THE NATURE OF THE MATERIALS IN THE LITHOSPHERE
=The rigid quality of our planet.=—For a long time it was supposed
that the solid earth constituted a crust only which was floated
upon a liquid interior. This notion was clearly an outgrowth of the
then generally accepted Laplacian hypothesis of the origin of the
universe, which assumed fluid interiors for the planets, the crust
being suggested by the winter crust of frozen water upon the surface of
our inland lakes. To-day the nebular hypothesis in the Laplacian form
is fast giving place to quite different conceptions, in which solid
particles, and not gaseous ones, are conceived to have built up the
lithosphere. The analogy with frozen water has likewise been abandoned
with the discovery that frozen rock, instead of floating, sinks in its
molten equivalent.
Yet even more cogent arguments have been brought forward to show that
whatever may be the state of aggregation within the earth’s core—and
it may be different from any now known to us—it nevertheless has many
of the properties recognized as belonging to solid and rigid bodies.
Provisionally, therefore, we may regard the earth’s core as rigid and
essentially solid. It was long ago pointed out by the late Lord Kelvin
that if our lithosphere were not more rigid than a ball of glass of the
same size, it would be constantly passing through periodic six-hourly
distortions of great amplitude in response to the varying attractions
of the moon. An equally striking argument emanating from the same high
authority is furnished by the well-known egg-spinning demonstration.
For illustration, Kelvin was accustomed to take two eggs, one boiled
and the other raw, and attempt to spin them upon their ends. For the
boiled, and essentially solid, egg this is easily accomplished, but
internal friction of the liquid contents of the raw egg quickly stops
any rotary motion which is imparted to it. Upon the same grounds it
is argued that had the earth’s interior possessed the properties of a
liquid, rotation must long since have ceased.
A stronger proof of earth rigidity than either of these has been lately
furnished by the instrumental study of earthquakes. With the delicate
apparatus which is now installed for the purpose, heavy earthquakes may
be sensed which have occurred anywhere upon the earth’s surface, the
earth movement sending its own message by the shortest route through
the core of the earth to the observing station. A heavy shock which
occurs in New Zealand is recorded in England, almost diametrically
opposite, in about twenty-one minutes after its occurrence. The laws
of wave propagation and their relation to the properties of the
transmitting medium are well known, and in order to explain such
extraordinary velocity it is necessary to assume that for such impulses
the earth’s interior is much more rigid than the finest tool steel.
=Probable composition of the earth’s core.=—In deriving views
concerning the nature of the earth’s interior we are greatly aided
by astronomical studies. The common origin long ago indicated for
the planets of the solar system and the sun has been confirmed by
the analysis of light with the aid of the spectroscope. It has thus
been found that the same chemical elements which we find in the earth
are present also in the sun and in the other stellar bodies. Again,
the group of planets of the solar system which are nearest to the
sun—Mercury, Venus, the Earth, and Mars—have each a high density, all
except Mars, the most distant, having specific gravities very closely
5½, that of Mars being about 4. This average specific gravity is also
that of the solid bodies, the so-called meteorites, which reach the
surface of our planet from the surrounding space. Yet though the earth
as a whole is thus found to have a specific gravity five and a half
times that of water, its surface shell has an average density of less
than half this value, or 2.7.
The study of meteorites has given us a possible clew to the nature of
the earth’s interior; for when both terrestrial and celestial rock
types are classified and placed in orderly arrangement, it is found
that the chemical elements which compose the two groups are identical,
and that these are united according to the same physical and chemical
laws. No new element has been discovered in the one group that has not
been found in the other, and though some compounds of these elements,
the minerals, occur in the earth’s crust that have not been found in
meteorites, and though some occur in meteorites which are not known
from the earth, yet of those which are common to both bodies there is
agreement, even to the minor details (Fig. 9). It is found, however,
that the commonest of the minerals in the earth’s shell are absent from
meteorites, as the commoner constituents of meteorites are wanting in
the earth’s crust. This observation would go far to show that we may in
the two cases be examining different portions of quite similar bodies;
and this view is strikingly confirmed when the rocks of the two groups
are arranged in the order of their densities (Fig. 9).
