THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS.
Near the high-road which passes between the towns of Eger and
Franzenbad in Bohemia, there rises a small hill known as the Kammerbühl
(see fig. 33), which has attracted to itself an amount of interest
and attention quite out of proportion to its magnitude or importance.
During the latter part of the last century and the earlier years of
the present one, the fiercest controversies were waged between the
partisans of rival schools of cosmogony over this insignificant hill;
some maintaining that it originated in the combustion of a bed of coal,
others that its materials were entirely formed by some kind of 'aqueous
precipitation,' and others again that the hill was the relic of a small
volcanic cone.
Among those who took a very active part in this controversy was
the poet Goethe, who stoutly maintained the volcanic origin of the
Kammerbühl, styling it 'a pocket edition of a volcano.' To Goethe
belongs the merit of having suggested a Very simple method by which the
controversies concerning this hill might be set at rest: he proposed
that a series of excavations should be undertaken around the hill, and
a tunnel driven right under its centre.
[Illustration: Fig. 33.--The Kammerbühl of Kammerberg, Bohemia.
(As seen from the south-west)]
[Sidenote: THE KAMMERBÜHL.]
The poet's friend, Count Caspar von Sternberg, determined to put this
project into execution. This series of excavations, which was completed
in 1837, has for ever set at rest all doubts as to the volcanic origin
of the Kammerbühl. A plug of basalt was found filling the centre of
the mass, and connected with a small lava-stream flowing down the side
of the hill; while the bulk of the hill was shown to be composed of
volcanic scoriæ and lapilli. The section fig. 34 will illustrate the
structure of the hill as revealed by these interesting excavations.
[Illustration: Fig. 34.--Section of the Kammerbühl, in Bohemia.
_a a._ Metamorphic rocks. _b._ Basaltic scoriæ. _c._ Solid plug
of basalt rising through the centre of the volcanic pile, _d d._
Lava-stream composed of the same rock. _e e._ Alluvial matter
surrounding the old volcano.
(The dotted lines indicate the probable former outline of the volcano.)]
[Sidenote: VOLCANOES DISSECTED BY DENUDATION.]
It can of course very seldom happen that actual mining operations, like
those undertaken in the case of the Kammerbühl, will be resorted to
in order to determine the structure of volcanic mountains. Geologists
have usually to avail themselves of less direct, but by no means less
certain, methods than that of making artificial excavations in order
to investigate the earth's crust. Fortunately it happens that what we
cannot accomplish ourselves, nature does for us. The action which we
call 'denudation' serves as a scalpel to dissect volcanic mountains
for us, and to expose their inner recesses to our view. Many portions
of the earth's surface are complete museums crowded with volcanic
'subjects,' exhibiting every stage of the process of dissection. In
some, rains and winds have stripped off the loose covering of cinders
and dust, and exposed the harder and more solid parts--the skeleton of
the mountain. In others, the work of destruction has proceeded still
further, and slowly wearing rivers or the waves of the sea may have cut
perfect, vertical sections of the mountain-mass. Sometimes the removal
of the materials of the volcanic mountain has gone on to such an extent
that its base and ground-plan are fully exposed. It only requires the
necessary skill in piecing together our observations on these dissected
volcanoes, in order to arrive at just views concerning the 'comparative
anatomy' of volcanoes. As the knowledge of the structure of animals
remained in the most rudimentary condition until the practice of
dissection was commenced, so our knowledge of volcanoes was likewise
exceedingly imperfect till geologists availed themselves of the
opportunities afforded to them of studying naturally dissected volcanic
mountains.
In some cases we may find that the sea has encroached on the base
of a volcanic hill, till one half of it has been washed away, and
the structure of the mass to its very centre is exposed to our view.
Thus in fig. 6 (page 43), it will be seen that there lies in front of
Vulcano a peninsula called Vulcanello, consisting of three volcanic
cones, united at their base, with the lava-streams which have flowed
from them. One half of the cone on the left-hand side of the picture
has been completely washed away by the sea, and a perfect section
of the internal structure of the cone is exposed. The appearances
presented in this section are shown in the sketch, fig. 35. Some
portions of the face of this section are concealed by the heaps of
fragments which have fallen from it, but enough is visible to convince
us that three kinds of structures go to make up the cone. In the first
place, we have the loose scoriæ and lapilli, which in falling through
the air have arranged themselves in tolerably regular layers upon the
sides of the cone.
[Illustration: Fig. 35.--Natural section or a Volcanic Cone in the
Island of Vulcano.
_a._ Crater. _b b._ Lava-streams. _c._ Dykes which have clearly formed
the ducts, through which the lava has risen to the crater. _d d._
Stratified volcanic scoriæ. _e._ Talus of fallen materials.]
In the second place, we have lava-streams which have been ejected from
the crater or from fissures on the flanks of the cone, and flowed down
its sides. And thirdly, we find masses of lava filling up cracks in
the cone; these latter are called 'dykes.' Of these three kinds of
structures most volcanic mountains are built up, but in different
cases the part played by these several elements may be very unequal.
Sometimes volcanoes consist entirely of fragmentary materials, at
others they are made up of lavas only, while in the majority of
cases they have been formed by alternations of fragmentary and fluid
ejections, the whole being bound together by dykes, which are masses of
lava injected into the cracks formed from time to time in the sides of
the growing cone.
