THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES.
That a volcanic vent, when once established, may display intense
activity during enormous periods of time, there cannot be the smallest
reason for doubting; for the accumulation of materials around some
existing volcanic centres must certainly have been going on during
many thousands, perhaps millions, of years. To us, whose periods of
observation are so circumscribed, it may therefore at first sight
appear a hopeless task to trace the 'life-history of a volcano,' to
discover the stages of its development, and to indicate the various
episodes which have occurred during the long periods it has been in
existence. But when it is remembered that we have the opportunity of
studying and comparing hundreds of such volcanoes, exhibiting every
varying phase of their development, we shall see that such an attempt
is by no means so unpromising as it at first sight appears to be. In
the present chapter, we shall give an account of the results which have
already been obtained by inquiries directed to this object.
[Sidenote: CYCLES OF VOLCANIC PHENOMENA.]
There is not the smallest room for doubt that during the past history
of our globe, exhibitions of subterranean energy have occurred at many
different parts of its surface. There is further evidence that at the
several sites where these displays of the volcanic forces have taken
place, the succession of the outbursts has run through a regular cycle,
gradually increasing in intensity to a maximum, and then as gradually
dying away.
A little consideration will show that the first portion of this cycle
of events is the one which it is most difficult to examine and study.
The products of the earlier and feeble displays of volcanic activity,
at any particular centre, are liable to be destroyed, or masked, during
the ejection of overwhelming masses of materials in the later stages
of its more matured energy. That the feeble displays of volcanic
force now exhibited in some localities will gradually increase in
intensity in the future, and eventually reach the grandest stage of
development, there can be no reason for doubting. But, unfortunately,
we are quite unable to discriminate these feeble manifestations, which
are the embryonic stages in the development of grand exhibitions of
the volcanic forces, from slight outbursts which die away and make no
farther sign.
From what has been proved concerning the true nature of volcanic
action, however, it is certain that the first step towards the
exhibition of such action, at any particular locality, must be the
production of an aperture in the earth's crust. Only by means of
such an aperture can the vapours, gases, and rocky materials reach
the surface, and give rise to the phenomena there displayed. There is
reason to believe that all such apertures are really of the nature of
fissures, or cracks, which have been opened through the superjacent
strata by the efforts of the repressed subterranean forces.
Some recent writers have, it is true, endeavoured to draw a distinction
between what they call 'fissure-eruptions,' and eruptions taking
place from volcanic cones. But all volcanic outbursts are truly
'fissure-eruptions'--the subterranean materials finding their way to
the surface through great cracks, which, in a more or less vertical
position, traverse the overlying rock-masses. It is true that in many
cases portions of these cracks soon get choked up, while other portions
become widened, and the volcanic energy is concentrated at such spots.
Thus the materials ejected from these fissures are usually emitted in
greatest quantities at one or more points along the fissure, and a
single great volcanic vent, or a row of smaller vents, is established
upon the line at which the fissure reaches the surface.
We have seen that the amount of explosive action taking place at
different volcanic vents varies according to the proportion of
imprisoned water contained in the lava. In the cases where there is
much explosive action, vast accumulations of scoriæ, lapilli, and
dust take place, and cones of great size are built up; but in those
cases where the explosive action is small the lavas flow quietly from
the vent, and only small scoriæ-cones are thrown up, these being
probably soon swept away by the lava-currents themselves or by denuding
agencies. But both kinds of eruption have equal claims to be called
'fissure-eruptions.'
[Sidenote: FORMATION OF VOLCANIC FISSURES.]
In the expansive force of great masses of imprisoned vapour, we have
a competent cause for the production of the fissures through which
volcanic outbursts take place. Such fissures are found traversing the
rocks lying above volcanic foci, and often extending to distances of
many miles, or even hundreds of miles, from the centres of activity.
Some of these cracks are found to be injected with fused materials from
below, others have been more or less completely filled with various
minerals that have been volatilized, or carried by superheated waters
from the deeper regions of the earth's crust. That many of the cracks
thus produced in the superjacent rocks, by the heaving forces of
imprisoned vapour seeking to escape, never reached the surface, we have
sufficient proof in many mining regions.
If we now transfer our attention from the deeper portions of the
earth's crust to the surface, we can well understand how the attempts
of the imprisoned vapours to force a passage for themselves through the
solid rock-masses would lead to shocks and jars among the latter. Each
of these shocks or jars would give rise, in the surrounding portions
of the earth's crust, to those vibrations which we know as earthquakes.
The close connection between most earthquakes and volcanic phenomena is
a fact that does not admit of the smallest doubt; and though it would
be rash to define all earthquakes as 'uncompleted efforts to establish
a volcano,' yet, in the efforts of the repressed subterranean forces to
find a vent by the production of fissures in the overlying rock-masses,
we have a cause competent to the production of those shocks which are
transmitted to such enormous distances as waves of elastic compression.
We have seen that the production of the fissure upon which the small
volcano of Monte Nuovo was thrown up was preceded by a succession
of earthquakes, which for a period of over two years terrified the
inhabitants of the district, and might have warned them of the
coming event. In the same manner, doubtless, the period before the
appearance of volcanic phenomena in a new area would be marked by
powerful subterranean disturbances within it, due to the efforts of the
imprisoned vapours to force for themselves a channel to the surface.
[Sidenote: NATURE OF FIRST EJECTIONS FROM FISSURES.]
