THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS
From what has been said in the preceding chapters it will be seen that
while some of the materials ejected from volcanic vents are, by the
movements of the air and ocean, distributed over every part of the
face of the globe, another, and by far the larger, part of the matter
so ejected, accumulates in the immediate vicinity of the vent itself.
By this accumulation of erupted materials, various structures are
built up around the orifices from which the ejections take place, and
the size and character of these structures vary greatly in different
cases, according to the quantity and nature of the ejected materials,
and the intensity of the eruptive forces by which they were thrown from
the orifice. We shall proceed in the present chapter to notice the
chief varieties in the forms and characters of the heaps of materials
accumulated round volcanic vents.
These heaps of materials vary in size from masses no bigger than a
mole-heap up to mountains like Etna, Teneriffe, and Chimborazo. The
size of volcanic mountains is principally determined by the conditions
of the eruptive action at the vent around which they are formed. If
this action exhausts itself in a single effort, very considerable
volcanic cones, like the Monte Nuovo with many similar hills in its
vicinity, and the Puys of Auvergne, may be formed; but if repeated
eruptions take place at longer or shorter intervals from the same vent,
there appears to be scarcely any limit to the size of the structures
which may, under such conditions, be formed. It is by this repeated
action from the same volcanic vent going on for thousands or even
millions of years, that the grandest volcanic mountains of the globe
have been built up. Such volcanoes have sometimes a diameter at their
base of from 30 to 100 miles, and an elevation of from 10,000 to 25,000
feet.
The _form_ of volcanic mountains is determined in part by the nature
of the materials ejected, and in part by the character of the eruptive
action.
From what has been said in the preceding chapter, it will be gathered
that the volcanoes built up by ejections of fragmentary materials
differ in many striking particulars from those formed by the outwelling
of lavas from volcanic vents. In a less degree, the volcanoes composed
of the same kind of volcanic materials also vary among themselves.
[Sidenote: CHARACTERS OF SCORIA-CONES.]
When masses of scoriæ in a semi-fluid condition are thrown to only a
little distance above the volcanic vent, so that they have not time to
assume a perfectly solid condition before they fall round the vent, the
rugged masses of lava unite to form heaps of most irregular shape. In
such cases, the falling fragments being in a semi-plastic state, stick
to the masses below, and do not tend to roll down the sides of the
heap. Irregular heaps of such volcanic scoriæ abound on the surfaces
of lava-streams, being piled up around each 'bocca' or vent which the
steam-jets escaping from the lava-currents form at their surfaces.
Such irregular accumulations of scoriæ were observed on the lavas of
Vesuvius during the eruptions of 1822, 1855, and 1872, and have also
been described in the case of many other volcanoes. In fig. 26 (p. 101)
we have given representations of a group of such irregular scoria-cones
which was observed by Schmidt on the Vesuvian lava of 1855. It will
be seen from this drawing that there is scarcely any limit to the
steepness of the sides of such scoria-heaps, in which the materials are
in an imperfectly solidified condition when they reach the ground.
But in the majority of cases, the scoriæ ejected from volcanic vents
are thrown to a great height, and are in a more or less perfectly
solidified condition when they fell to the ground again. In such cases
the fragments obey the ordinary mechanical laws of falling bodies,
rolling and sliding over one another, till they acquire a slope which
varies according to the size and form of the fragments. In this way
the great conical mounds are formed which are known as 'cinder-cones,'
or more properly as 'scoria-cones.' Scoria-cones usually vary in the
slope of their sides from 35° to 40° and may differ in size from mere
monticules to hills a thousand feet or more in height. Scoria-cones
of this character abound in many volcanic districts, as the Auvergne,
where they may be numbered by thousands. The materials forming such
scoria-cones vary in size from that of a nut to masses as large as a
man's head, and fragments of even larger dimensions are by no means
uncommon.
When the lava in a volcanic vent is perfectly glassy, instead of being
partially crystalline in structure, we find not scoriæ but pumice
ejected. In such cases, as in the Lipari Islands for example, we see
cones entirely built up of pumice. Such pumice-cones resemble in
the angle of their slope (see fig. 41, facing p. 124), the ordinary
scoria-cones, but are of a brilliant white colour, appearing as if
covered with snow.
[Sidenote: PRESERVATION OF SCORIA-CONES.]
Ordinary scoriæ are usually of a black colour when first ejected, but
after a short time the black oxide of iron (magnetite) which they
contain, attracts the oxygen of the air and moisture, and assumes the
reddish-brown colour of iron-rust. Under such circumstances the heaps
of black material gradually acquire the red-brown colour which is
characteristic of so many of the scoria-cones around Etna, and in the
Auvergne and the Eifel. The moisture of the air, and the rain falling
upon these loose cindery heaps, cause them to decompose upon their
surfaces; the action is facilitated by the growth of the lower forms
of vegetation, such as mosses and lichens, and thus at last a soil is
produced on the surfaces of these conical piles of loose materials
which may support an abundant vegetation. Cinder- or scoria-cones are
not uncommonly found retaining in a most perfect manner their regular,
conical form, the lips of their craters being sharp and unbroken as if
the cone were formed but yesterday, while their slopes may nevertheless
be covered with a rich soil supporting abundant grass and forest-trees.
