THE NATURE OF VOLCANIC ACTION.
The dose investigation of what goes on within a volcanic vent may
appear at first sight to be a task beset with so many difficulties and
dangers that we may be tempted to abandon it as altogether hopeless. At
the first recorded eruption of Vesuvius the elder Pliny lost his life
in an attempt to approach the mountain and examine the action which
was taking place there; and during the last great outburst of the same
volcano a band of Neapolitan students, whose curiosity was greater than
their prudence, shared the same fate.
But in both these cases the inquirers paid the penalty of having
adopted a wrong method. If we wish to examine the mode of working of a
complicated steam-engine, it will be of little avail for us to watch
the machinery when the full blast of steam is turned on, and the rapid
movements of levers, pinions, and slides baffle all attempts to follow
them, and render hopeless every effort to trace their connection with
one another. But if some friendly hand turn off the greater part of
the steam-supply, then, as the rods move slowly backwards and forwards,
as the wheels make their measured revolutions, and the valves axe
seen gradually opening and shutting, we may have an opportunity of
determining the relations of the several parts of the machine to one
another, and of arriving at just conclusions concerning the plan on
which it is constructed. Nor can we doubt that the parts of the machine
bear the same relation to one another, and that their movements take
place in precisely the same order, when the supply of steam is large as
when it is small.
Now, as we shall show in the sequel, a volcano is a kind of great
natural steam-engine, and our best method of investigating its action
is to watch it when a part of the steam-supply is cut off. It is
true that we cannot at will control the source of supply of steam to
a volcano, as we can in a steam-engine, but as some volcanoes have
usually only a small steam-supply, and nearly all volcanoes vary
greatly in the intensity of their action at different periods, we can,
by a careful selection of the object or the time of our study, gain all
those advantages which would be obtained by regulating its action for
ourselves.
Spallanzani appears to have been the first to perceive the important
fact, that the nature of volcanic action remains the same, however
its intensity may vary. Taking advantage of the circumstance that
there exists in the Mediterranean Sea a volcano--Stromboli--which
for at least 2,000 years has been in a constant and regular, but not
in a violent or dangerous, state of activity, he visited the spot,
and made the series of careful observations which laid the foundation
of our knowledge of the 'physiology of volcanoes.' Since the time
of Spallanzani, many other investigators have visited the crater
of Stromboli, and they have been able to confirm and extend the
observations of the great Italian naturalist, as to the character of
the action which is constantly taking place within it. We cannot better
illustrate the nature of volcanic action than by describing what has
been witnessed by numerous observers within the crater of Stromboli,
where it is possible to watch the series of operations going on by the
hour together, and to do so without having our judgment warped either
by an excited imagination or the sense of danger.
[Sidenote: APPEARANCE OF STROMBOLI FROM A DISTANCE.]
In the sketch, fig. 1, which was made on April 20, 1874, I have shown
the appearance which this interesting volcano usually presents, when
viewed from a distance. The island is of rudely circular outline, and
conical form, and rises to the height of 3,090 feet above the level of
the Mediterranean. From a point on the side of the mountain, masses
of vapour are seen to issue, and these unite to form a cloud over
the mountain, the outline of this vapour-cloud varying continually
according to the hygrometric state of the atmosphere, and the direction
and force of the wind. At the time when this sketch was made, the
vapour-cloud was spread in a great horizontal stratum overshadowing
the whole island, but it was clearly seen to be made up of a number
of globular masses, each of which, as we shall hereafter see, is the
product of a distinct outburst of the volcanic forces.
Viewed at night-time, Stromboli presents a far more striking and
singular spectacle. The mountain, with its vapour canopy, is visible
over an area having a radius of more than 100 miles. When watched from
the deck of a vessel anywhere within this area, a glow of red light is
seen to make its appearance from time to time above the summit of the
mountain; this glow of light may be observed to increase gradually in
intensity, and then as gradually to die away. After a short interval
the same appearances are repeated, and this goes on till the increasing
light of the dawn causes the phenomenon to be no longer visible. The
resemblance presented by Stromboli to a 'flashing light' on a most
gigantic scale is very striking, and the mountain has long been known
as 'the lighthouse of the Mediterranean.'
It must be pointed out, however, that in two very important particulars
the appearances presented by Stromboli differ markedly from those
rhythmical gleams exhibited by the 'flashing-lights' of our coasts.
In the first place, the intervals between successive flashes are very
unequal, varying from less than one minute to twenty minutes, or even
more; and in the second place, the duration and intensity of the red
glow above the mountain are subject to like variation, being sometimes
a momentary scarcely visible gleam, and at others a vivid burst of
light which illuminates the sky to a considerable distance round.
[Illustration: Fig. 1.--Stromboli, viewed from the North-west, April
1874.]