In the case of Monte Nuovo, we have seen that the fissure, when
produced, emitted water--at first in a cold, then in a boiling
condition--and, eventually, steam and scoriæ. It is probable that
through the first cracks which reached the surface, during the heaving
of the subterranean forces, water, charged with carbonic acid, flowed
abundantly, and that these cold springs, charged with carbonic acid
and carbonate of lime, would be succeeded by others which were hot
and contained silica in solution. In Hungary, the Western Isles of
Scotland, and many other volcanic districts, we find abundant evidence
that, before the eruption of lavas in the area, great masses of
travertine and siliceous sinter were formed by the action of cold and
hot springs.
As the volcanic action became more intense by the more perfect opening
of the fissures, the evolution of carbonic add gas would be succeeded
by the appearance of sulphurous acid, sulphuretted hydrogen, boracic
acid, and hydrochloric acid, which recent studies have shown to be
successively emitted from volcanic vents as the temperature within them
rises. At last lava or molten rock becomes visible within the fissures,
and the ejection of the frothy masses--scoriæ, pumice, lapilli and
dust--commences, and this is sometimes succeeded by the outflow of
currents of lava.
That volcanoes originate upon lines of fissure in the earth's crust
we have the most convincing proofs. Not only have such fissures been
seen in actual course of formation at Vesuvius, Etna, and other active
volcanoes, but a study of the volcanoes dissected by denudation affords
the most convincing evidence of the same fact. The remarkable linear
arrangement seen in groups of volcanoes, which is conspicuous to the
most superficial observer, confirms this conclusion.
[Illustration: Fig. 81.--Map of the volcanic group of the Lipari
Islands, illustrating the position of the lines of fissure on which the
volcanoes have been built up.]
[Sidenote: SHIFTING OF VENTS ALONG FISSURES.]
We have described the action going on at Stromboli as typical of that
which occurs at all volcanic vents. Stromboli is, however, one among
a group of islands all of which are entirely of volcanic origin. The
volcanoes of this group of islands, the Æolian or Lipari Islands, are
arranged along a series of lines which doubtless mark fissures in the
earth's crust. These fissures, as will be seen by the accompanying map
(fig. 81), radiate from a centre at which we have proofs of the former
existence of a volcano of enormous dimensions. It is a very interesting
fact, which the studies of Prof. Suess have established, that the
earthquakes which have so often desolated Calabria appear to have
originated immediately beneath this great centre of volcanic activity.
[Illustration: Fig. 82.--The Puy de Pariou in the Auvergne,
illustrating the shifting of the centre of eruption along a line of
fissures.]
When two volcanic cones are thrown up on the same line of fissure,
their full development is interfered with, and irregularities in their
form and characters are the consequence. In the plan (fig. 82) and the
section (fig. 83) an example is given of the results of such a shifting
of the centre of eruption along a line of fissure. By the second
outburst, one-half of the first-formed cone has been removed, and the
second-formed overlaps the first.
[Illustration: Fig. 83.--Ideal section of the Puy de Pariou.]
Sometimes a number of scoria- or tuff-cones are thrown up in such close
proximity to one another along a line of fissure, that they merge into
a long irregular heap on the summit of which a number of distinct
craters can be traced. An example of this kind was furnished by the
line of scoria-cones formed above the fissure which opened on the
flanks of Etna in 1865 (see fig. 84).
[Illustration: Fig. 84.--Fissure formed on the flanks of Etna during
the eruption of 1865.
_a._ Monte Frumento, an old parasitic cone. _b._ Line of fissure. _c,
c, c._ New scoria-cones thrown up on line of fissure. _d._ Lava from
same.]
[Illustration: Fig. 85.--Plan of the Island of Vulcano, based on the
map of the Italian Government.]
[Sidenote: SHIFTING OF ERUPTIONS ALONG FISSURES.]
Even in the case of great composite cones, however, we sometimes find
proofs of the centre of eruption having shifted its place along the
line of fissure. No better example of this kind could possibly be
adduced than that of the Island of Vulcano, with the peninsula of
Vulcanello, which is joined to it by a narrow isthmus (see the map,
fig. 81, p 192). In fig. 85 we have given an enlarged plan of this
island which will make its peculiar structure more intelligible (see
also the section given in fig. 77, No. 6, facing p. 178).
The south-eastern part of the island consists of four crater-rings,
one half of each of Which has been successively destroyed, through
the shifting of the centre of eruption towards the north-west, along
the great line of fissure shown in the general map (fig. 81). The
last formed of these four crater-rings is the one which is now most
complete, and culminates in Monte Saraceno (1581 ft.), _a_ in the
plan, the highest point in the island. The older crater-rings have
been in part removed by the inroads of the waters of the Mediterranean
on the shores of the island. In the centre of the great crater,
_b_, which we have just described, rises the present active cone of
Vulcano, 1,266 feet high, and having a crater, _c_, about 600 yards
in diameter and more than 500 feet in depth. From this cone, a great
stream of obsidian, _e_, flowed in the year 1775, and a small crater,
_d_, the Fossa Anticha, has been opened in the side of the cone. The
continuation of the same line of fissure is indicated by a ruined
tuff-cone, _f_, known as the Faraglione, and the three scoria-cones of
Vulcanello, _g, h_, which have been thrown up so close to one another
as to have their lower portions merged in one common mass, as shown in
fig. 86.
[Sidenote: SYSTEMS OF VOLCANIC FISSURES.]