It may at first sight seem difficult to understand how a loose mass of
scoriæ could have so long withstood the action of the rain and floods,
retaining so perfectly its even slopes and sharp ridges. A little
consideration will, however, convince us that it is the very loose and
pervious nature of the materials of which scoria-cones are composed,
which tends to their perfect preservation. The rain at once sinks into
their mass, before it has time to form rivulets and streams which would
wear away their surfaces and destroy the regularity of their outlines.
Scoria- and pumice-cones are frequently found to be acted upon by acid
vapours to such an extent that the whole of the materials is reduced
to a white pulverulent mass. In these cases the oxides of iron and the
alkalis have united with the sulphuric or hydrochloric or carbonic
acids, the compounds being carried away in solution by the rain-water
falling on the mass; the materials left are silica, the hydrated
silicate of alumina, and hydrated sulphate of lime (gypsum), all of
which are of a white colour.
Cinder- or scoria-cones, and pumice-cones, are often found raised
by the action of winds to a greater elevation on one side than the
other, in the manner already described. One side of the cone is often
seen to be more or less completely swept away by an outwelling stream
of lava, and thus breached cones are formed (see fig. 40, p. 123).
Not unfrequently we find a number of cones which are united more or
less completely at their bases, as in Vulcanello (fig. 6, p. 43), the
several vents being so near together that their ejections have mingled
with one another. Cones composed entirely of fragmentary materials
often show an approach to the beautifully curved slopes which we have
described as being so characteristic of volcanoes, as may be seen in
fig. 41, facing p. 124. In the case of scoria- and pumice-cones this
curvature is probably due to the rolling downnwards and outwards of the
larger fragments.
We have already pointed out that with the scoriæ there are often
ejected fragments torn from the sides of the volcanic vents. Sometimes
such fragments are so numerous as to make up a considerable portion of
the mass of the volcanic cones. Thus in the Eifel we find hills, of
by no means insignificant size, completely built up of small scoriæ
and broken fragments of slate torn from the rocks through which the
volcanic fissures have been opened. Occasionally we see that few or no
scoriæ have been ejected, and the volcanic vents are surrounded simply
by heaps of burnt slate.
The smaller fragmentary materials ejected from volcanic vents--such
as lapilli and dust--rest in heaps, having a different angle of slope
from those formed by scoriæ. In many cases, as we have seen, such
finely-divided materials descend in the condition of mud, which flows
evenly over the surface of the growing cone and consolidates in beds of
very regularly stratified 'tufa' or 'tuff.'
[Sidenote: CHARACTERS OF TUFF-CONES.]
The 'tuff-cones' thus formed differ in many important respects from the
scoria-cones already described. The slope of their sides varies from
15° to 30°, and is almost always considerably less than in scoria- and
pumice-cones. The tuff-cones undergo much more rapid degradation from
rain and moisture than do the scoria-cones; for, though the materials
of the former 'set,' as we have seen, into a substance of considerable
hardness, yet this substance, being much less pervious to water than
the loose scoria heaps, permits of the formation of surface-streams
which furrow and wear away the sides of the cones. Sometimes the sides
of the crater are found to be almost wholly removed by atmospheric
denudation, and only a shallow depression is found occupying the
site of the crater; such a case is represented in fig. 59. We not
unfrequently find the whole slopes of such cones to be traversed by a
series of radiating grooves passing from the summit to the base of the
mountains, these channels being formed by water, which has collected
into streams, flowing down the slopes of the mountains. The volcanic
cone, under these circumstances, frequently presents the appearance of
a partially opened umbrella. Owing to the impervious character of the
materials composing tuff-cones, their craters are frequently found to
be occupied by lakes.
[Illustration: Fig. 59.--Summit of the volcano of Monte Sant' Angelo in
Lipari exhibiting a crater with walls worn down by denudatioh.]
Tufas have usually a white or yellowish-brown colour, and these are the
colours exhibited by the cones composed of this material before they
become covered by vegetation. Tufas scoriæ, and lavas usually crumble
down to form a very rich soil, and many of the choicest wines are
produced from grapes grown on the fertile slopes of volcanic mountains.
When, however, as not unfrequently happens, the materials are finely
divided and incoherent, they are so easily driven about by the winds
that cultivation of any kind is rendered almost impossible. In the
Islands of Stromboli and Vulcano the gardens have to be surrounded by
high fences to prevent them from being overwhelmed by the ever-shifting
masses of volcanic sand.
[Sidenote: CHARACTERS OF LAVA-CONES.]
