THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS.
The escape of great quantities of steam and other gases from the midst
of a mass of fluid or semi-fluid lava gives rise to the formation of
vast quantities of froth or foam upon its surface. This froth or foam,
which is formed upon the surface of lava by the escape of gaseous
matters from within it, is made up of portions of the lava distended
into vesicles, in the same way that bubbles are formed on the surface
of water. It bears precisely the same relation to the liquid mass of
lava that the white crest of foam upon an advancing wave does to the
sea-water, from the bubbles of which it is formed.
This froth upon the surface of lavas varies greatly in character
according to the nature of the material from which it is formed. In
the majority of cases the lavas consist, as we have seen, of a mass
of crystals floating in a liquid magma, and the distension of such a
mass by the escape of steam from its midst gives rise to the formation
of the rough cindery-looking material to which the name of 'scoria'
is applied. But when the lava contains no ready-formed crystals, but
consists entirely of a glassy substance in a more or less perfect state
of fusion, the liberation of steam gives rise to the formation of the
beautiful material known as 'pumice.' Pumice consists of a mass of
minute glass bubbles; these bubbles have not usually, however, retained
their globular form, but have been elongated in one direction through
the movement of the mass while it was still in a plastic state.
The steam frequently escapes from lava with such violence that the
froth or scum on its surface is broken up and scattered in all
directions, as the foam crests of waves are dispersed by the wind
during a storm. In this way fragments of scoria or pumice are often
thrown to the height of many hundreds or thousands of feet into the
atmosphere, as we have seen is the case at Stromboli and Vesuvius.
Indeed, during violent eruptions, a continuous upward discharge of
these fragments is maintained, the ragged cindery masses hurtling one
another in the atmosphere, as they are shot perpendicularly upwards
to an enormous height and fall back into the vent; or they may rise
obliquely and describe curves so as to descend outside the orifice from
which they were ejected.
[Sidenote: FINENESS OF VOLCANIC DUST.]
During their upward discharge and downward fall, the cindery fragments
are by attrition continually reduced to smaller dimensions. The noise
made by these fragments, as they strike against one another in
the air during their rise and fall, is one of the most noteworthy
accompaniments of volcanic eruptions. It has been noticed that in many
cases there is a constant diminution in the size of the fragments
ejected during a volcanic outburst, this being doubtless due to the
friction of the masses as they are ejected and re-ejected from the
vent. Thus it is related by Mr. Poulett Scrope, who watched the
Vesuvian eruption of 1822, which lasted for nearly a month, that
during the earlier stages of the outburst fragments of enormous size
were thrown out of the crater, but by constant re-ejection these were
gradually reduced in size, till at last only the most impalpable dust
issued from the vent. This dust filled the atmosphere, producing in
the city of Naples 'a darkness that might be felt,' and so excessively
finely divided was it, that it penetrated into all drawers, boxes, and
the most closely fastened receptacles, filling them completely. Mr.
Whymper relates that, while standing on the summit of Chimborazo, he
witnessed an eruption of Cotopaxi, which is distant more than fifty
miles from the former mountain. The fine volcanic dust fell in great
quantities around him, and he estimated that no less than two millions
of tons must have been ejected during this slight outburst. Professor
Bonney has examined this volcanic dust from Cotopaxi, and calculates
that it would take from 4,000 to 25,000 particles to make up a grain in
weight.
Various names have been given by geologists to the fragments ejected
from volcanic vents, which, as we have seen, differ greatly in their
dimensions and other characters. Sometimes masses of more or less
fluid lava are flung bodily to a great height in the atmosphere.
During their rise and fall these masses are caused to rotate, and in
consequence assume a globular or spheroidal form. The water imprisoned
in these masses, during their passage through the atmosphere, tends to
expand into steam, and they become more or less completely distended
with bubbles. Such masses, which sometimes assume very regular and
striking forms, are known as 'volcanic bombs.' Many volcanic bombs have
a solid nucleus of refractory materials. The large, rough, angular,
cindery-looking fragments are termed 'scoriæ.' When reduced to the
dimensions of a marble or pea they are usually called by the Italian
name of 'lapilli.' The still finer materials are known as volcanic sand
and dust.
There are, however, two names which are frequently applied to these
fragmentary materials ejected from volcanoes, which are perhaps
liable to give rise to misconception. These are the terms 'cinders'
and 'ashes.' It must be remembered that the scoriæ or cindery-looking
masses are not, like the cinders of our fires, the product of the
partial combustion of a material containing inflammable gases, but are,
like the clinkers of furnaces and brick-kilns, portions of partially
vitrified and fused rock distended by gases. So, too, volcanic ashes
only resemble the ashes of our grates in being very finely divided;
they are not, like the latter, the incombustible residue of a mass
which has been burnt.
[Sidenote: VOLCANIC BOMBS AND PELE'S HAIR.]
The glassy lavas, when distended by escaping gases, give rise to the
formation of pumice, the white colour of which, as in the case of the
foam of a wave, is due to the reflection of a portion of the light in
its frequent passage from one medium to another--in this case from
air to glass, and from glass to air. The volcanic bombs formed from
glassy lavas are often of especially beautiful and regular forms.
Sometimes the passage of steam through a mass of molten glass produces
large quantities of a material resembling spun glass. Small particles
or shots of the glass are carried into the air and leave behind them
thin, glassy filaments like a tail. At the volcano of Kilauea in Hawaii
this filamentous volcanic glass is abundantly produced, and is known
as 'Pele's Hair'--Pele being the name of the goddess of the mountain.
Birds' nests are sometimes found composed of this beautiful material.
In recent years an artificial substance similar to this Pele's hair
has been extensively manufactured by passing jets of steam through the
molten slag of iron-furnaces; it resembles cotton-wool, but is made up
of fine threads of glass, and is employed for the packing of boilers
and other purposes.
The very finely-divided volcanic dust is often borne to enormous
distances from the volcano out of which it has been ejected. The force
of the steam-current carrying the fragments into the atmosphere is
often so great that they rise to the height of several miles above
the mountain. Here they may actually pass into the upper currents of
the atmosphere and be borne away to the distance of many hundreds
or thousands of miles. Hence it is not an unusual circumstance for
vessels at sea to encounter at great distances from land falling
showers of this finely divided, volcanic dust. We sometimes meet with
this far-travelled, volcanic dust under very unexpected circumstances.
