was not until 1056 that vigorous eruptions again began. From time to
time slight explosions occurred, none of which yielded lava flows; it
was not until the date last mentioned that this accompaniment of the
eruption began to appear. In 1636, after a repose of nearly a century
and a half, there came a very great outbreak, which desolated a wide
extent of country on the northwestern side of the cone. At this stage
in the history of the crater the volcanic flow began to attain the
sea. Washing over the edge of the old original crater of Monte Somma,
and thus lowering its elevation, these streams devastated, during the
eruption just mentioned and in various other outbreaks, a wide field
of cultivated land, overwhelming many villages. The last considerable
eruption which yielded large quantities of lava was that of 1872,
which sent its tide for a distance of about six miles.
Since 1636 the eruptions of Vesuvius have steadily increased in
frequency, and, on the whole, diminished in violence. In the early
years of its history the great outbreaks were usually separated by
intervals of a century or more, and were of such energy that the lava
was mostly blown to dust, forming clouds so vast that on two occasions
at least they caused a midnight darkness at Constantinople, nearly
twelve hundred miles away. This is as if a volcano at Chicago should
completely hide the sun in the city of Boston. In the present state of
Vesuvius, the cone may be said to be in slight, almost continuous
eruption. The old central valley which existed before the eruption of
79, and continued to be distinct for long after that time, has been
filled up by a smaller cone, bearing a relatively tiny crater of vent,
the original wall being visible only on the eastern and northern parts
of its circuit, and here only with much diminished height. On the
western face the slope from the base of the mountain to the summit of
the new cone is almost continuous, though the trained eye can trace
the outline of Monte Somma--its position in a kind of bench, which is
traceable on that side of the long slope leading from the summit of
the new cone to the sea. The fact that the lavas of Vesuvius have
broken out on the southwestern side, while the old wall of the cone
has remained unbroken on the eastern versant, has a curious
explanation. The prevailing wind of Naples is from the southwest,
being the strong counter trades which belong in that latitude. In the
old days when the Monte Somma cone was constructed these winds caused
the larger part of the ashes to fall on the leeward side of the cone,
thus forming a thicker and higher wall around that part of the crater.
From the nature of the recent eruptions of Vesuvius it appears likely
that the mountain is about to enter on a second period of inaction.
The pipes leading through the new cone are small, and the mass of this
elevation constitutes a great plug, closing the old crater mouth. To
give vent to a large discharge of steam, the whole of this great mass,
having a depth of nearly two thousand feet, would have to be blown
away. It seems most likely that when the occasion for such a discharge
comes, the vapours of the eruption will seek a vent through some other
of the many volcanic openings which lie to the westward of this great
cone. The history of these lesser volcanoes points to the conclusion
that when the path by way of Vesuvius is obstructed they may give
relief to the steam which is forcing its course to the surface. Two or
three times since the eruption of Pliny, during periods when Vesuvius
had long been quiet, outbreaks have taken place on Ischia or in the
Phlægræn Fields, a region dotted with small craters which lies to the
west of Naples. The last of these occurred in 1552, and led to the
formation of the beautiful little cone known as Monte Nuovo. This
eruption took place near the town of Puzzuoli, a place which was then
the seat of a university, the people of which have left us records of
the accident.
[Illustration: Fig. 16.--Diagrammatic sections through Mount Vesuvius,
showing changes in the form of the cone. (From Phillips.)]
The outbreak which formed Monte Nuovo was slight but very
characteristic. It occurred in and beside a circular pool known as the
Lucrine Lake, itself an ancient crater. At the beginning of the
disturbance the ground opened in ragged cavities, from which mud and
ashes and great fragments of hard rock were hurled high in the air,
some of the stones ascending to a height of several thousand feet.
With slight intermissions this outbreak continued for some days,
resulting in the formation of a hill about five hundred feet high,
with a crater in its top, the bottom of which lay near the level of
the sea. Although this volcanic elevation, being made altogether of
loose fragments, is rapidly wearing down, while the crater is filling
up, it remains a beautiful object in the landscape, and is also
noteworthy for the fact that it is the only structure of this nature
which we know from its beginning. In the Phlægræn Field there are a
number of other craters of small size, with very low cones about them.
These appear to have been the product of brief, slight eruptions. That
known as the Solfatara, though not in eruption during the historic
period, is interesting for the fact that from the crevices of the
rocks about it there comes forth a continued efflux of carbonic-acid
gas. This substance probably arises from the effect of heat contained
in old lavas which are in contact with limestone in the deep
under-earth. We know such limestones are covered by the lavas of
Vesuvius, for the reason that numerous blocks of the rock are thrown
out during eruptions, and are often found embedded in the lava
streams. It is an interesting fact that these craters of the
Phlægræn Field, lying between the seats of vigorous eruption on
Ischia and at Vesuvius, have never been in vigorous eruption. Their
slight outbreaks seem to indicate that they have no permanent
connection with the sources whence those stronger vents obtain their
supply of heated steam.
The facts disclosed by the study of the Vesuvian system of volcanoes
afford the geologist a basis for many interesting conclusions.
In the first place, he notes that the greater part of the cones, all
those of small size, are made up of finely divided rock, which may
have been more or less cemented by the processes of change which
go on within it. It is thus clear that the lava flows are
unessential--indeed, we may say accidental--contributions to the mass.
In the case of Vesuvius they certainly do not amount to as much as one
tenth of the elevation due to the volcanic action. The share of the
lava in Vesuvius is probably greater than the average, for during the
last six centuries this vent has been remarkably lavigerous.[8]
Observation on the volcanoes of other districts show that the Vesuvian
group is in this regard not peculiar. Of nearly two hundred cones
which the writer has examined, not more than one tenth disclose
distinct lavas.
[Footnote 8: I venture to use this word in place of the phrase
"lava-yielding" for the reason that the term is needed in the
description of volcanoes.]