[Illustration:
FIG. 9.—Diagram to show how terrestrial rocks grade into those of the
meteorites. 1, oxygen; 2, silicon; 3, aluminium; 4, alkali metals; 5,
alkaline earth metals; 6, iron, nickel, cobalt, etc.; _a_, granites and
rhyolites; _b_, syenites and trachytes; _c_, diorites and andesites;
_d_, gabbros and basalts; _e_, ultra-basic rocks; _f_, basic inclosures
in basalt, etc.; _g_, iron basalts of west Greenland; _h_, iron masses
of Ovifak, west Greenland; _a’-d’_, meteorites in order of density
(after Judd).]
In a broad way, density, structure, and chemical composition are all
similarly involved in the gradations illustrated by the diagram; and it
is significant that while there are terrestrial rocks not represented
by meteorites, the densest and the most unusual of the terrestrial
rocks are chemically almost identical with the less dense of the
celestial bodies.
=The earth a magnet.=—The denser, and likewise the more common, of
the meteorite rocks—the so-called meteoric irons—are composed almost
entirely of the elements iron, nickel, and cobalt. Such aggregates
are not known as yet from terrestrial sources, although transitional
types appear to exist upon the island of Disco off the west coast of
Greenland. If it were possible to explore the earth’s interior, would
such combinations of the iron minerals be encountered? Apart from the
surprising velocity of transmission of earthquake waves, the strongest
argument for an iron core to the lithosphere is found in the magnetic
property of the earth as a whole. The only magnetic elements known to
us are those of the heavy meteorites—iron, nickel, and cobalt,—and
the earth is, as we know, a great magnet whose northern pole in British
America and whose southern pole in Antarctica have at last been visited
by Amundsen and David, respectively. The specific gravity of iron is
7.15, and those of nickel and cobalt, which in the meteorites are
present in relatively small amounts, are 7.8 and 7.5, respectively.
Considering that the surface shell of the earth has a specific gravity
of 2.7, these values must be regarded as agreeing well with the
determined density of the earth (5.6) and the other planets of its
group (Mercury 5.7, Venus 5.4, Mars 4).
=The chemical constitution of the earth’s surface shell.=—The number
of the so-called chemical elements which enter into the earth’s
composition is more than eighty, but few of these figure as important
constituents of the portion known to us. Nearly one half of the mass
of this shell is oxygen, and more than a quarter is silicon. The
remaining quarter is largely made up of aluminium, iron, calcium,
magnesium, and the alkalies sodium and potassium, in the order named.
These eight constituent elements are thus the only ones which play any
important rôle in the composition of the earth’s surface shell. They
are not found there in the free condition, but combined in the definite
proportions characteristic of chemical compounds, and as such they are
known as _minerals_.
=The essential nature of crystals.=—A crystal we are accustomed to
think of as something transparent bounded by sharp edges and angles,
our ideas having been obtained largely from the gem minerals. This
outward symmetry of form is, however, but an expression of a power
which resides, so to speak, in the heart or soul of the crystal
individual—it has its own structural make-up, its individuality. No
more correct estimates of the comparison of crystal individualities
would be obtained by the study of outward forms alone of two minerals
than would be gained by a judgment of persons from the cut of their
clothing. Too often this outward dress tells only of the conditions by
which both men and crystals have been surrounded, and but little of the
power inherent in the individual. Many a battered mineral fragment with
little beauty to recommend it, when placed under suitable conditions
for its development, has grown into a marvel of beauty. Few minerals
are so mean that they have not within them this inherent power of
individuality which lifts them above the world of the amorphous and
shapeless.
[Illustration:
FIG. 10.—Comparison of a crystalline with an amorphous substance when
expanded by heat and when attacked by acid.]
Just as the real nature of a person is first disclosed by his behavior
under trying circumstances, so of a crystal it is its conduct under
stress of one sort or another which brings out its real character. By
way of illustration let us prepare a sphere from the mineral quartz—it
matters not whether we destroy the beautiful outlines of the crystal or
employ a battered fragment—and then prepare a sphere of similar size
and shape from a noncrystalline or amorphous substance like glass. If
now these two spheres be introduced into a bath of oil and raised to a
higher temperature, the glass globe undergoes an enlargement without
change of its form; but the crystal ball reveals its individuality
by expanding into a spheroid in which each new dimension is nicely
adjusted to this more complex figure (Fig. 10).
We may, instead of submitting the two balls to the “trial by fire”,
allow each to be attacked by the powerful reagent, hydrofluoric acid.