Even in volcanoes of the largest dimensions we sometimes find proofs of
the centre of eruption having shifted along the line of fissure. Lyell
showed that such a change in the position of the central axis of the
volcano had taken place in Etna, and the same phenomenon is exhibited
in the clearest manner' by some of the ancient volcanoes of the Inner
Hebrides, which have been dissected by the denuding forces.
[Illustration: Fig. 86.--Vulcanello, with its three craters.
_a._ The most recently-formed and perfect crater, _b_ and _c_. Older
craters, the walls of which have been partly removed by denudation,
_e._ Lava-currents proceeding from _b_. The section exposed in the
cliff at _d_ is represented in fig. 35, p. 116.]
In the case of the Lipari Islands, the fissures along which the
volcanic mountains have been thrown up radiate from a common centre,
and a similar arrangement can be traced in many volcanic regions,
especially those in which a great central volcano has existed. In other
cases, however, as in the Campi Phlegræi, the volcanic vents appear
to be formed along lines which assume a parallel arrangement, and
this doubtless marks the relative position of the original fissures
produced in the earth's crust when these volcanoes were formed. In some
other cases we find evidences of the existence of a principal fissure
from the sides of which smaller cracks originated. These three kinds of
arrangements of volcano-producing fissures are equally well illustrated
when we study those denuded districts, in which, as we have seen, the
ground-plans of volcanic structures are revealed to our view.
There is now good ground for believing that in volcanic vents, at which
long-continued eruptive action takes place, the lavas of different
chemical composition make their appearance in something like a definite
order. It had been remarked by Scrope and other geologists at the
beginning of the present century, that in many volcanic areas the acid
or trachytic lavas were erupted before the basic or basaltic.
Von Richthofen, by his studies in Hungary and the volcanic districts
of the Rocky Mountains, has been able to enunciate a law governing the
natural order of succession of volcanic products; and although some
exception to this law may be mentioned, it is found to hold good for
many other districts than those in which it was first determined.
In a great number of cases it has been found that the first erupted
rocks in a volcanic district are those of intermediate composition
which are known as andesites. These andesites, which are especially
characterised by the nature of their felspar, sometimes contain free
quartz and are then known as quartz-andesites or dacites, from their
abundance in Transylvania, the old Roman province of Dacia.
[Sidenote: ORDER OF ERUPTION OF VOLCANIC PRODUCTS.]
Von Richthofen suggests that another class of volcanic rocks, to which
he gives the name of 'propylites,' were in every case erupted before
the andesites, and in support of his views adduces the fact that in
many instances propylites are found underlying andesites. But the
propylites are, in chemical composition, identical with the andesites,
and like them present some varieties in which quartz occurs, and others
in which that mineral is absent. In their microscopic characters the
propylites differ from the andesites and dacites only in the fact that
the former are more perfectly crystalline in structure, being indeed in
many cases quite undistinguishable from the diorites or the plutonic
representatives of the andesites. The propylites also contain liquid
cavities, which the andesites and dacites as a rule do not, and the
former class of rocks, as Prof. Szabo well points out, are usually much
altered by the passage of sulphurous and other vapours, in consequence
of which they frequently contain valuable metallic ores.
The extrusion of these andesitic lavas is sometimes accompanied, and
sometimes preceded or followed, by eruptions of trachytic lavas--that
is, of lavas of intermediate composition which have a different kind of
felspar from that prevailing in the andesites.
In the final stages of the eruptive action in most volcanic districts
the lavas poured forth belong to the classes of the rhyolitic or acid,
and the basaltic or basic lavas.
These facts are admirably illustrated in the case of the volcanic
district of the Lipari Islands, to which we have had such frequent
occasion to refer. The great central volcano of this district, which
now in a ruined condition constitutes a number of small islets (see
the map, fig. 81, p. 192), is composed of andesitic lavas. The other
great volcanoes thrown up along the three radiating lines of fissure
are composed of andesitic and trachytic rocks. But all the more recent
ejections of the volcanoes of the district have consisted either of
rhyolites, as in Lipari and Vulcano, or of basalts, as in Stromboli and
Vulcanello.
Von Richthofen and the geologists who most strongly maintain the
generalisations which he has made concerning the order of appearance
of volcanic products, go much farther than we have ventured to do,
and insist that in all volcanic districts a constant and unvarying
succession of different kinds of lavas can be made out. It appears to
us, however, that the exceptions to the law, as thus precisely stated,
are so numerous as to entirely destroy its value.
The generalisation that in most volcanic districts the first ejected
lavas belong to the intermediate group of the andesites and trachytes,
and that subsequently the acid rhyolites and the basic basalts made
their appearance, is one that appears to admit of no doubt, and is
found to hold good in nearly all the volcanic regions of the globe
which have been attentively studied.
The Tertiary volcanic rocks of our own country, those of North
Germany, Hungary, the Euganean Hills, the Lipari Islands, and many
other districts in the Old World, together with the widespread volcanic
rocks of the Rocky Mountains in the New World, all seem to conform to
this general rule.
[Sidenote: THEORY OF VOLCANIC MAGMAS.]
In connection with this subject, it may be well to refer to the
ideas on the composition of volcanic rocks which were enunciated by
Bunsen, and the theoretic views based on them by Durocher. Bunsen
justly pointed out that all volcanic rocks might be regarded as
mixtures in varying proportions of two typical kinds of materials,
which he named the 'normal trachytic' and the 'normal pyroxenic'
elements respectively. The first of these corresponds very closely in
composition with the acid volcanic rocks or rhyolites, and the second
with the basic volcanic rocks or basalts. Durocher pointed out that
if quantities of these different materials existed in admixture, the
higher specific gravity of the basic element would cause it gradually
to sink to the bottom, while the acid element would rise to the top.