[Illustration: Fig. 2.--Map of tub Island of Stromboli. (Scale about
two inches to a mile.)]
[Sidenote: GENERAL FEATURES OF THE MOUNTAIN.]
Let us now draw near and examine this wonderful phenomenon of a
mountain which seemingly ever burns with fire, and yet is not consumed.
The general form of the Island of Stromboli will be gathered from an
inspection of the plan, fig. 2, which is copied from a map published by
the Italian Government. When we land upon the island, we find that it
is entirely built up of such materials as we know to be ejected from
volcanoes; indeed, it resembles on a gigantic scale the surroundings
of an iron furnace, with its heaps of cinders and masses of slag. The
irregularity in the form of the island is at once seen to be due to the
action of the wind, the rain, and the waves of the surrounding sea,
which have removed the loose, cindery materials at some points, and
left the hard, slaggy masses standing up prominently at others.
This great heap of cindery and slaggy materials rises, as we have said,
to a height of more than 3,000 feet above the sea-level, but even this
measurement does not give a just idea of its vast bulk. Soundings in
the sea surrounding the island show that the bottom gradually shelves
around the shores to the depth of nearly 600 fathoms, so that Stromboli
is a great conical mass of cinders and slaggy materials, having a
height of over 6,000 feet, and a base whose diameter exceeds four miles.
The general form and proportions of this mass will be better understood
by an examination of the section, fig. 3, which is also constructed
from the materials furnished by the map of the island issued by the
Italian Government. The same section, and the map, fig. 2, will serve
to make clear the position and relations of the point on the mountain
at which the volcanic activity takes place. At a spot on the north-west
slope of the mountain, about 1,000 feet below its summit, and 2,000
feet above the level of the sea, there exists a circular depression,
the present active 'crater' of the volcano; and leading down from this
to the sea there is a flat slope making an angle of about 35° with the
horizon, and known as the 'Sciarra.' The Sciarra is bounded by steep
cliffs, as shown in the sketch fig. 1, and the plan fig. 2.
[Illustration: Fig. 3.--Section through the Island of Stromboli from
n.w. to s.e.
_a._ Highest summit of the mountain, _c._ Cratère del Fossa, _b._ Point
overlooking the crater, _d._ Steep slope known as the Sciarra del
Fuoco. _e._ Continuation of the same slope beneath the level of the
sea. _f._ Steep cliffs of the Punta dell' Omo.]
[Sidenote: FORM AND FUNCTION OF THE CRATER.]
If we climb up to this scene of volcanic activity, we shall be able to
watch narrowly the operations which are going on there. On the morning
of the 24th of April, 1874, I paid a visit to this interesting spot in
order to get a near view of what was taking place. On reaching a point
upon the side of the Sciarra, from which the crater was in full view
before me, I witnessed, and made a sketch of, an outburst which then
took place, and this sketch has been reproduced in fig. 4. Before the
outburst, numerous light curling wreaths of vapour were seen ascending
from fissures on the sides and bottom of the crater. Suddenly, and
without the slightest warning, a sound was heard like that produced
when a locomotive blows off its steam at a railway-station; a great
volume of watery vapour was at the same time thrown violently into
the atmosphere, and with it there were hurled upwards a number of
dark fragments, which rose to the height of 400 or 500 feet above the
crater, describing curves in their course, and then falling back upon
the mountain. Most of these fragments tumbled into the crater with a
loud, rattling noise, but some of them fell outside the crater, and a
few rolled down the steep slope of the Sciarra into the sea. Some of
these falling fragments were found to be still hot and glowing, and in
a semi-molten condition, so that they readily received the impression
of a coin thrust into them.
[Illustration: Fig. 4.--The Crates of Stromboli as viewed from the side
of the Sciarra during an eruption on the morning of April 24, 1874.]
[Sidenote: APERTURES AT THE BOTTOM OF THE CRATER.]
But on the upper side of the crater, at the point marked 6, on the
section fig. 3, there exists a spot from which we can look down upon
the bottom of the crater, and view the operations taking place there.
This is the place where Spallanzani and other later investigators have
carried on their observations, and, when the wind is blowing from
the spectator towards the crater, he may sit for hours watching the
wonderful scene displayed before him. The black slaggy bottom of the
crater is seen to be traversed by many fissures or cracks, from most
of which curling jets of vapour issue quietly, and gradually mingle
with and disappear in the atmosphere. But besides these smaller cracks
at the bottom of the crater, several larger openings are seen, which
vary in number and position at different periods; sometimes only one of
these apertures is visible, at others as many as six or seven, and the
phenomena presented at these larger apertures are especially worthy of
careful investigation.
