed thence into a liquid, and later into
a solid state. Under this view, there are two hypotheses as to the
original distribution of internal heat, dependent on the mode of
solidification. According to the one, solidification began at the
surface after convection had brought the temperature of the whole
mass down nearly to the point of congelation; according to the other,
solidification began at the center at a high temperature, because of
pressure, and proceeded thence outwards. The former only has been much
developed in the literature of the subject, though the latter is now
generally regarded as the more probable.
Another view of the globe’s origin is that the earth was built up
gradually by the infall of matter, bit by bit, at such a rate that
though each little mass became hot as a result of its fall, it cooled
off before others fell on the same spot, the rain of matter not being
fast enough to heat up the whole mass to the melting-point. Under this
view, the internal heat arose chiefly from compression due to the
earth’s gravity.
A clear conception of the three hypotheses of thermal distribution
which rest on these two views of the origin of the earth is important
to the further discussion.
=1. Thermal distribution on the convection hypothesis.=—It was
formerly the prevailing opinion that the molten condition of the
earth persisted in the interior until after the crust had formed,
and that solidification proceeded from the surface downwards. It was
a natural corollary of this view that, previous to the beginning of
solidification, convection stirred the liquid mass from center to
circumference and equalized the temperature so that the whole mass
cooled down equably until it approached the point of solidification
and became too viscous for ready convection. The temperature should,
therefore, have been nearly the same from center to surface at the
stage just preceding incipient solidification. This conception forms
the basis of most discussions involving internal temperatures.[248]
The famous studies of Lord Kelvin are based on the assumption of a
uniform initial temperature of 7000° Fahr.[249] Other temperatures
have been assumed in similar studies by others, but the results do not
differ materially. On this hypothesis there would be no deep-seated
change of temperature until a temperature-gradient, extending to the
deeper horizons, had been developed by surface cooling. In the earliest
eras, the loss of heat would be felt solely in the outer zone. By
surface cooling, a temperature gradient would be slowly developed, and
gradually changed from age to age, as shown by the curved lines in
Fig. 450, each of which shows the temperature at the successive stages
stated in the legend. The computations for these curves were based
on the methods and assumptions of Lord Kelvin. The two lower curves
represent greater periods than those usually assigned by geologists to
the whole history of the earth. It will be seen that the modification
of the original temperature line extends only about 160 miles below the
surface for the 100,000,000-year period, only about 240 miles for the
237,000,000-year period, and only about 320 miles for the excessive
period of 600,000,000 years. The superficial nature of the whole
thermal problem under this hypothesis is thus made clear and impressive.
[Illustration: +Fig.+ 450.—Diagram showing the original distribution
of heat assumed by the convection hypothesis and the modifications
of this distribution near the surface in successive long periods.
The base-line of the figure represents divisions of the earth-radius
with center at the left and surface at the right. The vertical
lines represent temperatures ranging from 0° C. to 5000° C. The
assumed initial temperature 3900° C. (7000° F.) is represented by
the horizontal line TC, full at the left and dotted at the right to
indicate the original extension of the initial temperature to the
surface. The upper curve at the right shows how much the temperature
will have been modified at the end of 100,000,000 years, computed
according to the method of Lord Kelvin. The middle curve shows the
change at the end of 237,000,000 years, and the lower curve the
change at the end of 600,000,000 years. Similar curves may be found
in an article by Clarence King, Am. Jour. of Sci., XLV, 1893, p. 16.]
After the outer shell had cooled so as to be in approximate equilibrium
with the environment of the earth, it suffered practically no
contraction.
So also it appears from the diagram that there was practically no
contraction below 160 miles up to the end of the 100,000,000-year
period, because cooling had not yet reached that depth. Between these
two non-contracting horizons the greatest rate of contraction at the
close of the 100,000,000-year period lay about 60 miles below the
surface. The contraction of this middle zone, while the outermost shell
and the interior body remained constant, is held to have developed a
state of horizontal thrust in the outer shell, because this shell,
being too large for the shrinking subcrust, tended to settle, and
to crowd upon itself horizontally. The wrinkling and other modes of
deformation of the outer part of the earth are referred, under this
view, to the thrust so developed. This is the view which has been most
generally accepted.
=Level of no stress.=—As the outer shell is thus held to be in a state
of thrust while the zone below is in a state of shrinkage, there must
be, between these two zones, a _level of no stress_, where there is
neither compression nor stretching. Above this level, the thrust
increases to the surface, and below it, the stretching increases to the
depth of most rapid change of temperature, below which it decreases
and finally vanishes at the lower limit of temperature change. In the
earliest stages of cooling, the level of no stress must have been
near the surface, and must have descended gradually as the cooling
proceeded. The depth of this level has been repeatedly computed on
the basis of assumed times and rates of cooling. Fisher, assuming
the temperature of solidification to have been 4000° Fahr. and the
period of cooling 33,000,000 years, computed its depth at only ⁷⁄₁₀
of a mile below the surface.[250] T. Mellard Reade, with somewhat
different assumptions, placed it at 2 miles after 100,000,000 years
of cooling.[251] Davison (1897) placed it at 2.17 miles,[252] and G. H.
Darwin at 2 miles after the same period.[252] In a later computation,
based on the assumption that the coefficient of dilatation increases
with the temperature, Davison placed the level of no stress at 7.79
miles, and stated that if the coefficient of conductivity and the
initial heat also increased down wards, the zone would lie still
deeper. To suppose the initial heat to increase downwards, however, is
to abandon the hypothesis we are now considering. These computations
seem to show that, at the very utmost, the level of no stress, under
this hypothesis, lies at a very slight depth, and that the thrust zone
above is, therefore, very shallow. This should be kept constantly in
mind in all deductions drawn from this hypothesis. If the thickness
of the thrust zone be taken at 8 or 10 miles, it will apparently be
conceding to the view all that can legitimately be claimed for it.
[Illustration: +Fig.+ 451.—Diagram illustrating the internal
temperatures of the earth when it first became solid, under the
hypothesis that it solidified from the center outward, and assuming
that the fusing-point rose directly as the pressure, in accordance
with Barus’ experiments with diabase. The divisions of the base-line
represent fractions of the earth’s radius. The divisions of the
vertical lines represent pressures in atmospheres at the left,
and temperatures in degrees C. at the right. The lower curve, PC,
represents the interior pressures, ranging from one atmosphere at
the surface to 3,000,000 atmospheres at the center, derived from
Laplace’s law of density. The upper curve, FC, represents the
fusion-points of diabase at the various depths and pressures, and
hence the temperatures at which the interior would become solid at
the various depths, or, in other words, the initial temperatures of
the solid earth. The lower curve is derived from Slichter; the upper
is formed by directly plotting the temperatures given by Barus (Am.
Jour. Sci., 1893, p. 7).]
=2. Thermal distribution on the hypothesis of central
solidification.=—When the previous conception was first formed, the
effect of pressure on the melting-points of lavas was neglected, as
little or nothing was known on the subject. Experiment, however, has
shown that pressure, as a rule, raises the melting-points of lavas, and
out of this has grown the doctrine that the earth solidified first at
the center, where the pressure was greatest, and gradually congealed
outwards. Barus has shown that the melting-point of diabase,[253]
selected as a representative rock, rises directly with the pressure.
If this rate holds good to the center of the earth, the melting
temperature of diabase there would be 76,000° C. (136,800°F.). The
range of the experiment is, however, very small compared with the range
of the application, and little confidence can be felt in the special
numerical result reached. The rate of rise of the fusion-point may
be much changed as the extraordinary conditions of the deep interior
are invaded. Still there is good ground for the hypothesis that
solidification took place at some very high temperature at the center,
because of the very great pressure there. The inference then is that
when the temperature of the center of the supposed molten globe reached
the appropriate point, solidification began there, and that it took
place at lesser depths in succession as the appropriate temperatures
were reached. This view _excludes convection_ in the successive
zones from the center outward after the time when their temperatures
of solidification were reached, or after these were approached
sufficiently near to develop prohibitive viscosity. Some loss of heat
from these horizons would be suffered while the outer parts were
solidifying, but on account of the exceedingly slow conductivity of
rock, it is improbable that the amount of loss would be sufficient
to change the general character of the internal distribution of heat
previous to solidification at the surface, the time when the existing
phase of the earth’s history by hypothesis began. Fig. 451 shows the
theoretical distribution of heat under this view. The consequences of
this assumption are very important to geological theory and, carried
out to their logical consequences, lead to the conclusion that cooling
and shrinkage _affected the deep interior of the earth_, for the
high central heat must have been constantly passing out toward the
surface. Instead, therefore, of the contraction being concentrated
in and limited to the outer 200 miles or so, as under the preceding
hypothesis, it was deeply distributed. The contraction within the outer
zone would be less than under the preceding view, because the flow
of heat from within would partially offset the flow outwards, and a
corresponding part of the contraction would be distributed below.
COMPUTED PRESSURES, DENSITIES, AND TEMPERATURES WITHIN THE
EARTH BASED ON LAPLACE’s LAW.
+------------+--------------+----------+-------------+
| Distance | Pressure | | |
|from center | in megadynes | | Temperature |
|in terms of | per sq. | Density. | in |
| radius. | cm.[254] | | degrees C. |
+------------+--------------+----------+-------------+
| 1.00 | 0 | 2.80 | 0 |
| .95 | 97,000 | 3.37 | 320 |
| .90 | 215,000 | 3.95 | 1,110 |
| .85 | 353,000 | 4.54 | 2,190 |
| .80 | 510,000 | 5.13 | 3,470 |
| .75 | 684,000 | 5.71 | 4,880 |
| .70 | 874,000 | 6.28 | 6,350 |
| .65 | 1,077,000 | 6.84 | 7,860 |
| .60 | 1,289,000 | 7.38 | 9,360 |
| .55 | 1,507,000 | 7.90 | 10,830 |
| .50 | 1,727,000 | 8.39 | 12,250 |
| .45 | 1,944,000 | 8.84 | 13,590 |
| .40 | 2,154,000 | 9.26 | 14,840 |
| .35 | 2,353,000 | 9.64 | 15,980 |
| .30 | 2,535,000 | 9.98 | 17,000 |
| .25 | 2,698,000 | 10.27 | 17,880 |
| .20 | 2,836,000 | 10.51 | 18,610 |
| .15 | 2,947,000 | 10.70 | 19,190 |
| .10 | 3,029,000 | 10.84 | 19,610 |
| .05 | 3,078,000 | 10.92 | 19,870 |
| .00 | 3,095,000 | 10.95 | 19,950 |
+------------+--------------+----------+-------------+
=3. Thermal distribution under the accretion hypothesis.=—The accretion
hypothesis assumes that the internal heat was gradually developed from
the center outwards as the earth grew and the internal compression
was progressively developed. The heat, therefore, continued to rise
at the center as long as compression continued, or at least as long
as the compression was sufficient to generate heat faster than it
was conducted outwards. As the conduction of heat through rock is
exceedingly slow, the central heat may be assumed to have continued to
rise so long as the infall of matter caused appreciable compression.
In the same way, heat was generated progressively in the less central
parts, and these parts also received the heat that passed out from
beneath. It is assumed under this hypothesis that the degree of
interior compression stands in close relation to interior density,
for while there would probably be some segregation of heavier matter
toward the center and of lighter toward the surface by means of
volcanic action and internal rearrangement under stress differences,
the interior density is regarded as due mainly to compression. The
distribution of internal pressure and density generally accepted is
that of Laplace, who assumed that the increase of the density varies
as the square root of the increase of the pressure. This law gives a
distribution of density that accords fairly well with the phenomena of
precession of the equinoxes, which require that the higher densities
of the interior shall be distributed in certain proportions between
the center and the equatorial protuberance whose attraction by the sun
and moon causes precession. The increases in pressure, density, and
temperature have been computed as follows by Mr. A. C. Lunn,[255] the
average specific gravity of the earth being taken at 5.6, the surface
specific gravity at 2.8, and the specific heat at .2.