Carrying out this idea still further, he propounded the theory that
beneath the earth's solid crust there exist two magmas, the upper
consisting of light acid materials, the lower of heavy basic ones; and
he supposed that by the varying intensity of the volcanic forces we may
have sometimes one or the other magma erupted and sometimes varying
mixtures of the two.
The study of volcanic rocks in recent years has not lent much support
to the theoretic views of Durocher concerning the existence of two
universal magmas beneath the earth's crust; and there are not a few
facts which seem quite irreconcilable with such a theory. Thus we
find evidence that in the adjacent volcanic districts of Hungary and
Bohemia, volcanic action was going on during the whole of the latter
part of the Tertiary period. But the products of the contemporaneous
volcanic outbursts in adjacent areas were as different in character
as can well be imagined. The volcanic rocks all over Hungary present
a strong family likeness; the first erupted were trachytes, then
followed andesites and dacites in great abundance, and lastly rhyolites
and basalts containing felspar. But in Bohemia, the lavas poured out
from the volcanoes during the same period were firstly phonolites
and then basalts containing nepheline and leucite. It is scarcely
possible to imagine that such very different classes of lavas could
have been poured out from vents which were in communication with the
same reservoirs of igneous rock, and we are driven to conclude that the
Hungarian and Bohemian volcanoes were supplied from different sources.
[Sidenote: SEPARATION OF LAVAS IN RESERVOIRS.]
But the undoubted fact that in so many volcanic regions the eruption of
andesitic and trachytic rocks, which are of intermediate composition,
is followed by the appearance of the differentiated products, rhyolite
and basalt, which are of acid and basic composition respectively, lends
not a little support to the view that under each volcanic district a
reservoir of more or less completely molten rock exists, and that in
these reservoirs various changes take place during the long periods
of igneous activity. During the earlier period of eruption the heavier
and lighter elements of the contents of these subterranean reservoirs
appear to be mingled together; but in the later stages of the volcanic
history of the district, the lighter or acid elements rise to the top,
and the heavier or basic sink to the bottom, and we have separate
eruptions of rhyolite and basalt. We even find some traces of this
action being carried still further. Among the basalts ejected from the
volcanoes of Northern Germany, Bohemia, Styria, Auvergne, and many
other regions, we not unfrequently find rounded masses consisting of
olivine, enstatite, augite, and other heavier constituents of the rock.
These often form the centre of volcanic bombs, and are not improbably
portions of a dense mass which may have sunk to the bottom of the
reservoirs of basaltic materials.
In consequence of the circumstance that the eruption of lavas of
intermediate composition usually precedes that of other varieties, we
usually find the central and older portions of great volcanoes to be
formed of andesites, trachytes, or phonolites, while the outer and
newer portions of the mass are made up of acid or basic lavas. This is
strikingly exemplified in the great volcanoes of the Auvergne and the
Western Isles of Scotland, in all of which we find that great mountain
masses have, in the first instance, been built up by extrusions of lava
of the intermediate types, and that through this central core fissures
have been opened conveying basic lavas to the surface. From these
fissures great numbers of basaltic lava-streams have issued, greatly
increasing the height and bulk of the volcanic cones and deluging the
country all around.
The lavas of intermediate composition--the andesites, trachytes, and
phonolites--possess, as we have already seen, but very imperfect
liquidity as they flow from the volcanic vents. Hence we find them
either accumulating in great dome-shaped masses above the vent or
forming lava-streams which are of great bulk and thickness, but do not
flow far from the orifices whence they issue. The more fusible basaltic
lavas, on the other hand, spread out evenly on issuing from a vent, and
sometimes flow to the distance of many miles from it. This difference
in the behaviour of the intermediate and basic lavas is admirably
illustrated in the volcanic districts of the Auvergne and the Western
Isles of Scotland.
In other cases, like Vesuvius, we find that great volcanic cones of
trachytic tuff have been built up, and that these masses of fragmentary
trachytic materials have been surrounded and enclosed by the ejection,
at a later date, of great outbursts of basaltic lavas. In still
other cases, of which Rocca Monfina in Southern Italy constitutes an
excellent example, we find that a great crater-ring of trachytic tuffs
has been formed in the first instance, and in the midst of this a cone,
composed of more basic materials, has been thrown up.
[Sidenote: EXCEPTIONS TO THE GENERAL LAW.]
In all these volcanoes we see the tendency towards the eruption
of intermediate lavas in the first instance, and of basaltic and
acid lavas at a later date. Valuable, however, as are the early
generalisations of Scrope, and the more precise law enunciated by Von
Richthofen concerning the 'natural order of succession of volcanic
products,' we must not forget that there are to be found a considerable
number of exceptions to them. There are some volcanic centres from
which only one kind of lava has been emitted, and this may be either
acid, basic, or intermediate in composition; and on the other hand,
there are districts in which various kinds of lava have been ejected
from the same vents within a short period of time, in such a way as to
defy every attempt to make out anything like a law as to the order of
their appearance. Nevertheless the rules which we have indicated appear
to hold good in so great a number of cases that they are well worthy
of being remembered, and may serve as a basis on which we may reason
concerning the nature of the action going on beneath volcanic vents.
From the study of the external appearances of volcanic mountains,
combined with investigations of those which have been dissected by
denudation, we are able to picture to our minds the series of actions
by which the great volcanic mountains of the globe have been slowly and
gradually built up.