These larger apertures, if we study the nature of the action taking
place at them, may be divided into three classes. From those of the
first class, steam is emitted with loud, snorting puffs, like those
produced by a locomotive-engine, but far less regular and rhythmical
in their succession. In the second class of apertures masses of molten
material are seen welling out, and, if the position of the aperture
be favourable, flowing outside the crater; from this liquid molten
mass steam is seen to escape, sometimes in considerable quantities.
The openings of the third class present still more interesting
appearances. Within the walls of the aperture a viscid or semi-liquid
substance is seen slowly heaving up and down. As we watch the seething
mass the agitation within it is observed to increase gradually, and at
last a gigantic bubble is formed which violently bursts, when a great
rush of steam takes place, carrying fragments of the scum-like surface
of the liquid high into the atmosphere.
If we visit the crater by night, the appearances presented are found to
be still more striking and suggestive. The smaller cracks and larger
openings glow with a ruddy light. The liquid matter is seen to be red-
or even white-hot, while the scum or crust which forms upon it is of a
dull red colour. Every time a bubble bursts and the crust is broken up
by the escape of steam, a fresh, glowing surface of the incandescent
material is exposed. If at these moments we look up at the vapour-cloud
covering the mountain, we shall at once understand the cause of the
singular appearances presented by Stromboli when viewed from a distance
at night, for the great masses of vapour are seen to be lit up with a
vivid, ruddy glow, like that produced when an engine-driver opens the
door of the furnace and illuminates the stream of vapour issuing from
the funnel of his locomotive.
Let us now endeavour to analyse the phenomena so admirably displayed
before us in the crater of Stromboli. The three essential conditions
on which the production of these phenomena seems to depend are the
following: first, the existence of certain apertures or cracks
communicating between the interior and the surface of the earth;
secondly, the presence of matter in a highly heated condition beneath
the surface; and thirdly, the existence of great quantities of water
imprisoned in the subterranean regions--which water, escaping as steam,
gives rise to all those active phenomena we have been describing.
[Sidenote: CAUSE OF THE GLOWING LIGHT.]
We have said, at the outset, that there exists no analogy whatever
between the action which takes place in volcanoes and the operation of
burning or combustion. Occasionally, it is true, certain inflammable
substances are formed by the action going on within the volcano, and
these inflammable substances, taking fire, produce real flames. Such
flames are, however, in almost all cases only feebly luminous, and do
not give rise to any conspicuous appearances. What is usually taken for
flame during volcanic eruptions is simply, as we have already pointed
out, the glowing red-hot surface of a mass of molten rock, reflected
from a vapour-cloud hanging over it. The red glow observed over a
volcano in eruption is indeed precisely similar in its nature and
origin to that which is seen above London during a night of heavy fog,
and which is produced by the reflection of the gas-lights of the city
from the innumerable particles of water-vapour diffused through the
atmosphere. Fires, of course, occur when the molten and incandescent
materials poured out from a volcano come in contact with inflammable
substances, such as forests and houses, but in these cases the
combustion is quite a secondary phenomenon.
There is another popular delusion concerning volcanic action, which it
may be necessary to refer to and to combat. From the well-known fact
that sulphur or brimstone is found abundantly in volcanic regions, the
popular belief has arisen that this highly inflammable substance has
something to do with the production of the eruptions of volcanoes. In
school-books which were, until comparatively recent years, in constant
use in this country, the statement may be found that by burying certain
quantities of sulphur, iron-pyrites, and charcoal in a hole in the
ground, we may form a miniature volcano, and produce all the essential
phenomena of a volcanic eruption. No greater mistake could possibly be
made. The chemical reactions which take place when sulphur and other
substances are made to act upon each other differ entirely from the
phenomena of volcanic action. The sulphur which is found in volcanic
regions is the result and not the cause of volcanic action. Among the
most common substances emitted from volcanic vents along with the steam
are the two gases, sulphurous acid and sulphuretted hydrogen. When
these two gases come into contact with one another, chemical action
takes place, and the elements contained in them--oxygen, hydrogen,
and sulphur--are free to group themselves together in an entirely new
fashion; the consequence of this is that water and sulphuric acid (oil
of vitriol) are formed, and a certain quantity of sulphur is set free.
The water escapes into the atmosphere, the sulphuric acid combines with
lime, iron, or other substances contained in the surrounding rocks,
and the sulphur builds up crystals in any cavities which may happen to
exist in these rocks.
[Sidenote: VOLCANIC ACTION RESEMBLES BOILING.]
If, however, careful and exact observations, like those carried on
at Stromboli, compel us to reject the popular notions concerning
the supposed resemblance between volcanic action and the combustion
of sulphur or other substances, they nevertheless suggest analogies
with certain other simple and well-known operations. And in pursuing
these analogies, we are led to the recognition of some admirable
illustrations both of the attendant phenomena and of the true cause of
volcanic outbursts.