If we direct our attention in the first place to the fragmentary
ejections, we shall find that they affect a very marked and peculiar
arrangement, which is best exhibited in those volcanic cones composed
entirely of such materials.
[Sidenote: INTERNAL STRUCTURE OF VOLCANIC CONES.]
Everyone who examines volcanoes for the first time will probably be
struck by the regular stratification of materials of which they are
composed. Thus the tuffs covering the city of Pompeii are found to
consist of numerous thin layers of lapilli and volcanic dust, perfectly
distinct from one another, and assuming even the arrangement which we
usually regard as characteristic of materials that have been deposited
from a state of suspension in water. The fragmentary materials in
falling through the air are sorted, the finer particles being carried
farther from the vent than the larger and heavier ones. The force
of different volcanic outbursts also varies greatly, and sometimes
materials of different character are thrown out during successive
ejections. These facts will be illustrated by fig. 36, which is a
drawing of a section exposed in a quarry opened in the side of the
Kammerbühl. In this section we see that the falling scoriæ have been
arranged in rudely parallel beds, but the regular deposition of these
has been interrupted by the ejection of masses of burnt slate torn from
the side of the vent, probably during some more than usually violent
paroxysm of the volcano. In those volcanoes which are built up of tuffs
and materials which have fallen in the condition of a muddy paste, the
perfect stratification of the mass is often very striking indeed, and
large cones are found built up of thin uniformly-spread layers of more
or less finely-divided materials, disposed in parallel succession.
Such finely-stratified tuff-cones abound in the district of the Campi
Phlegræi.
[Illustration: Fig. 36.--Section in the side of the Kammerbühl, Bohemia.
_a a._ stratified basaltic scoriæ. _b b._ Bands made up of fragments
of burnt slate. _c._ Stratified basaltic scoriæ. _d d._ Pseudo-dykes
occupying lines of fault.]
[Sidenote: ARRANGEMENT OF FRAGMENTAL MATERIALS.]
If, in consequence of any subterranean movements, fissures are produced
in the sides of the cones formed of fragmentary materials, these often
become gradually filled with loose fragments from the sides of the
fissure, and in this manner 'pseudo-dykes' are formed. An example of
such pseudo-dykes is represented in fig. 36, where the beds composing
the volcanic cone of the Kammerbühl are seen to have been broken across
or faulted, and the fissures produced in the mass have been gradually
filled with loose fragments.
It is not difficult to imitate, on a small scale, the conditions which
exist at those volcanic vents from which only fragmentary materials
are ejected. If we take a board having a hole in its centre, into
which a pipe is inserted conveying a strong air-blast, we shall, by
introducing some light material like bran or sawdust into this pipe
cause an ejection of fragments, which will, when the board is placed
horizontally, fall around the orifice of the pipe and accumulate there
in a conical heap (fig. 37). It will be found necessary, as was shown
by Mr. Woodward, who performed the experiment before the Physical
Society, to adopt some contrivance, such as a screw, for forcing the
material into the air-pipe. If we alternately introduce materials of
different colours, like mahogany- and deal-sawdust into the pipe, these
materials will be arranged in layers which can be easily recognised,
and the mode of accumulation of the mass will be evident. By means of
a sheet of tin or cardboard we may divide this miniature volcanic cone
vertically into two portions, and if we sweep one of these away the
internal structure of the other half will be clearly displayed before
our eyes.
In this way we shall find that the conical heap of sawdust with the
hole in its centre has a very peculiar and definite arrangement of its
materials. It is made up of a number of layers each of which slopes in
opposite directions, towards the centre of ejection and away from that
centre. These layers are thickest along the line of the circle where
the change in slope takes place, and they thin away in the direction of
the two opposite slopes.
[Illustration: Fig. 37.--Experimental illustration of the mode of
Formation of Volcanic Cones composed of fragmental materials.]
[Sidenote: CAUSE OF THIS ARRANGEMENT.]
The cause of this peculiar arrangement of the materials is evident.
The sawdust thrown up by the air blast descends in a shower and tends
to accumulate in a circular heap around the orifice, the area of this
circular heap being determined by the force of the blast. Within this
circular area, however, the quantity of falling fragments is not
everywhere the same; along a circle surrounding the vent at a certain
distance, the maximum number of falling fragments will be found to
descend, and here the thickest deposit will take place. As this goes
on, a circular ridge will be formed, with slopes towards and away
from the centre of injection. As the ridge increases in height, the
materials will tend to roll down either one slope or the other, and
gradually a structure of the form shown in the figure will be piled up.
The materials sliding down the outer slope will tend to increase the
area of the base of the cone, while those which find their way down the
inner slope will fall into the vent to be again ejected.
[Illustration: Fig. 38.--Natural section of a Tuff-cone forming the
Cape of Misenum, and exhibiting the peculiar internal Arrangement
characteristic of volcanoes composed of fragmentary materials.]