The temperatures are shown graphically in Fig. 452, in which the
curves of pressure and density are also given. The nature of the curve
of temperature is such that, if the thermometric conductivity of the
material is uniform at all depths, the temperature _will fall in the
deeper portions and rise in the outer ones_, excluding the surface
portions subject to outside cooling. The curve indicates that the
rising temperature would affect somewhat more than 800 miles of the
outer part of the spheroid, or about half its volume, i.e. the inner
half during the initial period had a falling temperature and the
outer half, except the immediate surface, a rising temperature. This
introduces a very singular feature into the problem, for the outer
zone must shrink to fit the inner portion that is losing heat, while
its own material is expanding because of its increase of temperature.
A double distortional effect must result.[256] If the conductivity of
the dense interior is greater than that of the outer parts, the effect
is intensified. The redistribution of heat resulting from this unequal
flowage would in time change the curve so that more nearly equal
flowage would result. It would probably take a very long period for
this to be effected, on account of the very slow conductivity of rock.
The accretion hypothesis assumes that, during the growth of the
earth, large amounts of heat were carried by volcanic action from
deeper horizons to higher ones and to the surface, and that this
still continues at a diminished rate. It assumes that whenever the
interior heat raised any constituent of the interior matter above
its fusing-point under the local pressure, it passed into the liquid
state, _and was forced outwards by the stress differences to which
it was subjected_, unless its specific gravity was sufficiently
high to counterbalance them. It is conceived that the more fusible
portions were liquefied first, and that in so doing they absorbed the
necessary heat of liquefaction and began to work their way outward,
carrying their heat into higher horizons and temporarily checking the
development of more intense stresses in the lower horizons. They thus
served to keep the temperature there below the fusion-point of the
remaining more refractory substances. Meanwhile the extruded portions
were raising the temperatures of the higher horizons into which
they were intruded or through which they were forced to pass. There
was thus, it is thought, an automatic action that tended to reduce
the heat-curve to the fusion-curve. _The actual curve of internal
temperature may, therefore, be practically the fusion-curve._ This
is identical with the curve supposed to arise from solidification by
pressure from the center outward under the molten hypothesis, except
so far as the two would vary as the result of variations in the
distribution of matter, which would not be quite the same under the two
hypotheses. The curve of fusion deduced by an extension of the results
of Barus’ experiment has been given. It is necessary to recognize
that the rate of rise of the fusion-point may, and very likely
does, change in the deep interior. The curve given represents much
higher temperatures in the central parts than those given by Lunn’s
computations from compression, which seem inherently more probable than
the higher ones.
[Illustration: +Fig.+ 452.—Diagram illustrating the distribution of
temperature under the accretion hypothesis (neglecting the heat from
infall and other external sources). The divisions of the base-line
represent fractions of the earth’s radius. The vertical divisions
represent both pressure in megadynes per sq. cm., nearly the same as
atmospheres per sq. in., at the left, and temperatures in degrees C.
at the right. It is to be noted that the temperature scale is 2000°
C. per division, while that of Fig. 451 is 5000° C. per division.
The upper curve at the left, PC, is the pressure curve. The middle
curve, DC, is the density curve, beginning at 2.8 at the surface
and reaching nearly 11 at the center. The lower curve, TC, is the
temperature curve, rising from the surface temperature, 0° C., at
the right, to 20,000° C. at the center. It is to be noted that the
portion of this curve at the left representing the deeper part of
the earth is convex upwards, while the portion at the right is
concave. It will be seen that the gradient increases from the center
to a point between .6 and .7 radius, and then decreases, and that
between .8 radius and the surface, a distance of about 800 miles, the
decrease is notable. This means that with an equal coefficient of
conductivity the flow from the center outward to .6 or .7 radius will
be faster than the flow from .8 radius to the surface, neglecting
the immediate surface effects of external cooling. These curves were
worked out by Mr. Lunn.]
As astronomical and seismic evidences strongly favor the view that
the earth is rigid throughout, they lend support to the view that
the interior retains its rigidity by the extrusion of liquid matter
practically as fast as it is formed, and that this progressive
extrusion adjusts the temperature to that which is consistent with
solidity.
The bearing of this conception becomes evident on consideration. The
shrinkage of the earth from loss of heat by conduction and by the
extrusion of molten rock, affects the deep interior as well as the
more superficial zones. _It is even possible that the shrinkage may
originate chiefly in the deeper zones._ The postulated transfer of
fluid rock from the deeper parts to the more superficial ones lessens
the heat in the former, and adds to that in the latter. The postulated
greater flow of heat from the deeper half to the outer half, than from
the latter outward, gives a concordant result. If the conductivity
of the deeper and denser material is appreciably greater than that
of the more superficial and less dense material, as seems probable,
this effect is intensified. The distribution of compressibility at
the existing state of condensation may possibly be such that more
new heat is generated by shrinkage in the outer parts than in the
inner. Neither of these conceptions can be affirmed as actually taking
place. They merely lie within the range of reasonable hypothesis in
the present state of experimental data. What the real truth is must be
left to further research. Present effort may be regarded as temporarily
successful if it forms consistent conceptions of the applicable
hypotheses, and of their consequences.
=Recombination of material.=—One other peculiarity of the accretion
hypothesis must be recalled here. The incoming bodies must probably be
assumed to have fallen in promiscuous order, and hence to have been
indiscriminately mingled in the growing earth. As they became buried
deeper and deeper and their temperatures and pressures were raised,
much recombination, chemical and physical, may be presumed to have
followed. As already noted, these changes would probably give increased
density in the main. The material being, however, in a solid state, the
rearrangement would be slow and its persistence in time indeterminate,
and it may yet be far from complete. It is not improbable, therefore,
under this hypothesis, that some notable part of the recent shrinkage
of the earth has been due to the continued rearrangement of its
heterogeneous internal matter. This would not be equally so in an
earth derived from a molten mass, for the required adjustments of
the material should have taken place while in the fluid state before
solidification.
=Comparison of the hypotheses.=—By comparing the three hypotheses of
the early states of the earth’s temperature, it will be seen that
there is a radical difference, thermally, between the first and
the last two. The first assumes a nearly uniform distribution of
internal temperature, and hence, owing to the exceedingly slow rate
of conduction, limits the movements and deformations of the crust, so
far as dependent on heat, to very superficial horizons. The second
and third views agree in postulating changes of temperature in the
deep portions, as well as in the superficial, and hence involve the
central portion of the earth in the great movements and deformations.
It is not to be supposed that this of itself necessarily increases the
sum-total of the effects of contraction, for, given a certain loss of
heat from the surface, it may be relatively immaterial whether this
loss arose from a large reduction of temperature in a shallow zone, or
a small reduction of temperature in a deep zone, for, except as the
coefficient of expansion varies, the total shrinkage would be the same.
But _the difference in distribution_ makes a radical difference in the
resulting movements, for, in the first case, the movements are in a
weak superficial shell that cannot accumulate great stresses, and hence
must yield practically as fast as the stresses arise, while, in the
second case, the stress-accumulating power of the thick segments may be
great, and the stresses may gather for long periods and give rise to
great cumulative results at long intervals. In this respect the last
two views have much in common, though they differ in other important
particulars.
With this general background of hypothesis, we may now turn to the
direct evidences of the distribution of internal temperature which
observations near the surface afford. Unfortunately they are limited to
a mere film, as it were, little more than ¹⁄₄₀₀₀ of the radius of the
earth.
OBSERVED TEMPERATURES IN EXCAVATIONS.
As the earth is penetrated below the zone of seasonal changes by wells,
mines, tunnels, and other excavations, the temperature is almost
invariably found to rise. The rate of rise, however, is far from
uniform. If we set aside as exceptional the unusually rapid rise near
volcanoes and in other localities of obvious igneous influence, the
highest rates are still six times the lowest. A large number of records
have been collated by the Committee on Underground Temperatures, of the
British Association for the Advancement of Science. These range from
1° F. in less than 20 feet to 1° F. in 130 feet, with an average of 1°
F. in 50 to 60 feet, which has usually been taken as representative.
The more recent deep borings that have been carefully measured with due
regard to sources of error indicate a slower rate of rise. Some of the
more notable records are as follows:[1]
Depth. Rate of rise.
Sperenberg bore (Germany) 3492 feet. 1° F. in 51.5 feet.
Schladeback bore (Germany) 5630 „ 1° F. in 67.1 „
Cremorne bore (N. S. Wales) 2929 „ 1° F. in 80 „
Paruschowitz bore (Upper Silesia) 6408 „ 1° F. in 62.2 „
Wheeling well (W. Va.) 4462 „ 1° F. in 74.1 „
St. Gothard tunnel (Italy-Switzerland) 5578 „ 1° F. in 82 „
Mt. Cenis tunnel (France-Italy) 5280 „ 1° F. in 79 „
Tamarack mine (N. Mich.) 4450 „ 1° F. in 100 „[257]
Calumet and Hecla mine (N. Mich.) 4939 „ 1° F. in 103 „[257]
Ditto, between 3324 feet and 4837 feet 1° F. in 93.4 „
It is to be noted that even these selected records vary a hundred per
cent. Very notable variations are found in the same mine or well, and
often much difference is found in adjacent records, especially those of
artesian wells. Some of these are explainable, but the full meaning of
other variations is yet to be found.
=Explanations of varying increment.=—Certain apparent variations are
merely due to _inequalities of topography_. The isogeotherms, or planes
of equal underground temperature, do not normally rise and fall with
every local irregularity of the surface, but more nearly strike an
average. A well on a bluff 500 feet high would probably reach nearly
the same temperature at 1000 feet, as a well 500 feet deep in the
adjacent valley, giving a gradient twice as great in the one case as in
the other.
In interpreting the temperatures of artesian flows, regard must be
had to the _depths of rock under which the waters have passed_, as
well as the depths at the location of the wells. Darton has found[258]
unusually high and varying temperatures in the artesian wells of the
Dakotas, some part of which may be due to this cause, though a full
explanation of their singular variations is not yet reached.
=The permeation and circulation of water= affect the temperature in two
important ways: (1) wet rocks are better conductors than dry ones, and
(2) the convective movement of water is a means of conveying heat from
lower to higher horizons. As the circulation of underground water is
very unequal, much irregularity of thermal distribution in the upper
zones probably arises from this source. The _general_ effect of water
circulation is to reduce the thermal gradient where the circulation
is relatively rapid, as it is near the surface and in the main
thoroughfares of circulation, and hence to cause a relatively rapid
rise in the gradient just below the zone of effective water influence.
Some records conform to this theoretical deduction, but in general it
is masked by other influences.
=Chemical action=, especially oxidation, carbonation, hydration,
solution, and precipitation, modify the normal temperature gradient,
but how effectively is not well determined. With little doubt the
first three mentioned above raise the temperature, while solution
and precipitation in some large measure offset each other.[259] The
sum-total is probably an appreciable rise in temperature. It has even
been conjectured that the heat of volcanic action is due to chemical
combination in the lower reaches of water circulation, but this is
obviously an over-estimate.
=Differences in the conductivity of rock= are an obvious source of
varying underground temperature gradients. If an outer formation
conducts heat more freely than those below, it tends to lower the
gradient within itself and to cause a relative rise in the gradient
just below. If a lower formation is more conductive than that above, it
tends to lower the gradient within itself, and to raise it in the one
above, because it carries heat to the outer one faster than the latter
carries it away.
The =compression= to which rocks have been subjected affects their
temperature. At the surface the variation from this source is chiefly
dependent on the lateral thrust suffered.
When allowances are made for all these and other known causes of local
variation of temperature, it is still not clear that a uniform average
gradient remains as the true conception. If the earth were once a
molten spheroid, there would be a strong presumption that, aside from
local variations, there would be a normal curve applicable to all
regions. On the other hand, if the internal heat has arisen chiefly
from compression, and if the compression has varied in different
regions, as the inequalities of the surface render probable, there
would be no such definite normal curve in the accessible zone of the
earth, but rather a varying rate in different regions. In either case,
the later movements, compressions and strains of the crust, must modify
the original thermal gradients.