The common glass under the attack of the acid remains as it was before,
a sphere, but with shrunken dimensions. The crystal, on the other hand,
is able to control the action of the solvent, and in so doing its
individuality is again revealed in a beautifully etched figure having
many curving outlines—it is as though the crystal had possessed a soul
which under this trial has been revealed. This glimpse into the nature
of the crystal, so as to reveal its structural beauty, is still more
surprising when the crystal is taken from the acid in the early stages
of the action and held close beneath the eye. Now the little etchings
upon the surface display each the individuality of the substance, and
joining with their neighbors they send out a beautifully symmetrical
and entirely characteristic picture (Fig. 11).
[Illustration:
FIG. 11.—“Light figure” seen upon an etched surface of a crystal of
calcite (after Goldschmidt and Wright).]
=The lithosphere a complex of interlocking crystals.=—To the layman
the crystal is something rare and expensive, to be obtained from
a jeweler or to be seen displayed in the show cases of the great
museums. Yet the one most striking quality of the lithosphere which
separates it from the hydrosphere and the atmosphere is its crystalline
structure,—a structure belonging also to the meteorite, and with
little doubt to all the planets of the earth group. A snowflake caught
during its fall from the sky reveals all the delicate tracery of
crystal boundary; collected from a thick layer lying upon the ground,
it appears as an intricate aggregate of broken fragments more or less
firmly cemented together. And so it is of the lithosphere, for the
myriads of individuals are either the ruins of former crystals, or
they are grown together in such a manner that crystal facets had no
opportunity to develop.
Such mineral individuals as once possessed the crystal form may have
been broken and their surfaces ground away by mutual attrition under
the rhythmic beating of the waves upon a shore or in the continuous
rolling of pebbles on a stream bed, until as battered relics they
are piled away together in a bed of sand. Yet no amount of such rough
handling is sufficient to destroy the crystal individuality, and if
they are now surrounded with conditions which are suitable for their
growth, their individual nature again becomes revealed in new crystal
outlines. Many of our sandstones when turned in the bright sunlight
send out flashes of light to rival a bank of snow in early spring.
These bright flashes proceed from the facets of minute crystals formed
about each rounded grain of the sand, and if we examine them under a
lens, we may note the beauty of line formed with such exactness that
the most delicate instruments can detect no difference between the
similar angles of neighboring crystals (Fig. 12).
[Illustration:
FIG. 12.—Battered sand grains which have taken on a new lease of life
and have developed a crystal form. _a_, a single grain grown into an
individual crystal; _b_, a parallel growth about a single grain; _c_,
growth of neighboring grains until they have mutually interfered and so
destroyed the crystal facets—the common condition within the mass of a
rock (after Irving and Van Hise).]
This individual nature of the crystal is believed to reside in a
symmetrical grouping of the chemical molecules of the substance into
larger and so-called “crystal molecules.” The crystal quality belongs
to the chemical elements and to their compounds in the solid condition,
but not to ordinary mixtures of them.
=Some properties of natural crystals, minerals.=—No two mineral
species appear in crystals of the same appearance, any more than two
animal species have been given the same form; and so minerals may be
recognized by the individual peculiarities of their crystals. Yet
for the reason that crystals have so generally been prevented from
developing or retaining their characteristic faces, in the vast number
of instances it is the behavior, and not the appearance, of the mineral
substance which is made use of for identification.
When a mineral is broken under the blow of a hammer, instead of
yielding an irregular fracture, like that of glass, it generally
tends to part along one or more directions so as to leave plane
surfaces. This property of _cleavage_ is strikingly illustrated
for a single direction in the mineral mica, for two directions in
feldspar, and for three directions in calcite or Iceland spar. Other
properties of minerals, such as hardness, specific gravity, luster,
color, fusibility, etc., are all made use of in rough determinations
of the minerals. Far more delicate methods depend upon the behavior
of minerals when observed in polarized light, and such behavior is
the basis of those branches of geological science known as optical
mineralogy and as microscopical petrography. An outline description of
some of the common minerals and the means for identifying them will be
found in appendix A.
=The alterations of minerals.=—By far the larger number of minerals
have been formed in the cooling and consequent consolidation of molten
rock material such as during a volcanic eruption reaches the earth’s
surface as lava. Beginning their growth at many points within the
viscous mass, the individual crystals eventually may grow together and
so prevent a development of their crystal faces.