Volcanic cones composed of scoriæ, dust, &c. are found to have exactly
the same internal structure as is exhibited by the miniature cone of
sawdust. The more or less regular layers of which they are made up dip
in opposite directions, away from and towards the vent, and thin out
in the direction of their dip (see fig. 38). In small cones the crater
or central cavity is of considerable size in proportion to the whole
mass, but as the cone grows upwards and outwards, the dimensions of
the crater remain the same, while the area of the base and the height
of the cone are continually increasing. This is the normal structure
of volcanic cones formed of fragmentary materials, though, as we shall
hereafter show, many irregularities are often produced by local and
temporary causes.
[Illustration: Fig. 39.--Section of a small Scoria-cone formed within
the crater of Vesuvius in the year 1885, illustrating the filling-up of
the central tent of the cone by subsequent ejections.]
In some cases the central vent of a volcanic scoria-cone may be
filled up by subsequent ejections. A beautiful example of this kind
was observed by Abich, in the case of a small cone formed within the
crater of Vesuvius in 1835, and is represented in fig. 39.
[Illustration: Fig. 40.--Volcanic Cones composed of Scoriæ, and
breached on one side by the outflow of lava-currents.]
[Sidenote: BREACHED CONES.]
Many cones formed in the first instance of scoriæ, tuff, and pumice may
give rise to streams of lava, before the vent which they surround sinks
into a state of quiescence. In these cases, the liquid lava in the vent
gives off such quantities of steam that masses of froth or scoriæ are
formed, which are ejected and accumulate around the orifice. When the
force of the explosive action is exhausted, the lava rises bodily in
the crater, which it more or less completely fills. But, eventually,
the weaker side of the crater-wall yields beneath the pressure of the
liquid mass, and this part of the crater and cone is swept away before
the advancing lava-stream. Examples of such 'breached cones' abound in
Auvergne and many other volcanic districts (see fig. 40). A beautiful
example of a cone formed of pumice, which has been breached by the
outflow of a lava-stream of obsidian, occurs in the Lipari Islands, at
the Rocche Rosse. It is this locality which supplies the whole world
with pumice (see fig. 41).
[Illustration: Fig. 41.--Campo Bianco, in the Island of Lipari. A
Pumice-cone breached by the Outflow of an Obsidian Lava-current.]
It is often surprising to find how volcanic cones composed of loose
materials, such as tuffs, scoriæ, or pumice, retain their distinctive
forms, and even the sharpness of their outlines, during enormous
periods of time. Thus, in the scoria-cones which abound in the
Auvergne, and were, in all probability, formed before the historical
period, the sharp edges of the craters appear to have suffered scarcely
any erosion, and the cones are as perfect in their outlines as though
formed but yesterday. It is probable that the facility with which these
cindery heaps are penetrated by the rain which falls upon them is the
cause why they are not more frequently washed away.
[Illustration: Fig. 42.--Volcanic Cones in Auvergne which have suffered
to some extent from atmospheric denudation.]
Sometimes, however, scoria-cones are found reduced by atmospheric
waste to mere heaps of cinders, in which the position of the crater is
indicated only by a slight depression, as in fig. 42.
[Sidenote: CONES COMPOSED OF LAVA.]
When but little explosive action takes place at the volcanic vent, and
only fluid lava is ejected, mountains are formed differing very greatly
in character from the cones composed of fragmentary materials.
If the lavas be of very perfect liquidity, like those erupted in the
Sandwich Islands, they flow outwards around the vent to enormous
distances. By the accumulation of materials during successive
outbursts, a conical mass is built up which has but a slight elevation
in proportion to the area of its base. Thus in Hawaii we find great
volcanic cones, composed of very fluid lavas, which have a height of
nearly 14,000 feet with a diameter of base of seventy miles. In these
Hawaiian mountains the slope of the sides rarely exceeds 6° to 8°.
But if, on the other hand, the lavas be of much more viscid
consistency, the character of the volcanic cones which are produced by
their extrusion will be very different. The outwelling material will
tend to accumulate and heap itself up around the vent. By successive
ejections the first-formed shell is forced upwards and outwards, and
a steep-sided protuberant mass is formed, exhibiting in its interior
a marked concentric arrangement. Dr. Ed. Reyer, of Grätz, has devised
a very ingenious method for reproducing on a miniature scale the
characteristic features of these eruptions of viscid lavas. He takes a
quantity of plaster of Paris reduced to a pasty consistence, which he
forces through a hole in a board. The plaster accumulates in a great
rounded boss about the orifice through which it has been forced. If the
plaster have some colouring matter introduced into it, the mass, on
being cut across, will exhibit in the disposition of its colour-bands
the kind of action which has gone on during its extrusion, fig. 43.
[Illustration: Fig. 43.--Experimental illustration of the Mode of
Formation of volcanic cones composed of viscid lavas.]
[Illustration: Fig. 44.--The Grand Puy of Sarcoui, composed of
trachyte, rising between two breached scoria-cones (Auvergne).]
There are many volcanic cones which exhibit clear evidence of having
thus been formed by the extrusion of a viscid mass of lava through
a volcanic fissure. Among such we may mention the domitic Puys of
Auvergne, fig. 44, many andesitic volcanoes in Hungary, the phonolite
hills of Bohemia, and the so-called 'mamelons' of the Island of
Bourbon. See figs. 45 and 46. When the interior of these masses is
exposed by natural or artificial sections, they are all found to
exhibit the onion-like structure which occurs in the plaster models.