Thus, in the spring of 1875 I had occasion to visit Prof. Vom Rath of
Bonn, who showed me a quantity of fine volcanic dust which had during
the past winter fallen in considerable quantities in certain parts
of Norway. This dust, upon microscopic examination, proved to be so
similar to what was known to be frequently ejected from the Icelandic
volcanoes that a strong presumption was raised that volcanic outbursts
had been going on in that island. On returning to England I found that
the first steamer of the season had just reached Leith from Iceland,
bringing the intelligence that very violent eruptions had taken place
during the preceding months.
[Sidenote: DISPERSION OF PUMICE AND VOLCANIC DUST.]
This finely-divided volcanic dust is thus carried by the winds and
spread over every part of the ocean. Everyone is familiar with the
fact that pumice floats upon water; this it does, not because it is a
material specifically lighter than water, but because cavities filled
with air make up a great part of its bulk. If we pulverise pumice, we
find the powder sinks readily in water, but the rock in its natural
condition floats for the same reason that an iron ship does--because
of the air-chambers which it encloses. When this pumice is ejected from
a volcano and falls into a river or the ocean, it floats for a long
time, till decomposition causes the breaking down of the thin glassy
partitions between the air chambers, and causes the admission of water
into the latter, by which means the whole mass gets water-logged. Near
the Liparis and other volcanic islands the sea is sometimes covered
with fragments of pumice to such an extent that it is difficult for a
boat to make progress through it, and the same substance is frequently
found floating in the open ocean and is cast up on every shore.
During the year 1878 masses of floating pumice were reported as
existing in the vicinity of the Solomon Isles, and covering the surface
of the sea to such extent that it took ships three days to force their
way through them. Sometimes these masses of pumice accumulate in such
quantities along coasts that it is difficult to determine the position
of the shore within a mile or two, as we may land and walk about on
the great floating raft of pumice. Now, recent deep-sea soundings,
carried on in the 'Challenger' and other vessels, have shown that the
bottom of the deepest portion of the ocean, far away from the land, is
covered with these volcanic materials which have been carried through
the air or floated on the surface of the ocean. To these deeper parts
of the ocean no sediments carried down by the rivers are borne, and the
remains of calcareous organisms are, in these abysses, soon dissolved;
under such conditions, therefore, almost the only material accumulating
on the sea bottom is the ubiquitous wind- and wave-borne volcanic
products. These particles of volcanic dust and fragments of pumice by
their disintegration give rise to a clayey material, and the oxidation
of the magnetite, which all lavas contain, communicates to the mass a
reddish tint. This appears to be the true origin of those masses of
'red-clay' which, according to recent researches, are found to cover
all the deeper parts of the ocean, but which probably attain to no
great thickness.
But while some portion of the materials ejected from volcanoes may
thus be carried by winds and waves, so as to be dispersed over every
part of the land and the ocean-bed, another, and in most cases by far
the largest, portion of these ejections falls around the volcanic vent
itself. It is by the constant accumulation of these ejected materials
that such great mountain masses as Etna, Teneriffe, Fusiyama, and
Chimborazo have been gradually built up around centres of volcanic
action.
There are cases in which the formation of volcanic mountains on a small
scale has actually been observed by trustworthy witnesses. There are
other cases in which volcanic mountains of larger size can be shown
to have increased in height and bulk by the fall upon their sides and
summits of fragmentary materials ejected from the volcanic vent. In all
cases the examination of these mountain-masses leads to the conclusion
that they are entirely built up of just such materials as we constantly
see thrown out of volcanoes during eruption.
[Sidenote: FORMATION OF VOLCANIC MOUNTAINS.]
Thus we are led to the conclusion that all volcanic mountains are
nothing but heaps of materials ejected from fissures in the earth's
crust, the smaller ones having been formed during a single volcanic
outburst, the larger ones being the result of repeated eruptions from
the same orifice which may, in some cases, have continued in action for
tens or hundreds of thousands of years.
No observer has done such useful work in connection with the study of
the mode of formation of volcanic mountains as our countryman, Sir
William Hamilton, who was ambassador at Naples from 1764 to 1800, and
made the best possible use of his opportunities for examining the
numerous volcanoes in Southern Italy.
A little to the west of the town of Puzzuoli on the Bay of Naples there
stands a conical hill rising to the height of 440 feet above the level
of the Mediterranean, and covering an area more than half a mile in
diameter. Now we have the most conclusive evidence that in ancient
times no such hill existed on this site, which was partly occupied by
the Lucrine Lake, and the fact is recognised in the name which the hill
bears, that of Monte Nuovo, or the 'New Mountain.' See fig. 10.
Sir William Hamilton rendered admirable service to science by
collecting all the contemporary records relating to this interesting
case, and he was able to prove, by the testimony of several
intelligent and trustworthy witnesses, that during the week following
the 29th of September, 1538, this hill had gradually been formed of
materials ejected from a volcanic vent which had opened upon this site.
[Illustration: Fig. 10. Monte Nuovo (440 ft. high) on the shores of the
Bay of Naples.]
[Sidenote: HISTORY OF THE FORMATION OF MONTE NUOVO.]
The records collected by Hamilton with others which have been
discovered since his death prove most conclusively the following
facts. During more than two years, the country round was affected
by earthquakes, which gradually increased in intensity and attained
their climax in the month of September 1538; on the 27th and 28th of
that month these earthquake shocks are said to have been felt almost
continuously day and night. About 8 o'clock on the morning of the 29th,
a depression of the ground was noticed on the site of the future hill,
and from this depression, water, which was at first cold and afterwards
tepid, began to issue. Four hours afterwards the ground was seen to
swell up and open, forming a gaping fissure, within which incandescent
matter was visible. From this fissure numerous masses of stone, some of
them 'as large as an ox,' with vast quantities of pumice and mud, were
thrown: up to a great height, and these falling upon the sides of the
vent formed a great mound. This violent ejection of materials continued
for two days and nights, and on the third day a very considerable hill
was seen to have been built up by the falling fragments, and this hill
was climbed by some of the eye-witnesses of the eruption. The next day
the ejections were resumed, and many persons who had ventured on the
hill were injured, and several killed by the falling stones. The later
ejections were however of less violence than the earlier ones, and seem
to have died out on the seventh or eighth day after the beginning of
the outburst. The great mass of this considerable hill would appear,
according to the accounts which have been preserved, to have been built
up by the materials which were ejected during two days and nights.