In the first instance the eruptions appear to have taken place at
several points along a line of fissure, but gradually all of these
would become choked up except one which became the centre of
habitual eruption. From this opening, ejections, firstly of lavas of
intermediate composition, and afterwards of basic materials, would take
place, until a volcano of considerable dimensions was built up around
it. But at last a point would be reached in the piling up of this
cone, when the volcanic forces below would be inadequate to the work
of raising the liquid lava through the whole length of the continually
upward-growing tube of the volcano. Under these circumstances the
expansive force of the imprisoned steam would find it easier to rend
asunder the sides of the volcanic cone than to force the liquid
material to the summit of the mountain. If these fissures reached the
surface explosive action would take place, in consequence of the escape
of steam from the glowing mass, and scoria-, tuff-, and lava-cones
would be formed above the fissure. In this way, as we have already
pointed out, the numerous 'parasitic cones' which usually abound on
the flanks of the greater volcanic mountains have been formed. The
extrusion of these masses of scoriæ and lava on the flanks of the
mountain tends, not only to increase the bulk of the mass, but to
strengthen and fortify the sides. For by the powerful expansive force
at work below, every weak place in the cone is discovered and a fissure
produced there; but by the extrusion of material at this fissure, and
still more by the consolidation of the lava in the fissure, the weak
place is converted into one of exceptional strength.
[Sidenote: INTRUSIVE MASSES BENEATH VOLCANOES.]
As the sides of the cone are thus continually repaired and strengthened
they are rendered more capable of withstanding the heaving forces
acting from below, and these forces can then only find vent for
themselves by again raising the liquefied lava to the central orifice
of the mountain. Many volcanoes, like Etna, exhibit this alternation of
eruptive action from the crater at the summit of the mountain, and from
fissures opened upon its flanks, the former tending to raise the height
of the volcanic pile, the latter to increase its bulk.
But at last a stage will be reached when the volcanic forces are
no longer able either to raise the lava up the long column of the
central vent on the one hand, or to rend asunder the strongly-built
and well-compacted flanks of the mountain on the other. It is probably
under these conditions, for the most part, that the lavas find their
way between the masses of surrounding strata and force them asunder in
the way that we have already described.
In the case of the more fluid basaltic lavas, as was pointed out so
long ago by Macculloch, the liquefied materials may find their way
between the strata to enormous distances from the volcanic centre. Such
extended flat sheets of igneous rock retain their parallelism with the
strata among which they are intruded over large areas, and did not
probably produce any marked phenomena at the surface.
But in the case of less fluid lavas, such as those of intermediate or
acid composition, for example, the effect would be far otherwise. Such
lavas, not flowing readily from the centre of eruption, would tend to
form great bulky lenticular masses between the strata which they forced
asunder, and, in so doing, could not fail to upheave and fissure the
great mountain-mass above. Vast lenticular masses of trachytic rock,
thus evidently forced between strata, have been described by Mr. G.
K. Gilbert, as occurring in the Henry Mountains of Southern Utah, and
by him have been denominated 'laccolites,' or stone-cisterns. Whether
the great basaltic sheets, like those described by Macculloch, and
those more bulky lenticular reservoirs of rock of which Mr. Gilbert
has given us such an admirable account, were in all cases connected
with the surface, may well be a matter for doubt. It is quite possible
that, in some cases, liquefied masses of rocky materials in seeking
to force their way to the surface only succeeded in thus finding a
way for themselves between the strata, and their energy was expended
before the surface was reached and explosive action took place. But it
is an undoubted fact that beneath many of the old volcanoes, of which
the internal structure is now revealed to us by the action of denuding
forces, great intrusive sheets and laccolites abound; and we cannot
doubt that beneath volcanoes now in a state of eruption, or in those
which have but recently become extinct, similar structures must be in
course of formation.
[Sidenote: EFFECTS OF INTRUSION BENEATH CONES.]
That great upheaving forces have operated on volcanoes, subsequently
to the accumulation of their materials, we have sufficient evidence in
the Val del Bove of Etna, the Caldera of Palma, the Corral of Madeira,
&c. In all of these cases we find a radial fissure ('barranco') leading
into a great crateral hollow; and these radial fissures are of such
width and depth that their origin can only be referred to a disruptive
force like that which would be exercised by the intrusion of masses of
more or less imperfectly fluid material between the subjacent strata.
These facts, of course, lend no countenance to the views formerly held
by many geologists, both in Germany and France, that the materials
of which volcanoes are built up were deposited in an approximately
horizontal position, and were subsequently blown up like a gigantic
bubble. In Etna, Palma, and Madeira we find abundant proofs that the
mass existed as a great volcanic cone before the production of the
fissures (barrancos), which we have referred to the force exercised
during the intrusion of great igneous masses beneath them.
But besides the horizontally-disposed intrusive sheets and laccolites,
great, radiating, vertical fissures are produced by the heaving forces
acting beneath those volcanic centres which have been closed up and
'cicatrised' by the exudation from them of subterranean materials.
These vertical intrusions, which we call dykes, like the horizontal
ones, differ in character, according to the nature of the materials
of which they are composed. Dykes of acid and intermediate lava are
usually of considerable width, and do not extend to great distances
from the centres of eruption. Dykes composed of the more-liquid, basic
lavas, on the other hand, may extend to the distance of hundreds of
miles from the central vent. The way in which comparatively narrow,
basaltic dykes are found running in approximately straight lines
for such enormous distances is a very striking fact, and bears the
strongest evidence to the heaving and expanding forces at work at
volcanic centres, during and subsequently to the extrusion of the
igneous products at the surface.