=Gradients projected.=—It is not probable that these gradients, even
when corrected for local variations, continue unmodified to the
center of the earth. If they did, 1° F. in 60 feet continued to the
earth’s center would give 348,000° F., and 1° F. in 100 feet would
give 209,000° F. It is much more probable that the rates of rise fall
away below the superficial zone. If water circulation in the fracture
zone is the most efficient agency cooperating with conductivity in the
outward conveyance of heat, as seems probable, the gradient in that
zone should rise at an abnormal rate, and hence the average gradient
in the deeper portions not affected by this circulation should be
lower. It will be recalled that the central temperature deduced from
an extension of Barus’ fusion curve is 136,800° F. (76,000° C.),
which, high as it is, gives a lower average gradient than the surface
observations. The computations from compression by Lunn, giving a
central temperature of 36,000° F. (20,000° C.), imply a still lower
average rate, while the convection hypothesis postulates no sensible
increase at all below 200 or 300 miles.
----------------+------------------+-------------+-----+-----------+----------
Average material| | Mineral | | |
of crust | Norm minerals | equivalent | | Linear | Volume
(Clarke’s | calculated from | (C.I.P.W. |Axis.| expansion.|expansion.
tables).[260] |Clarke’s average. | system). | | |
----------------+------------------+-------------+-----+-----------+----------
SiO₂ 58.59 |Quartz 11.4|Quartz | |+.00001206 |.00003618
Al₂O₃ 15.04 |Orthoclase 17.2| | _a_ |+.00001906 |
Fe₂O₃ 3.94 |Albite 27.3|Anorthite | _b_ |-.000002035|
FeO 3.48 |Anorthite 17.8| | _c_ |-.000001495|.00001553
CaO 5.29 |Diopside 6.8| | _a_ |+.000008125|
MgO 4.49 |Hypersthene 10.2|Diopside | _b_ |+.000016963|.0000234
K₂O 2.90 |Magnetite and | | _c_ |-.000001707|
Na₂O 3.20 | Ilmenite 6.8|Augite | _a_ |+.000013856|
TiO₂ .55 |Minor | (used for | | |
| constituents | hypersthene)| | |
Minor | omitted 2.5| | | |
constituents | ——————| | _b_ |+.00000272 |.0000245
omitted 2.52 | 100.00|Magnetite | _c_ |+.00000791 |
—————— | | | |+.000009540|.00002862
100.00 | | | | |
----------------+------------------+-------------+-----+-----------+----------
=The amount of loss of heat.=—The amount of loss of interior heat
which the earth suffers may be estimated by that which is observed to
be passing outward through the rock, or by computing the amount which
should be conveyed outwards with the estimated gradients and with the
conductivity of rock as determined by experiment. The latter method
is usually employed in general problems. Taking the mean thermometric
conductivity of rock as 0.0045, the gradient as 1° C. in 30 meters,
the average specific heat of rock as 0.5 small calories per cubic
centimeter, it is computed that in 100,000,000 years the loss of heat
would amount to 45° C. (81° F.) for the whole body of the earth.[261]
Tait makes the more conservative estimate of 10° C. (18° F.) in the
same period.[262] This is an exceedingly small result, and emphasizes
the low conductivity of rock.
=The amount of shrinkage from loss of heat.=—To compute the amount of
shrinkage for a given amount of cooling, the average coefficient of
expansion of rock is required. This has been experimentally determined
by several investigators. By combining the determinations of others
with his own, T. Mellard Reade found the _linear_ coefficient to be
.000005257 per 1° F., equivalent to .00002838 per 1° C. per _volume_.
In this the proportions of the different rocks in the crust were
roughly estimated. To secure an independent result from the best
available estimate of what constitutes the average rock, W. H. Emmons
has reduced Clarke’s average of the chemical constituents of the
crust to the norm minerals under the new system of Cross, Iddings,
Pirsson, and Washington (see p. 454) and made a weighted average of the
conductivities of these, as shown in the following table:
--------------+-----------+---------+-----------+--------------
| | | Volume | Volume
|Percentages| Sp. Gr. |proportions| proportions
| of norm | of norm | of norm | of temp.
| minerals. |minerals.| minerals. | 1° C. higher.
--------------+-----------+---------+-----------+--------------
Quartz | 11.4 | 2.66 | 4.28 | 4.2801548504
Feldspars[263]| 62.3 | 2.7 | 23.07 |23.0703582771
Diopside | 6.8 | 3.3 | 2.06 | 2.0600482040
Hypersthene | 10.2 | 3.45 | 2.95 | 2.9500722750
Magnetite | 6.8 | 5.17 | 1.3 | 1.3000372060
| ———— | | ————— |-------------
Total | 97.5 | | 33.66 |33.6606708125
--------------+-----------+---------+-----------+--------------
Subtracting the stated volume from the volume at a temperature of 1°
C. higher, the difference is found to be .0006708125, which divided by
the volume gives .0000199, which is the coefficient of expansion of the
theoretical, average, surface rock of the earth.
With this coefficient, the radial shrinkage resulting from an average
loss of 10° C. (18° F.), (Tait’s estimate), is a little over a quarter
of a mile (.2572); and for a loss of 45° C. (81° F.), (estimate of
Daniell’s Physics), a little over a mile (1.1574). The shortening of
the circumference for 10° C. loss is 1.6 miles, and for 45° C., 7.27
miles. Computations based on the coefficient of expansion adopted by
Reade give 2.35 miles circumferential shortening for a loss of 10° C.
and 10.5 miles for a loss of 45° C. In both these cases, the whole
contraction is assumed to take a vertical direction, and hence these
are maximum results. They are exceedingly small.
Unless there is a very serious error in the estimated rate of thermal
loss, or in the coefficients of expansion, cooling would seem to be a
very inadequate cause for the shrinkage which the mountain foldings,
overthrust faults, and other deformations imply. This inadequacy has
been strongly urged by Fisher[264] and by Dutton.[265] In view of the
apparent incompetency of external loss of heat, the possibilities of
distortion from other causes invite consideration.
OTHER SOURCES OF DEFORMATION.
=Transfer of internal heat.=—It is theoretically possible that
deformation of the subcrust may result from the internal transfer of
heat without regard to external loss. It has already been shown (p.
539) that under certain possible conditions more heat would flow from
the inner parts to higher horizons than would be conveyed through
these latter to the surface and there lost, and that, as a result,
the temperatures of the inner parts might be falling, while those of
the outer parts (except the surface) might be rising. With the more
conservative coefficient of expansion previously given, a lowering of
the average temperature of the inner half of the earth 500° C. and the
raising, by transfer, of the outer half to an equal amount would give a
lateral thrust of about 83 miles, which is about the order of magnitude
thought to be needed. It is not affirmed that this takes place, but
some transfer of this kind is among the theoretical possibilities under
the accretion hypothesis. The process could not continue indefinitely;
but, for aught that can now be affirmed, it may still be in progress.
=Denser aggregation of matter.=—As already noted, matter under intense
pressure tends to aggregate itself in the forms that give the greatest
density. If the earth were built up of heterogeneous matter arranged at
haphazard, the material would probably readjust itself more or less,
as time went on, into combinations of greater and greater density.
This process may be one of the important sources of shrinkage, for an
average change of density of 1 percent., affecting the matter of the
whole globe, would probably meet all the demands of deformation since
the beginning of the Paleozoic period.
=Extravasation of lavas.=—It is obvious that if lavas are forced out
from beneath the crust and spread upon it, a compensating sinking of
the crust will follow. This, however, is rather a mode than an ulterior
cause, for a cause must be found for the extrusion of the lavas, and
this cause may be one of the other agencies recognized, such as a
transfer of heat, a reorganization of matter, or a change of pressure.
The more practical question, however, relates to its competency. Can
the amount of lava that has been extruded have had any very appreciable
effect on the descent of the crust? The great Deccan flow is credited
with an area of 200,000 square miles, and a thickness of 4000 to 6000
feet. Vast as this is for a lava-flow, it would form a layer only
about 5 feet thick when spread over the whole surface of the globe,
and hence the sinking to replace it would cause a lateral thrust, on
any great circle, of about 31 feet only. It requires a very generous
estimate of the lavas poured out between any two great mountain-making
periods since the beginning of the well-known stratigraphic series to
cause a horizontal thrust of any appreciable part of that involved
in mountain-making. The case is different, however, if we go back to
the Archean era, in which the proportion of extrusive and intrusive
rocks is very high. Very notable distortion may then be assigned to
the extravasation of lavas. The outward movement of lava must also be
credited with some transfer of heat from lower to higher horizons, and
this is probably one of the agencies that have produced the relatively
high underground temperatures in the outer part of the earth.
If lavas are thrust into crevices of the crust they contribute to its
extension, but causes for the crevices and for the intrusion must be
found, and these are probably only expressions of one or another of the
more general agencies.
=Change in the rate of rotation.=—As previously noted, the tide acts
as a brake on the rotation of the earth. The oblateness of the present
earth is accommodated to its present rate of rotation. It is assumed
that such accommodation has always obtained, and that if the rotation
has changed, the form of the earth has changed also. Now, the more
oblate the spheroid, the larger its surface shell and the less the
total force of gravity. Hence if the earth’s rotation has diminished,
its crust must have shrunk, because the form of the spheroid has
become more compact, and the increase of gravity has increased its
density. There is at present a water-tide chiefly generated in the
southern ocean, and irregularly distributed to more northerly waters.
This irregularity interferes with its systematic action as a brake,
and its average effects are difficult of estimation. The water-tides
of past ages are still more uncertain, as they must have depended on
the configuration and continuity of the oceans. There are geological
grounds for the belief that the southern ocean was interrupted by land
during portions of the past at least, and it is unknown whether there
were elsewhere ocean-belts well suited to the generation of large
tides. The ocean-tide, therefore, furnishes a very uncertain basis
for estimating the retardation of rotation.[266] The theoretical case
rests largely on the assumption of an effective body-tide. The earth
doubtless has some body-tide, but whether it is sufficiently great to
be effective, and whether its position, which depends on its promptness
in yielding and in resilience, is favorable to the retardation of
rotation, are yet open questions. The existence of an appreciable
body-tide has not yet been proved by observation.
G. H. Darwin, assuming that the earth is viscous enough to give a
body-tide of appreciable value and of effective position, has deduced
a series of former rates of rotation of the earth and has computed the
corresponding distances of the moon.[267] C. S. Slichter has shown that
the lessening of the area of the surface and the increase of the force
of gravity corresponding to these assigned changes of rotation are
large, and that if the changes were actually experienced they must have
involved much distortion of the crust.[268] These distortions would,
however, be of a peculiar nature, and should thereby be detectible, if
they were realized; for in passing from a more oblate to a less oblate
spheroid, the equatorial belt shrinks, and the polar tracts rise and
become more convex. Wrinkles should, therefore, mark the equatorial
belts, and tension the high latitudes. Slichter has computed that in
a change from a rotation period of 3.82 hours to the present one, the
equatorial belt must shorten 1131 miles and the meridional circles
lengthen 495 miles. If we take Heim’s estimate of the crust-shortening
involved in forming the Alps—74 miles—as a standard, the 1131 miles
of equatorial shortening would be sufficient for the formation of 15
mountain ranges of Alpine magnitude. If, as some geologists urge, the
estimate of mountain folding is too great, the quotient would be still
larger. These ranges should run across the equator and be limited to
about 33° N. and S. latitude. The high-latitude tension would be
sufficient to cause the earth to gape more than two hundred miles at
the poles, if there were simple ideal shrinkage. The amounts and the
distribution of thrust and shrinkage are shown in Fig. 453. If the
change of rotation were no more than from 14 hours to the present rate,
there would still be 52 miles of thrust in the equatorial belt, and
40 miles of shortage in the meridional circles. There are no clear
signs of such a remarkable distribution of thrust and tension as this
hypothesis requires. Mountains are about as abundant and as strong
north of 33°, the neutral line, as south of it, and they extend to
high latitudes. The Archean rocks, in which this agency should have
been most effective because of their early formation, are crumpled
and crushed in the high latitudes much the same as in low latitudes.