There are some cones which are composed in part of scoriæ and in part
of tufa. Hence we are sometimes at a loss whether to group them with
the one class of cones or the other. But in the majority of cases,
scoria- and tuff-cones present the sufficiently well-marked and
distinctive characters which we have described.
Lava-cones differ quite as greatly in their forms as do the cones
composed of fragmentary materials, the variations being principally
determined by the degree of liquidity of the lavas.
We sometimes find that outwelling masses of lava, when issuing in small
quantities from a vent, accumulate in cauliflower-shaped masses, or
sometimes in the form of a column, or bottle. Professor J. D. Dana
describes many such fantastically-formed masses of lava as being found
in Hawaii, one of which is represented in fig. 25 (p. 100).
When the lava issues from the vent in great quantities it tends to
flow on all sides of it, and to build up a great conical heap above
the orifice. If the lava be very liquid it flows to great distances,
resting at a very slight slope. Thus we find that the volcanoes of
Hawaii have been built up of successive ejections of very liquid lava,
which have formed cones having a slope of only 6° to 8°, but of such
enormous dimensions that the diameter of their bases is seventy miles
and their height 14,000 feet.
[Illustration: Fig. 60.--Outlines of Lava-cones.
1. Mauna Loa, in Hawaii. Composed of very fluid lava. 3. The
Schlossberg of Teplitz, Bohemia. Composed of very imperfectly fluid or
viscid lava.]
If, on the other hand, the lava be viscid, or very imperfectly liquid
in character, it tends to accumulate immediately around the vent; fresh
ejections force the first extruded matter outwards, in the manner so
well illustrated by Dr. Reyer's experiments, and at last a more or less
steep-sided bulbous mass is formed over the vent. Such bulbous masses,
composed of imperfectly fluid lavas, occur in many volcanic districts,
and constitute hills of considerable size. From the tendency of matters
thus extruded to choke up the vents, however, these volcanoes composed
of viscid lavas cannot be expected to attain the vast dimensions
reached by some of those composed of very liquid lavas. The difference
in the forms of lava-cones composed of very fluid or of somewhat
viscid materials is illustrated in fig. 58. When the interior of such
steep-sided volcanic mountains composed of viscid materials is exposed
by the action of denuding forces, the peculiar internal structure we
have described is displayed by them. In the Chodi-Berg of Hungary,
a great bulbous mass of andesitic rock, this endogenous structure is
admirably displayed. It is also well seen in the excavation of the hill
of the Grand Sarcoui, a similar mass, composed of altered trachyte,
which has been erupted in the midst of a scoria-cone in the Auvergne.
See fig. 44 (p. 126).
[Sidenote: CHARACTERS OF COMPOSITE CONES.]
Most of the great volcanic mountains of the globe belong to the
class of 'composite cones,' and are built up by alternate ejections
of fluid lava and fragmentary materials. The slope of the sides in
such composite cones is subject to a wide range of variation, being
determined in part by the degree of liquidity of the lavas, in part
by the nature of the fragmentary materials ejected, and in part by
the proportions which the fragmentary and lava-ejections bear to one
another.
But there is another set of causes which tends to modify the form
and character of these composite, volcanic cones. As we have already
pointed out, the sides of such cones are liable to be rent asunder from
time to time, and the fissures so produced are injected with masses of
liquid lava from below. These fissures, rent in the sides of volcanic
cones, often reach the surface and eruptive action takes place, giving
rise to the formation of a cone, or series of cones, upon the line
of the fissure (fig. 61). Such small cones thrown up on the flanks
of a great volcanic mountain are known as 'parasitic cones'; though
subordinate to the great mountain mass, they may be in themselves of
considerable dimensions. Among the hundreds of parasitic cones which
stud the flanks of Etna, there are some which are nearly 800 feet in
height.
[Illustration: Fig. 61.--Diagram illustrating the formation of
Parasitic Cones along lines of fissure formed on the flanks of a great
volcanic mountain.]
[Illustration: Fig. 62.--Outline of Etna, as seen from Catania.]
[Sidenote: FORMATION OF PARASITIC CONES.]
The building up of parasitic cones upon the flanks of a volcanic
mountain tends, of course, to destroy its regular conical form. This
may be well seen in Etna, which, by the accumulation of materials
upon its flanks, has become a remarkably 'round-shouldered' mountain.
(See figs. 62 and 63.) At the same time it must be remembered that
materials erupted from the central vent tend to fill up the hollows
between these parasitic cones, and thus to restore to the mountain its
regularly conical form.
[Illustration: Fig. 63.--Outline of Etna, as seen from the Val del
Bronte.]
[Illustration: Fig. 64.--Plan of the Volcano forming the Island of
Ischia.
_a, a, a._ The semi-circular crater-ring of Epomeo.
_b, c, d._ Lava-currents which have flowed from the principal crater.