These basaltic dykes occur in such prodigious numbers around some
volcanic vents, that the whole of the stratified rocks in the immediate
vicinity are broken up by a complete network of them, crossing and
interlacing in the most complicated fashion. Farther away from the
vents, similar dykes are found in smaller numbers, evidently radiating
from the same centre, and sometimes extending to a distance of more
than a hundred miles from it. Nowhere can we find more beautiful
illustrations of such dykes than in the Western Isles of Scotland. When
composed of materials which do not so easily undergo decomposition as
the surrounding rocks, they stand up like vast walls; but when, on the
other hand, they are more readily acted on by atmospheric moisture than
are the rocks which enclose them, they give rise to deep trenches with
vertical sides, which render the country almost impassable.
[Sidenote: STRUCTURE OF INTRUSIVE MASSES.]
The lava consolidating in these horizontal intrusions (sheets and
laccolites), and the vertical intrusions (dykes), is usually more
crystalline in structure than the similar materials poured out at the
surface. In the same dyke or sheet, when it is of great width, we
often find every variation--from a glassy material formed by the rapid
cooling of the mass where it is in contact with other rocks, to the
perfectly crystalline or granitic varieties which form the centre of
the intrusion. It is in these dykes and other intrusions that we find
the most convincing evidence of the truth of the conclusions, which
we have enunciated in a former chapter, concerning the dependence of
the structure of an igneous rock upon the conditions under which it
has consolidated. One material is found, under varying conditions,
assuming the characters of obsidian, rhyolite, quartz-felsite, or
granite; another, under the same set of conditions, taking the form of
tachylyte, basalt, dolerite, and gabbro.
That these great intrusive masses, sheets and dykes, in their passage
between the sedimentary rocks sometimes find places where the overlying
strata are of such thinness or incoherence that the liquefied rocks are
able to force a way for themselves to the surface, we have the clearest
proof. In some dykes we find the rock in their upper portions losing
its compact character and becoming open and scoriaceous, showing that
the pressure had been so far diminished as to allow of the imprisoned
water flashing into steam.
All round great volcanoes which have become extinct we frequently find
series of small volcanic cones, which have evidently been thrown up
along the lines where the great lava-filled fissures, which we have
been describing, have reached the surface and given rise to explosive
action there. The linear arrangement of these small cones, which are
thrown up in the plains surrounding vast volcanic mountains that have
become extinct, is very striking. The numerous 'puys' of the Auvergne
and adjoining volcanic regions of Central France are for the most part
small scoria- and lava-cones which were thrown up along great lines
of fissure radiating from the immense, central, volcanic mountains of
the district, after they had become extinct. These scoria-cones and
the small lava-streams which flow from them, as was so well shown by
Mr. Scrope, mark the latest efforts of the volcanic forces beneath the
district before they finally sank into complete extinction. In the
Western Isles of Scotland, as I have elsewhere shown, we can study the
formation of these later-formed cones in the plains around extinct
volcanic mountains, with the additional advantage of having revealed
to us, by the action of the denuding forces, their connection with the
great radiating fissures.
It has been shown that the several stages in the decline of each
volcanic outburst is marked by the appearance at the vent of certain
acid gases. In the same way, after the ejection of solid materials
from a volcanic vent has come to an end, certain gaseous substances
continue to be evolved; and as the temperature at the vents declines,
the nature of the volatile substances emitted from them undergoes a
regular series of changes.
[Sidenote: ORDER OF EMISSION OF VOLCANIC GASES.]
M. Fouqué, by a careful series of analyses of the gases which he
collected at different gaseous vents, or fumaroles as they are called,
in the crater of Vulcano, has been able to define the general relations
which appear to exist between the temperature at a volcanic orifice
and the volatile substances which issue from it. He found that in
fumaroles, in which the temperature exceeded 360° centigrade, and
in which in consequence strips of zinc were fused by the stream of
issuing gas, the analysis of the products showed sulphurous acid and
hydrochloric add to be present in large quantities, and sulphuretted
hydrogen and carbonic acid in much smaller proportions. Around these
excessively heated fumaroles, the lips of which often appear at night
to be red-hot, considerable deposits of sulphide of arsenic, chloride
of iron, chloride of ammonium, boracic acid, and sulphur were taking
place.
It was found, however, that as the temperature of the vent declined,
the emission of the sulphurous acid and hydrochloric acid diminished,
and the quantity of sulphuretted hydrogen and carbonic acid mingled
with them was proportionately increased.
In the same way it appears to be a universal rule that when a volcanic
vent sinks into a condition of temporary quiescence or complete
extinction the powerfully acid gases, hydrochloric acid and sulphurous
acid, make their appearance in the first instance, and at a later stage
these are gradually replaced by sulphuretted hydrogen and carbonic acid.
Of these facts we find a very beautiful illustration in the Campi
Phlegræi near Naples. With the exception of Monte Nuovo, the volcano
which has most recently been in a state of activity in that district
is the Solfatara. From certain apertures in the floor of the crater of
the Solfatara there issue continually watery vapours, sulphurous acid,
sulphuretted hydrogen, hydrochloric acid, and chloride of ammonium.