Furthermore, if there had been appreciable change in the form of the
earth to accommodate itself to a slower rotation, the water on the
surface, being the most mobile element, should have gathered toward
the poles, and the less mobile solid earth should have protruded about
the equator, but the distribution of land and water, present and past,
gives no clear evidence of this. The equatorial belt contains a less
percentage of land than the area north of it and more than that south
of it. It varies but slightly from the average for the whole globe.
[Illustration: +Fig.+ 453.—Polar projection of the earth’s hemisphere
showing the theoretical high-latitude tension and low-latitude
compression involved in a change of rotation from 3.82 hours to
the present rate. The figure is drawn to true scale as seen from
a point above the pole, and in consequence the equatorial tract
is foreshortened. The black triangles show compression reduced in
length by foreshortening; the white show tension in essentially true
proportions to the high-latitude areas. The neutral line between the
areas of compression and of stretching lies at 33° 20′ latitude.]
While the doctrine of tidal retardation is theoretically sound, and
while the relations of the moon to the earth have probably been
appreciably affected by tidal action, geological evidence indicates
that it has not been sufficiently effective in producing crustal
deformations to be clearly detected by its own distinctive results.
This may be due (1) to the fact that there are compensating agencies
that tend to acceleration of rotation, and (2) to the probable fact
that the central rigidity of the earth is too high to give a very
effective body-tide. Hence the process of retardation may have been too
slow to have been geologically appreciable in the known period. The
recent estimates of the effective rigidity of the earth are greater
than former ones, and they may need to be modified yet further in the
same direction.
=Distribution of rigidity.=—An important consideration in this
connection is the distribution of interior rigidity. It is certain
that the rigidity of the outermost part, taken as a mass, is somewhat
less than that of rock of an average surface type, for it is fissured,
and there is no reason to suppose that the rigidity of the rock next
below the fissure zone rises at once to the rigidity of steel, and
hence if the average rigidity of the whole earth is equal to that of
steel, a portion of the interior must have a rigidity much higher than
steel. There is probably some law of increase from surface to center,
and there are theoretical grounds for thinking that it is in some way
connected with the laws of pressure, density, compressibility, and
temperature. All of these factors probably affect rigidity, but in
different ways. The modulus of rigidity of steel is about 770 × 10⁶
grms. per sq. cm. Milne and Gray[269] found that of granite to be 128 ×
10⁶. The ratio of the rigidity of steel to that of rock is, therefore,
about 6 : 1. If it be assumed that the rigidity increases in depth
directly as the density, the rigidity will nowhere reach that of steel,
being only about two-thirds as much at the center. If it be assumed
that the rigidity increases as the squares of the density ratios, the
following values are obtained:
---------------+----------------+---------+----------+------------
Distances from | | | Density |
center in terms| Densities under| Density | ratios | Deduced
of radius. | Laplace’s law.| ratios. | squared. | rigidities.
---------------+----------------+---------+----------+------------
1.00 | 2.8 | 1 | 1 | 0.16 Steel
.75 | 5.7 | 2 | 4 | 0.6 „
.50 | 8.39 | 3 | 9 | 1.5 „
.25 | 10.27 | 3.7 | 13.7 | 2.3 „
.00 | 10.95 | 3.9 | 15.2 | 2.5 „
These values seem fairly consistent with the apparent requirements of
the case.
If the distribution of rigidity were of this nature, the _average_
rigidity would be much less than that of steel, for more than half
the volume lies in the outer division, between 1.00 and .75 radius,
and yet the effective resistance to tidal deformation would be high,
for, according to G. H. Darwin,[270] the tidal stress-differences are
eight times as great in the center as at the surface. The rigidity
would, therefore, be distributed so as to be much more effective
in resistance than if it were uniform. The suggestion arises here
that the tidal stresses and other analogous stresses arising from
astronomical sources may be in themselves the causes of some such
distribution of rigidity as this. The tidal stresses are rhythmical
and give rise to a kind of kneading of the body of the earth, small in
measure to be sure, but persistent and rapidly recurrent. Since these
stress-differences at the center are eight times those at the surface,
and since also the gravitative stress at the center is 3,000,000 times
that at the surface, there is a series of persistently recurring
stress-differences, greatest at the center and declining outwards,
superposed on enormous static stresses, also intensest at the center
and declining outwards. Now, if the earth material were once made up
of a mixture of minerals of different fusibility, some of which became
more mobile (whether fluid or viscous) than others under the rising
temperature of the interior, it seems that the more mobile portion must
have tended to move from the regions of greater stress-differences to
those of lesser stress-differences. The persistence and the rhythmical
nature of the tidal stress-differences seem well suited to aid the
mobile parts in gradually working their way outwards. At the same time
the more solid and resistant portions should remain behind, and thus
come to constitute the dominant material of the central regions where
stress-differences were greatest, and so, as it were, concentrate
rigidity there. The process may still be in action.
If it be assumed that the rhythmical stresses have thus developed a
resistance to deformation proportional to their intensity, we may
combine this with density to form the basis of another hypothetical
distribution of rigidity, as follows:
---------------+-----------+---------+----------------+------------
Distances from | Densities | Density | Ratios adjusted|
center in terms| under | ratios. | to stress | Deduced
of radius. | Laplace´s | | differences. | rigidities.
| Law. | | (1:8) |
---------------+-----------+---------+----------------+------------
1.00 | 2.8 | 1 | 1 | 0.16 Steel
.75 | 5.7 | 2 | 3.5 | 0.58 „
.50 | 8.39 | 3 | 5.4 | 0.90 „
.25 | 10.27 | 3.7 | 7 | 1.16 „
.00 | 10.95 | 3.9 | 8 | 1.33 „
The average rigidity is here also much less than that of steel, but its
distribution is such as to render it ideally fitted to resist tidal
distortion.
These hypothetical distributions of rigidity have no claims to special
value in themselves, for the grounds on which they are based are quite
inadequate, but they are not without importance in giving tangible form
to considerations that bear vitally not only on tidal problems, but on
many others connected with the internal constitution and dynamics of
the earth.
_Sphericity as a factor in deformation._
It is obvious that if the earth shrinks, its crust must become too
large for the reduced spheroid, and must be compressed or distorted
to fit the new form. The amount of distortion required for any given
shrinkage is easily computed from the ratio of the radius to the
circumference of a sphere, which is approximately 1 : 6.28. If, for
example, the radius shortens 5 miles, each great circle must on the
average be compressed, wrinkled, or otherwise distorted to the extent
of about 31 miles, or, in reversed application, if the mountain
foldings on any great circle together show a shortening of 100 miles,
the appropriate radial shortening is 16 miles. The ratio of 1 : 6+
furnishes a convenient check on hypotheses that assign specific thrusts
to specific sinkings of adjacent segments. A segment 3000 miles across,
for example, such as the bottom of the North Atlantic basin, sinking
three miles, about the full depth of the basin, would give a lateral
thrust of about 2.2 miles, a little over a mile on each side, a
trivial amount compared with the foldings on the adjacent continental
borders.
=The influence of the domed form of the surface.=—Because of the
spheroidal form of the earth, each portion of the crust is ideally an
arch or dome. When broad areas like the continents are considered, it
is the dome rather than the arch that is involved, and in this the
thrust is ideally toward all parts of the periphery. It is probably for
this reason that mountain ranges so often follow curved or angulated
lines, or outline rude triangles or polygons. The sigmoidal courses
of the ranges of southern Europe, the looped chains of the eastern
border of Asia, and the curved ranges of the Antillean region, are
notable examples. The border ranges of the Americas, of the Thibetan
plateau, and of other great segments, illustrate the polygonal
tendency. The general distribution of the great ranges is such that a
nearly equal portion of crustal crumpling is thrown across each great
circle, as theory demands. The common generalization that mountain
ranges run chiefly in oblique directions, as northeast-southwest,
northwest-southeast, is but a partial view of the more general fact
that the lines of distortion must lie in all directions to accommodate
the old crust to the new geoid, if there be equable contraction in all
parts.
=Theoretical strength of domes of earth-dimensions.=—As the domed form
of the crust has played an important part in theories of deformation,
it is important to form quantitative conceptions of the strength of
ideal domes having the figure and dimensions of segments of the earth’s
crust. According to Hoskins,[271] a dome corresponding perfectly to the
sphericity of the earth, formed of firm crystalline rock of the high
crushing strength of 25,000 pounds to the square inch, and having a
weight of 180 pounds to the cubic foot, would, if unsupported below,
sustain _only ¹⁄₅₂₅ of its own weight_.[272] This result is essentially
independent of the extent of the dome, and also of its thickness,
provided the former is continental and the latter does not exceed a
small fraction of the earth’s radius. If this ideal case be modified
by supposing the central part of the spherical dome to rise above the
average surface, the supporting power will not be materially changed
unless the central elevation is a considerable fraction of the radius
of the dome. Assuming a central elevation of two miles—to represent
the protrusion of the continental segments—the results for domes of
different horizontal extent are as follows:[273]
THEORETICAL STRENGTH OF IDEAL DOMES ARCHED TWO MILES ABOVE
THE AVERAGE SURFACE OF THE SPHERE.
+--------------+---------------------+----------------------+
| Diameter of | Multiplier of ¹⁄₅₂₅ | Proportion of its own|
| given dome | i.e. the supporting | weight sustained by |
|arched 2 miles| proportion of a | given dome arched |
| above sphere.| spherical dome. | 2 miles above sphere.|
+--------------+---------------------+----------------------+
| 3,000 miles | 1.006 | 1/522 |
| 400 ” | 1.396 | 1/376 |
| 240 ” | 2.11 | 1/249 |
| 160 ” | 3.49 | 1/150 |
| 80 ” | 10.97 | 1/48 |
+--------------+---------------------+----------------------+
From this table it will be seen that for domes of continental
dimensions the supporting strength _equals only a very small fraction
of the dome’s own weight_. Increasing the thickness of the shell
increases its actual supporting power, but the proportion is somewhat
less when the whole sphere is concerned. The problem has not been
worked out for domes of limited extent. For rough estimates, where
the dimensions of the dome are of continental magnitude, each mile of
thickness may be taken as supporting a layer of about 10 feet of its
own material. If the hypothetical level of no stress be placed at 8
miles depth, the shell above this, by reason of its domed shape, could
relieve its own pressure on that below to an amount equal only to the
weight of about 80 feet of rock over its surface, even if its form and
structure were ideal. If the shell were thick enough (817 miles) to
embrace one-half the volume of the earth, its supporting power would
be a little more than the weight of one and one-half miles of rock. As
the radius of the earth is less than 4000 miles, the extreme supporting
power reckoned on this basis would be only about 8 miles of rock-depth.
It is interesting, if not significant, to observe that this depth
barely reaches the minimum shrinkage that will serve, according to
current estimates, to account for the crustal shortening of the great
mountain-making periods. It is as if the shrinkage stresses accumulated
to the full extent of the stress-resisting power of the whole sphere,
and then collapsed. It is not safe, however, to give much weight to
this coincidence, for higher densities and probably higher resistances
to distortion come into play in the deeper horizons. If these
resistances are proportional to the higher densities of the interior,
the deductions would remain the same. If the effective rigidity of the
earth as a whole is that of steel, as deduced by Kelvin and Darwin
from tidal and other observations, or twice that of steel, as inferred
by Milne from the transmission of seismic vibrations, the supporting
power of the body of the earth dependent on its sphericity would be
appreciably higher.