An inspection of the old inner wall of Monte Somma in that portion
where it is best preserved, on the north side of the Atria del
Cavallo, or Horse Gulch--so called for the reason that those who
ascended Vesuvius were accustomed to leave their saddle animals
there--we perceive that the body of the old cone is to a considerable
extent interlaced with dikes or fissures which have been filled with
molten lava that has cooled in its place. It is evident that during
the throes of an eruption, when the lava stands high in the crater,
these rents are frequently formed, to be filled by the fluid rock. In
fact, lava discharges, though they may afterward course for long
distances in the open air, generally break their way underground
through the cindery cone, and first are disclosed at the distance of a
mile or more from the inner walls of the crater. Their path is
probably formed by riftings in the compacted ashes, such as we trace
on the steep sides of the Atria del Cavallo, as before noted. For the
further history of these fissures, we shall have to refer to facts
which are better exhibited in the cone of Ætna.
The amount of rock matter which has been thrown forth from the
volcanoes about the Bay of Naples is very great. Only a portion of it
remains in the region around these cones; by far the greater part has
been washed or blown away. After each considerable eruption a wide
field is coated with ashes, so that the tilled grounds appear as if
entirely sterilized; but in a short time the matter in good part
disappears, a portion of it decays and is leached away, and the most
of the remainder washes into the sea. Only the showers, which
accumulate a deep layer, are apt to be retained on the surface of the
country. A great deal of this powdered rock drifts away in the wind,
sometimes in great quantities, as in those cases where it darkened the
sky more than a thousand miles from the cone. Moreover, the water of
the steam which brought about the discharges and the other gases which
accompanied the vapour have left no traces of their presence, except
in the deep channels which the rain of the condensing steam have
formed on the hillsides. Nevertheless, after all these subtractions
are made, the quantity of volcanic matter remaining on the surface
about the Bay of Naples would, if evenly distributed, form a layer
several hundred feet in thickness--perhaps, indeed, a thousand feet in
depth--over the territory in which the vents occur. All this matter
has been taken in relatively recent times from the depths of the
earth. The surprising fact is that no considerable and, indeed, no
permanent subsidence of the surface has attended this excavation. We
can not believe that this withdrawal of material from the under-earth
has resulted in the formation of open underground spaces. We know full
well that any such, if it were of considerable size, would quickly be
crushed in by the weight of the overlying rocks. We have, indeed, to
suppose that these steam-impelled lavas, which are driven toward the
vent whence they are to go forth in the state of dust or fluid, come
underground from distances away, probably from beneath the floors of
the sea to the westward.
Although the shores of the Bay of Naples have remained in general with
unchanged elevation for about two thousand years, they have here and
there been subjected to slight oscillations which are most likely
connected with the movement of volcanic matter toward the vents where
it is to find escape. The most interesting evidence of this nature is
afforded by the studies which have been made on the ruins of the
Temple of Serapis at Puzzuoli. This edifice was constructed in
pre-Christian times for the worship of the Egyptian god Serapis, whose
intervention was sought by sick people. The fact that this divinity of
the Nile found a residence in this region shows how intimate was the
relation between Rome and Egypt in this ancient day. The Serapeium was
built on the edge of the sea, just above its level. When in modern
days it began to be studied, its floor was about on its original
level, but the few standing columns of the edifice afford indubitable
evidence that this part of the shore has been lowered to the amount of
twenty feet or more and then re-elevated. The subsidence is proved by
the fact that the upper part of the columns which were not protected
by the _débris_ accumulated about them have been bored by certain
shellfish, known as _Lithodomi_, which have the habit of excavating
shelters in soft stone, such as these marble columns afford. At
present the floor on which the ruin stands appears to be gradually
sinking, though the rate of movement is very slow.
Another evidence that the ejections may travel for a great distance
underground on their way to the vent is afforded by the fact that
Vesuvius and Ætna, though near three hundred miles apart, appear to
exchange activities--that is, their periods of outbreak are not
simultaneous. Although these elements of the chronology of the two
cones may be accidental, taken with similar facts derived from other
fields, they appear to indicate that vents, though far separated from
each other, may, so to speak, be fed from a common subterranean
source. It is a singular fact in this connection that the volcano of
Stromboli, though situated between these two cones, is in a state of
almost incessant activity. This probably indicates that the last-named
vent derives its vapours from another level in the earth than the
greater cones. In this regard volcanoes probably behave like springs,
of which, indeed, they may be regarded as a group. The reader is
doubtless aware that hot and cold springs often escape very near
together, the difference in the temperature being due to the depth
from which their waters come forth.
As the accidents of volcanic explosion are of a nature to be very
damaging to man, as well as to the lower orders of Nature, it is fit
that we should note in general the effect of the Neapolitan eruptions
on the history of civilization in that region. As stated above, the
first Greek settlements in this vicinity--those on the island of
Ischia--were much disturbed by volcanic outbreaks, yet the island
became the seat of a permanent and prosperous colony. The great
eruption of 79 probably cost many hundred lives, and led to the
abandonment of two considerable cities, which, however, could at small
cost have been recovered to use. Since that day various eruptions have
temporarily desolated portions of the territory, but only in very
small fields have the ravages been irremediable. Where the ground was
covered with dust, it has in most places been again tillable, and so
rapid is the decay of the lavas that in a century after their flow has
ceased vines can in most cases be planted on their surfaces. The city
of Naples, which lies amid the vents, though not immediately in
contact with any of them, has steadfastly grown and prospered from the
pre-Christian times. It is doubtful if any lives have ever been lost
in the city in consequence of an eruption, and no great inconvenience
has been experienced from them. Now and then, after a great ash
shower, the volcanic dust has to be removed, but the labour is less
serious than that imposed on many northern cities by a snowstorm.
Through all these convulsions the tillage of the district has been
maintained. It has ever been the seat of as rich and profitable a
husbandry as is afforded by any part of Italy. In fact, the ash
showers, as they import fine divided rock very rich in substances
necessary for the growth of plants, have in a measure served to
maintain the fertility of the soil, and by this action have in some
degree compensated for the injury which they occasionally inflict.
Comparing the ravages of the eruptions with those inflicted by war,
unnecessary disease, or even bad politics, and we see that these
natural accidents have been most merciful to man. Many a tyrant has
caused more suffering and death than has been inflicted by these rude
operations of Nature.