_e, f, g, h._ Plateaux formed by ancient lava-currents.
_k._ Montagnone. }
_l._ Monte Rotaro. }
_m._ Monte Tabor. } Parasitic cones and craters on the slopes
_n._ Castiglione. } of the mountain.
_o._ Lago di Bagno. }
_p._ The Cremate. }
_r._ Lava-stream of the Arso, which flowed from the Cremate in 1301.
_x, x, x._ Raised beaches on the shores of the island, showing that it
has recently undergone elevation.
]
The Island of Ischia is a good example of a great volcanic cone the
flanks of which are covered with numerous small parasitic cones. While
the great central volcano has evidently been long extinct, and one
side of its crater-wall is completely broken down, some of the small
parasitic cones around its base have been formed within the historical
period--one of them as recently as the year 1302. Fig. 64 is a plan of
the Island of Ischia, showing the numerous parasitic cones scattered
over the slopes of the principal cone.
[Illustration: Fig, 65.--A primary Parasitic Cone with a secondary one
at its base--Ischia.
_a._ Monte Rotaro. _b._ Monte Tabor. _c._ Lava-stream flowing from the
latter.]
In one case we find that a parasitic cone, the Monte Rotaro, has itself
a similar smaller cone, which is parasitic to it, at its foot; this
secondary parasitic cone gives off a small lava-stream of trachyte,
which has flowed down to the sea. (See fig. 65.)
[Illustration: Fig. 66.--Scoria-cone near Auckland, New Zealand, with a
lava-current flowing from it.
The strata beneath the volcanic cone are exposed in the sea-cliff, and
exhibit proofs of depression having taken place.]
[Illustration: Fig. 67.--Section of rocks below the ancient triassic
volcano of Predazzo in the Tyrol.
The position of the strata _a b c_, etc., indicates a central
subsidence.]
[Sidenote: SUBSIDENCE BENEATH VOLCANIC VENTS.]
Most great volcanic mountains exhibit a tendency towards a subsidence
of their central portions, which may take place either during or
subsequently to their period of activity. When we examine the strata
upon which a volcano has been built up, but which are now exposed to
our study by denuding forces, we usually find that they incline towards
the centre of the eruptive activity. (See figs. 66 and 67.) Two causes
may contribute to bring about this result. In the first instance,
we must remark that the piling up of materials around the volcanic
vent causes the subjacent strata to be subjected to a degree of
pressure far is excess of that which acts upon the surrounding rocks.
And secondly, it must be borne in mind that the continual removal
of material from below the mountain must tend to the production of
hollows, into which the overlying strata will sink. The effect of this
central subsidence is to give to the flanks of volcanic cones those
beautifully curved outlines which constitute so striking a feature in
Vesuvius (see fig. 17, p. 87), Fusiyama (see fig. 77, No. 1, facing p.
178), and many other volcanic mountains.
There seems, at first sight, to be scarcely any limit to the dimensions
which these great composite volcanic cones may attain: the lateral
eruptions tending to enlarge the area of the base of the mountain,
and, by the injection of the fissures, to knit together and strengthen
its structure, while the central eruptions continually increase the
elevation of the mass. Great, however, as is the force which is
concerned in the production of our terrestrial volcanoes, it has its
limits; and, at last, the piling up of materials will have gone on
to such an extent, that the active forces beneath the volcano are no
longer competent either to raise materials to the elevated summit of
the mountain or to tear asunder its strengthened and fortified flanks.
Under these circumstances, the volcanic forces, if they have not
already exhausted themselves, will be compelled to find weak places in
the district surrounding the volcano, at which fissures may be produced
and the phenomena of eruption displayed.
[Sidenote: SHIFTING OF VOLCANIC FOCI.]
Some volcanic cones exhibit evidence that during the series of
eruptions by which they have gradually been built up, the centre of
volcanic action has shifted to another point within the mountain. Thus
Lyell has shown, in the case of Etna, that during the earlier periods
in the history of the mountain the piling up of materials went on
around a centre which is now situated at a distance of nearly four
miles from the present focus of eruptive activity. Some of our old
British volcanoes, of which the denuded wrecks exist in the Western
Isles of Scotland, show similar evidence of a shifting of the axis of
eruption.
One of the most conspicuous features of a volcanic cone is the great
depression or crater found at its summit. In describing the internal
structure of volcanic cones, we have seen how these craters are
produced and acquire their inverted conical form, by the slipping and
rolling back of materials towards the centre of eruptive action.
Almost all volcanic cones exhibit craters, but in those which are
formed entirely by the outwelling of viscid lavas the central
depression is often slight and inconspicuous, and occasionally
altogether wanting. It frequently happens, however, that eruptive
action has ceased at the centre of a volcano, and its summit-crater
may by denudation be entirely destroyed, while new and active craters
are formed upon its flanks. Stromboli furnishes us with an admirable
example of this kind (see fig. 1, facing p. 10). Other volcanoes may
exhibit several craters, one at the summit of the mountain and others
upon its flanks. Of this we find a good example in Vulcano (fig. 6, p.