It would seem clear from the foregoing considerations that something
more than the mere crust of the earth has been involved in the great
deformations. Indeed it is not clear that the fullest resources of
stress-accumulation which the spheroidal form of the earth affords are
sufficient to meet the demands of the problem, unless the rigidity of
the earth be taken at a much higher value than that of surface-rock,
and this is perhaps an additional argument for the high rigidities
inferred from tides and seismic waves.
In view of the doubtful competency of even the thickest segments to
accumulate the requisite stresses, there is need to consider modes
of differential stress-accumulation other than those dependent on
sphericity.
=Stress-accumulation independent of sphericity.=—The principle of the
dome is brought into play whenever an interior shell shrinks away, or
tends to shrink away, from an outer one which does not shrink. In this
case, there is a free outer surface and a more or less unsupported
under surface toward which motion is possible. The dome may, therefore,
yield by crushing or by contortion. The computations given above are
for cases of this kind. But where the thickness becomes great and
the dome involves a large part or even all of a sector of the earth,
freedom of motion beneath is small, and to readjust the matter to a new
form, strains must be developed widely throughout the sector, and must
involve regions where the pressure is extremely great on all sides,
and crushing in the usual sense impossible. Assuming the correctness
of the modern doctrine that such pressure increases rigidity, instead
of the older doctrine that it gives plasticity, it becomes reasonable
to assume that stress-differences would be distributed throughout the
mass, and bring into play a large portion of its stress-accumulating
competency. When the mass yielded, it would not be by crushing, but by
“flowage,” which would be more or less general throughout the mass.
It might, however, be partially concentrated, as, for example, on the
borders of sectors of different specific gravity.
Stress-differences may arise from physical changes within the rock
itself. Whenever there is a re-aggregation of matter, or a change
of any kind which involves change of volume, a change of stress
is liable to be involved. It may be of the nature of relief or of
intensification. In an earth built up by the haphazard infall of
matter, a very heterogeneous mass must result, and the subsequent
changes may be supposed to be intimately distributed through the
mass, being slight at any point, but present at innumerable points.
An immeasurable number of small stress-differences may, therefore, be
developed throughout the mass. Until these overmatch the effective
strength of the mass, they may continue to accumulate. These are not
necessarily connected with stresses that arise from sphericity, and may
work more or less independently of them. It is not improbable that the
great stress-accumulating power of the globe finds an essential part of
its explanation in supplemental considerations of this kind, and not
wholly in its spheroidal form.
=The actual configuration of the surface.=—The foregoing computations
relative to the power of shells of the earth to sustain pressures
are based on ideal forms and structures that are not realized in
fact. How far the earth fails to conform to these conditions must
now be considered. When compared with the earth as a whole, the
inequalities of its surface are trivial. If the great dynamic forces
acted through the whole or the larger part of the body of the earth,
the configuration of the surface can be supposed to have done little
more than influence the location of the surface deformations and their
special phases. But if the forces were limited to a crust of moderate
thickness, the configuration of the surface is a matter of radical
importance.
=Concave tracts.=—There is need, therefore, to inquire if any
considerable breadth of the crust is outwardly plane or concave, for
the principle of the dome is obviously not applicable to a plane or
concave surface. To be a source of fatal weakness, the concavity must
be broad enough to cause the planes of equal cooling, the isogeotherms,
to be concave to considerable depths. For example, if the hypothetical
level of no stress is eight miles below the surface, as computed on
certain assumptions, the concave portion must be so broad that the
isogeotherms will also be concave outward at something near that
depth; in other words, the main part of the zone of thrust must be
concave. A narrow concavity at the surface, such as an ordinary valley
in a portion of the crust that has the average convexity, would not
seriously depress the isogeotherms, or affect the zone of thrust, but
a valley several times eight miles (level of no stress) in breadth
would. For inspecting the surface of the earth in this regard, it is
convenient to know what amounts of fall below the level surface give
a true plane for given distances. These are shown in the following
table:[274]
+--------+---------------------------+----------------+
| | Length of normal to chord | Average fall of|
| Length | at middle point in | true plane from|
| of arc |------------+--------------+ level plane per|
| in | | | mile, in feet.|
| miles. | Feet. | Fathoms. | Greater fall |
| | | |gives concavity.|
+--------+---------------------------+----------------+
| 25 | 100.3 | 16.7 | 8. |
| 50 | 432. | 72. | 17.3 |
| 75 | 913.4 | 152.2 | 24.3 |
| 100 | 1,684. | 280.7 | 33.7 |
| 150 | 3,748.8 | 624.8 | 49.9 |
| 200 | 6,674. | 1,112.3 | 66.7 |
| 250 | 10,369.9 | 1,728.3 | 82.9 |
| 300 | 14,942. | 2,490.3 | 99.6 |
| 400 | 26,664. | 4,444. | 133.3 |
| 500 | 41,659. | 6,943. | 166.6 |
+--------+---------------------------+----------------+
Applying these criteria to the surface of the lithosphere, _it is found
that concave tracts from 100 to 300 miles in breadth are not uncommon_.
The more notable of these are shown in black on the accompanying map,
Fig. 454, and two typical ones are shown in cross-section in Figs.
455 and 456. It is to be observed that concave tracts border the
continents very generally. They are connected with the descent from
the continental shelf to the abysmal basins, and are unsymmetrical.
Notable concavities are found in some of the great valleys on the
continental platforms. The basins of Lake Superior, Michigan, Huron,
and Ontario are in part concave; so are Puget Sound, the Adriatic,
and the Dead Sea; so also are the valleys of California, of the Po,
and of the Ganges, when the adjacent mountains are included. Some of
the “deeps” of the bottom of the ocean are notably concave. Fig.
455, a cross-section of the Challenger Deep, _drawn to true scale and
convexity_, shows the nature of the phenomenon. The breadth is here
300 miles, and the depression below a true plane is 11,400 feet. The
lower line of the figure shows the approximate position and form of the
normal isogeotherm about ten miles below the surface. Assuming equal
conductivity in all parts, it is clear that the isogeotherms must be
concave upwards for a considerable distance below ten miles. Unless
the shell of thrust is much more than ten miles thick, these concave
portions should yield as fast as cooling below them permits, and _no
stresses arising from convexity could be accumulated_.
[Illustration: +Fig.+ 454.—Map of the world, showing in black the chief
submarine concavities of the lithosphere. (Prepared by W. H. Emmons.)]
[Illustration: +Fig.+ 455.—Section of the Challenger Deep from an
island on the Caroline plateau, _a_, to an island on the Ladrone
plateau, _b_, _drawn to a true scale_, showing the real concavity
of the surface of the lithosphere for a breadth of 300 miles. The
upper line represents the sea surface, a natural level. The next
line below represents a true plane, eliminating the curvature of the
sea surface. The third line represents the bottom of the deep. By
comparison with the line above, its true concavity may be seen. The
lowest line represents an isogeotherm at about 10 miles below the
surface; i.e. appreciably below “the level of no stress,” as usually
computed, showing that the whole thrust zone is concave outwards, if
it is limited to surface cooling as usually computed. (Prepared by W.
H. Emmons.)]
[Illustration: +Fig.+ 456.—Section through the Atlantic coastal plain,
the continental shelf, and a portion of the abysmal bottom, _drawn
to a true scale_, showing that the surface of the lithosphere
drops below a true plane tangent to the continental shelf and the
ocean-bottom. The upper line represents the surface of the coastal
plain at the left and of the ocean at the right. The lower line
represents the sea-bottom, and the middle line a true plane tangent
to the shelf and the sea-bottom. The breadth of the concave tract
varies from 100 to 150 miles. (Prepared by W. H. Emmons.)]
These concavities of surface are so extensive and so widely distributed
over the globe that _no part of the outer shell can be supposed to
be capable of accumulating notable stresses_ unless rigidly attached
to the earth-body below. In other words, so far as sphericity is
concerned, the crust must ease all its stresses nearly as fast as they
accumulate, if, as usually assumed, it rests on a contracting or mobile
substratum.
Surface cooling under these conditions should give only feeble thrusts,
developed and eased nearly constantly. Such movements should be
admirably adapted to give those gentle, nearly constant subsidences
that furnish the nice adjustments of water-depth required for the
accumulation of thick strata in shallow water, and those slow upward
warpings that renew the feeding-grounds of erosion, the necessary
complement of the deposition. These gentle, nearly constant movements
mark every stage of geological history, and constitute one of its
greatest though least obtrusive features. _But if superficial stresses
arising in this way are eased in producing these effects, they cannot
accumulate to cause the great periodic movements._
Even where the crust is not concave, it is so warped and so traversed
by folds and fault-planes that its resistance to thrust is relatively
low, and it should, therefore, warp easily and at many points, if the
thrust be confined to a superficial crust.
=General conclusion.=—When to the weakness of the crust, as computed
under ideal conditions, there is added the weakness inherent in these
concave and warped tracts, the conclusion seems imperative that while
the crust is the pliant subject of minor and nearly constant warpings,
such as are everywhere implied in the stratigraphic series, it is
wholly incompetent to be the medium of those great deformations
which occur at long intervals and mark off the great eras of geologic
history. These great deformations apparently involve the whole, or a
large part, of the body of the earth, and seem to require a very high
state of effective rigidity.
=General references on crustal movements.=—Babbage, Jour. Geol.
Soc., Vol. III (1834), p. 206; Lyell, Principles of Geology, Vol.
II, p. 235; Mallet, Phil. Trans. (1873), p. 205; Reade, Origin of
Mountain Ranges, and Evolution of Earth Structure; Fisher, Physics
of the Earth’s Crust; Dutton, Greater Problems of Physical Geology,
Bull. Phil. Soc. of Washington, Vol. XI, p. 52, also Amer. Jour. of
Sci., Vol. VIII (1874), p. 121, and Geology of the High Plateaus
of Utah (1880); Jamieson, Quar. Jour. Geol. Soc. (1882), and Geol.
Mag. (1882), pp. 400 and 526; Heim, Mechanismus der Gebirgsbildung;
Marjerie and Heim, Les Dislocations de l’Écorce terrestre (1888);
Shaler, Proc. Boston Soc. Nat. Hist., Vol. XVII, p. 288; Dana,
Manual of Geol., 4th ed., p. 345 et seq.; Woodward, Mathematical
Theories of the Earth, Smithsonian Rept. for 1890, p. 196; Willis,
The Mechanics of the Appalachian Structures, 13th Ann. Rept. U.
S. Geol. Surv., Pt. II (1893), pp. 211–282; LeConte, Theories of
Mountain Origin, Jour. Geol. Vol. I (1893), p. 542; Gilbert, Jour.
Geol., Vol. III (1895), p. 333, and Bull. Phil. Soc. of Washington,
Vol. XIII (1895), p. 31; Van Hise, Earth Movements, Trans. Wis.
Acad. Sci., Arts and Let., Vol. II (1898), pp. 512–514; Estimates
and Causes of Crustal Shortening, Jour. Geol., Vol. VI (1898), pp.
29–31; Relations of Rock Flowage to Mountain Making, Mon. XLVII,
U. S. Geol. Surv. (1904), pp. 924–931; A. Geikie, Text-book of
Geology, 4th ed., pp. 672–702.
CHAPTER X.
THE EXTRUSIVE PROCESSES.
=Outward movements.=—In the preceding chapters movements toward the
center have been considered. The complementary processes of outward
movement now invite attention. Without doubt these are mainly but a
resultant of the centripetal actions. For each pound of material moved
outwards an equivalent is quite surely moved inwards. Notwithstanding
this, the outward movements have a peculiar nature of their own, and
serve a function of radical importance in the economy of the globe.
Some minor phases have been incidentally considered, such as the upward
flow of springs and deep-seated waters, but here the descending and
ascending factors are alike, and are closely and obviously connected.
VULCANISM.
The great example of ascensive action is the movement of fluid rock
from the interior outwards. The term vulcanism will be used to embrace
not only volcanic phenomena in the narrower sense, but all outward
forcing of molten material, whether strictly extrusive or merely
ascensive.