No one can look down on the mass of seething material in violent
agitation within the fissures at the bottom of the crater of Stromboli,
without being forcibly reminded of the appearances presented by liquids
in a state of boiling or ebullition. The glowing material seems to be
agitated by two kinds of movements, the one whirling or rotatory, the
other vertical or up-and-down in its direction. The fluid mass in this
way appears to be gradually impelled upwards, till it approaches the
lips of the aperture, when vast bubbles are formed upon its surface,
and to the sudden bursting of these the phenomena of the eruption are
due.
Now if we take a tall narrow vessel and fill it with porridge or
some similar substance of imperfect fluidity, we shall be able, by
placing it over a fire, to imitate very closely indeed the appearances
presented in the crater of Stromboli. As the temperature of the mass
rises, steam is generated within it, and in the efforts of this steam
to escape, the substance is set in violent movement. These movements of
the mass are partly rotatory and partly vertical in their direction; as
fresh steam is generated in the mass its surface is gradually raised,
while an escape of the steam is immediately followed by a fall of the
surface. Thus an up-and-down movement of the liquid is maintained, but
as the generation of steam goes on faster than it can escape through
the viscid mass, there is a constant tendency in the latter to rise
towards the mouth of the vessel. At last, as we know, if heat continues
to be applied to the vessel, the fluid contents will be forced up to
its edge and a catastrophe will occur; the steam being suddenly and
violently liberated from the bubbles formed on the surface of the mass,
and a considerable quantity of the material forcibly expelled from
the vessel. The suddenness and violence of this catastrophe is easily
accounted for, if we bear in mind that the escaping steam acts after
the manner of a compressed spring which is suddenly released. Steam
is first formed at the bottom of the vessel which is in contact with
the fire; but here it is under the pressure of the whole mass of the
liquid, and moreover, the viscidity of the substance tends to retard
the union of the steam bubbles and their rise to the surface of the
mass. But when the pressure is relieved by the bursting of bubbles at
the surface, the whole of the generated steam tends to escape suddenly.
[Sidenote: ESCAPE OF STEAM-BUBBLES FROM LAVA.]
Now within the crater of Stromboli we have precisely the necessary
conditions for the display of the same series of operations. In the
apertures at the bottom there exists a quantity of imperfectly fluid
materials at a higher temperature, containing water entangled in its
mass. As this water passes into the state of steam it tends to escape,
and in so doing puts the whole mass into violent movement. When the
steam rises to the surface, bubbles are formed, and the formation of
these bubbles is promoted by the circumstance that the liquid mass,
where exposed to the atmosphere, becomes chilled, and thereby rendered
less perfectly fluid. By the bursting of these bubbles the pressure is
partially relieved, and a violent escape of the pent-up steam takes
place through the whole mass. Equilibrium being thus restored, there
follows a longer or shorter interval of quiescence, during which steam
is being generated and collected within the mass, and the series of
operations which we have described then recommences.
There is one other consideration which must be borne in mind in
connection with this subject. It is well known that if water be
subjected to sufficiently great pressure it may be raised to a very
high temperature and still retain its liquid condition. When this
pressure is removed, however, the whole mass passes at once into the
condition of steam or water-gas; and the gas thus formed at high
temperatures has a proportionably high tension. In a Papin's digester
water confined in a strong vessel is raised to temperatures far above
its ordinary boiling-point, and from any opening in such a vessel the
steam escapes with prodigious violence. Now, at considerable depths
beneath the earth's surface, and under the pressure of many hundreds
or thousands of feet of solid rock, water still retaining its liquid
condition may become intensely heated. When the pressure is relieved by
the formation of a crack or fissure in the superincumbent mass of rock,
the escape of the superheated steam will be of very violent character,
and may be attended with the most striking and destructive results. In
the existence of high temperatures beneath the earth's surface, and the
presence in the same regions of imprisoned water capable of passing
into the highly elastic gas which we call steam, we have a cause fully
competent to produce all the phenomena which we have described as
occurring at Stromboli.
It may at first sight appear that the grand and terrible displays
of violence witnessed during a great volcanic eruption differ
fundamentally in their character and their origin from those feeble
outbursts which we are able to examine closely and analyse rigorously
at Stromboli. But that such is not the case a few simple considerations
will soon convince us.
[Sidenote: STROMBOLI COMPARED WITH VESUVIUS.]
Although Stromboli usually displays the subdued and moderate activity
which we have been describing, yet the intensity of the action going
on within it is subject to considerable variation. Occasionally the
violence of the outbursts is greatly increased--the roaring of the
steam-jets may be heard for many miles around, considerable streams of
incandescent liquefied rock flow down the Sciarra into the sea, and the
explosions in the crater are far more frequent and energetic, cinders
and fragments of rock being scattered all over the island and the
surrounding seas.