From the point of view of the naturalist, Ætna is vastly more
interesting than Vesuvius. The bulk of the cone is more than twenty
times as great as that of the Neapolitan volcano, and the magnitude of
its explosions, as well as the range of phenomena which they exhibit,
incomparably greater. It happens, however, that while human history of
the recorded kind has been intimately bound up with the tiny Vesuvian
cone, partly because the relatively slight nature of its disturbances
permitted men to dwell beside it, the larger Ætna has expelled culture
from the field near its vent, and has done the greater part of its
work in the vast solitude which it has created.[9]
[Footnote 9: In part the excellent record of Vesuvius is due to the fact
that since the early Christian centuries the priests of St. Januarius,
the patron of Naples, have been accustomed to carry his relics in
procession whenever an eruption began. The cessation of the outbreak has
been written down to the credit of the saint, and thus we are provided
with a long story of the successive outbreaks.]
Ætna has been in frequent eruption for a very much longer time than
Vesuvius. In the odes of Pindar, in the sixth century before Christ,
we find records of eruptions. It is said also that the philosopher
Empedocles sought fame and death by casting himself into the fiery
crater. There has thus in the case of this mountain been no such long
period of repose as occurred in Vesuvius. Though our records of the
outbreaks are exceedingly imperfect, they serve to show that the vent
has maintained its activity much more continuously than is ordinarily
the case with volcanoes. Ætna is characteristically a lava-yielding
cone; though the amount of dust put forth is large, the ratio of the
fluid rock which flows away from the crater is very much greater than
at Vesuvius. Nearly half the cone, indeed, may be composed of this
material. Our space does not permit anything like a consecutive story
of the Ætnean eruptions since the dawn of history, or even a full
account of its majestic cone; we can only note certain features of a
particularly instructive nature which have been remarked by the many
able men who have studied this structure and the effects of its
outbreak.
The most important feature exhibited by Ætna is the vast size of its
cone. At its apex its height, though variable from the frequent
destruction and rebuilding of the crater walls, may be reckoned as
about eleven thousand feet. The base on which the volcanic material
lies is probably less than a thousand feet above the sea, so that the
maximum thickness of the heap of volcanic ejections is probably about
two miles. The average depth of this coating is probably about five
thousand feet, and, as the cone has an average diameter of about
thirty miles, we may conclude that the cone now contains about a
thousand cubic miles of volcanic materials. Great as is this mass,
it is only a small part of the ejected material which has gone forth
from the vent. All the matter which in its vaporous state went forth
with the eruption, the other gases and vapours thus discharged, have
disappeared. So, too, a large part of the ash and much of the lava has
been swept away by the streams which drain the region, and which in
times of eruption are greatly swollen by the accompanying torrential
rains. The writer has estimated that if all the emanations from the
volcano--solid, fluid, and gaseous--could be heaped on the cone, they
would form a mass of between two and three thousand cubic miles in
contents. Yet notwithstanding this enormous outputting of earthy
matter, the earth on which the Ætnean cone has been constructed has
not only failed to sink down, but has been in process of continuous,
slow uprising, which has lifted the surface more than a thousand feet
above the level which it had at the time when volcanic action began in
this field. Here, even more clearly than in the case of Vesuvius, we
see that the materials driven forth from the crater are derived not
from just beneath its foundation, but from a distance, from realms
which in the case of this insular volcano are beneath the sea floors.
It is certain that here the migration of rock matter, impelled by the
expansion of its contained water toward the vent, has so far exceeded
that which has been discharged through the crater that an uprising of
the surface such as we have observed has been brought about.
[Illustration: _Mount Ætna, seen from near Catania. The imperfect
cones on the sky line to the left are those of small secondary
eruptions._]
There are certain peculiarities of Mount Ætna which are due in part to
its great size and in part to the climatal conditions of the region in
which it lies. The upper part of the mountain in winter is deeply
snow-clad; the frozen water often, indeed, forms great drifts in the
gorges near the summit. Here it has occasionally happened that a layer
of ashes has deeply buried the mass, so that it has been preserved for
years, becoming gradually more inclosed by the subsequent eruptions.
At one point where this compact snow--which has, indeed, taken on the
form of ice--has been revealed to view, it has been quarried and
conveyed to the towns upon the seacoast. It is likely that there are
many such masses of ice inclosed between the ash layers in the upper
part of the mountain, where, owing to the height, the climate is very
cold. This curious fact shows how perfect a non-conductor the ash beds
of a volcano are to protect the frozen water from the heat of the
rocks about the crater.
The furious rains which beset the mountain in times of great eruptions
excavate deep channels on its sides. The lava outbreaks which attend
almost every eruption, and which descend from the base of the cinder
cone at the height of from five to eight thousand feet above the sea,
naturally find their way into these channels, where they course in the
manner of rivers until the lower and less valleyed section of the cone
is reached.
Such a lava flow naturally begins to freeze on the surface, the lava
at first becoming viscid, much in the manner of cream on the surface
of milk. Urged along by the more fluid lava underneath, this viscid
coating takes a ropy or corrugated form. As the freezing goes deeper,
a firm stone roof may be formed across the gorge, which, when the
current of lava ceases to flow from the crater, permits the lower part
of the stream to drain away, leaving a long cavern or scries of caves
extending far up the cone. The nature of this action is exactly
comparable to that which we may observe when on a frosty morning after
rain we may find the empty channels which were occupied by rills of
water roofed over with ice; the ice roofs are temporary, while those
of lava may endure for ages. Some of these lava-stream caves have been
disclosed, in the manner of ordinary caverns, by the falling of their
roofs; but the greater part are naturally hidden beneath the
ever-increasing materials of the cone.