[Sidenote: INTERNAL STRUCTURE OF LAVA-CONES.]
[Illustration: Fig. 45.--Volcanic Cone (Mamelon) composed of very
viscid lava. (Island of Bourbon.)]
[Illustration: Fig. 46.--Another Mamelon in the Island of Bourbon, with
a crater at its summit.]
But while some volcanoes are composed entirely of the fragmentary
ejections and others are wholly formed by successive outflows of lava,
the majority of volcanoes, especially those of larger dimensions, are
built up of alternations of these different kinds of materials.
[Illustration: Fig. 47.--Cliff-section in the Island of Madeira,
showing how a composite volcano is built up of lava-streams, beds of
scoriæ, and dykes.]
[Sidenote: NATURAL SECTIONS OF CONES.]
The structure of these composite cones may be understood by an
inspection of the accompanying fig. 47, which shows the appearances
presented in a cliff on the coast of the Island of Madeira. We see
that the mass is made up of numerous layers of volcanic scoriæ,
alternating with sheets of lava. The latter, which are represented
in transverse section in the drawing, are seen to thin out on either
side, and to vary greatly in breadth. Besides the alternating masses
of scoriæ and the lava-sheets, there are seen in the section, bands
of a bright-red colour, which are represented in the drawing by black
lines. These are layers of soil, or volcanic dust, which, by the
passage of a lava-stream over their surface, have been burnt so as
to acquire a brick-red colour. These bands of red material, to which
the name of 'laterite' has been frequently applied, very commonly
occur in sections of composite volcanic cones. Crossing the whole of
the horizontally-disposed masses in the section, we find a number of
'dykes,' which are evidently great cracks filled with lava from below.
Some of these run vertically through the cliffs, others obliquely. In
some cases the lava, rising to fill a dyke, has flowed as a lava-stream
at the surface. Last of all, we must call attention to the fact that
the section exhibits evidence of great movements having taken place
subsequently to the accumulation of the whole of the materials. A
great crack has been produced, on one side of which the whole mass has
subsided bodily, giving rise to the phenomenon which geologists call a
'fault.'
In the section, fig. 27, p. 104, copied from a drawing of a sea-cliff
in the Island of Vulcano, a transverse section of a lava-stream is
represented on a somewhat larger scale. The upper and under surface of
the lava-stream is seen to have a scoriaceous structure, but the thick
central mass is compact, and divided by regular joint-planes. This
section also illustrates the fact that, before the lava-stream flowed
down the sides of the mountain, a valley had been cut by meteoric
agencies on the flanks of the volcano, the dykes which traverse the
lower beds of tuff being abruptly truncated.
In mountain ravines, upon the slopes of ancient volcanoes, and in the
cliffs of volcanic islands, we are often able to study the way in which
these great mountain masses are built up of alternating lava-currents,
beds of volcanic agglomerate, scoriæ, tuff and dust, and intersecting
dykes. In fig. 48, the features above described are illustrated by a
section in the sides of the great volcano of Mont Dore.
[Illustration: Fig. 48.--Section seen at the cascade. Bains du Mont
Dore.]
[Illustration: Fig. 49.--Section in the Island of Ventotienne, showing
a great stream of andesitic lava overlying stratified tuffs.]
[Sidenote: SECTIONS IN THE PONZA ISLANDS.]
In figs. 49, 50, 51, and 52, we have given drawings of portions of the
sea-cliffs in several of the Ponza Islands, a small volcanic group off
the Italian coast.
[Illustration: Fig. 50.--Cliff on the south side of the Island of San
Stephano.
_a._ Trachyte lava-stream, with a scoriaceous upper surface overlaid by
stratified tuffs, _b_.]
[Illustration: Fig. 51.--The headland of Monte della Guardia, in the
Island of Ponza.
_a._ Columnar trachyte. _b._ Stratified tuffs. _c._ Pumiceous
agglomerates. _d._ Dyke of rhyolite.]
[Illustration: Fig. 52.--Western side of the same headland, as seen
from the north side of Luna Bay.
_a._ Trachyte lava. _b._ Stratified tuffs. _c._ Dykes of rhyolite, with
their edges passing into pitchstone. _d._ Pumiceous agglomerate.]
[Illustration: Fig. 53.--Sea-cliff at Il Capo, the north-east point of
Salina showing stratified agglomerates traversed by numerous dykes, the
whole being unconformably overlaid by stratified aqueous deposits.]
Fig, 53 represents a cliff-section in the island of Salina, one of the
Liparis, exhibiting evidence that a series of volcanic agglomerates
traversed by dykes of Andesite have been denuded and covered by a
recent stratified deposit.
[Sidenote: PART PLAYED BY DYKES IN CONE-BUILDING.]