Another class of minerals are deposited from solution in water within
the cavities and fissures of the rocks; and if this process ceases
before the cavities have been completely closed, the minerals are
found projecting from the walls in a beautiful lining of crystal—the
_Krystallkeller_ or “crystal cellar.” It is from such pockets or veins
within the rocks that the valuable ores are obtained, as are the
crystals which are displayed in our mineral cabinets.
[Illustration:
FIG. 13.—Crystal of garnet developed in a schist with grains of quartz
included because not assimilated.]
There is, however, a third process by which minerals are formed,
and minerals of this class are produced within the solid rock as a
product of the alteration of preëxisting minerals. Under the enormous
pressures of the rocks deep below the earth’s surface, they are as
permeable to the percolating waters as is a sponge at the surface.
Under these conditions certain minerals are dissolved and their
material redeposited after traveling in the solution, or solution
and redeposition of mineral matter may go on together within the mass
of the same rock. One new mineral may have been produced from the
dissolved materials of a number of earlier species, or several new
minerals may be the result of the alteration of a preëxisting mineral
with a more complex chemical structure. Where the new mineral has been
formed “in place”, it has sometimes been able to utilize the materials
of all the minerals which before existed there, or it may have been
obliged to inclose within itself those earlier constituents which it
could not assimilate in its own structure (Fig. 13).
[Illustration:
FIG. 14.—A crystal of augite within the mass of a rock altered in
part to form a rim of the minerals hornblende and magnetite. Note the
original outline of the augite crystal.]
At other times a crystal which is imbedded in rock has been attacked
upon its surface by the percolating solutions, and the dissolved
materials have been deposited in place as a crown of new minerals
which steadily widens its zone until the center is reached and the
original crystal has been entirely transformed (Fig. 14). It is
sometimes possible to say that the action by which these changes have
been brought about has involved a nice adjustment of supply of the
chemical constituents necessary to the formation of the new mineral or
minerals. In rocks which are aggregates of several mineral species, a
newly formed mineral may appear only at the common margin of certain of
these species, thus showing that they supply those chemical elements
which were necessary to the formation of the new substance (Fig. 15).
Thus it is seen that below the earth’s surface chemical reactions are
constantly going on, and the earlier rocks are thus locally being
transformed into others of a different mineral constitution.
[Illustration:
FIG. 15.—A new mineral (hornblende) forming as an intermediate
“reaction rim” between the mineral having irregular fractures (olivine)
and the dusty white mineral (lime-soda feldspar).]
Near the earth’s surface the carbon dioxide and the moisture which are
present in the atmosphere are constantly changing the exposed portions
of the lithosphere into carbonates, hydrates, and oxides. These
compounds are more soluble than are the minerals out of which they were
formed, and they are also more bulky and so tend to crack off from
the parent mass on which they were formed. As we are to see, for both
of these reasons the surface rocks of the lithosphere succumb to this
attack from the atmosphere.
In connection with those wrinklings of the surface shell of the
lithosphere from which mountains result, the underlying rocks are
subjected to great strains, and even where no visible partings are
produced, the rocks are deformed so that individual minerals may be
bent into crescent-shaped or S-shaped forms, or they are parted into
one or more fragments which remain imbedded within the rock.
READING REFERENCES FOR CHAPTER III
Theories of origin of the earth:—
THOMSON and TAIT. Natural Philosophy. 2d ed. Cambridge, 1883, pp. 422.
T. C. CHAMBERLIN. Chamberlin and Salisbury’s Geology, vol. 2, pp. 1-81.
Rigidity of the earth:—
LORD KELVIN. The Internal Condition of the Earth as to Temperature,
Fluidity, and Rigidity, Popular Lectures and Addresses, vol. 2, pp.
299-318; Review of evidence regarding the physical condition of the
earth, _ibid._, pp. 238-272.
HOBBS. Earthquakes (Appleton, New York, 1907), Chapters xvi and xvii.
Composition of the earth’s core and shell:—
O. C. FARRINGTON. The Preterrestrial History of Meteorites, Jour.
Geol., vol. 9, 1901, pp. 623-236.
E. S. DANA. Minerals and How to Study Them (a book for beginners in
mineralogy). Wiley, New York, 1895.
On the nature of crystals:—
VICTOR GOLDSCHMIDT. Ueber das Wesen der Krystalle, Ostwalds Annalen
der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419.