Monte Nuovo is a hill of truncated conical form, which rises to the
height of 440 feet above the waters of the Mediterranean, and is now
covered with thickets of stone-pine. The hill is entirely made up
of volcanic scoriæ, lapilli, and dust, and the sloping sides have
evidently been produced by these fragmentary materials sliding over one
another till they attained the angle of rest; just as happens with the
earth and stones tipped from railway-waggons during the construction of
an embankment. In the centre of this conical hill is a vast circular
depression, with steeply sloping sides, which is of such depth that its
bottom is but little above the sea-level. This cup-shaped depression is
the 'crater' of the volcano, and it has evidently been formed by the
explosive action which has thrown out the materials immediately above
the vent, and caused them to be accumulated around it.
[Illustration: Fig. 11.--Map of the district around Naples, showing
Monte Nuovo and the surrounding volcanoes of older date.]
The district lying to the west of Naples, in which the Monte Nuovo is
situated, contains a great number of hills, all of which present a most
striking similarity to that volcano. All these hills are truncated
cones, with larger or smaller circular depressions at their summits,
and they axe entirely composed of volcanic scoriæ, lapilli, and dust.
Some of these hills are of considerably larger dimensions than the
Monte Nuovo, while others are of smaller size, as shown in the annexed
map, fig. 11. No stranger visiting the district, without previous
information upon the subject, would ever suspect the fact that, while
all the other hills of the district have existed from time immemorial,
and are constantly mentioned in the works of Greek and Roman writers,
this particular hill of Monte Nuovo came into existence less than 350
years ago.
[Sidenote: OLDER VOLCANOES OF THE CAMPI PHLEGRÆI.]
The evidently fused condition of the materials of which these hills are
built up is a dear sign of the volcanic action which has taken place
in it; and this feet was so fully recognised by the ancients that they
called the district the Campi Phlegræi, or 'the Burning Fields,' and
regarded one of the circular depressions in it as the entrance to Hades.
It is impossible for anyone to examine this district without being
convinced that all the numerous cones and craters which cover it
have been formed by the same agency as that by which Monte Nuovo was
produced. We have shown that there is the most satisfactory historical
evidence as to what that agency was.
Now volcanic cones with craters in their centres occur in great numbers
in many parts of the earth's surface. In some districts, like the
Auvergne, the Catacecaumene in Asia Minor, and certain parts of New
Zealand, these volcanic cones occur by hundreds and thousands. In some
instances, these volcanic cones have been formed in historic times,
but in the great majority of cases we can only infer their mode of
origin from their similarity to others of which the formation has been
witnessed.
Most of the smaller volcanic hills, with their craters, have been
thrown up during a single eruption from a volcanic fissure; but, as
Hamilton conclusively proved, the grandest volcanic mountains must
have been produced by frequent repetitions of similar operations upon
the same site. For not only are these great volcanic piles found to be
entirely composed of materials which have evidently been ejected from
volcanic vents, but, when carefully watched, such mountains are found
undergoing continual changes in form, by the addition of materials
thrown out from the vent, and falling upon their sides.
This fact will be well illustrated by a comparison of the series of
drawings of the summit of Vesuvius which were made by Sir William
Hamilton in 1767, and which we have copied in fig. 12. During the
earlier months of that year the summit of the mountain was seen to be
of truncated form, a great crater having been originated by the violent
outbursts of the preceding year. This condition of the mountain-top
is represented in the first figure of the series. The drawing made by
Hamilton, on July 8, shows that not only was the outer rim of the great
crater being modified in form by the fall of materials upon it, but
that in the centre of the crater a small cone was being gradually built
up by the quiet ejections which were taking place.
[Illustration: Fig. 12.--Outlines of the Summit of Vesuvius during the
Eruption of 1767.]
[Sidenote: CHANGES IN FORM OF VESUVIUS.]
If we compare the drawings made at successive dates, we shall find that
the constant showers of falling materials were not only raising the
edge of the great crater but were at the same time increasing the size
of the small cone inside the crater. By the end of October the small
cone had grown to such an extent that its sides were confluent with
those of the principal cone, which had thus entirely lost its truncated
form and been raised to a much greater height. The comparison of these
drawings will be facilitated by the dotted lines, which represent
the outline of the top of the mountain at the preceding observation;
so that the space between the dotted and the continuous line in each
drawing shows the extent to which the bulk of the cone had increased in
the interval between two observations.
But, although the general tendency of the action going on at volcanic
mountains is to increase their height and bulk by the materials falling
upon their summits and aides, it must be remembered that this action
does not take place by any means continuously and regularly. Not only
are there periods of rest in the activity of the volcano, during which
the rain and winds may accomplish a great deal in the way of crumbling
down the loose materials of which volcanic mountains are largely built
up, but sudden and violent eruptions may in a very short time undo the
slow work of years by blowing away the whole summit of the mountain at
once. Thus, before the great eruption of 1822, the cone of Vesuvius, by
the almost constant ejection of ashes during several years, had been
raised to the height of more than 4,000 feet above the level of the
sea; but by the terrible outburst which then took place the cone was
reduced in height by 400 feet, and a vast crater, which had a diameter
of nearly a mile, and a depth of nearly 1,000 feet (see fig. 13), was
formed at the top of the mountain. The enormous quantity of material
thus removed was either distributed over the flanks of the mountain,
or, when reduced to a finely comminuted condition, was carried by the
wind to the distance of many miles, darkening the air, and coating the
surface of the ground with a thick covering of dust.
[Illustration: Fig. 13.--Crater of Vesuvius formed during the eruption
in 1822. (It was nearly 1 mile in diameter and 1,000 ft. deep.)]