In the formation of these great composite cones, a minor but by no
means insignificant part is played by the dykes, or lava-filled
fissures, which are seen traversing the mass in all directions. That
dyke-fissures often reach the surface of a volcanic cone, and that
the material which injects them then issues as a lava-stream, is
illustrated by fig. 54. The formation of these cracks in a volcanic
cone, and their injection by liquid lava, must of course distend
the mountainous mass and increase its volume. If we visit the great
crater-walls of Somma in Vesuvius, and of the Val del Bove in Etna,
we shall find that the dykes are so numerous that they make up a
considerable portion of the mass. When the loose scoriæ and tuffs are
removed by denudation, these hard dykes often stand up prominently like
great walls, as represented in fig. 55. Even in such cases as these,
however, it is doubtful whether the bulk of all the dykes put together
exceeds one-tenth of that of the lavas and fragmentary materials.
[Illustration: Fig. 54.--Section observed in the Val del Bove, Etna,
showing a basaltic dyke, from the upper part of which a lava-current
has flowed.]
[Illustration: Fig. 55.--Basaltic Dykes projecting from masses of
stratified scoriæ in the sides of the Val del Bove, Etna.]
Hence we are led by an examination of the internal structure of
volcanic mountains to conclude that scoriæ- and tuff-cones, and cones
formed of very liquid lavas, increase by an _exogenous_ mode of growth,
all new materials being added to them from without; in the cones formed
of very viscid lavas, on the other hand, the growth is _endogenous_,
taking place by successive accretions within it. The composite cones
owe their origin to both the _exogenous_ and the _endogenous_ modes of
growth, but in a much greater degree to the former than the latter. The
layers of scoriæ, tuff, and dust, and the successive lava-streams are
added to the mass from without, and the lava forming the dykes from
within it.
[Sidenote: THEORY OF ELEVATION CRATERS.]
There are doubtless cases in which, when a tuff-cone is formed, a mass
of very viscid lavas is extruded into its interior, and the mass is
distended like a gigantic bubble. But inasmuch as the very viscid lavas
do not appear to give rise to scoriæ to anything like the same extent
as the more liquid kinds, such 'cupolas,' as they have been called
by some German geologists, are probably not very numerous, and may
be regarded as constituting the exception rather than the rule. The
idea which was formerly entertained by some geologists that all great
volcanic mountains were formed of masses originally deposited in a
horizontal position, and subsequently blown up into a conical form, has
been effectually disposed of by the observations of Lyell and Scrope.
The condition of the great fluid masses which underlie volcanic vents
is another point on which much light has been thrown by the study
of naturally-dissected volcanoes. In some cases, as was shown by
Hochstetter during his admirable researches among the New Zealand
volcanoes, the rising lavas form a great chamber for themselves in the
midst of a volcanic cinder-cone, taking the place of loose materials
which are re-ejected from the vent, or have been re-fused and absorbed
into the mass of lava itself. From this central reservoir of lava,
eruptions are kept up for some time, but when the volcano sinks
into a state of quiescence the lava slowly consolidates. In such
slowly solidified masses of lava, very beautiful groups of radiating
columns are often exhibited Northern Germany abounds with examples
of such basaltic masses, which have once formed the centres of great
cinder-cones; but in consequence of the removal of the loose materials
and the surrounding strata by denudation, these central reservoirs of
the volcanoes have been left standing above the surface, and exhibit
the peculiar arrangements of the columns formed in them during the
process of cooling.
[Sidenote: INTRUSIVE LAVA-SHEETS.]
But in the majority of the more solidly-built composite volcanoes no
such liquid reservoir can be formed within the volcanic cone itself.
Under these circumstances, the lavas, especially those of more liquid
character, tend to force passages for themselves among the rocks
through which they are extruded. Wherever a weak point exists, there
such lavas will find their way, and as the planes of stratification
in sedimentary rocks constitute such weak places, we constantly find
sheets of lava thus inserted between beds of aqueous origin. The areas
over which these intrusive sheets of rock sometimes extend may be very
great, but the more fusible, basic lavas (basalt, &c.) usually form
much more widely-spreading sheets than the less fusible, acid lavas.
In some cases these great intrusive sheets are found extending to a
distance of twenty or thirty miles from the centre at which they were
ejected, and they often follow the bedding of the strata with which
they are intercalated in so regular a manner, that it is difficult for
an observer to believe at first sight that they can have been formed in
the way which we have described. A closer examination will generally
reveal the fact that while these intrusive lava-sheets retain their
parallelism with the strata among which they have been intruded, over
considerable areas, yet they sometimes break across, or send offshoots
into them, as shown in fig. 56. In all cases, too, the rocks lying
above and below such sheets will be found to be more or less baked and
altered, and this affords a very convincing evidence of the intrusion
of the igneous mass between the strata so altered.
[Illustration: Fig. 56.--Sheets of Igneous Rock (Basalt) intruded
between beds of sandstone, clay, and limestone. (Island of Skye.)]
That in the case of most great volcanic mountains, or systems of
mountains, vast reservoirs of liquid lava must exist in the earth's
crust far below the surface, there can be little room for doubt.
Whether such fluid masses are in direct or indirect communication with
a great central reservoir, even supposing such to exist, is a totally
different question. In many cases the outburst of volcanoes in more or
less close proximity has been observed to take place simultaneously,
while in others the commencement of the eruption of one volcano has
coincided with the lapse into quiescence of another in its vicinity.