The lava-stream caves of Ætna are not only interesting because of
their peculiarities of form, which we shall not undertake to describe,
but also for the reason that they help us to account for a very
peculiar feature in the history of the great cone. On the slopes of
the volcano, below the upper cindery portion, there are several
hundred lesser cones, varying from a few score to seven hundred feet
in height. Each of these has its appropriate crater, and has evidently
been the seat of one or more eruptions. As the greater part of these
cones are ancient, many of them being almost effaced by the rain or
buried beneath the ejections which have surrounded their bases since
the time they were formed, we are led to believe that many thousands
of them have been formed during the history of the volcano. The
history of these subsidiary cones appears to be connected with the
lava caves noted above. These caverns, owing to the irregularities of
their form, contain water. They are, in fact, natural cisterns, where
the abundant rainfall of the mountain finds here and there storage.
When, during the throes of an eruption, dikes such as we know often to
penetrate the mountain, are riven outward from the crater through the
mass of the cone, and filled with lava, the heated rock must often
come in contact with these masses of buried water. The result of this
would inevitably be the local generation of steam at a high
temperature, which would force its way out in a brief but vigorous
eruption, such as has been observed to take place when these
peripheral volcanoes are formed. Sometimes it has happened that after
the explosion the lava has found its way in a stream from the fissure
thus opened. That this explanation is sufficient is in a measure shown
by observations on certain effects of lava flows from Vesuvius. The
writer was informed by a very judicious observer, a resident of
Naples, who had interested himself in the phenomena of that volcano,
that the lava streams when they penetrated a cistern, such as they
often encounter in passing over villages or farmsteads, vaporized the
water, and gave rise, through the action of the steam, to small
temporary cones, which, though generally washed away by the further
flow of the liquid rock, are essentially like those which we find on
Ætna. Such subsidiary, or, as they are sometimes called, parasitic
cones, are known about other volcanoes, but nowhere are they so
characteristic as on the flanks of that wonderful volcano.
A very conspicuous feature in the Ætnean cone consists of a great
valley known as the Val del Bove, or Bull Hollow, which extends from
the base of the modern and ever-changeable cinder cone down the flanks
of the older structure to near its base. This valley has steep sides,
in places a thousand or more feet high, and has evidently been formed
by the down-settling of portions of the cone which were left without
support by the withdrawal from beneath them of materials cast forth in
a time of explosion. In an eruption this remarkable valley was the
seat of a vast water flood, the fluid being cast forth from the crater
at the beginning of the explosion. In the mouths of this and other
volcanoes, after a long period of repose, great quantities of water,
gathering from rains or condensed from the steam which slowly escapes
from these openings, often pours like a flood down the sides of the
mountains. In the great eruption of Galongoon, in Java, such a mass of
water, cast forth by a terrific explosion, mingled with ashes, so that
the mass formed a thick mud, was shot forth with such energy that it
ravaged an area nearly eighty miles in diameter, destroying the
forests and their wild inhabitants, as well as the people who dwelt
within the range of the amazing disaster. So powerfully was this water
driven from the crater that the districts immediately at the base of
the cone were in a manner overshot by the vast stream, and escaped
with relatively little injury.
When it comes forth from the base of the cinder cone, or from one of
the small peripheral craters, the lava stream usually appears to be
white hot, and to flow with almost the ease of water. It does not
really have that measure of fluidity; its condition is rather that of
thin paste; but the great weight of the material--near two and a half
times that of water--causes the movement down the slope to be speedy.
The central portion of the lava stream long retains its high
temperature; but the surface, cooling, is first converted into a tough
sheet, which, though it may bend, can hardly be said to flow. Further
hardening converts these outlying portions of the current into hard,
glassy stone, which is broken into fragments in a way resembling the
ice on the surface of a river. It thus comes about that the advancing
front of the lava stream becomes covered, and its motion hindered by
the frozen rock, until the rate of ongoing may not exceed a few feet
an hour, and the appearance is that of a heap of stone slowly rolling
down a slope. Now and then a crevice is formed, through which a thin
stream of liquid lava pours forth, but the material, having already
parted with much of its heat, rapidly cools, and in turn becomes
covered with the coating of frozen fragments. In this state of the
stream the lava flow stands on all sides high above the slope which it
is traversing; it is, in fact, walled in by its own solidified parts,
though it is urged forward by the contribution which continues to flow
in the under arches. In this state of the movement trifling accidents,
or even human interference, may direct the current this way or that.
Some of the most interesting chapters in the history of Ætna relate to
the efforts of the people to turn these slow-moving streams so that
their torrents might flow into wilderness places rather than over the
fields and towns. In the great flow of 1669, which menaced the city of
Catania, a large place on the seashore to the southeast of the cone, a
public-spirited citizen, Señor Papallardo, protecting himself and his
servants with clothing made of hides, and with large shields, set
forth armed with great hooks with the purpose of diverting the course
of the lava mass. He succeeded in pulling away the stones on the
flank of the stream, so that a flow of the molten rock was turned in
another direction. The expedient would probably have been successful
if he had been allowed to continue his labours; but the inhabitants of
a neighbouring village, which was threatened by the off-shooting
current which Papallardo had created, took up arms and drove him and
his retainers away. The flow continued until it reached Catania. The
people made haste to build the city walls on the side of danger higher
than it was before, but the tide mounted over its summit.
Although the lavas which come forth from the volcano evidently have a
high temperature, their capacity for melting other rocks is relatively
small. They scour these rocks, because of their weight, even more
energetically than do powerful torrents of water, but they are
relatively ineffective in melting stone. On Ætna and elsewhere we may
often observe lavas which have flowed through forests. When the tide
of molten rock has passed by, the trees may be found charred but not
entirely burned away; even stems a few inches in diameter retain
strength enough to uphold considerable fringes and clots of the lava
which has clung to them. These facts bear out the conclusion that the
fluidity of the heated stone depends in considerable measure on the
water which is contained, either in its fluid or vaporous state,
between the particles of the material.