[Sidenote: EARLY HISTORY OF VESUVIUS.]
The volcano of Vesuvius, although of somewhat insignificant dimensions
when compared with the grander volcanic mountains of the globe,
possesses great interest for the student of Vulcanology, inasmuch as
being situated in the midst of a thickly populated district and in
close proximity to the city of Naples, it has attracted much attention
during past times, and there is no other volcano concerning which we
have so complete a series of historical records. The present cone of
Vesuvius, which rises within the great encircling crater-ring of Somma,
has a height of about 1,000 feet. But there is undoubted evidence that
this cone, to the top of which a railway has recently been constructed
for the convenience of tourists, has been entirely built up during the
last 1,800 years, and, what is more, that during this period it has
been many times almost wholly destroyed and reconstructed.
Nothing is more certain than the bet that the Vesuvius upon which the
ancient Romans and the Greek settlers of Southern Italy looked, was a
mountain differing entirely in its form and appearance from that with
which we are familiar. The Vesuvius known to the ancients was a great
truncated cone, having a diameter at its base of eight or nine miles,
and a height of about 4,000 feet. The summit of this mountain was
formed by a circular depressed plain, nearly three miles in diameter,
within which the gladiator Spartacus, with his followers, were
besieged by a Roman army. There is no evidence that at this time the
volcanic character of the mountain was generally recognised, and its
slopes are described by the ancient geographers as being clothed with
fertile fields and vineyards, while the hollow at the top was a waste
overgrown with wild vines.
[Illustration: Fig. 14.--Crater of Vesuvius in 1756. (From a drawing
made on the spot)]
But in the year 79 a terrible and unexpected eruption occurred, by
which a vast, crateral hollow was formed in the midst of Vesuvius, and
all the southern side of the great rim surrounding this crater was
broken down. Under the materials ejected during this eruption, the
cities of Pompeii, Herculaneum, and Stabiæ were overwhelmed and buried.
Numerous descriptions and drawings enable us to understand how in the
midst of the vast crater formed in the year 79 the modern cone has
gradually been built up. Fresh eruptions are continually increasing the
bulk, or raising the height of the Vesuvian cone.
The accompanying drawings made by Sir William Hamilton enable us to
understand the nature of the changes which have been continually
taking place at the summit of Vesuvius. The drawing fig. 14 shows the
appearance presented by the crater in the year 1756.
[Illustration: Fig. 15.--The Summit of Vesuvius in 1767. (From an
original drawing.)]
[Sidenote: VESUVIUS IN MODERN TIMES.]
At this time we see that inside the crater a series of cones had been
built up one within the other from which lava issued, filling the
bottom of the crater and finding its way through a breach in its walls,
down the side of the cone. It is evident that the ejected materials
falling on the sides of the innermost cone would tend to enlarge the
latter till its sides became confluent with the cone surrounding it,
and if this action went on long enough, the crater would be entirely
filled up and a perfect cone with only a small aperture at the top
would be produced. But from time to time, grand and paroxysmal
outbursts have occurred at Vesuvius, which have truncated the cone,
and sometimes formed great, cup-shaped cavities, reaching almost to its
base, like that shown in fig. 13.
In 1767 the crater of Vesuvius, as shown in fig. 15, contained a single
small cone in a state of constant spasmodic outburst, like that of
Stromboli.
[Illustration: Fig. 16.--Summit of Vesuvius in 1848.]
In 1843, we find that the crater of Vesuvius contained three such small
cones arranged in a line along its bottom as depicted in fig. 16.
These drawings of the summit of Vesuvius give a fair notion of the
changes which have been continually going on there during the whole of
the historical period. Ever and anon a grand outburst, like that of
1822, has produced a vast and deep crater such as is represented in
fig. 13, and then a long continuance of quiet and regular ejections
has built up within the crater small cones like those shown in figs.
14, 15 and 16, till at last the great crater has been completely filled
up, and the cone reconstructed.
[Illustration: Fig. 17.--Outlines of Vesuvius, showing its Form at
different periods of its history.]
[Sidenote: CHANGES IN OUTLINE OF VESUVIUS.]
In the series of outlines in fig. 17, we have endeavoured to illustrate
the succession of changes which has taken place in Vesuvius during
historical times. In the year 79 one side of the crater-wall of the
vast mountain-mass was blown away. Subsequent ejections built up the
present cone of Vesuvius within the great encircling crater-wall of
Somma, and the form of this cone and the crater at its summit have
been undergoing continual changes during the successive eruptions of
eighteen centuries.
What its _future_ history may be we can only conjecture from analogy.
It may be that a long continuance of eruptions of moderate energy may
gradually raise the central cone till its sides are confluent with
those of the original mountain; or it may be that some violent paroxysm
will entirely destroy the modern cone, reducing the mountain to the
condition in which it was after the great outburst of 79. On the other
hand, if the volcanic forces under Vesuvius are gradually becoming
extinct (but of this we have certainly no evidence at present), the
mountain may gradually sink into a state of quiescence, retaining its
existing form.
The series of changes in the shape of Vesuvius, which are proved by
documentary evidence to have been going on during the last 2,000 years,
probably find their parallel in all active volcanoes. In all of these,
as we shall hereafter show, the activity of the vents undergoes great
vicissitudes. Periods of continuous moderate activity alternate with
short and violent paroxysmal outbursts and intervals of complete rest,
which may in some cases last for hundreds or even thousands of years.
During the periods of continuous moderate activity, the crater of the
volcano is slowly filled up by the growth of smaller cones within it;
and the height of the mountain is raised. By the terrible paroxysmal
outbursts the mountain is often completely gutted and its summit blown
away; but the materials thus removed from the top and centre of the
mass are for the most part spread over its aides, so that its bulk
and the area of its base are thereby increased. During the intervals
of rest, the sides of the mountain which are so largely composed of
loose and pulverulent materials are washed downwards by rains and
driven about by winds. Thus all volcanoes in a state of activity are
continually growing in size every ejection, except in the case of those
where the materials are in the finest state of subdivision, adding to
their bulk; the area of their bases being increased during paroxysmal
outbursts, and their height during long-continued moderate eruptions.