43).
[Illustration: Fig. 68.--Cotopaxi (19,600 feet), as seen from a
distance of ninety miles.]
When a volcano has been built up by regular and continuous eruptions
from the same volcanic vent, the size of the crater remains the same,
while the volcano continually grows in height and in the diameter of
its base. The size of the crater will be determined by the eruptive
force at the volcanic centre, the size of the mountain by the duration
of the volcanic activity and the quantity of material ejected. In the
earliest stage of its history, such a volcano will resemble Monte
Nuovo, which has a crater reaching down almost to the base of the
mountain; in the later stages of its history, such a volcano will
resemble Cotopaxi (fig. 68) and Citlaltepetl (fig. 69), in which the
crater, though of far greater absolute dimensions than that of Monte
Nuovo, bears but a small proportion to the vast cone at the summit of
which it is situated.
[Illustration: Fig. 69.--Citlaltepetl, or the Pic d'Orizaba, in Mexico
(17,370 feet), as seem from the forest of Xalapa.]
[Sidenote: ORIGIN OF VOLCANIC CRATERS.]
In the great majority of volcanoes, however, eruptive action does not
go on by any means regularly and continuously, but terrible paroxysmal
outbursts occur, which suddenly enlarge the dimensions of the crater to
an enormous extent.
In the year 1772, there occurred a volcanic eruption in the Island
of Java, which is perhaps the most violent and terrible that has
happened within the historical period. A lofty volcanic cone, called
Papandayang, 9,000 feet high, burst into eruption, and, in a single
night, 30,000,000,000 cubic feet of materials were thrown into the
atmosphere, falling upon the country around the mountain where no less
than forty villages were buried. After the eruption, the volcano was
found to have been reduced in height from 9,000 to 5,000 feet, and
to present a vast crater in its midst, which had been formed by the
ejection of the enormous mass of materials.
Many similar cases might be cited of the removal of a great part of a
mountain-mass by a sudden, paroxysmal outburst. In some cases, indeed,
the whole mass of a mountain has been blown away during a terrific
eruption, and the site of the mountain is now occupied by a lake. This
is said to have been the case with the Island of Timor, where an active
volcano, which was visible from a distance of 300 miles at sea, has
entirely disappeared.
The removal of the central portion of great volcanic mountains by
explosive action, gives rise to the formation of those vast, circular,
crater-rings of which such remarkable examples occur in many volcanic
districts. These crater-rings present a wall with an outer slope
agreeing with that of the volcanic cone of which they originally formed
a part, but with steep inner cliffs, which exhibit good sections of the
beds of tuff, ash, and lava with the intersecting dykes of which the
original volcano was built up. Near Naples, one of these crater-rings,
with sloping outer sides and steep inner ones, is employed to form the
royal game-preserve of Astroni, the only entrance to the crater being
closed by gates.
[Sidenote: FORMATION OF CRATER-LAKES.]
As these crater-rings are usually composed of materials more or
less impervious to water, they often become the site of lakes. The
beautiful circular lake of Laach, in the Rhine Provinces, with the
numerous similar examples of Central Italy--Albano, Nemi, Bracciano,
and Bolsena--the lakes of the Campi Phlegræi (Agnano, Avernus, &c.),
and some similar lakes in the Auvergne, may be adduced as examples of
crater-rings which have become the site of lakes.
[Illustration: Fig. 70.--Lac Paven, in the Auvergne.
_a._ Scoriæ. _b._ Basalt.]
One of the most beautiful of the crater-lakes in the Auvergne is
Lac Paven (fig. 70), which lies at the foot of a scoria-cone, Mont
Chalme, and is itself surrounded by masses of ejected materials. The
crater-lake of Bagno, in Ischia (fig. 71), has had a channel cut
between it and the sea, so that it serves as a natural harbour. The
lake of Gustavila, in Mexico (fig. 72), is an example of a crater-lake
on a much larger scale.
In many of these crater-rings the diameter of the circular space
enclosed by them is often very great indeed as compared with the height
of the walls.
[Illustration: Fig. 71.--The crater-lake called Lago del Bagno, in
Ischia, converted into a harbour.]
[Illustration: Fig. 72.--Lake of Gustavila, in Mexico.
(The terraces round the lake have been artificially formed.)]
[Sidenote: DIMENSIONS OF CRATER-LAKES.]
Two of the largest crater-rings in the world are found in Central
Italy, and are both occupied by lakes, the circular forms of which
must strike every observer. The Lago di Bracciano, which lies to
the north-west of Rome, is a circular lake six and a half miles in
diameter, surrounded by hills which at their highest point rise to the
height of 1,486 feet above the sea, while the surface of the waters
of the lake is 640 feet above the sea-level. The Lago di Bolsena is
somewhat less perfectly circular in outline than the Lago di Bracciano;
it has a length from north to south of ten-and-a-quarter miles and a
breadth from east to west of nine miles; the surface of the waters of
this lake is 962 feet above that of the waters of the Mediterranean.