On the other hand, the remarkable case of the volcanoes of Hawaii
seems to indicate that two vents in close proximity may be supplied
from perfectly distinct reservoirs of lava. The active craters of
Mauna Loa and Kilauea are situated at the heights of 14,000 and 4,000
feet respectively above the sea level; yet the former is sometimes
in a state of violent activity, with which the latter shows no signs
of sympathy whatever. We shall, in a future chapter, adduce evidence
that the liquid lavas in underground reservoirs may undergo various
stages of change in the enormous periods of time during which habitual
volcanic vents are supplied from them.
We have already shown that the character assumed by a mass of fused
material in cooling varies greatly according as the cooling takes place
rapidly at the surface or slowly under enormous pressure. In the former
case a glassy base is formed containing a greater or smaller number
of crystallites or embryo crystals, in the latter the whole rock is
converted into a mass of fully-developed crystals.
[Sidenote: CONSOLIDATION OF LAVAS AT GREAT DEPTHS.]
The lavas which are poured out at the surface consist, as we have
seen, of a glassy magma in which a greater or smaller number of
crystals are found which have been borne up from below. The great
dykes and intrusive sheets consist for the most part of a mass of
small or imperfectly developed crystals in which a number of large and
perfectly formed crystals are embedded. Such rocks are said to have a
'porphyritic' structure. The rocks formed by the consolidation of the
liquid masses in the underground reservoirs are found to be perfectly
crystallised, the crystals impressing one another on every side and
making up the whole mass to the exclusion of any paste or magma between
them. The crystals in those rocks which have consolidated at these
vast depths exhibit evidence, in their enclosed watery solutions and
liquefied carbonic acid, of the enormous pressures under which they
must have been consolidated. The lavas, the more or less porphyritic
rocks of the dykes and sheets, and the perfectly crystalline (granitic)
rocks of the underground reservoirs pass into one another, however, by
the most insensible gradations.
We sometimes find examples of volcanoes which, by the action of
denuding forces, have had their very foundations exposed to our view.
Such examples occur in the Western Isles of Scotland, in the Euganean
Hills near Padua in Northern Italy, and in many other parts of the
earth's surface. In these cases we are able to trace the ground-plan of
the volcanic pile, and to study the materials which have consolidated
deep beneath the surface in the very heart of the mountain.
In studying these 'basal wrecks' of old volcanoes it is always
necessary to bear in mind that the appearance and general characters of
a volcanic rock may be completely disguised by chemical changes going
on within it. It is through want of attention to this fact that so many
mistakes were made by the Wernerian school of geologists who declared
that they could find no analogy between the basaltic rocks of the globe
and the products of active volcanoes, and were hence led to refer the
origin of the former to some kind of 'aqueous precipitation.'
Many of the hard and crystalline marbles which are employed as
ornamental stones were originally loose masses of shells and corals,
as we easily perceive when we examine the polished faces. But these
incoherent heaps of organic _débris_ have been converted into a compact
and solid rock in consequence of the mass being penetrated by water
containing carbonate of lime in solution. Crystals of this substance
were deposited in every cavity and interstice of the mass, and thus
the accumulation of separate organisms was gradually transformed to a
material of great solidity and hardness.
[Sidenote: FORMATION OF AMYGDALOIDS.]
In precisely the same way loose heaps of scoriæ, lapilli, or pumice
may, by the passage through them of water containing various substances
in solution, have their vesicles filled with crystals, and thus be
converted into the hardest and most solid of rock-masses. Similarly
the scoriaceous portions of lava-streams have their vesicles filled
with crystalline substances deposited from a state of solution, and
are thus converted into a solid mass which may at first sight appear
to offer but little resemblance to the vesicular materials of recent
lava-streams. To these vesicular rocks which have their cavities filled
with crystalline substances geologists apply the name of amygdaloids
(_L. amygdalus_, an almond). The cavities in lava-rocks are usually
more or less elongated, owing to the movement of the mass while in
a still plastic state, and the crystalline materials filling these
cavities take the almond-like shape; hence the name.
When the amygdaloids and altered fragmentary ejections of volcanoes
are studied microscopically, their true character is at once made
manifest. The exposure of faces of these altered volcanic rocks to the
weathering influences of the atmosphere, in many cases also causes
their true nature to be revealed, the crystalline materials filling
the interstices and vesicles of the mass are dissolved away by the
rain-water containing carbonic acid, and the rock regains its original
cavernous structure and appearance. But this repeated passage of water
through volcanic rock-masses may result in the removal of so large a
portion of their materials that the remainder crumbles down into the
condition of a clay or mud.
In the basal wrecks of volcanoes, of which we have spoken, we usually
find only small and fragmentary remains of the great accumulations
of loose and scoriaceous materials which originally constituted the
bulk of the mountain mass. In the centre of the ground-plan of such
a denuded volcano we find great masses of highly crystalline or
granitic rock, which evidently occupy vast fissures broken through
the sedimentary or other rocks upon which the volcanic pile has been
reared. These highly crystalline rocks exhibit, as we have shown,
clear evidence of having been consolidated from a state of fusion
with extreme slowness and under enormous pressure, but their ultimate
chemical composition is identical with that of the lavas which have
been ejected from the volcano.