[Sidenote: DEVIATIONS FROM CONICAL FORM.]
We have pointed out that the conical form of volcanic mountains is
due to the slipping of the falling materials over one another till
they attain the angle at which they can rest. There are, however, some
deviations from this regular conical form of volcanoes which it may be
well to refer to.
The quantity of rain which falls during volcanic eruptions is often
enormous, owing to the condensation of the great volumes of steam
emitted from the vent. Consequently the falling lapilli and dust often
descend upon the mountain, not in a dry state but in the condition
of a muddy paste. Many volcanic mountains have evidently been built
up by the flow of successive masses of such muddy paste over their
surfaces. Some volcanic materials when mixed with water have the
property of rapidly 'setting' like concrete. The ancient Romans and
modern Italians, well acquainted with this property of certain kinds of
volcanic dust and lapilli, have in all ages employed this 'puzzolana,'
as it is called, as mortar for building. The volcanic muds have often
set in their natural positions, so as to form a rock, which, though
light and porous, is of tolerably firm consistency. To this kind of
rock, of which Naples and many other cities are built, the name of
'tuff' or 'tufa' is applied. A similar material is known in Northern
Germany as 'trass.'
The cause of the 'setting' of puzzolana and tufa is that rain-water
containing a small proportion of carbonic acid acts on the lime in the
volcanic fragments, and these become cemented together by the carbonate
of lime and the free silica, which are thus produced in the mass.
When a strong wind is blowing during a volcanic outburst, the materials
may be driven to one side of the vent, and accumulate there more
rapidly than on the other. Thus lop-sided cones are formed, such
as may frequently be observed in some volcanic districts. In areas
where constant currents of air, like the trade-winds, prevail, all
the scoria-cones of the district may thus be found to be unequally
developed on opposite sides, being lowest on those from which the
prevalent winds blow, and highest on the sides towards which these
winds blow.
[Sidenote: ANGLE OF SLOPE IN VOLCANIC CONES.]
The examination of any careful drawing, or better still of the
photograph, of a volcanic cone, will prove that the profile of such
cones is not formed by straight lines, but by curves often of a
delicate and beautiful character. The delineations of the sacred
volcano of Fusiyama, which are so constantly found in the productions
of Japanese artists, must have familiarised everyone with the elegant
curved lines exhibited by the profiles of volcanoes. The upper slope
of the mountain is comparatively steep, often exhibiting angles of
30° to 35°, but this steepness of slope gradually diminishes, till it
eventually merges in the surrounding plains. The cause of this elegant
form assumed by most volcanic mountains is probably two-fold. In the
first place we have to remember that the materials falling upon the
flanks of the mountain differ in size and shape, and some will rest
on a steeper slope than others. Thus, while some of the materials
remain on the upper part of the mountains, others are rolling outwards
and downwards. Hence we find that those cones which are composed of
uniform materials have straight sides. But in some cases, we shall
see hereafter, there has certainly been a central subsidence of the
mountain mass, and it is this subsidence which has probably given rise
to the curvature of its flanks.
We have hitherto considered only the methods by which the froth or
foam, which accumulates on the surface of fluid lava, is dispersed.
But in many cases not only is this scum of the lava ejected from the
volcanic vent by the escaping steam, but the fluid lava itself is
extruded forcibly, and often in enormous quantities.
The lava in a volcanic vent is always in a highly heated, usually
incandescent, condition. Seen by night, its freshly exposed surface
is glowing red, sometimes apparently white-hot. But by exposure to
the atmosphere the surface is rapidly chilled, appearing dull red by
night, and black by day. Many persons are surprised to find that a
flowing stream of lava presents the appearance of a great mass of rough
cinders, rolling along with a rattling sound, owing to the striking of
the clinker-like fragments against each other. When viewed by night,
the gleaming, red light between these rough, cindery masses betrays the
presence of incandescent materials below the chilled surface of the
lava-stream.
No fact in connection with lavas is more striking than the varying
degrees of liquidity presented by them in different cases. While some
lava-streams seem to resemble rivers, the material flowing rapidly
along, filling every channel in its course, and deluging the whole
country around, others would be more fitly compared to glaciers,
creeping along at so slow a rate that the fact of their movement can
only be demonstrated by the most careful observation. Even when falling
over a precipice such lavas, owing to their imperfect liquidity, form
heavy, pendent masses like a 'guttering' candle, as is shown by fig.
18, which is taken from a drawing kindly furnished to me by Capt. S. P.
Oliver, R.A. The causes of these differences in the rate of motion of
lava-streams we must proceed to consider.
[Illustration: Fig. 18.--Cascade of Lava tumbling over a cliff in the
Island of Bourbon.]
[Sidenote: TEMPERATURE OF LAVA-STREAMS.]
There can be no doubt that the temperature of lavas varies greatly in
different cases. This is shown by the fact that while some lavas are in
a state of complete fusion, similar to that of the slags of furnaces,
and like the latter, such lavas on cooling form a glassy mass, others
consist of a liquid magma in which a larger or smaller number of
crystals are found floating. In these latter cases the temperature of
the magma must be below the fusing-point of the minerals which exist
in a crystalline condition in its midst. It has indeed been suggested
that the whole of the crystals in lavas are formed during the cooling
down of a completely fused mass; but no one can imagine that the
enclosed crystals of quartz, felspar, leucite, olivine, &c., have
been so formed, such crystals being sometimes more than an inch in
diameter. The microscopic examination of lavas usually enables us to
discriminate between those complete crystals which have been formed at
great depths and carried up to the surface, and the minute crystalline
particles and microliths which have been developed in the glassy mass
during cooling. Crystals of the former class, indeed, exhibit abundant
evidence, in their liquid cavities and other peculiarities, that they
have not been formed by simple cooling from a state of fusion, but
under the combined action of heat, the presence of water and various
gases, and intense pressure.