The lake of Bolsena, like that of Bracciano, is surrounded by hills
composed of volcanic materials; the highest points of this ring of
hills rise to elevations of 684, 780, and 985 feet respectively above
the waters of the lake.
In these great circular lakes of Bolsena and Bracciano, as well as in
the smaller ones of Albano, Nemi, and the lakes of Frascati in the same
district, the vast circular spaces enclosed by them, the gradual outer
slope of the ring, and the inner precipices which bound the lake, all
afford evidence of the explosive action to which they owe their origin.
But while the vast crater-rings we have mentioned are frequently found
to be occupied by lakes, there are many other similar crater-rings
which remain dry, either from the materials of which they are composed
being of more pervious character, or from rivers having cut a channel
through the walls of the crater, in this way draining off its waters.
Thus in the Campi Phlegræi, while we have the craters of Agnano and
Avernus forming complete circular lakes, Astroni has only a few
insignificant lakelets on its floor, and the Pianura, the Piano di
Quarto, which have each a diameter of three or four miles, with many
others, remain perfectly dry. In the vicinity of the great crater-lakes
of Central Italy we find the crater-ring of the Vallariccia, which has
evidently once been a lake but is now drained, its floor being covered
with villages and vineyards.
[Sidenote: CRATER-RINGS SURROUNDING CONES.]
A comparison of these vast crater-rings leads us to the conclusion that
in the majority of cases, if not in every instance, they are composed
almost entirely of volcanic tuff and dust. In the case of the more
solidly-built composite volcanic cones, the volcanic forces, as we have
seen, produce fissures in the mass, and along these fissures parasitic
cones are thrown up, the tension of the mass of imprisoned vapours
below the mountain being thus from time to time relieved. But in the
case of a volcanic cone composed of loose fragmentary materials, such
temporary relief is impossible. The cracks, as soon as they originate,
will be filled up and choked by the falling in of materials from above
and at their sides. In this way the eruptive action will be continually
repressed, till at last the imprisoned vapours acquire such a high
state of tension that the outburst, when it occurs, is of the most
terrible character, and the whole central mass of the volcano is blown
into the air. It may often seem surprising that the ejection of such
vast masses of material from the centre of a volcanic cone does not
effect more in the way of raising the height of the crater-walls. But
it must be remembered that, in the case of craters of such vast area,
the majority of the ejected materials must fall back again within
its circumference. By repeated ejections these materials will at last
be reduced to such an extreme state of comminution that they can be
borne away by the winds, and spread over the country to the distance of
hundreds or thousands of miles. After great volcanic outbursts enormous
areas are thus found covered with fine volcanic dust to the depth of
many inches or feet.
[Illustration: Fig. 73.--Peak of Teneriffe in the Canary Islands
(12,182 ft.), surrounded by great crater-rings.]
Sometimes, as in the case of the Lago di Bracciano, the eruptive forces
appear to have entirely exhausted themselves in the prodigious outburst
by which the great crater was produced. But in other cases, as in that
of the Lago di Bolsena, the eruptive action was resumed at a later
date, and small tuff-cones were thrown up upon the floor of the crater;
these now rise as islands above the surface of the lake. In other
cases, again, the eruptive action was resumed after the formation of
the great crater-ring, with such effect that bulky volcanic cones were
built up in the midst of the crater-ring which surrounds them like a
vast wall; examples of this are exhibited in the extinct volcanoes of
Rocca Monfina and Monte Albano. Some of the grandest volcanoes of the
globe, such as Teneriffe (fig. 73), the volcanoes of Mauritius and
Bourbon (figs. 74 and 75), and many others that might be cited, are
thus found to be surrounded by vast crater-rings. Vesuvius itself is
surrounded by the crater-ring of Somma (fig. 76).
[Illustration: Fig. 74.--The volcano of Bourbon, rising in the midst of
a crater-ring four miles in diameter.]
[Illustration: Fig. 75.--The volcano of Bourbon, as seen from another
point of view, with three concentric crater-rings encircling its base.]
[Sidenote: BASALTIC CONES IN TRACHYTIC CRATER-RINGS.]
This formation of cone within crater, often many times repeated, is
very characteristic of volcanoes. The craters mark sudden and violent
paroxysmal outbursts, the cones are the result of more moderate but
long-continued ejection. Sometimes, as at Vesuvius in 1767 (fig. 15,
p. 85), we find a nest of craters and cones which very strikingly
exemplifies this kind of action.
[Illustration: Fig. 76.--Vesuvius, as seen from Sorrento, half
encircled by the crater-ring of Somma.]