The philosophy of this ascensive action, taken as a whole, is simple.
In the effort at concentration under the powerful action of the
earth’s gravity, the material of high specific gravity is urged more
strongly toward the center, volume for volume, than that of less
specific gravity, and as gravity is perpetually active, it follows that
whenever any movement, molecular or molar, takes place which permits a
readjustment of the positions of the two kinds of matter, the heavier
sinks toward the center and the lighter rises, or at least tends to
do so. So also where there are stress-differences, the mobile matter
tends to flow from the regions of greater stress toward those of lesser
stress. In so far as any portion of the interior becomes liquid, it
is free to move up or down according to the balance of stress brought
to bear upon it, and adapts itself to any line of least resistance
available to it. As a natural result, therefore, the portion of the
interior which becomes fluid most largely participates in the outward
movement. In so far as molecular action permits a readjustment of
material, there is a tendency, even in the solid state, for the lighter
material to move upwards and the heavier downwards, and for the more
stressed portions to move toward points of less stress; but this takes
place with extreme slowness. In so far as the materials of the interior
diffuse themselves through each other, the same laws hold good, but
they are modified by the special principles that control diffusion.
The outward diffusion of interior gases may be a factor of appreciable
importance, but this cannot be affirmed at present.
=Phases of vulcanism.=—The forcing of fluid rock outward assumes two
general phases, which, however, merge into each other; and these
main phases take on various sub-phases. The first phase embraces
those outward movements of fluid rock which do not reach the surface.
The lavas, after ascending to the vicinity of the surface, intrude
themselves into the outer formations of the earth and congeal
underground (plutonic). The second phase embraces those outward
movements in which the fluid rock reaches the surface and gives rise
to eruptive phenomena (volcanic). The first is _intrusive_, the
second _extrusive_; the first constitutes _irruptions_, the second
_eruptions_.[275] The fundamental nature of the two is the same, but
the extrusions usually take on special phases because of the relief
of pressure at the surface of the earth, and because of the action of
surface-waters in contact with the heated lavas. Just where the lavas
come from, and how they find their way through the deep-lying compact
zone below the zone of fracture, may better be considered later. When
they reach the zone of fracture, they usually either take advantage
of fissures already formed, or force passageways for themselves by
fracture. There is little evidence that they bore their way through
the rocks by melting, though they appear to round out their channels
in some way into pipes, ducts, and other tubular forms when they flow
through them for long periods of time.
1. _Intrusions._
Fluid rock forced into fissures and solidified there forms _dikes_;
forced into chimney-like passages, it forms _pipes_ or _plugs_;
insinuated between beds, it forms _sills_; bunched under strata
so as to arch them upwards, it forms _laccoliths_; massed in great
aggregations underground, it constitutes _batholiths_, as already
described (pp. 394 and 500). Lavas sometimes crowd aside the adjacent
rocks so far as to cause them to take a concentric form about the
intruded mass. This is not uncommon in the oldest formations, and is
probably not infrequent in the deeper horizons where the pressures
are very great. Some part of this may, however, be due to later
deformations. Nearer the surface, usually, the beds are merely lifted
as in forming the sills, or are bowed upwards, as in the laccoliths, or
faulted as in _bysmaliths_ (p. 500).
=The heating action= on the adjacent rock varies greatly with the
mass and temperature of the intruded lava. Thin dikes and sills
often produce little effect, while greater and hotter masses notably
metamorphose the adjacent rock. In some cases marked effects are due
to a thin stream of lava flowing through a fissure for a long period,
and so maintaining a high temperature. In the least effective cases,
the adjacent rock usually shows some signs of baking. In the marked
cases, there is more or less new crystallization. The surrounding rock
commonly shows some evidence of material derived from the lavas; less
often the lava shows some evidence of having received material from
the adjacent rock. But since the lavas do not usually bore their way
through the strata in the zone of fracture, nor melt the adjacent rock,
the constitution of the lavas is not appreciably changed by the kinds
of rock which they penetrate. On the other hand, the intrusions often
show the effects of rather rapid cooling by contact with the adjacent
rock, (_a_) by a less coarse crystallization near the rock-walls, and
sometimes (_b_) in a segregation of the material.
2. _Extrusions._
When molten rock is forced to the surface it gives rise to the most
intense and impressive of all geological phenomena. The energies
acquired in the interior under great compression here find sudden
relief. Occluded gases often expand with extreme violence, hurling
portions of the lavas to great heights and shattering them into
fragments constituting “smoke,” ash, cinders, bombs, and other
pyroclastic material. Much of the explosive violence of volcanoes
has been attributed to the contact of surface-waters with the hot
rising lava, but the function of this kind of action has probably been
exaggerated.
There are two phases of extrusion often quite strongly contrasted.
The one is explosive ejection, often attended with great violence;
the other, a quiet out-welling of the lava, with little more than
ebullition. More or less closely related to these differences are two
classes of conduits, (_a_) the one, great fissures, out of which the
lava pours in great volume and spreads forth over wide tracts, often in
broad thin sheets; (_b_) the other, restricted openings, often pipes,
ducts, or limited fissures, from which the extrusion is usually much
less abundant, and hence it more largely congeals near the orifice,
forming cones. Flows from the former constitute massive eruptions;
those from the latter, the more familiar volcanic eruptions. There is
no radical difference between them, and the two classes blend. The
extent of the spreading of lava into thin sheets is due more to the
mass and the fluidity than to the form of the outlet. The stupendous
outflows of certain geologic periods appear to have issued mainly from
extended fissures, doubtless because these better accommodated the
outbursting floods.
=_a._ Fissure eruptions.=—The chief known fissure eruptions of recent
times are the vast basaltic floods of Iceland. Most of the eruptions
of historic times are of the volcanic type; but at certain times in
the past there were prodigious outpourings, flow following flow until
layers thousands of feet thick covering thousands of square miles were
built up. One of these occurred in Tertiary times in Idaho, Oregon, and
Washington, where some 200,000 square miles were covered with sheets
of lava, aggregating in places 2000 feet or more in thickness. Earlier
than this, in Cretaceous times, there were enormous flows on the Deccan
plateau of India, covering a like area to a depth of 4000 to 6000 feet.
Still earlier than this, in Keweenawan times, an even more prolonged
succession of lava-flows covered nearly all the area of the Lake
Superior basin, and extended beyond it, and built up a series of almost
incredible thickness, the estimates reaching 15,000 to 25,000 feet.
In these cases there is little evidence of explosive or other violent
action. There are few beds of ash, cinders, and similar pyroclastic
material. The inference is, therefore, that the lavas welled out rather
quietly and spread themselves rather fluently over the surrounding
country. For the most part these wide-spreading flows are composed of
basic material, which is more easily fusible and more highly fluent at
a given temperature than the acidic lavas. The latter are more disposed
to form thick embossments near the point of extrusion.
Massive outflows of this class constitute by far the greatest phenomena
of the extrusive type, though they are not now the dominant type.
It has been sometimes thought that the more local volcanic type of
extrusion followed the more massive fissure type as a phase of decline;
but this has not been substantiated.
[Illustration: +Fig.+ 457.—Lava-flow near the Jordan craters, Malheur
Co., Oregon. Though not of the gigantic order, it illustrates the
general aspect of massive lava-flows. (Russell, U. S. Geol. Surv.)]
=_b._ Volcanic eruptions.=—In the types of eruption prevailing at
the present time, the lavas are forced out through ducts or perhaps
short fissures or sections of fissures, and build up cones about the
vents, the eruptive action maintaining craters in the centers of the
cones. The essential feature of a volcano is the issuance of hot rock
and gas from a local vent. A mountain is the usual result, but the
mountain is secondary and not usually present in the first stages; the
localized eruption is the primary and necessary factor. The amount of
rock matter ejected is not necessarily great. Compared to the massive
extrusions of fissure eruptions, it is usually rather trivial; but the
volcano makes up in demonstrativeness what it lacks in massiveness of
product.
[Illustration: +Fig.+ 458.—The volcano Colima, Mexico, in eruption.
March 24, 1903. (José Maria Arreola, per Frederick Starr.)]
_c._ =Intermediate phenomena.=—On the border-line between the intrusive
and the extrusive phenomena there are special cases of interest. There
appear to be certain instances in which the intrusion comes so near
the surface as to develop explosive phenomena without the extrusion
of lava. From the nature of the case this is an interpretation rather
than a demonstration. It is certain, however, that occasional violent
explosions take place where no lava comes in sight. This sometimes
occurs in old volcanic formations, and sometimes in regions of
undisturbed horizontal strata. In the former case the phenomena may be
due to the intrusion of a fresh tongue of lava below, or it may be due
to the penetration of surface-waters to hot rocks that have remained
uncooled from previous volcanic action, and the development, by such
contact, of a volume of confined steam sufficient to produce the
explosion. A case of this doubtful kind occurred at Bandai-San in Japan
in 1888, where there was a sudden and violent explosion which blew away
a considerable part of the side of a volcanic mountain which had not
been in eruption for at least a thousand years. The mass and violence
of the exploded material was such as to fill the air with ashes and
débris in a fashion altogether similar to a typical volcanic eruption.
A large tract of adjacent country was devastated, and many lives lost.
The whole action, however, was concentrated in the initial explosion,
and within a few hours the cloud of ashes had disappeared and the
phenomenon was ended. An examination of the disrupted area revealed no
signs of liquid lava.
An example of the latter class is Coon Butte in Arizona.[276] This
consists of a rim of fragmental material encircling a crater-like pit
from which the fragments were obviously forced by violent explosion.
The pit is in ordinary sedimentary strata, and the material of the rim
is composed of the disrupted fragments of the sedimentary rock ejected
from the pit. There are no signs of igneous material, but there was
igneous action in the vicinity. Fragments of a meteorite were found
on the rim and in the vicinity, but this association appears to be
accidental. Computation shows that the volume of the material of the
rim closely matches the size of the pit. The source of the explosion
is not demonstrable, and it may be an error to connect it with an
intrusion of lava below; but since intrusions rise to various degrees
of nearness to the surface, and in innumerable cases reach the surface,
there is every reason to entertain the conception of a class of
intrusions which develop explosive phenomena by close approach to the
surface, without actually reaching it.
[Illustration: +Fig.+ 459.—Photograph of a portion of the moon taken at
Lick Observatory.]
=Lunar craters.=—There are grounds for thinking that the remarkable
craters of the moon, assuming that they are truly volcanic,[277] may
belong to this class, for they are very similar to the Coon Butte
pit. The capacities of the lunar craters, so far as they can be
estimated, seem to equal, if they do not in many cases exceed, the
volume of matter in their rims. They do not appear usually to be
great cones of accumulated material with relatively small craters,
like the typical products of terrestrial volcanoes. Besides, there
are no clear evidences of lava-streams. The radiating tracts once
interpreted as such have been shown by increased telescopic power
and the resources of photography to be at least something other than
lava-streams. They are vaguely defined tracts which run over heights
and depths indifferently, and are plausibly interpreted as lines of
débris projected to extraordinary distances because of the absence of a
lunar atmosphere, and because of the low force of the moon’s gravity.
Since the moon now has no appreciable atmosphere or surface-waters, and
since it is doubtful whether it ever possessed either on account of its
probable inability to hold atmospheric gases or the vapor of water in
the form of an envelope about it, owing to its low gravity, there is
reason to suppose that the external matter of the moon derived from the
explosions of the multitude of lunar volcanoes would remain in a loose,
incoherent condition, from the absence of dissolving and cementing
agencies. It is reasonable to suppose that lava-tongues arising from
the deeper interior would have a higher specific gravity, even in their
heated condition, than this porous covering of the moon, and that
therefore they would almost universally become intrusions rather than
extrusions, or at most they would not rise beyond the bottom of the
craters they had produced by explosion. This seems to furnish at least
a plausible explanation of the prevailing differences between the large
lunar craters encircled by mere rims and the much smaller terrestrial
craters seated in relatively large cones.