If we consider the Italian volcanoes as a whole, we find that they lie
in a long, discontinuous line extending from the northern part of the
valley of the Po, within sight of the Alps, to Ætna, and in
subterranean cones perhaps to the northern coast of Africa. At the
northern end of the line we have a beautiful group of extinct
volcanoes, known as the Eugean Mountains. Thence southward to southern
Tuscany craters are wanting, but there is evidence of fissures in the
earth which give forth thermal waters. From southern Tuscany southward
through Rome to Naples there are many extinct craters, none of which
have been active in the historic period. From Naples southward the
cones of this system, about a dozen in number, are on islands or close
to the margin of the sea. It is a noteworthy fact that the greater
part of these shore or insular vents have been active since the dawn
of history; several of them frequently and furiously so, while none of
those occupying an inland position have been the seat of explosions.
This is a striking instance going to show the relation of these
processes to conditions which are brought about on the sea bottom.
Ætna is, as we have noticed, a much more powerful volcano than
Vesuvius. Its outbreaks are more vigorous, its emanations vastly
greater in volume, and the mass of its constructions many times as
great as those accumulated in any other European cone. There are,
however, a number of volcanoes in the world which in certain features
surpass Ætna as much as that crater does Vesuvius. Of these we shall
consider but two--Skaptar Jokul, of Iceland, remarkable for the volume
of its lava flow, and Krakatoa, an island volcano between Java and
Sumatra, which was the seat of the greatest explosion of which we have
any record.
The whole of Iceland may be regarded as a volcanic mass composed
mainly of lavas and ashes which have been thrown up by a group of
volcanoes lying near the northern end of the long igneous axis which
extends through the centre of the Atlantic. The island has been the
seat of numerous eruptions; in fact, since its settlement by the
Northmen in 1070 its sturdy inhabitants have been almost as much
distressed by the calamities which have come from the internal heat as
they have been by the enduring external cold. They have, indeed, been
between frost and fire. The greatest recorded eruption of Iceland
occurred in 1783, when the volcano of Skaptar, near the southern
border of the island, poured forth, first, a vast discharge of dust
and ashes, and afterward in the languid state of eruption inundated a
series of valleys with the greatest lava flow of which we have any
written record. The dust poured forth into the upper air, being finely
divided and in enormous quantity, floated in the air for months,
giving a dusky hue to the skies of Europe, which led the common people
and many of the learned to fear that the wrath of God was upon them,
and that the day of judgment was at hand. Even the poet Cowper, a man
of high culture and education, shared in this unreasonable view.
The lava flow in this eruption filled one of the considerable valleys
of the island, drying up the river, and inundating the plains on
either side. Estimates which have been made as to the volume of this
flow appear to indicate that it may have amounted to more than the
bulk of the Mont Blanc.
This great eruption, by the direct effect of the calamity, and by the
famine due to the ravaging of the fields and the frightening of the
fish from the shores which it induced, destroyed nearly one fifth of
the Icelandic people. It is, in fact, to be remembered as one of the
three or four most calamitous eruptions of which we have any account,
and, from the point of view of lava flow, the greatest in history.
Just a hundred years after the great Skaptar eruption, which darkened
the skies of Europe, the island of Krakatoa, an isle formed by a small
volcano in the straits of Java, was the seat of a vapour explosion
which from its intensity is not only unparalleled, but almost
unapproached in all accounts of such disturbances. Krakatoa had long
been recognised as a volcanic isle; it is doubtful, however, if it had
ever been seen in eruption during the three centuries or more since
European ships began to sail by it until the month of May of the year
above mentioned. Then an outbreak of what may be called ordinary
violence took place, which after a few days so far ceased that
observers landed and took account of the changes which the convulsion
had brought about. For about three months there were no further signs
of activity, but on the 29th of August a succession of vast explosions
took place, which blew away a great part of the island, forming in its
place a submarine crater two or three miles in diameter, creating
world-wide disturbances of sea and air. The sounds of the outbreak
were heard at a distance of sixteen hundred miles away. The waves of
the air attendant on the explosion ran round the earth at least once,
as was distinctly indicated by the self-recording barometers; it is
possible, indeed, that, crossing each other in their east and west
courses, these atmospheric tides twice girdled the sphere. In effect,
the air over the crater was heaved up to the height of some tens of
thousands of feet, and thence rolled off in great circular waves, such
as may be observed in a pan of milk when a sharp blow pushes the
bottom upward.
The violent stroke delivered to the waters of the sea created a vast
wave, which in the region where it originated rolled upon the shores
with a surf wall fifty or more feet high. In a few minutes about
thirty thousand people were overwhelmed. The wave rolled on beyond its
destructive limits much in the manner of the tide; its influence was
felt in a sharp rise and fall of the waters as far as the Pacific
coast of North America, and was indicated by the tide gauges in the
Atlantic as far north as the coast of Europe.
Owing to the violence of the eruption, Krakatoa poured forth no lava,
but the dust and ashes which ascended into the air--or, in
other words, the finely divided lava which escaped into the
atmosphere--probably amounted in bulk to more than twenty cubic miles.
The coarser part of this material, including much pumice, fell upon
the seas in the vicinity, where, owing to its lightness, it was free
to drift in the marine currents far and wide throughout the oceanic
realm. The finer particles, thrown high into the air, perhaps to the
height of nearly a hundred thousand feet--certainly to the elevation
of more than half this amount--drifted far and wide in the
atmosphere, so that for years the air of all regions was clouded by
it, the sunrise and sunset having a peculiar red glow, which the dust
particles produce by the light which they reflect. In this period, at
all times when the day was clear, the sun appeared to be surrounded by
a dusky halo. In time the greater part of this dust was drawn down by
gravity, some portion of it probably falling on every square foot of
the earth. Since the disappearance of the characteristic phenomena
which it produced in the atmosphere, European observers have noted the
existence of faint clouds lying in the upper part of the air at the
height of a hundred miles or more above the surface. These clouds,
which were at first distinctly visible in the earliest stage of dawn
and in the latest period of the sunset glow, seemed to be in rapid
motion to the eastward, and to be mounting higher above the earth. It
has been not unreasonably supposed that these shining clouds represent
portions of the finest dust from Krakatoa, which has been thrown so
far above the earth's attraction that it is separating itself from the
sphere. If this view be correct, it seems likely that we may look to
great volcanic explosions as a source whence the dustlike particles
which people the celestial spaces may have come. They may, in a word,
be due to volcanic explosions occurring on this and other celestial
spheres.