When, as frequently happens, the volcano, after pouring out one kind
of lava for a certain period, has changed the nature of its ejections,
and given rise to materials of different composition, we find clear
evidence of the fact in studying the basal wreck or ground-plan of
the volcano. A great intrusive crystalline mass, of the same chemical
composition as the first-extruded lava, is found to be rent asunder and
penetrated by a similarly crystalline mass having the composition of
the lavas of the second period. Thus, in the volcanoes of the Western
Isles of Scotland, which are reduced by the action of denudation to
this condition of basal wrecks, we find that rhyolites, trachytes, and
andesites were ejected during the earlier periods of their history, and
basalts during the later periods.
[Illustration: Fig. 57.--Plan of the Dissected Volcano of Mull, in the
Inner Hebrides.]
[Illustration:
_a_ Rocks on which the Volcano has been built up.
_b_ Great intrusive masses of acid and intermediate rocks.
_c_ Lara currents of basalt which have flowed from _d_.
_d_ Intrusive masses of gabbros & dolerite.
_e_ Lava currents which have flowed from _b_.
_f_ Volcanic tuffs and agglomerates.
Fig. 58.--Section of the Volcano along the line _A B_.]
[Sidenote: ANCIENT VOLCANO OF MULL.]
We perceive on studying the ground-plan of these volcanoes that
great masses of granite, syenite, and diorite--the crystalline
representatives of the first-extruded lavas--are penetrated by
intrusions of gabbro--the granitic form of the later-ejected lavas.
These features are admirably illustrated by the ruined volcano now
constituting the Island of Mull, one of the Inner Hebrides, a plan
of which is given in fig. 57, and a section in fig. 58. This volcano
probably had a diameter at its base of nearly thirty miles, and a
height of from 10,000 to 12,000 feet, but is now reduced to a group of
hills few of which exceed 3,000 feet in height.
From these great intrusive masses of highly crystalline rocks there
proceed in every direction great spurs or dykes, which are evidently
the radiating fissures formed during the outwelling of igneous
materials from below, injected by these fluid substances. The rock
forming these dykes is often less perfectly crystalline than that which
constitutes the centre of the mass, and we may indeed detect among the
materials of these dykes examples of every variety of structure, from
the perfectly crystalline granite to the more or less glassy substance
of lavas. Besides the vertical or oblique dykes we also find horizontal
sheets, which, passing from these central masses, have penetrated
between the surrounding strata, often, as we have seen, to enormous
distances.
For the sake of simplicity, we have spoken of these ground-plans, or
basal wrecks of volcanoes, as constituting a flat plain; as a matter of
fact, however, the unequal hardness of the materials composing volcanic
mountains causes them to assume, under the influence of denuding
agencies, a very rugged and uneven surface. The hard crystalline
materials filling the central vent stand up as great mountain groups;
each large dyke, by the removal of the surrounding softer materials, is
left as a huge wall-like mass, while the remnants of lava-streams are
seen constituting a number of isolated plateaux.
The great Island of Skye is the basal wreck of another volcano which
was also in eruption during Tertiary times; probably, many millions
of years ago. This immense volcano had originally a diameter at its
base of about thirty miles, and a height of 12,000 to 15,000 feet,
and must have been comparable to Etna or Teneriffe in its dimensions.
At the present time, there is nothing left of this vast pile but the
highly crystalline granites and gabbros filling up the great fissures
through which the eruption of igneous materials took place. These, worn
by denudation into rounded dome-like masses and wild rugged peaks,
constitute the Red Mountains and Coolin Hills of Skye, which rise to
the height of more than 3,000 feet above the sea-level. From these
great, central masses of crystalline rocks, innumerable radiating dykes
may be found rising through the surrounding rock-masses, with isolated
patches of the scoriæ and lapilli ejected from the volcano, which have
here and there escaped removal by denudation. Along what were the
outskirts of this great mountain-mass are found flat-topped hills,
built up of lava-streams, only small portions of which have escaped
removal by denudation.
[Sidenote: RESERVOIRS BENEATH VOLCANOES.]
But this wearing away of the structure of a volcanic cone by the
denuding forces may proceed even one stage farther, and we may then
have revealed for our inspection and study the mass of originally fluid
materials, from which one or more volcanoes have been fed, cooled and
consolidated in their original reservoir. There are many examples of
masses of granitic or highly crystalline rocks, having precisely the
same composition as the different varieties of lavas, which are found
lying in the midst of the sedimentary rocks, and sending off into
these rocks veins and dykes of the same composition with themselves.
No one who has carefully studied the appearances presented by volcanic
mountains in different stages of dissection, by the action of denuding
forces, can avoid recognising these great granitic masses as the cooled
reservoirs from which volcanoes have in all probability been supplied
during earlier periods of the earth's history.
The eruption of these great masses of incandescent rock, impregnated
with water and acid gases, through strata of limestone, sandstone,
clay, coal, &c., may be expected to produce striking and wonderful
chemical changes in the latter. Nor are we disappointed in these
anticipations. Whenever we examine the sedimentary materials around
volcanic vents, we find that, in contact with the once-fused materials,
they everywhere exhibit remarkable evidences of the chemical action
to which they have been subjected. The limestones are converted into
statuary marble, the sandstones pass into quartzite, the days assume
the hardness and lustre of porcelain, while the coals have lost their
volatile ingredients and assumed a form like coke or graphite. And
these changes are found to extend in many cases to the distance of many
hundreds of yards from the planes of junction between the igneous and
the sedimentary materials.