VOLCANOES.
=Number of volcanoes.=—It is impracticable to state exactly the
number of volcanoes that are active at the present time, because most
volcanoes are periodic, and become active at more or less distant
periods, and it is impossible to say whether a given volcano that may
be now quiescent has really become extinct or is only enjoying its
customary period of rest. It is quite safe to include at least 300 in
the active list, and the number may reach 350 or more. The numbers that
have been active so recently that their cones have not been entirely
worn away is several times as great.
_Distribution of Volcanoes._
1. =In time.=—In the earliest known ages igneous action appears to
have been very general, if not practically universal. No area of the
earliest (Archean) rocks is now known which is not formed chiefly of
rocks that appear to have been either intruded or extruded. Rocks which
can reasonably be assigned to the hypothetical molten globe, if there
be such, are not here included. It is probable that the surface of the
early earth was as thickly occupied with points of extrusion as the
surface of the moon appears to be. In the ages between the Archean and
the present, the distribution of volcanic action over the surface seems
to have been in a general way much what it is to-day; that is, certain
areas were volcanically active at times, while other and larger areas
were measurably free from any outward expressions of igneous action.
This is not equally true of all ages, as will be seen in the historical
studies that follow. There were periods when volcanic activity seems
to have been widespread and energetic, and others when it was limited
both in amount and distribution. The known facts do not indicate a
steady decline in volcanic activity, but rather a periodicity; at
least this is so for the portion of the globe that is now well enough
known geologically to warrant conclusions. One of the greatest of the
volcanic periods falls within the Cenozoic era, just preceding the
present geological period, and the volcanic activity of the present is
perhaps but a declining phase of that time.
2. =Relative to land and sea.=—At present the active volcanoes are
chiefly distributed about the borders of the continents, and, less
notably, within the great oceanic basins. On this account the sea has
often been supposed to have some connection with volcanic action, and
the presence of chlorine in the volcanic emanations has been cited
in support of this position. When critically examined, however, the
argument from distribution is not very strong; for the volcanoes are
not distributed equally or proportionately about the several oceans,
as if dependent on them. Volcanoes are especially numerous around and
within the Pacific, the greatest of the oceans, and this might seem a
favorable instance, but they are also numerous around and within the
Mediterranean, a relatively small body of water. Volcanoes are not
especially abundant in or about the margins of the Atlantic.
[Illustration: +Fig.+ 460.—Volcanoes in the Pacific. Jones Relief
Globe. (Photo. by R. T. Chamberlin.)]
If volcanoes were dependent upon proximity to the sea, the relation
should be close in the past as well as in the present, but this does
not seem to be true. There has recently been much volcanic activity
in the plateau region of western America at long distances from the
Pacific basin. Even on the plains east of the Rocky Mountains notable
volcanic action took place. There were also volcanoes in the interior
of Asia and of Africa.
3. =Relative to crustal deformations.=—The distribution of present
and recent volcanoes is much more suggestively associated with those
portions of the crust _that have undergone notable changes_ in position
in comparatively recent times. The great “world-ridge” stretching from
Cape Horn to Alaska and thence onwards along the east coast of Asia
is a striking instance, for it is dotted throughout with active and
recently extinct volcanoes. The tortuous zone of mountainous wrinkles
that borders the Mediterranean and stretches thence eastward to the
Polynesian Islands is another notable volcanic tract. These two belts
include the greater number of existing and recent volcanoes on the
land, while the great basins associated with them embrace the chief
oceanic volcanoes.
[Illustration: +Fig.+ 461.—Active volcanic area at the junction of the
continental segments of North and South America, and of the abysmal
segments of the Atlantic and Pacific. Jones Relief Globe. (Photo. by
R. T. Chamberlin.)]
There is perhaps some significance in the fact that the most active
regions of vulcanism to-day lie _at the angular junctions of the great
earth-segments_. The Antillean and Central American volcanic region,
that has recently been so demonstrative, lies where the southern angle
of the North American continental block joins the northern angle of
the South American continental block, and where the western angle
of the North Atlantic abysmal segment closely approaches one of the
eastern angles of the great Pacific abysmal segment. The complex and
very active Java-Philippine volcanic region lies where the southeastern
angle of the great Asian segment projects toward the Australian block,
and where the western angle of the Pacific block approaches the
northeastern angle of the Indian oceanic segment. The active Alaskan
volcanic area lies at the angles of the North American, Asian, Pacific,
and Arctic segments. The Mediterranean volcanic area falls less notably
under this generalization, but it lies where the continental blocks of
Europe and Africa come into peculiar relations to each other on either
side of the remarkable Mediterranean trough. The eastern angle of the
North Atlantic segment is near by, but not in very close relations.
The Icelandic region, small but vigorous, lies near the junction of
the North American, European, North Atlantic, and Arctic segments,
and the New Zealand volcanic region is somewhat less closely related
to the approach of the Australian, Antarctic, Pacific, and Southern
oceanic segments. Nearly all of these angular conjunctions involve
two depressed segments joining two relatively elevated segments. This
relationship suggests a causal connection between the intensified
movements at these angular conjunctions and the intensified volcanic
action of these regions. There are enough volcanoes, however, that do
not fall into these groups, or apparently into any other grouping, to
suggest that the development of volcanoes is not wholly dependent on
any surface relationship, but that it is connected with deep-seated
causes that are indeed modified, but not wholly controlled, by surface
conditions, or even by the movements of the master segments of the
earth’s crust.
[Illustration: +Fig.+ 462.—Active volcanic area at the junction of the
continental segments of Asia and Australia, and the abysmal segments
of the Pacific and Indian oceans. Jones Relief Globe. (Photo. by R.
T. Chamberlin.)]
4. =In latitude.=—The distribution of volcanoes appears to have no
specific relation to latitude. Mounts Erebus and Terror, amid the
ice-mantle of Antarctica, and Mount Hecla in Iceland, as well as
the numerous volcanoes of the Aleutian chain, give no ground for
supposing that volcanoes shun the frigid zones. On the other hand,
the numerous volcanoes of the equatorial zone do not imply that they
avoid the torrid belt. Their distribution appears to be independent of
latitude. This is not cited because of any supposed effects of external
temperature, for that must be trivial, but because it bears on the
question whether strains are now arising from the supposed _slackening
of the earth’s rotation_, which have any connection with volcanic
action. If the oblateness of the earth is decreasing, the equatorial
belt must be sinking and growing shorter, and hence must be under
lateral pressure, while the polar caps must be rising, and increasing
their curvatures, and should be under tension. These conditions, if
real, might be supposed to have something to do with the extrusion of
lava. Nothing in the present or the past distribution of igneous action
seems to afford much support to this hypothetical inference.
5. =In curved lines.=—In the Antilles, the Aleutian Islands, the
Kurile Islands, and in other instances, there is a notable linear
arrangement of volcanoes with appreciable curvature. It has been noted
that the convexity of the curves is turned toward the adjacent ocean.
In some cases, however, there is a notable linear arrangement without
appreciable curvature, as in the Hawaiian range, in the recently
extinct line of cones of the Cascade Range, and in others. Less often,
volcanoes are bunched irregularly, as in some of the groups of volcanic
islands of the Pacific (Fig. 460).
_Relations of Volcanoes._
1. =Relations to rising and sinking surfaces.=—So far as observations
cover this point, the area immediately adjacent to active volcanoes
is rising (Dutton). This is shown by raised beaches, terraces, coral
deposits, etc. Whether this is wholly due to the expansional effect
of the heating of the subterrane by the rising lava, or whether it
has a wider significance, is not known. If a broader view is taken,
it does not appear that there are sufficient data to connect volcanic
action exclusively with either the rising or the sinking of the general
surface. It is certain that the great mountain ranges and plateaus in
which so much of the more recent volcanic action has taken place have
been recently elevated relatively, but they have also undergone more or
less of oscillation, involving some relative depression. The question
whether the Pacific basin as a whole has been relatively elevated or
depressed in modern times is a mooted one. Darwin[278] and Dana,[279]
as the result of their early studies on its coral deposits and on other
phenomena, concluded that the Pacific was a sinking area, but this view
has been recently challenged by Murray[280] and Agassiz[281] with at
least some measure of success. From the fiords on the borders of the
Pacific and other physical phenomena, the inference has been drawn that
relative sinking of the land has recently taken place. Raised beaches
on the coasts are interpreted as indicating a relative rise of the land
or a sinking of some ocean basin, for the withdrawal of the waters can
only be the result of increasing the capacity of the oceanic basin as
a whole. The most probable view is that the general areas of present
and recent volcanic action are partly rising areas and partly sinking
areas, and that movement of either kind may be connected with the
extrusion of the lavas. The rising and sinking are but complementary
phases of a deformation of the earth’s body, and involve a readjustment
of stresses within the body of the earth. These stresses are possibly
an essential factor in eruptions.
2. =Relations to one another.=—A most significant feature of volcanic
action is the degree of concurrence or of independence of action in
adjacent volcanoes. In some instances they act as though in sympathy,
as in the recent outburst in Martinique and Saint Vincent, and the
concurrent symptoms of activity in other places. On the other hand,
the independence of neighboring vents is sometimes extraordinary. The
group of volcanoes near the center of the Mediterranean, of which
Vesuvius and Etna are the most conspicuous examples, usually act with
measurable independence of one another, an eruption in the one not
being habitually coincident with an eruption in the others. But the
most conspicuous instance of independence is found in the great craters
of Mauna Loa and Kilauea in Hawaii. They are only about twenty miles
apart, the one on the top and the other on the side of the same great
mountain mass. The crater of Mauna Loa is about 10,000 feet higher
than the crater of Kilauea, and yet, while the latter has been in
constant activity as far back as its history is known, the former is
periodic. The case is the more remarkable because of the greatness of
the ejections. The outflow of Mauna Loa in 1885 formed a stream from
three to ten miles in width, and forty-five miles in length, with a
probable average thickness of 100 feet, and some of its other outflows
were of nearly equal greatness; indeed its outflows are among the
most massive that have issued from volcanoes in recent times. Besides
this massiveness there have been extraordinary movements of the lava
within the crater, if the testimony of witnesses may be trusted. But
throughout these great movements in the higher crater, the lava-column
of Kilauea, 10,000 feet lower, continued its quiet action without
sensible effects from its boisterous neighbor. The bearing of such
extraordinary independence upon the sources of volcanic action is very
cogent, for the lavas are of the same type, both being basalts, that of
Mauna Loa being notably basic and probably as high in specific gravity
as that in Kilauea. No difference in specific gravity that could at
all account for a difference in height of 10,000 feet can be presumed,
unless their ducts remain separate to extraordinary depths. Nor does
it appear possible that a superior amount of gas within the column of
Mauna Loa could account for such an extraordinary difference in height,
for the hydrostatic pressure of such a column is not far from 10,000
pounds to the square inch. Even if the difference in the heights of
the columns could be explained by differences in specific gravity,
the agitation of the one should be communicated to the other, and an
outflow of the one, particularly an outflow by a breakage through
its walls sufficient to lower its surface hundreds of feet, as has
repeatedly occurred in Kilauea, should change the surface of the other
proportionately, if they were in hydrostatic equilibrium. It seems
a necessary inference, therefore, that the two lava-columns have no
connection with each other or with a common reservoir. The tops of some
lava-columns stand about 20,000 feet above the sea, while others emerge
on the sea-bottom far below sea-level. The total vertical range is,
therefore, probably between 30,000 and 40,000 feet, a difference which
tells its own story as to their relative independence.
[Illustration: +Fig.+ 463.—Surface of lava-flow of 1881, from Mauna
Loa, as seen back of Hilo, Hawaii. (Photo. by Calvin.)]
3. =Unimportant coincidences.=—Eruptions seem to be somewhat more
liable to occur at times of high atmospheric pressure than at low,
doubtless because the increased atmospheric weight on a large area of
the adjacent crust aids in forcing out the lava or the volcanic gases.