The question suggested above as to the possibility of volcanic
ejections throwing matter from the earth beyond the control of its
gravitative energy is one of great scientific interest. Computations
(not altogether trustworthy) show that a body leaving the earth's
surface under the conditions of a cannon ball fired vertically upward
would have to possess a velocity at the start of at least seven miles
a second in order to go free into space. It would at first sight seem
that we should be able to reckon whether volcanoes can propel earth
matter upward with this speed. In fact, however, sufficient data are
not obtainable; we only know in a general way that the column of
vapour rises to the height of thirty or forty thousand feet, and this
in eruptions of no great magnitude. In an accident such as that at
Krakatoa, even if an observer were near enough to see clearly what was
going on, the chance of his surviving the disturbance would be small.
Moreover, the ascending vapours, owing to their expansion of the steam
in the column, begin to fly out sideways on its periphery, so that the
upper part of the central section in the discharge is not visible from
the earth.
It is in the central section of the uprushing mass, if anywhere, that
the dust might attain the height necessary to put it beyond the
earth's attraction, bringing it fairly into the realm of the solar
system, or to the position where its own motion and the attraction of
the other spheres would give it an independent orbital movement about
the sun, or perhaps about the earth. We can only say that observations
on the height of volcanic ejections are extremely desirable; they can
probably only be made from a balloon. An ascension thus made beyond
the cloud disk which the eruption produces might bring the observer
where he could discern enough to determine the matter. Although the
movements of the rocky particles could not be observed, the colour
which they would give to the heavens might tell the story which we
wish to know. There is evidence that large masses of stone hurled up
by volcanic eruption have fallen seven miles from the base of the
cone. Assuming that the masses went straight upward at the beginning
of their ascent, and that they were afterward borne outwardly by the
expansion of the column, computations which have a general but no
absolute value appear to indicate that the masses attained a height of
from thirty to fifty miles, and had an initial velocity which, if
doubled, might have carried them into space.
Last of all, we shall note the conditions which attend the eruptions
of submarine volcanoes. Such explosions have been observed in but a
few instances, and only in those cases where there is reason to
believe that the crater at the time of its explosion had attained to
within a few hundred feet of the sea level. In these cases the
ejections, never as yet observed in the state of lava, but in the
condition of dust and pumice, have occasionally formed a low island,
which has shortly been washed away by the waves. Knowing as we do that
volcanoes abound on the sea floor, the question why we do not oftener
see their explosions disturbing the surface of the waters is very
interesting, but not as yet clearly explicable. It is possible,
however, that a volcanic discharge taking place at the depth of
several thousand feet below the surface of the water would not be able
to blow the fluid aside so as to open a pipe to the surface, but would
expend its energy in a hidden manner near the ocean floor. The vapours
would have to expand gradually, as they do in passing up through the
rock pipe of a volcano, and in their slow upward passage might be
absorbed by the water. The solid materials thrown forth would in this
case necessarily fall close about the vent, and create a very steep
cone, such, indeed, as we find indicated by the soundings off certain
volcanic islands which appear only recently to have overtopped the
level of the waters.
As will be seen, though inadequately from the diagrams of Vesuvius,
volcanic cones have a regularity and symmetry of form far exceeding
that afforded by the outlines of any other of the earth's features.
Where, as is generally the case, the shape of the cone is determined
by the distribution of the falling cinders or divided lava which
constitutes the mass of most cones, the slope is in general that known
as a catenary curve--i.e., the line formed by a chain hanging between
two points at some distance from the vertical. It is interesting to
note that this graceful outline is a reflection or consequence of the
curve described by the uprushing vapour. The expansion in the
ascending column causes it to enlarge at a somewhat steadfast rate,
while the speed of the ascent is ever diminishing. Precisely the same
action can be seen in the like rush of steam and other gases and
vapours from the cannon's mouth; only in the case of the gun, even of
the greatest size, we can not trace the movement for more than a few
hundred feet. In this column of ejection the outward movement from the
centre carries the bits of lava outwardly from the centre of the
shaft, so that when they lose their ascending velocity they are drawn
downward upon the flanks of the cone, the amount falling upon each
part of that surface being in a general way proportional to the
thickness of the vaporous mass from which they descend. The result is,
that the thickest part of the ash heap is formed on the upper part of
the crater, from which point the deposit fades away in depth in every
direction. In a certain measure the concentration toward the centre of
the cone is brought about by the draught of air which moves in toward
the ascending column.
Although, in general, ejections of volcanic matter take place through
cones, that being the inevitable form produced by the escaping steam,
very extensive outpourings of lava, ejections which in mass probably
far exceed those thrown forth through ordinary craters, are
occasionally poured out through fissures in the earth's crust. Thus in
Oregon, Idaho, and Washington, in eastern Europe, in southern India,
and at some other points, vast flows, which apparently took place from
fissures, have inundated great realms with lava ejections. The
conditions which appear to bring about these fissure eruptions of lava
are not yet well understood. A provisional and very probable account
of the action can be had in the hypothesis which will now be set
forth.
Where any region has been for a long time the seat of volcanic action,
it is probable that a large amount of rock in a more or less fluid
condition exists beneath its surface. Although the outrushing steam
ejects much of this molten material, there are reasons to suppose that
a yet greater part lies dormant in the underground spaces. Thus in the
case of Ætna we have seen that, though some thousands of miles of
rock matter have come forth, the base of the cone has been uplifted,
probably by the moving to that region of more or less fluid rock. If
now a region thus underlaid by what we may call incipient lavas is
subjected to the peculiar compressive actions which lead to
mountain-building, we should naturally expect that such soft material
would be poured forth, possibly in vast quantities through fault
fissures, which are so readily formed in all kinds of rock when
subject to irregular and powerful strains, such as are necessarily
brought about when rocks are moved in mountain-making. The great
eruptions which formed the volcanic table-lands on the west coast of
North America appear to have owed the extrusion of their materials to
mountain-building actions. This seems to have been the case also in
some of those smaller areas where fissure flows occur in Europe. It is
likely that this action will explain the greater part of these massive
eruptions.