As we have already seen, the different lavas vary greatly in their
degrees of fusibility. The basic lavas, containing a low percentage
of silica, are much more fusible than the acid lavas, which contain
a high percentage of silica. When the basic lavas are reduced to a
complete state of fusion their liquidity is sometimes very perfect, as
is the case at Kilauea in Hawaii, where the lava is thrown up into jets
and fountains, falling in minute drops, and being drawn out into fine
glassy threads. On the other hand, the less fusible acid lavas appear
to be usually only reduced to the viscous or pasty condition, which
artificial glasses assume long before their complete fusion. Of this
fact I have found many proofs in the Lipari Islands, where such glassy,
acid lavas abound. In fig. 6 (page 43) a lava-stream is represented on
the side of the cone of Vulcano.
[Sidenote: IMPERFECTLY FLUID LAVAS.]
This lava is an obsidian--that is to say, it is of the add type and
completely glassy--but its liquidity must have been very imperfect,
seeing that the stream has come to a standstill before reaching the
bottom of a steep slope of about 35°. In fig. 19 there is given a side
view of the same stream of obsidian, from which it will be seen that
it has flowed slowly down a steep slope and heaped itself up at the
bottom, as its fluidity was not complete enough to enable it to move on
a slighter incline. An examination of the interior of such imperfectly
fluid lavas affords fresh proofs of the slow and tortuous movements
of the mass. Everywhere we find that the bands of crystallites and
sphærulites are, by the movement of the mass, folded and crumpled and
puckered in the most remarkable manner, as is illustrated in figs. 20
and 21. Similar appearances occur again and again among the vitreous
and semi-vitreous acid lavas of Hungary.
[Illustration: Fig. 19.--Lava-stream (obsidian) in the Island of
Vulcano showing the imperfect liquidity of the mass.]
[Illustration: Fig. 20.--Interior of a Rhyolitic Lava-stream in the
Island of Lipari, showing broad sigmoidal folds produced by the slow
movements of the mass.]
[Illustration: Fig. 21.--Interior of a Rhyolitic Lava-stream in the
Island of Lipari, showing the complicated crumplings and puckerings
produced by the slow movements of the mass.]
[Sidenote: RATE OF MOVEMENT OF VESUVIAN LAVAS.]
But, although the temperature of lava-streams and the fusibility of
their materials may in some cases account for their condition of
either perfect liquidity or viscidity, it is clear that in other
instances there must be some other cause for this difference. Thus
it has been found that at Vesuvius the lavas erupted in modern times
have all a striking similarity to one another in chemical composition,
in the minerals which they contain, and in their structure. They are
all basic lavas, which when examined by the microscope are seen to
consist of a more or less glassy magma, in the midst of which numerous
crystals of augite, leucite, olivine, magnetite, and other minerals
are scattered. Yet nothing can be more strikingly different than the
behaviour of the lavas poured out from Vesuvius at various periods. In
some cases the lava appears to be in such a perfectly liquid condition
that, issuing from the crater, it has been described as rushing down
the slope of the cone like a stream of water, and such exceedingly
liquid lavas have in some cases flowed to the distance of several
miles from the base of the mountain in a very short time. But other
Vesuvian lavas have been in such a viscid condition that their rate
of movement has been so extremely slow as to be almost imperceptible.
Such lava-streams have continued in movement during many years, but the
progress has been so slow (often only a few inches in a day) that it
could only be proved by means of careful measurements.
If we examine some of these Vesuvian lavas which have exhibited such
striking differences in their rate of flow, we shall find that they
present equally marked differences in the character of their surfaces.
The lava-current of 1858 was a remarkable example of a slow-flowing
stream, and its surface, as will be seen in fig. 22, which is taken
from a photograph, has a very marked and peculiar character. A
tenacious crust seems to have formed on the surface, and by the further
motion of the mass this crust or scum has been wrinkled and folded
in a very remarkable manner. Sometimes this folded and twisted crust
presents a striking resemblance to coils of rope. Precisely similar
appearances may be observed on the surface of many artificial slags
when they flow from furnaces, and are seen to be due to the same
cause, namely, the wrinkling up of the chilled surface-crust by the
movement of the liquid mass below. Lavas which present this appearance
are frequently called 'ropy lavas'; an admirable example of them is
afforded in the lava-cascade of the Island of Bourbon represented in
fig. 18 (page 93).
But lavas in which the rate of flow has been very rapid, exhibit
quite a different kind of surface to that of the ropy lavas. The
Vesuvian lava-stream of 1872 was remarkable for the rapidity of its
flow, and its surface presents a remarkable contrast to that of the
slow-moving lava of 1858. The surface of the lava-current of 1872 is
covered with rough cindery masses, often of enormous dimensions, and
it is exceedingly difficult to traverse it, as the ragged projecting
fragments tear the boots and lacerate the skin. The appearance
presented by this lava-stream is illustrated by fig. 23, which is also
taken from a photograph.
[Illustration: Fig. 22.--Vesuvian Lava-stream of 1858, exhibiting the
peculiar 'Ropy' Surfaces of Slowly Moving Currents.
(_From a Photograph._)]
[Illustration: Fig. 23.--Vesuvian Lava-stream of 1872, exhibiting the
Rough Cindery Surfaces characteristic of Rapidly Flowing Currents.
(_From a Photograph._)]
[Sidenote: VESUVIAN LAVA-STREAM OF 1872.]
Now it is found that those lava-streams which move slowly and present
ropy surfaces give off but little steam during their flow, while those
lava-streams which flow more rapidly and present a rough and cindery
appearance give off vast quantities of steam. The extraordinary amount
of vapour given off from the lava-streams which flowed from Vesuvius
in 1872 is illustrated in the photograph copied in fig. 5 (facing page
24), in which the three lava-currents are each seen to be surmounted
by enormous vapour-clouds rising to the height of several thousands
of feet above them, and mingling with the column that issued from
the central vent. By the escape of this enormous quantity of steam
the surface of the lava was thrown into rugged cindery projections,
and in some places little cones were formed upon it, which threw out
small scoriæ and dust. The quantity of vapour was, in fact, so great,
that little parasitical volcanoes were formed on the surface of the
lava-stream. Some of these miniature volcanoes were of such small
dimensions that they were carried away on boards to be employed as
illustrations in the lecture-rooms of the University of Naples.