On the other hand, volcanoes like Vesuvius, which are sometimes the
scene of eruptions on the very grandest scale, at others subside into a
temporary state of moderate activity quite similar in character to that
which is the normal condition of Stromboli. Thus, shortly before the
great eruption of Vesuvius in April 1872, a small cone was formed near
the edge of the crater, and during some months observers could watch,
in ease and safety, a series of small explosions taking place, quite
similar in their character and attendant phenomena to those which we
have described as occurring at Stromboli. French geologists are in the
habit of defining the condition of activity in a volcano by speaking
of the more quiet and, regular state as the 'Strombolian stage,' and
the more violent and paroxysmal as the 'Vesuvian stage'; but the two
conditions are, as we have seen, presented by the same volcano at
different periods, and pass into one another by the most insensible
gradations.
We must now proceed to compare the grand and terrible appearances
presented during a great eruption with those more feeble displays which
we have been describing, to show that in all their essential features
these different kinds of outbursts are identical with one another, and
must be referred to the action of similar causes.
The volcanic eruption which has been most carefully studied in recent
times is that which we have already referred to as occurring at
Vesuvius, in the month of April 1872. With the exception, perhaps, of
that which took place in October 1822, this eruption was the grandest
which has broken out at Vesuvius during the present century. Owing to
the circumstance of its proximity to the great city of Naples, Vesuvius
has always been the most carefully watched of all volcanoes, and in
recent years the erection of an observatory, provided with instruments
for recording the smallest subterranean tremors affecting the mountain,
has facilitated the carrying on of those continuous and minute
observations which are so necessary for exact scientific inquiry.
[Illustration: Fig 5. Vesuvius in Eruption, as seen from Naples, April
26, 1872. (_From a photograph_)]
[Sidenote: VESUVIUS ERUPTION OF 1872.]
On the occasion of this outburst, the aid of instantaneous photography
was first made available for obtaining a permanent record of the
appearances displayed at volcanic eruptions. In fig. 5 we have one of
these photographs, which was taken at 5 o'clock P.M. on April 26, 1872,
transferred to a wood-block and engraved. In examining it we feel sure
that we are not being misled by any exaggeration or error on the part
of the artist. Vesuvius rises to the height of nearly 4,000 feet above
the level of the sea, and an inspection of the photograph proves that
the vapours and rock-fragments were thrown to the enormous height of
20,000 feet, or nearly four miles, into the atmosphere.
The main features of this terrifying outburst were as follows. For
more than a twelvemonth before, the activity of the forces at work
within the mountain appeared to be gradually increasing, and the great
eruption commenced on April 24, attained its climax on the 26th, and
began to die out on the following day. During the eruption the bottom
of the crater was entirely broken up, and the sides of the mountain
were rent by fissures in all directions. So numerous were these
fissures and cracks that liquid matter appeared to be oozing from every
part of its surface, and, as Professor Palmieri, who witnessed the
outburst from the observatory, expressed it, 'Vesuvius sweated fire.'
One of the fissures was of enormous size, extending from the summit to
far beyond the base of the cone; the scar left by this gigantic rent
being plainly visible at the present day.
From the great opening or crater at the summit, and from some of the
fissures on the sides of the mountain, enormous volumes of steam rushed
out with a prodigious roaring sound, the noise being so terrific that
the inhabitants of Naples, five miles off, fled from their houses
and spent the night in the open streets. Although this roaring sound
appeared at a distance to be continuous, yet those upon the mountain
could perceive that it was produced by detonations or explosions
rapidly following one another. Each of these explosions was accompanied
by the formation of a great globe of white vapour, which, rising into
the atmosphere, swelled the bulk of the vast cloud overhanging the
mountain. An inspection of the photographs (see fig. 5) shows that the
great vapour-cloud over Vesuvius was made up of the globular masses
ejected at successive explosions. Each of these explosive upward
rushes of steam carried along with it a considerable quantity of solid
fragments, and these fell in great numbers all over the surface of
the mountain, breaking the windows of the observatory, and making it
dangerous to be out of doors.
We have said that lava, or molten rock, appeared to be issuing from
the very numerous cracks formed all over the flanks of the mountain.
But at three points this molten rock issued in such quantities as to
form great, fiery floods, which rushed down the sides of the mountain,
and flowed to a considerable distance beyond its base. The largest
of these lava-floods overwhelmed and destroyed the two villages of
Massa di Somma and San Sebastiano, besides many country houses in the
neighbourhood.
[Sidenote: STEAM EMITTED FROM LAVA-CURRENT.]