This can only be effective when other forces have almost accomplished
the result, and would doubtless have completed it a little later had
not the atmospheric wave supplied the little remaining pressure needed.
Eruption seems also to be more common when the tidal strains favor it,
for like reasons. In the same class are probably to be put the effects
of heavy rains, whether they act by gravity or by giving rise to steam.
Such agencies are to be regarded as mere incidents of no moment in
the real causation of vulcanism, but of some value in determining the
precise moment of action. This is not to be understood as inconsistent
with the view that the periodic stresses of the body-tides of the
earth are important factors in vulcanism, as elsewhere explained, but
merely that the special time of surface-eruption is only incidentally
connected with the water-tides.
[Illustration: +Fig.+ 464.—Crater of Kilauea.]
=Periodicity.=—Most volcanoes are periodic in their stages of action.
Long dormant periods intervene between eruptive periods. Volcanoes
supposed to be extinct occasionally awaken with terrific violence.
Sometimes also they awaken quietly. This larger periodicity yet awaits
an explanation, but it very likely means a temporary exhaustion of the
supply of gas or of lava, or of both, to which the active stage is due.
_Formation of Cones._
=Lava-cones.=—The lava usually flows away from the vent in short
streams which solidify before running far. As the lava-streams flow in
different directions at different times, the total effect is a low cone
formed of radiating tongues surrounding the point of exit. Occasionally
the streams run a dozen or a score of miles, but such cases, except in
the gigantic volcanoes of Hawaii and a few others, are rare. Often the
streams congeal before they reach much beyond the base of the cone, and
quite often while they are yet on its slope. So far, therefore, as the
volcanic cone is formed of lava, it has a radiate structure made up of
a succession of congealed lava-streams. In these cases the slopes are
low, because the fluidity of the lava prevents the development of high
gradients. It is, however, rather the exception than the rule, that
the cone is made up mainly of lava-streams, though the great Hawaiian
volcanoes are of this class.
[Illustration: +Fig.+ 465.—Typical cinder-cone, Clayton valley, Cal.
(Turner, U. S. Geol. Surv.)]
=Cinder-cones.=—The larger portion of the lava blown into the air by
the expanding gas-bubbles falls back in the immediate vicinity of the
vent and builds up a cinder-cone. From the nature of the case, this
often takes on a beautiful symmetry and assumes a steep slope (Fig.
465). The ragged cinders lend themselves readily to the formation of
an acute cone, quite different from the flatter cone formed by lavas.
Sometimes the cinders are still plastic when they fall, and weld
themselves together and hold their places even on very steep slopes,
but usually they have already hardened before they reach the surface.
[Illustration: +Fig.+ 466.—Spatter-cone and cavern. Kilauea, Hawaii.
(Photo. by Libbey.)]
[Illustration: +Fig.+ 467.—Hollow spatter-cone. Oregon. (Russell, U. S.
Geol. Surv.)]
=Subordinate cones.=—Small or temporary vents formed as offshoots from
the main vents often give rise to secondary or “parasitic” cones.
These are sometimes numerous, as in the case of Etna, and they may be
so important that the mountain becomes a compound cone. A still more
subordinate variety consists of “spatter-cones” formed by small mildly
explosive vents that spatter forth little dabs of lava which form
chimneys, or cones, and sometimes completely curved domes over vents
(Figs. 466 and 467). Spatter-cones often arise from the lava-flows
themselves.
=Composite cones.=—From most existing volcanoes there issue both
lava-flows and fragmental ejecta, and the resulting cones are composite
in material. The lava more frequently breaks through the side of the
cone than overflows its summit, and this gives rise to irregularities
of form and structure. The cones are also subject to partial
destruction both by the outbursts of lava and by the explosions, and
perhaps also by migration of the vents. As a result, many volcanic
regions show old, partially destroyed craters, together with new and
more perfect ones, and the history of volcanic action in a region may
often be read in the succession of cone formations.
The form of the cone, when composed chiefly of lava, is also affected
by the mass of the outflow and by its fluidity. The larger the outflow
at a given time, other things being equal, the wider it distributes
itself and the flatter is the cone. As a rule, the basic lavas are more
fluid than the acidic, and the cones of basic lavas are flatter than
the cones of acidic lavas.
=Extra-cone distribution.=—In violent eruptions, the steam, accompanied
with much ash, is shot up to great heights, often rolling outwards
in cumulus or cauliflower-like forms (Fig. 458). In the more violent
explosions these columns are projected several miles. In the phenomenal
case of Krakatoa the projection was estimated at seventeen miles. The
steam, by reason of its great expansion and its contact with the colder
regions of the upper air, is quickly condensed, and prodigious floods
of rain frequently accompany the eruption. This rain, carrying down a
portion of the ash and gathering up much that had previously fallen,
gives rise to _mud-flows_, which in some cases constitute a large
part of the final deposit. These mud-flows chiefly lodge on the lower
slopes of the volcano or adjacent to its base, and give rise to rather
flat cones, sometimes designated as _tufa-cones_ to distinguish them
from cinder-cones formed by the direct fall of fragmental material.
Mud-flows appear also to be formed by the ejection of mud and water
that had gathered in quiescent craters during intervals between stages
of eruption.
A portion of the finer exploded material floats away in the air to
greater or less distances, and forms widespread _tufa-deposits_. In.
some cases beds of volcanic ash of appreciable thickness (as those
of Nebraska)[282] are found far from any known volcanic center. The
extremely fine ash from the great explosion of Krakatoa floated several
times around the earth in the equatorial belt and spread northward into
the temperate zones.
Illustration: +Fig.+ 468.—Mt. Shasta, a typical extinct cone, furrowed
by erosion, but retaining its general form. (Diller, U. S. Geol.
Surv.)]
LAVAS.
=Their nature.=—In the chapter on the Origin and Descent of Rocks, the
nature of lavas and of the rocks derived from them has been discussed
(Chapter VII). In view of prevalent misconceptions, it may be repeated,
for the sake of emphasis, that lavas are mutual _solutions_ of mineral
matter in mineral matter, rather than simply melted rock. Into this
mutual solution there enter not only rock materials, but gases. The
distinction between mutual solutions and simple molten rock cannot be
rigorously made, but it is at least essential to know that the minerals
do not necessarily crystallize from lavas in the order of their melting
temperatures, or in any uniform order, but rather in the order in
which saturation of the several mineral constituents happens to be
reached in the given mutual solution. Thus quartz, which has a very
high melting-point, is often one of the last minerals to crystallize.
The mutual solutions are exceedingly complex, embracing a wide range
of chemical substances, but the chief of them, as already stated, are
silicates of aluminum, potassium, sodium, calcium, magnesium, and iron,
with minor ingredients of nearly all known substances, in greater or
less proportion. The old idea of lavas as simply melted rock is not,
however, wholly to be abandoned. The mode of solidifying is often
simply that of molten matter freezing. If lava be suddenly cooled, the
congelation is essentially the solidification of a melted substance.
The result is a glassy body, every part of which has essentially the
same composition that the liquid had. Usually, however, even in this
case, the gases escape in part. If the cooling is slower, the various
substances in the mixture crystallize out into minerals in the order in
which they severally reach saturation. This involves the principle that
solubility is dependent on temperature, and that as the temperature
sinks the degree of solubility declines, and the saturation-point for
some constituents of the solution is reached earlier than for others.
With sufficiently slow cooling, all the material will pass into the
solid state by the crystallizing of the several minerals in succession.
This does not mean that two or more minerals may not be forming at the
same time, for crystals often interfere with each other’s growth. It
does, however, involve the doctrine that some substances may complete
their crystallization while the surrounding material is yet in the
fluid condition. In most igneous rocks nearly perfect crystals of
certain minerals are common, while other minerals, crystallizing later,
are compelled to adapt themselves to the space left. This conception is
supported by the fact that lavas, while still in the fluid condition,
often contain well-formed crystals, and these crystals sometimes make
up a considerable percent. of the flowing mass, just as water in
certain conditions may be filled with crystals of ice. So also crystals
after having been formed may be redissolved in part, doubtless because
of changes in the nature of the magma due to undetermined conditions
which may arise in the process of crystallization, or from the
accession of gas, or from new material dissolved from the walls of the
passageway.
[Illustration: +Fig.+ 469.—Lobular form of lava-flow, “Pahoehoe.”
(Dutton, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 470.—Terminal portion of a rough lava-flow,
“_aa_.” Cinder Buttes, Idaho. (Russell, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 471.—Lava flowing over a precipice near Hilo,
Hawaiian Islands.]
=Consanguinity and succession of lavas.=—The lavas that are poured
forth at different stages in the succession of eruptions of a given
region are usually not the same, as might naturally be expected, but
form a curious series the members of which are related to one another.
Iddings has called this relation _consanguinity_.[283] No universal
law of succession has yet been established, and perhaps none exists;
but Richthofen[284] many years ago announced a definite order for the
Tertiary flows of western America which seems to hold fairly well
in its general aspects, though not everywhere completely realized,
so far as surface observation goes. Richthofen’s order is: (1) lavas
of neutral types, (2) lavas of acid types, (3) lavas of basic types,
(4) lavas of more acid types, and (5) lavas of more basic types. The
special varieties of rock vary, and even the general order is often
apparently defective. The defects are sometimes assigned to the
concealment of some of the outflows. While this may be true in some
cases, it is not unlikely that in others there is a real failure of the
sequence. At any rate, the sequence can only be regarded as a rough
generalization. It is supposed to be due to magmatic differentiation
caused by the differences of temperature to which the different parts
are subjected underground, by differences of specific gravity and
fluidity which result from changes of temperature, and probably by
other causes.
=Temperatures of lavas.=—Accurate determinations of the temperatures
in the center of the lava-columns, where they have been least reduced
by contact with the rock-walls, have not yet been made, but it is
clear from the whiteness of the lavas that their temperatures are
often appreciably above the melting-point. This is also a necessary
inference from the length of time they remain fluid, notwithstanding
the great surface contact of the column in its miles of ascent, the
conversion of contact water into steam, and the expansion and escape
of the gases. In cases where determination has been practicable (and
they certainly do not represent the maximum temperatures) it has been
found that the melting-points of silver, about 960° C., and of copper,
about 1060° C., are reached. In connection with overflows, it has
been found that brass is decomposed into its component metals, the
copper actually crystallizing. Silver has been sublimed, and made to
redeposit itself in crystalline form. This implies much more than the
bare melting temperatures. Even the fine edges of flints have been
fused. It is, therefore, probably safe to assume that the original
temperatures of the lavas as they rise to the surface sometimes reach
considerably beyond 2000° Fahr. (1093° C.), and may perhaps even attain
3000° Fahr. or more. Even these temperatures must be somewhat below
the original subterranean temperatures of the lavas, because some heat
must necessarily be lost in rising, partly by contact with the walls
of the colder rocks through which they pass, probably for as much as a
score of miles at least, and partly from the expansion of the gases
within them. If any considerable part of these gases is derived from
waters which joined the lava in its upward course in the fracture zone,
the energy consumed in raising the water to the high temperatures
of the lavas must be subtracted from the original heat, and must be
a further source of reduction of temperature. It is important to
emphasize this point in view of its bearing upon the origin of the
lavas. It has been suggested that lavas may be due to an aqueo-igneous
fusion, a kind of fusion which may take place at comparatively moderate
temperatures. It seems obvious, however, from the phenomena themselves,
that temperatures as high as ordinary dry fusion, and perhaps even
higher, are attained. It is clear also that the maintenance of the
liquid condition in a constant state of ebullition for a long period
of time implies a large surplus of heat above that necessary for
liquefaction simply. This is especially true if the ebullition comes
from surface-waters penetrating to and becoming absorbed in the
lava-column below. This process must tend rapidly to exhaust the heat
in the column of lava. If, on the other hand, the gases are derived
from the deep interior, and the ebullition at the surface is due to
their escape, they may bring up new supplies of heat to counteract the
cooling effects of their expansion.