The arrangement of the materials forced out from fissures on the
surfaces of lava-streams by the disengaged vapours and gases depends
on the degree of fluidity of the lava, and the force of the escaping
steam-jets. In very viscous lavas the materials may issue quietly,
forming great concentric masses like coils of rope; such were described
by Mr. Heaphy as occurring in New Zealand (see fig. 24).
[Illustration: Fig. 24.--Concentric Folds on mass of cooled Lava.]
In other cases the lava, if somewhat more liquid, may in issuing
quietly without great outbursts of steam, accumulate in great
bottle-shaped masses, which have been compared to 'petrified
fountains.' Cases of this kind have been described by Professor Dana as
occurring on the slopes of Hawaii (see fig. 25).
[Illustration: Fig. 25.--Mass of cooled Lava formed over a spiracle on
the slopes of Hawaii.]
[Sidenote: MINIATURE CONES ON LAVA-STREAMS.]
When the steam escapes with explosive violence from a spiracle
('bocca') on the surface of a lava-stream, minute cinder cones,
like those described as being formed in 1872, are the result. Fig.
26 represents a group of miniature cones thrown up on the Vesuvian
lava-stream of 1855: it is taken from a drawing by Schmidt.
[Illustration: Fig. 26.--Group of small Cones thrown up on the Vesuvian
Lava-current of 1856.]
Some of these appear like burst blisters or bubbles, while others
are built up of scoriaceous masses which have been ejected from the
aperture and have become united while in a semi-fluid condition. Other
examples of these spiracles or bocche on the surfaces of lava-currents
may be seen in the figs. 22 and 23, which are copied from photographs.
The facts we have described all point to the conclusion that the
presence of large quantities of water imprisoned in a mass of lava
contributes greatly to its mobility. And this conclusion is supported
by so many other considerations that it is now very generally accepted
by geologists. The condition of this imprisoned water in lavas is one
which demands further investigation at the hands of physicists. It has
been suggested, with some show of reason, that the water may exist
in the midst of the red-hot lava as minute particles in the curious
'spheroidal condition' of Boutigny, and that these flash into steam as
the lava flows along.
Lava, when extruded from a volcanic crater in a more or less completely
fluid state, flows down the side of the cone, and then finds its way
along any channel or valley that may lie in its course, obeying in its
movements all the laws of fluid bodies. The lava-currents thus formed
are sometimes of enormous dimensions, and may flood the whole country
for many miles around the vent.
Lava-streams have been described, which have flowed for a distance of
from fifty to a hundred miles from their source, and which have had
a breadth varying from ten to twenty miles. Some lava-streams have a
thickness of 500 feet, or even more. These measures will give some
idea of the enormous quantities of material brought from the earth's
interior by volcanic action and distributed over its surface. The mass
of lava which flowed out during an eruption off Reykjanes in Iceland,
in the year 1783, has been calculated to be equal in bulk to Mont Blanc.
There are many parts of the earth's surface, such as the Western Isles
of Scotland and the North-east of Ireland, the Deccan of India, and
large tracts in the Rocky Mountains, where successive lava-sheets have
been piled upon one another to the height of several thousands of feet,
and cover areas of many hundreds or even thousands of square miles.
[Sidenote: FEATURES OF LAVA-STREAMS.]
The more fusible basic lavas are as a general rule more liquid in
character than any others, and it is these very liquid lavas that are
usually found forming plateaux built up of successive lava-streams. The
less liquid lavas, like those of Hungary and Bohemia, are not usually
found flowing to such distances from the vent, but form dome-shaped
mountain-masses.
Lava-streams usually exhibit in their upper and under surfaces a
scoriaceous texture due to the escape of steam from the upper surface,
portions of the cindery masses so formed falling off from the end of
the stream, and being rolled over by the stream so as to form its
base. The thickness of this scoriaceous upper and lower part of a
lava-stream varies according to the quantity of steam imprisoned in
it; but all thick lava-streams have a compact central portion which
is composed of hard, solid rock. Very good examples of the internal
structure of lava-streams may sometimes be examined in the sea-cliffs
of volcanic islands. In fig. 27 we have given a copy of a drawing made
while sailing round the shores of Vulcano. The scoriaceous portions of
lava-streams are sometimes employed, as at Volvic in the Auvergne, as
a building material, or as at Neidermendig in the Eifel and in Hungary
for mill-stones; the compact portions are employed for building and
paving, and for road metal. The rock of some of the modern lava-streams
of Vesuvius is largely quarried for paving the streets of Naples.
This solid portion of the lava-streams in slowly cooling down from its
highly-heated condition undergoes contraction, and in consequence is
rent asunder by a number of cracks. Sometimes these cracks assume a
wonderfully regular arrangement, and the rock may be broken up into
very symmetrical masses.
[Illustration: Fig. 27.--Natural section of a Lava-stream in the Island
of Vulcano, showing the compact central portion and the scoriaceous
upper and under surfaces.]
[Sidenote: COLUMNAR STRUCTURE OF LAVAS.]
If we imagine a great sheet of heated material, like a lava-stream,
slowly cooling down, it is evident that the contraction which must
take place in it will tend to produce fissures breaking up the mass
into prisms. A little consideration will convince us what the form of
these prisms must be. There are only three regular figures into which
a surface can be divided, namely, equilateral triangles, squares, and
regular hexagons; the first being produced by the intersection of
sets of six lines radiating at angles of 60° from certain centres;
the second by the intersection of sets of _four_ lines radiating from
centres at angles of 90°; and the third from sets of _three_ lines
radiating from centres at an angle of 120°. It is evident that a less
amount of contractile force will be required to produce the sets of
_three_ cracks rather than those of four or six cracks; or, in other
words, the contractile force in a mass will be competent to produce the
cracks which give rise to hexagons rather than those which form squares
or triangles. This is no doubt the reason why the prisms formed by the
cooling of lava, as well as those produced during the drying of starch
or clay, are hexagonal in form.
The hexagonal prisms or columns formed by contraction during the
consolidation of lavas vary greatly in size, according to the rate of
cooling, the nature of the materials, and the conditions affecting the
mass. Sometimes such columns may be found having a diameter of eight
or ten feet and a length of five hundred feet, as in the Shiant Isles
lying to the north of the Island of Skye; in other cases, as in certain
volcanic glasses, minute columns, an inch or two in length and scarcely
thicker than a needle, are formed; and examples of almost every
intermediate grade between these two extremes may sometimes be found.