A very marked and interesting feature exhibited by these three
lava-floods was the quantity of watery vapour which they gave off
during their flow. All along their course, enormous volumes of steam
were evolved from them, as will be seen by an inspection of the
photograph. Indeed, such was the abundance and tension of the steam
thus escaping from the surfaces of the lava-currents that it forced
the congealing rock up into great bubbles and blisters, and gave rise
to the formation of innumerable miniature volcanoes, varying in size
from a beehive to a cottage, some of which remained in a state of
independent activity for a considerable time.
So far, what we have described as taking place at Vesuvius, in April
1872, has been only the repetition on a £Eur grander scale of the
three kinds of action which we have shown to be constantly taking
place at Stromboli; namely, the formation of cracks or fissures in the
earth's surface, the escape of steam with explosive violence from these
openings, often propelling rock-fragments into the atmosphere, and the
outwelling, under the influence of this compressed steam, of masses of
molten materials.
There were some other appearances presented at the great outburst at
Vesuvius, which do not seem at first sight to find any analogies in
the manifestations of the more feeble action continually going on at
Stromboli.
Before and during the great outbreak of April 1872, Vesuvius itself
and the whole country round were visited with earthquake-shocks, or
tremblings of the ground. The sensitive instruments in the Vesuvian
Observatory showed the mountain daring the eruption to be in a constant
state of tremor. These earthquakes are not, as is commonly supposed,
actual upheavings of the earth's surface, but are vibrations propagated
through the solid materials of which the earth is built up. We cannot
stamp our feet upon the ground without giving rise to such vibrations,
though our senses may not be sufficiently acute to perceive them.
The explosive escape of steam from a crack is a cause sufficiently
powerful to produce a shock which is propagated and may be felt for
a considerable distance round. Even on Stromboli an observer at the
edge of the crater may notice that each explosive outburst of steam
is accompanied by a perceptible tremor of the ground, and in the case
of Vesuvius the violent shocks produced by the escape of far larger
volumes of steam give rise to proportionately stronger vibrations. The
nature and origin of those far more terrible and destructive shocks
which sometimes accompany, and more frequently precede, great volcanic
eruptions, we shall consider in the sequel.
[Sidenote: CAUSE OF LIGHTNING DURING ERUPTIONS.]
Another striking phenomenon which was exhibited in the great eruption
of Vesuvius in 1872 was the vivid display of lightning accompanied
by thunder. The uprushing current of steam and rock-fragments forms
a vertical column, but as the steam condenses it spreads out into a
great horizontal cloud which is seen to be made up of the great globes
of vapour emitted at successive explosions. When there is little or
no wind the vertical column with a horizontal cloud above it bears a
striking resemblance to the stone-pine trees which form so conspicuous
a feature in every Neapolitan landscape. Around this column of vapour
the most vivid lightning constantly plays and adds not a little to the
grand and awful character of the spectacle of a volcanic eruption,
especially when it is viewed by night.
In the eruption of 1872 a strong wind blowing from the north-west
destroyed the usual regular appearance of this 'pine-tree appendage' to
the mountain, which is so well known to, and dreaded by the inhabitants
of Naples; the cloud, as will be seen from the photograph (fig. 5,
_facing_ p. 24), was blown on one side, and most of the falling
fragments took the same direction.
It is well known that when high-pressure steam is allowed to escape
through an orifice, electricity is abundantly generated by the
friction, and Sir William Armstrong's hydro-electric machine is
constructed on this principle. Every volcano in violent eruption is
a very efficient hydro-electric machine, and the uprushing column
is in a condition of intense electrical excitation. This result is
probably aided by the friction of the solid particles as they are
propelled upwards and fall back into the crater. The restoration of
the condition of electrical stability between this column and the
surrounding atmosphere is attended with the production of frequent
lightning-flashes and thunder-claps, the found of the latter being
usually, however, drowned in the still louder roar of the uprushing
steam-column.
The discharge of Buch large quantities of steam into the atmosphere
soon causes the latter to be saturated with watery vapour, and there
follows an excessive rainfall; long-continued rain and floods were an
accompaniment of the great Vesuvian outbreak of 1872, as they have
been of almost all great volcanic eruptions. The Italians, indeed,
dread the floods which follow an eruption more than the fiery streams
of lava which accompany it--for they have found the mud-streams (_lave
di fango_), formed by rain-water sweeping along the loose volcanic
materials, to be more widely destructive in their effects than the
currents of molten rock (_lave di fuoco_).
Besides the phenomena which we have now described as accompanying a
great volcanic outburst, many others have undoubtedly been recorded
by apparently trustworthy authorities. But, in dealing with the
descriptions of these grand and terrible events, we must always be on
our guard against accepting as literal facts, the statements made by
witnesses, often writing at some distance from the scene of action, and
almost always under the influence of violent excitement and terror.