The action of these substances upon one another, and upon the volcanic
rocks through which they pass, gives rise to the formation of certain
chemical products which, from a very early period, have been collected
on account of their commercial value. The action of these add gases
upon the surrounding rocks is very marked; efflorescent deposits of
various sulphates and chlorides take place in all the crevices and
vesicles of the rock; sulphur and sulphide of arsenic are also formed
in considerable quantities; and the trachytic tuffs, deprived of their
iron-oxide, alkaline earths and alkalies, which are converted into
soluble sulphates and chlorides, are reduced to a white, powdery,
siliceous mass. Many volcanoes, which have sunk into a state of
quiescence or extinction like the Solfatara of Naples, exhibit the same
tendency to give off great quantities of the powerfully-acid gases
which act upon the surrounding rocks, and deprive them of their colour
and consistency. Such volcanoes are said by geologists to have sunk
into the 'solfatara stage.'
[Sidenote: SOLFATARA-STAGE OF VOLCANOES.]
At the Lake of Agnano and some other points in the Campi Phlegræi,
however, we find fissures from which the less-powerfully acid gases,
sulphuretted hydrogen and carbonic acid, issue. These gases as they,
are poured forth from the vents are found to be little, if at all,
above the temperature of the atmosphere. Sulphuretted hydrogen is an
inflammable gas, and in the so-called salses and mud-volcanoes, at
which it is ejected in considerable quantities, it not unfrequently
takes fire and bums with a conspicuous flame. Carbonic acid on account
of its great density tends to accumulate in volcanic fissures and
craters rather than to mingle with the surrounding atmosphere. At the
so-called Grotto del Cane, beside the Lago Agnano, it is the custom to
show the presence of this heavy and suffocating gas by thrusting a dog
into it, the poor animal being revived, before life is quite extinct,
by pouring cold water over it. At the Büdos Hegy or 'stinking hill' of
Transylvania, carbonic acid and sulphuretted hydrogen are emitted in
considerable quantities, and it is possible to take a bath of the heavy
gas, the head being kept carefully above the constant level of the
exhalations.
Although the stories of the ancient Avernian lake, across which no bird
could fly without suffocation, and of the Guevo Upas, or Poison Valley
of Java, which it has been said no living being can cross, may not
improbably be exaggerations of the actual facts, yet there is a basis
of truth in them in the existence of old volcanic fissures and craters
which evolve the poisonous sulphuretted hydrogen and carbonic acid
gases.
Besides the gases which we have already named, and which are the most
common at and characteristic of volcanic vents, there are some others
which are not unfrequently emitted. First among these we must mention
boracic acid, which, though not a remarkably volatile substance, is
easily carried along in a fine state of division in a current of steam.
At Monte Cerboli and Monte Rotondo in Tuscany, great quantities of
steam jets accompanied by sulphuretted hydrogen and boracic acid issue
from the rocks, and these jets being directed into artificial basins of
water, the boracic acid is condensed and is recovered by evaporation.
We have already noticed that boracic add is evolved with the gases at
Vulcano and other craters; and the part which this substance plays in
volcanic districts is shown by the fact that many of the rocks, filling
old subterranean volcanic reservoirs, are found to be greatly altered
and to have new minerals developed in their midst through the action
upon them of boracic acid.
Ammonia and various compounds of carbon, nitrogen, and hydrogen are
among the gases evolved from volcanic vents. In some cases these gases
may be produced by the destructive distillation of organic materials
in the sedimentary rocks through which volcanic outbursts take place.
But it is far from impossible that under the conditions of temperature
and pressure which exist at the volcanic foci, direct chemical union
may take place between substances, which at the surface appear to be
perfectly inert in each other's presence.
When the temperature at volcanic fissures is no longer sufficiently
high to cause water to issue in the condition of vapour or steam, as is
the case at the 'stufas' which we have described, it comes forth in the
liquid state. Water so issuing from old volcanic fissures may vary in
its temperature, from the boiling point downwards.
[Sidenote: GEYSERS AND HOT-SPRINGS.]
When the water issues at a temperature little removed from the boiling
point, it is apt to give rise to intermittent springs or geysers, the
eruptions of which exhibit a remarkable analogy with those of ordinary
volcanoes. Geysers may indeed be described as volcanoes in which heated
water, instead of molten rock, is forced out from the vent by the
escaping steam. They occur in great abundance in districts in which the
subterranean action is becoming dormant or extinct, such as Iceland,
the North Island of New Zealand, and the district of the National Park
in the Rocky Mountains.
Many attempts have been made to explain the exact mechanism by which
the intermittent action of geysers is produced, but it is not at all
probable that any one such explanation will cover all the varied
phenomena exhibited by them. Like volcanic outbursts, geyser eruptions
doubtless originate in the escape of bubbles of steam through a liquid
mass, and this liberation of steam follows any relief of pressure.
In districts where vast masses of lava are slowly cooling down from
a state of incandescence, and surface waters are finding their way
downwards while subterranean waters are finding their way upwards,
there can be no lack of the necessary conditions for such outbursts.
Sometimes the eruptions of geysers take place at short and regular
intervals, at other times they occur at wide and irregular intervals
of time. In some cases the outbursts take place spontaneously, and at
others the action can be hastened by choking up the vent with stones or
earth.
Other hot springs, like the Strudel of Carlsbad, rise above the surface
in a constant jet, while most of them issue quietly and flow like
ordinary springs.
Although the violent and paroxysmal outbursts of volcanic mountains
arrest the attention, and powerfully impress us with a sense of the
volcanic activity going on beneath the earth's surface, yet it may
well be doubted whether the quantity of heat, which the earth gets rid
of by their means, at all approaches in amount that which is quietly
dissipated by means of the numerous 'stufas,' gaseous exhalations, and
thermal springs which occur in such abundance all over its surface.