The largest columns are those which are formed in very slowly cooling
masses.
The columnar structure is exhibited by all kinds of lava, and indeed
in other rock-masses which have been heated by contact with igneous
masses and gradually cooled. The rocks which display the structure in
greatest perfection, however, are the basalts.
Mr. Scrope first called attention to the fact that the upper and lower
portions of lava-streams sometimes cool in very different ways, and
hence produce columns of dissimilar character. The lower portion of the
mass parts with its heat very slowly, by conduction to the underlying
rocks, while the upper portions radiate heat more irregularly into
the surrounding atmosphere. Hence we often find the lower portions of
thick lava-streams to be formed of stout, vertical columns of great
regularity; while the upper part is made up of smaller and less regular
columns, as shown in fig. 28.
[Illustration: Fig. 28.--Section of a Lava-stream exposed on the side
of the river Ardèche, in the south-west of France.]
The remarkable grotto known as Fingal's Cave in the Island of Staffa
has been formed in the midst of a lava-stream such as we have been
describing; the thick vertical columns, which rise from beneath the
level of the sea, are divided by joints and have been broken away by
the action of the sea; in this way a great cavern has been produced,
the sides of which are formed by vertical columns, while the roof is
made up of smaller and interlacing ones. The whole structure bears some
resemblance to a Gothic cathedral; the sea finding access to its floor
of broken columns, and permitting the entrance of a boat during fine
weather. Similar, though perhaps less striking, structures are found in
many other parts of the globe wherever basaltic and other lava-streams
exhibit the remarkable columnar structure as the result of their slow
cooling. Portions of basaltic columns are often employed for posts
by the road-sides, as in Central Germany and Bohemia, or for paving
stones, as in Pompeii and at the Monte Albano near Rome.
[Illustration: Fig. 29.--Portion of a Basaltic Column from the Giant's
Causeway, exhibiting both the ball-and-socket and the tenon-and-mortise
structures.]
[Sidenote: OTHER JOINT-STRUCTURES IN LAVAS.]
Occasionally basaltic lava-streams exhibit other curious structures in
addition to the columnar. Thus some basaltic columns are found divided
into regular joints by equidistant, curved surfaces, the joints thus
fitting into one another by a kind of ball-and-socket arrangement.
Sometimes we find processes projecting from the angles of the curved
joint-surfaces, which cause the blocks to fit together as with a tenon
and mortise. This kind of structure is admirably displayed at the
Giant's Causeway, Co. Antrim, in the North of Ireland. A portion of a
basaltic column from this locality is represented in fig. 29.
[Illustration: Fig. 30.--Vein of green Pitchstone, at Chiaja di Luna
in the Island of Ponza, breaking up into regular columns, and into
spherical masses with a concentric series of joints.]
While the ordinary columnar structures are very common in basalts,
the ball-and-socket and tenon-and-mortise structures are exceedingly
rare. The question of the mode of origin of these remarkable structures
has given rise to much discussion, and the opinions of geologists and
physicists are by no means unanimous upon the subject.
Sometimes we find masses of lava traversed by curved joints, and
occasionally we find curious combinations of curved and plane joints,
giving rise to appearances scarcely less remarkable than those
presented by the columns of the Giant's Causeway. Some of the more
striking examples of this kind have been described and explained by
Professor Bonney.
[Illustration: Fig. 31.--Illustration of the 'Perlitic structure' in
glassy Rocks.
a. Perlltic structure, as seen in a lava from Hungary.
b. The same structure, artificially produced in Canada Balsam
during cooling.
]
[Sidenote: PERLITIC-STRUCTURE IN LAVAS.]
In the Ponza Islands there occurs a remarkable example of a columnar
pitchstone, which is also traversed by a member of curved concentric
joints, causing the rock to break up into pieces like the coats of an
onion. This remarkable rock-mass is represented in fig. 30.
A very similar structure is often seen in certain glassy lavas, when
they are examined in thin sections under the microscope. Such glassy
lavas exhibit the peculiar lustre of mother-of-pearly doubtless in
consequence of the interference of light along the cracks. Lavas
exhibiting this character are known to geologists as 'perlites.' The
perlitic structure has been produced artificially by Mr. Grenville Cole
in Canada Balsam, and by MM. Fonqué and Michel Lévy, in chemically
deposited silica. See fig. 31.
A thick lava-stream must take an enormous period to cool down--probably
many hundreds or even thousands of years. It is possible to walk over
lava-streams in which at a few inches below the surface the rock is
still red-hot, so that a piece of stick is lighted if thrust into a
crack. Lava is a very bad conductor of heat, and loose scoriæ and
dust are still worse conductors. During the eruption of Vesuvius in
1872, masses of snow which were covered with a thick layer of scoriæ,
and afterwards by a stream of lava, were found three years afterwards
consolidated into ice, but not melted. The city of Catania is
constantly supplied with ice from masses of snow which have been buried
under the ejections of Etna.
During the cooling down of lavas, the escape of steam and various
gases gives rise to the deposition of many beautiful crystalline
substances in the cavities and on the surfaces of the lava. Deposits
of sulphur, specular-iron, tridymite, and many other substances are
often thus produced, and the colour and appearance of the rock-masses
are sometimes completely disguised by these surface incrustations, or
by the decomposition of the materials of the lava by the action of the
add gases, and vapours upon it.
[Sidenote: SINKING OF SURFACES OF LAVA-STREAMS.]
Very frequently the surface of a lava-stream becomes solid, while the
deeper portions retain their fluid condition; under such circumstances
the central portions may flow away, leaving a great hollow chamber or
cavern. In consequence of this action, we not unfrequently find the
upper surface of a lava-current exhibiting a depression, due to the
falling in of the solidified upper portions when the liquid lava has
flowed away and left it unsupported, as in fig. 32.
[Illustration: Fig. 32.--Transverse section of a Lava stream. (The
dotted line indicates the original surface.)]