The desire to administer to the universal love of the marvellous, and
the tendency to exaggeration, will usually account for many of the
wonderful statements contained in such records; and, even where the
witness is accurately relating events which he thinks passed before his
eyes, we must remember that it is probable he may have had neither the
opportunity nor the capacity for exact observation.
The more carefully we sift the accounts which have been preserved of
great volcanic outbursts, the more are we struck by the fact that the
appearances described can be resolved into a few simple operations, the
true character of which has been distorted or disguised by the want of
accurate observation on the part of the witnesses.
[Sidenote: SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS.]
We are thus led to the conclusion that the grand and terrible
appearances displayed at Vesuvius and other volcanoes in a state of
violent eruption do not differ in any essential respect from the
phenomena which we have witnessed accompanying the miniature outbursts
of Stromboli. And we are convinced, by the same considerations, that
the forces which give rise to the feeble displays in the latter case
would produce, if acting with greater intensity and violence, all the
magnificent spectacles presented in the former.
In Vesuvius and Stromboli alike, the active cause of all the phenomena
exhibited is found to be the escape of steam from the midst of
masses of incandescent liquefied rock. The violence, and therefore
the grandeur and destructive effects of an eruption, depend upon the
abundance and tension of this escaping steam.
There is one respect in which volcanic phenomena are especially
calculated to excite the fear and wonder of beholders--namely, in the
sudden and apparently spontaneous character of their manifestations.
Eclipses were regarded as equally portentous with volcanic eruptions
till astronomers learned not only to explain the causes which gave
rise to them, but even to predict to the second the times of their
occurrence. If we were able in like manner to warn the inhabitants of
volcanic regions of the approach of a grand eruption, the fear and
superstition with which these events are now regarded would doubtless
be in great part dispelled. The power of prediction is alike the
crucial test and the crowning triumph of a scientific theory.
But, although natural philosophers are able to assign the causes to
which the grand operations of volcanoes are due, and also to explain
all the varied appearances which accompany them, they have not as yet
so far mastered the laws which govern volcanic action as to be able to
predict the periods of their manifestation.
That these operations, like all others going on upon the globe, are
governed by great natural laws we cannot for a moment doubt. And that,
in all probability, more careful and exact observation and reasoning
will at some future time lead us to the recognition of these laws,
every student of nature is sanguine. But at the present time, it must
be confessed, we are very far indeed from being able to afford that
crowning proof of the truth of our theories of volcanic action which is
implied in the power of predicting the period and degree of intensity
of their manifestations.
[Sidenote: ERUPTIONS AND THE INTERVALS BETWEEN THEM.]
There are, however, some observations which lead us to hope that the
time may not be far distant when we shall have so £Eur obtained a
knowledge of the conditions on which volcanic action depends as to be
able to form some judgment as to its manifestations in the future at
any particular locality. But we must recollect that these conditions
axe very numerous and complicated, and that some of them may lie almost
entirely outside our sphere of observation; hence hasty attempts in
this direction, such as have recently been made, are to be deprecated
by every true lover of science.
Concerning the eruptions that have taken place at those volcanic
centres which have been known from a remote antiquity, we have records
from which we can determine the intervals separating these outbursts
and their relative violence. A critical examination of these records
leads to the following conclusions:--
(1.) A long period of quiescence is generally followed by an eruption
which is either of long duration or of great violence.
(2.) A long-continued, or very violent eruption is usually followed by
a prolonged period of repose.
(3.) Feeble and short eruptions usually succeed one another at brief
intervals.
(4.) As a general rule, the violence of a great eruption is inversely
proportional to its duration.
It will be seen that these general conclusions are in perfect harmony
with the theory that volcanic outbursts are due to the accumulation
of steam at volcanic centres, and that the tension of this imprisoned
gas eventually overcomes the repressing forces which tend to prevent
its manifestation. Before astronomers had learnt to determine all
the conditions on which the production of eclipses depends, they had
found that these phenomena succeed one another at regular intervals.
The discovery of such astronomical cycles was a great advance in our
knowledge of the heavenly bodies, and in the same way the determination
of these general relations between the intensity and duration of
volcanic outbursts and the intervals of time which separate them may
be regarded as the first step towards the discovery of the laws which
govern volcanic activity.
In the actual determination of the conditions upon which the occurrence
of volcanic eruptions depends, it must be confessed, however, that
very little has as yet been done. This is in part due to the fact that
some at least of these conditions lie beyond the limits of direct
observation. But it must also be admitted, on the other hand, that
little has been as yet accomplished towards the careful and systematic
observation of those phenomena which may, and probably do, exert an
influence in bringing about volcanic outbursts.