For while the former are intermittent in their action, and powerful
outbursts are interrupted by long periods of rest, the action of the
latter, though feeble, is usually continuous.
[Sidenote: EFFECTS OF HOT-SPRINGS.]
Most people may regard the hot spring of Bath as a very slight
manifestation of volcanic activity. This spring issues at a constant
temperature of 49° C, or 120° Fahr. As, however, no less than 180,000
gallons of water issue daily from this source, we may well understand
how great is the amount of heat of which the earth's crust is relieved
by its agency. It may indeed be doubted whether its action in this way
is not at least equal to that of a considerable volcano which, though
so much more violent, is intermittent in its action.
Nor are thermal springs by any means ineffective agents in bringing
materials from the interior of the earth's crust and depositing it
at the surface. The Bath spring contains various saline substances,
principally sulphates and chlorides, in solution in its waters. These
are quietly carried by rivers to the sea, and are lost to our view. The
spring has certainly maintained its present condition since the time
of the Romans, and I find that if the solid materials brought from the
interior of the earth during the last 2,000 years had been collected,
they would form a solid cone equal in height to Monte Nuovo. Yet we
usually regard the Campi Phlegræi as a powerfully-active volcanic
district, and the subterranean action in our own country as quite
unworthy of notice.
When we remember the fact that on the continent of Europe the hot and
saline springs may be numbered by thousands, and that they especially
abound in districts like Hungary, the Auvergne, the Rhine provinces,
and Central Italy, where volcanic action has recently become extinct,
we shall be able to form some slight idea of the work performed
by these agents, not only in relieving the earth's crust of its
superfluous heat, but in transporting materials in a state of solution
from the interior of that crust and depositing them at the surface. The
vast deposits of siliceous sinter and of travertine also bear witness
to the effects produced by hot and mineral springs.
Nor is the work of these springs confined to the surface. Mr. John
Arthur Phillips has shown that metallic gold and the sulphide of
quicksilver (cinnabar) have been deposited with the silica and other
minerals formed on the sides of a fissure from which hot springs issue
at the surface. There cannot be any doubt that the metallic veins or
lodes, which are the repositories of most of the metals employed in the
arts, have been formed in cracks connected with great volcanic foci,
the transfer of the various sulphides, oxides, and salts which fill
the vein having been effected either by solution, sublimation, or the
action of powerful currents of steam.
As the igneous activity of the district declines, the temperature
of the issuing gases and waters diminishes with it, until at last
the volcanic forces appear to wholly abandon that region and to be
transferred to another.
Yet even after all or nearly all indications of the volcanic agencies
cease to make themselves visible at the surface, occasional tremblings
of the earth's crust show that perfect equilibrium has not been
restored below, but that movements are taking place which result in
shocks that are transmitted through the overlying and surrounding
rock-masses as earthquake vibrations.
[Sidenote: NATURE OF VOLCANIC CYCLES.]
Such is the cycle of changes which appears to take place at each
district of the earth's surface, as it successively becomes the scene
of volcanic activity.
The invasion of any particular area of the earth's surface by the
volcanic forces appears to be heralded by subterranean shocks causing
earthquake vibrations. Presently the origination of fissures is
indicated by the rise of saline and thermal springs, and the issuing
of carbonic acid and other gases at the surface. As the subterranean
activity becomes more pronounced, the temperature of the springs and
emitted gases is found to increase, and at last a visible rent is
formed at the surface, exposing the incandescent materials below.
From this open fissure which has thus been formed, the gas and vapours
imprisoned in the incandescent rock-materials escape with such violence
as to disperse the latter in scoriæ and dust, or to cause them to
well out in great streams as lava-flows. Usually the action becomes
concentrated at one or several points at which the ejected materials
accumulate to form volcanic cones.
Sometimes the volcanic activity dies away entirely after these cones
are thrown up along the line of fissure, but at others some such
centre becomes for a longer or shorter time the habitual vent for the
volcanic forces in the district, and by repeated ejections of lavas and
fragmentary materials at longer or shorter intervals the cone increases
both in height and bulk.
When the height of the cone has grown to a certain extent, it becomes
more easy for the volcanic energies below to rend the sides of the
cone than to raise the molten materials to its summit. In this way
lateral or parasitic cones are thrown up on the flanks of the volcanic
mountain, the mass being alternately elevated and strengthened by the
ejections from the summit and sides respectively.
When the volcanic energies no longer suffice to raise the fluid
materials to the summit, nor to rend the sides of the volcano, fissures
with small cones may be formed in the plains around the great central
volcano.
At last, however, this energy diminishes so far that rock materials
can no longer be forced to the surface, the fissures become sealed up
by consolidating lava, and the volcanic cones fall into a condition of
extinction and decay.
The existence of heated materials at no great depth from the surface
is indicated by the outburst of gases and vapours, the formation of
geysers, mud-volcanoes, and ordinary thermal springs. But as the
underlying rocks cool down, the issuing jets of gas and vapour lose
their high temperature and diminish in quantity, the geysers and
mud-volcanoes become extinct, and the thermal springs lose their
peculiar character or disappear, and thus all manifestations of the
igneous energies in the district gradually die away.
[Sidenote: DURATION OF VOLCANIC CYCLES.]
Such a cycle of changes probably requires many hundreds of thousands,
or even many millions, of years for its accomplishment; but by the
study of volcanoes in every stage of their growth and decline we are
able to reconstruct even the minutest details of their history.