Among the most interesting effects resulting from the extrusion of
masses of incandescent rock, charged with water and various gases,
through beds of limestone, clay, sandstone, &c., we may mention the
production of those beautiful crystalline minerals which adorn our
museums and are so highly prized as gems. By far the larger part of
these beautiful minerals have been formed, directly or indirectly, by
volcanic agencies.
These gems and beautiful minerals are, for the most part, substances
of every-day occurrence, which entirely owe their beauty to the
crystalline forms they have assumed. The diamond is crystallised
carbon, the ruby and sapphire are crystallised alumina, the amethyst
and a host of other gems are crystallised silica; and in almost all
cases the materials of gems are common and widely diffused, it is only
in their finely crystalline condition that they are rare and therefore
valuable.
[Sidenote: FORMATION OF VOLCANIC MINERALS.]
Crystals are formed during the slow deposition of a substance, either
by the evaporation of a liquid in which it is dissolved, by its
volatilisation, or its cooling from a state of fusion. In many cases it
can be shown that the formation of large and regular crystals is aided
if the work goes on with extreme slowness and under great pressure.
By sealing up various substances in tubes containing water which can
be kept at a high temperature, minute crystals of many well-known
minerals have been artificially formed by chemists. Part of the water
converted into steam has formed a powerful spring, which, reacting
upon the remainder of the liquid in the tube, has subjected it to
enormous pressure, and under these conditions of extreme pressure and
temperature, chemical actions take place of which we have no experience
under ordinary circumstances. The experiments of Mr. Hannay seem to
prove that when carbon is separated from certain organic substances
at a high temperature and under great pressure, it may crystallise in
the form of the diamond. And the recent discovery of diamonds in the
midst of materials filling old volcanic vents in South Africa seems
to show that this was in many cases the mode in which the gem was
originated. Even under the conditions which prevail at the earth's
surface, however, minute and unnoticed chemical actions taking place
during long periods of time, produce most remarkable results. This has
been well illustrated by M. Daubrée, who has shown that in the midst of
masses of concrete which the Romans built up around the hot springs of
Plombières and other localities, many crystalline minerals have been
formed, in the course of 2,000 years, by the action of the waters upon
the ingredients of the concrete.
But most of the crystals of minerals which have been thus artificially
formed are of minute, indeed often of microscopic, dimensions. In the
underground reservoirs beneath volcanoes, however, we have all the
necessary conditions for the formation of crystals of minerals on a
far grander scale. High temperatures, pressures far greater than any
we can command at the earth's surface, the action of superheated steam
and many acid gases on the various constituents of both igneous and
sedimentary rocks, and, above all, time of almost unlimited duration;
these constitute such a set of conditions as may fairly be expected
to result in the formation of crystals, similar to those artificially
produced but of far greater size and beauty.
If we visit those parts of the earth's surface where great masses of
fused volcanic rock have slowly cooled down in contact with sedimentary
materials, we shall not be disappointed in our expectations. Diamonds,
rubies, sapphires, emeralds, topazes, garnets, and a host of equally
beautiful, if less highly prized, crystalline substances, are found in
such situations, lying in the subterranean chemical laboratories in
which they have been formed, but now, by the action of denuding forces,
revealed to our view.
In some cases it is not necessary to penetrate to these subterranean
laboratories in order to find these beautiful gems and other
crystallised minerals; for the steam jets which issue from volcanic
fissures carry up fragments of rock torn from the side of the vent,
and in the cavities and fissures of such ejected masses beautiful
crystallised products are often found. Such rock-fragments containing
minerals finely crystallised are found abundantly on the flanks of
Vesuvius and other active volcanoes, and among the materials of the
Laacher See and other extinct volcanoes.
[Sidenote: FORMATION OF MINERAL-VEINS.]
But it is not only the finely crystallised minerals and gems which
we owe to volcanic action. The various metallic minerals have nearly
all been brought from deep-seated portions of the earth's crust and
deposited upon the sides of rock-fissures by the agency of the same
volcanic forces. It is these forces which have, in the first instance,
opened the cracks through the solid rock masses; and, in the second
place, have brought the metallic sulphides, oxides, and salts--either
in fusion, in solution, or in a vaporised condition--from the
deep-seated masses within the earth, causing them to crystallise upon
the sides of the fissures, and thus form those metallic lodes and veins
which are within reach of our mining operations.
There is still one other important class of minerals which owe the
existence, though indirectly, to volcanic agencies. The cavities of
igneous rocks, especially the vesicles formed by the escape of steam,
constitute, when filled with water, laboratories in which complicated
chemical reactions take place. The materials of the lava are gradually
dissolved and re-crystallised in new combinations. By this means the
most beautiful examples of such minerals as the agates, the onyxes,
the rock-crystals, the Iceland-spars, and the numerous beautiful
crystals classed together as 'Zeolites' have been formed. No one can
visit a large collection of crystalline minerals without being struck
with the large number of beautiful substances which have thus been
formed as secondary products from volcanic materials.