It need not be supposed that the rock beneath these countries, which
when forced out became lava, was necessarily in the state of perfect
fluidity before it was forced through the fissures. Situated at great
depth in the earth, it was under a pressure so great that its
particles may have been so brought together that the material was
essentially solid, though free to move under the great strains which
affected it, and acquiring temperature along with the fluidity which
heat induces as it was forced along by the mountain-building pressure.
As an illustration of how materials may become highly heated when
forced to move particle on particle, it may be well to cite the case
in which the iron stringpiece on top of a wooden dam near Holyoke,
Mass., was affected when the barrier went away in a flood. The iron
stringer, being very well put together, was, it is said, drawn out by
the strain until it became sensibly reddened by the motion of its
particles, and finally fell hissing into the waters below. A like
heating is observable when metal is drawn out in making wire. Thus a
mass of imperfectly fluid rock might in a forced journey of a few
miles acquire a decided increase of temperature.
Although the most striking volcanic action--all such phenomena,
indeed, as commonly receives the name--is exhibited finally on the
earth's surface, a great deal of work which belongs in the same group
of geological actions is altogether confined to the deep-lying rock,
and leads to the formation of dikes which penetrate the strata, but do
not rise to the open air. We have already noted the fact that dikes
abound in the deeper parts of volcanic cones, though the fissures into
which they find their way are seldom riven up to the surface. In the
same way beneath the ground in non-volcanic countries we may discover
at a great depth in the older, much-changed rock a vast number of
these crevices, varying from a few inches to a hundred feet or more in
width, which have been filled with lavas, the rock once molten having
afterward cooled. In most cases these dikes are disclosed to us
through the down-wearing of the earth that has removed the beds into
which the dikes did not penetrate, thus disclosing the realm in which
the disturbances took place.
Where, as is occasionally the case in deep mines, or on some bare
rocky cliff of great height, we can trace a dike in its upward course
through a long distance, we find that we can never distinctly discover
the lower point of its extension. No one has ever seen in a clear way
the point of origin of such an injection. We can, however, often
follow it upward to the place where there was no longer a rift into
which it could enter. In its upward path the molten matter appears
generally to have followed some previously existing fracture, a joint
plane or a fault, which generally runs through the rocks on those
planes. We can observe evidence that the material was in the state of
igneous fluidity by the fact that it has baked the country rocks on
either side of the fissure, the amount of baking being in proportion
to the width of the dike, and thus to the amount of heat which it
could give forth. A dike six inches in diameter will sometimes barely
sear its walls, while one a hundred feet in width will often alter the
strata for a great distance on either side. In some instances, as in
the coal beds near Richmond, Va., dikes occasionally cut through beds
of bituminous coal. In these cases we find that the coal has been
converted into coke for many feet either side of a considerable
injection. The fact that the dike material was molten is still further
shown by the occurrence in it of fragments which it has taken up from
the walls, and which may have been partly melted, and in most cases
have clearly been much heated.
Where dikes extend up through stratified beds which are separated from
each other by distinct layers, along which the rock is not firmly
bound together, it now and then happens, as noted by Mr. G.K. Gilbert,
of the United States Geological Survey, that the lava has forced its
way horizontally between these layers, gradually uplifting the
overlying mass, which it did not break through, into a dome-shaped
elevation. These side flows from dikes are termed laccolites, a word
which signifies the pool-like nature of the stony mass which they form
between the strata.
In many regions, where the earth has worn down so as to reveal the
zone of dikes which was formed at a great depth, the surface of the
country is fairly laced with these intrusions. Thus on Cape Ann, a
rocky isle on the east coast of Massachusetts, having an area of about
twenty square miles, the writer, with the assistance of his colleague,
Prof. R.S. Tarr, found about four hundred distinct dikes exhibited on
the shore line where the rocks had been swept bare by the waves. If
the census of these intrusions could have been extended over the whole
island, it would probably have appeared that the total number exceeded
five thousand. In other regions square miles can be found where the
dikes intercepted by the surface occupy an aggregate area greater than
that of the rocks into which they have been intruded.
Now and then, but rarely, the student of dikes finds one where the
bordering walls, in place of having the clean-cut appearance which
they usually exhibit, has its sides greatly worn away and much melted,
as if by the long-continued passage of the igneous fluid through the
crevice. Such dikes are usually very wide, and are probably the paths
through which lavas found their way to the surface of the earth,
pouring forth in a volcanic eruption. In some cases we can trace their
relation to ancient volcanic cones which have worn down in all their
part which were made up of incoherent materials, so that there remains
only the central pipe, which has been preserved from decay by the
coherent character of the lava which filled it.
The hypothesis that dikes are driven upward into strata by the
pressure of the beds which overlie materials hot and soft enough to be
put in motion when a fissure enters them, and that their movement
upward through the crevice is accounted for by this pressure, makes
certain features of these intrusions comprehensible. Seeing that very
long, slender dikes are found penetrating the rock, which could not
have had a high temperature, it becomes difficult to understand how
the lava could have maintained its fluidity; but on the supposition
that it was impelled forward by a strong pressure, and that the energy
thus transmitted through it was converted into heat, we discover a
means whereby it could have been retained in the liquid condition,
even when forced for long distances through very narrow channels.
Moreover, this explanation accounts for the fact which has long
remained unexplained that dikes, except those formed about volcanic
craters, rarely, if ever, rise to the surface.