[Sidenote: INFLUENCE OF ATMOSPHERIC CONDITIONS.]
In the Lipari Islands there has prevailed a belief, from the very
earliest period of history, that the feeble eruptions of Stromboli
are in some way dependent upon the condition of the atmosphere.
These islands were known to the ancients as the Æolian Isles, from
the fact that they were once ruled over by a king of the name of
Æolus. It seems not improbable that Æolus was gifted with natural
powers of observation and reasoning far in advance of those of his
contemporaries. A careful study of the vapour-cloud which covers
Stromboli would certainly afford him information concerning the
hygrometric condition of the atmosphere; the form and position
assumed by this vapour-cloud would be a no less perfect index of the
direction and force of the wind; and, if the popular belief be well
founded, the frequence and violence of the explosions taking place
from the crater would indicate the barometric pressure. From these
data an acute observer would be able to issue 'storm-warnings' and
weather-prognostics of considerable value. In the vulgar mind, the
idea of the prediction of natural events is closely bound up with that
of their production; and the shrewd weather-prophet of Lipari was
after his death raised to the rank of a god, and invested with the
sovereignty of the winds.
Whether the popular idea that the outbursts of Stromboli are regulated
by atmospheric conditions has any foundation is still open to grave
doubt. It seems to be certain, however, that during autumn and winter
the more violent paroxysms of the volcano occur, and that in summer
the action which takes place is far more regular and equable. It would
be of the greatest benefit to science if an observatory were erected
beside the crater of Stromboli, where a careful record might be kept of
all atmospheric changes, and of the synchronous manifestations of the
volcanic forces.
A little consideration will show that it is a by no means unreasonable
supposition that variations in atmospheric pressure may exercise a very
important influence in bringing about volcanic outbursts. Changes in
the barometer to the extent of two inches within a very short period
are not uncommon occurrences. A very simple calculation will show that
the fall of the mercury in the barometer to the extent of two inches
indicates the removal of a weight of two millions of tons from each
square mile of the earth's surface where this change takes place. Now,
if we suppose, as we have good ground for doing, that under volcanic
areas vast quantities of superheated water are only prevented from
flashing into steam by the superincumbent pressure, a relief of this
pressure to the extent of two millions of tons on every square mile
could scarcely fail to produce very marked effects. The way in which
explosions in fiery coal-mines generally follow closely upon sudden
falls in the atmospheric pressure is now well known; and coal-mine
explosions and volcanic outbursts have this in common, that both
result from the sudden and violent liberation of subterranean gases.
There are not a few apparently well-authenticated accounts of volcanic
and earthquake phenomena following closely on peculiar atmospheric
conditions, and the whole question of the relation of the volcanic
forces to atmospheric pressure, as Spallanzani himself so long ago
pointed out, is deserving of a most careful and rigorous investigation.
[Sidenote: SUPPOSED TIDAL EFFECTS.]
There is one other consideration which has frequently been urged as
worthy of especial attention, in dealing with the question of the
exciting causes of volcanic outbursts. If volcanoes were, as was at one
time almost universally supposed, in direct communication with a great
central mass of liquefied materials, or even if any large reservoirs
of such liquids existed beneath volcanic districts, as others have
imagined, then the different mobility of the solid and liquid portions
of the earth's mass would give rise to tidal effects similar to those
occurring in the surface waters of the globe. Under such circumstances,
volcanic outbursts, like the tides, would be determined by the relative
positions of the sun and moon to our globe. It is certain, however,
that no very direct relation has yet been established between the
lunar periods and those of volcanic outbursts, though recent close
observations upon the crater of Vesuvius, by Professor Palmieri, do
seem to lend support to the view that such relations may exist.
At the present time, therefore, it must be admitted that vulcanologists
have only just commenced those series of exact and continuous
observations which are necessary to determine the conditions that
regulate the appearance of volcanic phenomena. The study of the laws
of volcanic action is yet in its infancy. But the establishment of
observatories on Vesuvius and Etna 18 fall of promise for the future,
and when we consider the advances which have been made, during the last
one hundred years, in our knowledge of the true nature of volcanic
action, we need not despair that the extension of the same methods
of inquiry will lead to equally important results concerning the
conditions which determine and the laws which govern it.
In the meanwhile, it is no small gain to have established the fact that
volcanic phenomena, divested of all those wonderful attributes with
which superstition and the love of the marvellous have surrounded them,
are operations of nature obeying definite laws, which laws we may hope
by careful observation and accurate reasoning to determine; and that
the varied appearances, presented alike in the grandest and feeblest
outbursts, can all be referred to one simple cause--namely, the escape,
from the midst of masses of molten materials, of imprisoned steam or
water-gas.