We shall point out, hereafter, that at most volcanic centres the
ejection of trachytic lavas precedes that of the basaltic lavas. Now it
is these trachytic lavas which principally give rise to the formation
of the light lapilli of which tuff-cones are formed. Hence it is that
we so frequently find, as in the case of Vesuvius, Rocca-Monfina, and
many other volcanoes, that a great crater-ring, largely composed of
tuffs, encloses a cone built up of more basic lavas.
In fig. 77 we have shown by a series of outline sections the various
forms assumed by volcanoes in consequence of the different kinds of
eruptive action going on in them:--
1. Is an outline of Fusiyama, an almost perfect cone, with a small
crater at its summit. The sides of this volcano admirably illustrate
the beautiful double curves characteristic of volcanic cones.
2. Hverfjall in Iceland, a volcanic cone with a large crater, reaching
almost to its base.
3. The crater-lake of Bracciano, in which the area of the crater is out
of all proportion to the height of the crater-walls.
4. Rocca-Monfina, in Southern Italy, a tuff-cone of large dimensions,
in the midst of which an andesitic lava-cone has been built up.
5. Teneriffe, in the Canary Islands, in which a perfect volcanic cone
has been built up in the centre of an encircling crater-ring.
6. Vulcano, in the Lipari Islands, in which, by the shifting of the
centre of volcanic activity along a line of fissure, a series of
overlapping volcanic cones has been produced.
[Illustration: Fig. 77.--Outlines of various Volcanoes, illustrating
the different relations of the craters to cones.]
[Sidenote: SUBMARINE VOLCANOES.]
While speaking of the varieties of form assumed by volcanic cones and
craters, we must not forget to notice the effects which are produced
by denuding forces upon them. In the case of submarine volcanoes, like
the celebrated island called by the English Graham Isle, by the French
Isle Julie, and by the Germans the Insel Ferdinandez (fig. 78), which
was thrown up off the coast of Sicily in 1831, it was evident that
volcanic outbursts taking place at some depth below the level of the
sea gradually piled up a cone of scoriæ with a crater in its midst.
By constant accessions to its mass, this scoria-cone was eventually
raised above the sea-level, but the action of the waves upon the loose
materials soon destroyed the crater-walls and eventually reduced the
island to a shoal. It is evident that in all cases in which eruptions
take place beneath the sea-level, and the loose materials are exposed
during their accumulation to the beating of the sea-waves, the form
of the volcanic cone so produced will be greatly modified by the
interaction of the two sets of opposed causes, the eruptive forces from
below and the distributive action of the sea-waves.
[Illustration: Fig. 78.--Island thrown up in the Mediterranean Sea in
July and August 1881.
(The view was taken in the month of September, after the sides of the
crater had been washed away by the waves.)]
Craters when once formed are often rent across, along the line of the
fissure above which they are thrown up. Thus the crater of Vesuvius was
in 1872 rent completely asunder on one side, so that it was possible
to climb through the fissure thus produced and reach the bottom of the
crater. Streams flowing down the sides of the crater, and escaping
through such a rent, may in the end greatly modify the form and
disguise the characters of a volcanic crater. Of this kind of action we
have a striking example in the Val del Bove of Etna.
Volcanoes, as we shall point out in the sequel, are after their
extinction frequently submerged beneath the waters of the ocean. The
sea entering the craters, eats back their cliff-like sides and enlarges
their areas. Such denuded waters are called 'calderas,' the channels
into them 'barrancos.'
Sometimes the action of the waves upon a partially submerged volcano
has led to the cutting back of its slopes into steep cliffs, at the
same time that the crater-ring is enlarged. In such cases we have left
a more or less complete rocky ring, composed of alternating lavas and
fragmentary materials. Of such a ruined crater-ring, the Island of St.
Paul in the South Atlantic affords an admirable example.
When the action of denudation has gone still further, all the lavas and
tuffs composing the cone may be completely removed and nothing left but
masses of the hard and highly-crystalline rocks which have cooled down
slowly in the heart of the volcano. An example of this kind is afforded
to us by St. Kilda, the remotest member of the British Archipelago.
But although the majority of volcanic craters are clearly formed by
explosive action, there are some craters, like those of Kilauea in
Hawaii, which probably owe their origin to quite a different set of
causes. In this case the explosive action at the vent is but slight,
and the crater, which is of very irregular form, appears to have
originated in a fissure, which has been slowly enlarged by the liquid
lavas encroaching upon and eating away its sides. Such craters as
these, however, appear to be comparatively rare.
Besides the great volcanic mountains composed of lava, scoriæ, tuff and
ash, there are other structures which are formed around volcanic vents
even when these do not eject molten rock-masses. The water which issues
in these cases either as steam or in a more or less highly heated
condition frequently carries materials in suspension or solution, and
these sometimes accumulate in considerable quantities around the vent.
[Sidenote: FORMATION OF MUD-VOLCANOES.]
When fissures are formed in the midst of loose argillaceous materials,
such as are frequently produced by the decomposition of volcanic rocks,
the waters which issue through them are sometimes so charged with muddy
matter that this accumulates to form cones having all the general
characters of volcanic mountains, and which occasionally rise to the
height of 250 feet. The gases and vapours which issue from these
'mud-volcanoes' are those which are known to be emitted from volcanic
vents at which the action going on is not very intense. Daubeny and
others have suggested that these mud-volcanoes may be the result of
actions which have little or no analogy with those which take place
at ordinary volcanic vents, and that the combustion of subterranean
beds of sulphur and similar causes would be quite competent to their
production. But inasmuch as these mud-volcanoes are almost always
situated in regions in which the more powerful volcanic action has
only recently died away, and the gases and vapours emitted by them are
very similar in character to those which issue from volcanoes, there
does not appear to be any good reason for doubting that they should be
classed as truly volcanic phenomena.
Mud-volcanoes are found in Northern Italy near Modena, in Sicily near
Girgenti, on the shores of the Sea of Azof and the Caspian, in Central
America, and in other parts of the globe. The gas frequently escapes
from them with such violence that mud is thrown into the air to the
height of several hundreds of feet. Sometimes this gas is inflammable,
consisting of sulphuretted hydrogen, hydrogen, or some hydrocarbons,
and these gases occasionally take fire, so that true flames issue from
these mud-volcanoes. In other cases the mud-volcanoes appear to be
formed by either hot or cold springs containing large quantities of
suspended materials, and the liquid mud issues from the vent without
any violent eruptive action.
[Illustration: Fig. 79.--Sinter-cones surrounding the orifices of
Geysers.
1. Basin of the Great Geyser, Iceland. 2. Hot spring cone. 3. Old
Faithful. 4. The Great Geyser. 5. Liberty Cap. (2, 3, 4 and 5 are in
the Yellowstone Park district of the Rocky Mountains.)]
[Sidenote: FORMATION OF SINTER-CONES.]
The soluble materials which waters issuing from volcanic vents deposit
on their sides are chiefly silica and carbonate of lime.
Hot springs, whether intermittent or constant, often contain large
quantities of silica in solution. The solution of this silica is
effected, at the moment of its separation from combination with the
alkali or alkaline earths, during the decomposition of volcanic rocks,
and is favoured by the presence of alkaline carbonates in the water,
and the high temperature and the pressure under which it exists in the
subterranean regions. When the water reaches the surface and, being
relieved from pressure, begins to cool down the silica is deposited.
By this deposited silica the basins around the geysers of Iceland are
formed. Sometimes conical structures are built up around the vents of
hot springs by the deposition of silica from their waters. Examples
of this kind abound in the National Park of Colorado, where they have
received fanciful names, such as the Beehive, Liberty Gap, &c. This
deposited silica is known to geologists as sinter. The forms of some of
the structures which surround the orifices of geysers is shown in fig.
79. The 'Liberty Cap' is an extinct geyser-cone fifty feet high and
twenty feet in diameter.
Hot and cold springs rising in volcanic regions are often highly
charged with carbonic acid, and in passing through calcareous rocks
dissolve large quantities of carbonate of lime. Upon exposure to the
atmosphere, the free carbonic acid escapes and the carbonate of lime
is deposited in the form known as 'travertine.' Such springs occur
in great numbers in many volcanic regions. In the Auvergne great
rock-masses occur formed of carbonate of lime deposited from a state
of solution and taking the form of natural aqueducts and bridges. In
Carlsbad the numerous hot springs have deposited masses of pisolitic
rock (Strudelstein) which have filled up the whole bottom of the
valley, and upon these deposits the town itself is mainly built.
In Central Italy the deposits of travertine formed by calcareous
springs are of enormous extent and thickness: St. Peter's and all the
principal buildings of Rome being constructed of this travertine or
'Tibur-stone.'
[Sidenote: FORMATION OF SINTER-TERRACES.]
When springs charged with silica or carbonate of lime rise upon the
slope of a hill composed of loose volcanic materials, they give rise
to the remarkable structures known as sinter- and travertine-terraces
(see fig. 80). The water flowing downwards from the vent forms a
hard deposit upon the lower slope of the hill, while the continual
deposition of solid materials within the vent tends to choke it up.
As a new vent cannot be forced by the waters through the hard rock
formed below, it is originated a little higher up. Thus the site of the
spring is gradually shifted farther and farther back into the hill. As
deposition takes place along the surfaces over which this water flows,
terraces are built up enclosing basins. Of structures of this kind we
have remarkable examples in the sinter-terraces of Rotomahana in New
Zealand and the travertine-terraces of the Gardiner's River in the
Yellowstone Park district of the Rocky Mountains.
[Illustration: Fig. 80.--Diagram illustrating the mode of formation of
Travertine and Sinter Terraces on the sides of a hill of tuff.]