The materials contained in dikes differ exceedingly in their chemical
and mineral character. These variations are due to the differences in
Nature of the deposits whence they come, and also in a measure to
exchanges which take place between their own substance and that of the
rocks between which they are deposited. This process often has
importance of an economic kind, for it not infrequently leads to the
formation of metalliferous veins or other aggregations of ores, either
in the dike itself or in the country rock. The way in which this is
brought about may be easily understood by a familiar example. If flesh
be placed in water which has the same temperature, no exchange of
materials will take place; but if the water be heated, a circulation
will be set up, which in time will bring a large part of the soluble
matter into the surrounding water. This movement is primarily
dependent on differences of temperature, and consequently differences
in the quantity of soluble substances which the water seeks to take
up. When a dike is injected into cooler rocks, such a slow circulation
is induced. The water contained in the interstices of the stone
becomes charged with mineral materials, if such exist in positions
where it can obtain possession of them, and as cooling goes on, these
dissolved materials are deposited in the manner of veins. These veins
are generally laid down on the planes of contact between the two kinds
of stone, but they may be formed in any other cavities which exist in
the neighbourhood. The formation of such veins is often aided by the
considerable shrinkage of the lava in the dike, which, when it cools,
tends to lose about fifteen per cent of its volume, and is thus likely
to leave a crevice next the boundary walls. Ores thus formed afford
some of the commonest and often the richest mineral deposits. At
Leadville, in Colorado, the great silver-bearing lodes probably were
produced in this manner, wherein lavas, either those of dikes or those
which flowed in the open air, have come in contact with limestones.
The mineral materials originally in the once molten rock or in the
limy beds was, we believe, laid down on ancient sea floors in the
remains of organic forms, which for their particular uses took the
materials from the old sea water. The vein-making action has served to
assemble these scattered bits of metal into the aggregation which
constitutes a workable deposit. In time, as the rocks wear down, the
materials of the veins are again taken into solution and returned to
the sea, thence perhaps to tread again the cycle of change.
In certain dikes, and sometimes also, perhaps, in lavas known as
basalts, which have flowed on the surface, the rock when cooling, from
the shrinkage which then occurs, has broken in a very regular way,
forming hexagonal columns which are more or less divided on their
length by joints. When worn away by the agencies of decay, especially
where the material forms steep cliffs, a highly artificial effect is
produced, which is often compared, where cut at right angles to the
columns, to pavements, or, where the division is parallel to the
columns, to the pipes of an organ.
What we know of dikes inclines us to the opinion that as a whole they
represent movements of softened rock where the motion-compelling agent
is not mainly the expansion of the contained water which gives rise to
volcanic ejection, but rather in large part due to the weight of
superincumbent strata setting in motion materials which were somewhat
softened, and which tended to creep, as do the clays in deep coal
mines. It is evident, however; it is, moreover, quite natural, that
dike work is somewhat mingled with that produced by the volcanic
forces; but while the line between the two actions is not sharp, the
discrimination is important, and occurs with a distinctness rather
unusual on the boundary line between two adjacent fields of phenomena.
* * * * *
We have now to consider the general effects of the earth's interior
heat so far as that body of temperature tends to drive materials from
the depths of the earth to the surface. This group of influences is
one of the most important which operates on our sphere; as we shall
shortly see, without such action the earth would in time become an
unfit theatre for the development of organic life. To perceive the
effect of these movements, we must first note that in the great
rock-constructing realm of the seas organic life is constantly
extracting from the water substances, such as lime, potash, soda, and
a host of other substances necessary for the maintenance of
high-grade organisms, depositing these materials in the growing
strata. Into these beds, which are buried as fast as they form, goes
not only these earthy materials, but a great store of the sea water as
well. The result would be in course of time a complete withdrawal into
the depths of the earth of those substances which play a necessary
part in organic development. The earth would become more or less
completely waterless on its surface, and the rocks exposed to view
would be composed mainly of silica, the material which to a great
extent resists solution, and therefore avoids the dissolving which
overtakes most other kinds of rocks. Here comes in the machinery of
the hot springs, the dikes, and the volcanoes. These agents, operating
under the influence of the internal heat of the earth, are constantly
engaged in bearing the earthy matter, particularly its precious more
solvent parts, back to the surface. The hot springs and volcanoes work
swiftly and directly, and return the water, the carbon dioxide, and a
host of other vaporizable and soluble and fusible substances to the
realm of solar activity, to the living surface zone of the earth. The
dikes operate less immediately, but in the end to the same effect.
They lift their materials miles above the level where they were
originally laid, probably from a zone which is rarely if ever exposed
to view, placing them near the surface, where the erosive agents can
readily find access to them.
Of the three agents which serve to export earth materials from its
depths, volcanoes are doubtless the most important. They send forth
the greater part of the water which is expelled from the rocks.
Various computations which the writer has made indicate that an
ordinary volcano, such as Ætna, in times of most intense explosion,
may send forth in the form of steam one fourth of a cubic mile or
more of water during each day of its discharge, and in a single great
eruption may pour forth several times this quantity. In its history
Ætna has probably returned to the atmosphere some hundred cubic miles
of water which but for the process would have remained permanently
locked up in its rock prison.
The ejection of rock material, though probably on the average less in
quantity than the water which escapes, is also of noteworthy
importance. The volcanoes of Java and the adjacent isles have, during
the last hundred and twenty years, delivered to the seas more earth
material than has been carried into those basins by the great rivers.
If we could take account of all the volcanic ejections which have
occurred in this time, we should doubtless find that the sum of the
materials thus cast forth into the oceans was several times as great
as that which was delivered from the lands by all the superficial
agents which wear them away. Moreover, while the material from the
land, except the small part which is in a state of complete solution,
all falls close to the shore, the volcanic waste, because of its fine
division or because of the blebs of air which its masses contain, may
float for many years before it finds its way to the bottom, it may be
at the antipodes of the point at which it came from the earth. While
thus journeying through the sea the rock matter from the volcanoes is
apt to become dissolved in water; it is, indeed, doubtful if any
considerable part of that which enters the ocean goes by gravitation
to its floor. The greater portion probably enters the state of
solution and makes its way thence through the bodies of plants and
animals again into the ponderable state.
If an observer could view the earth from the surface of the moon, he
would probably each day behold one of these storms which the volcanoes
send forth. In the fortnight of darkness, even with the naked eye, it
would probably be possible to discern at any time several eruptions,
some of which would indicate that the earth's surface was ravaged by
great catastrophes. The nearer view of these actions shows us that
although locally and in small measure they are harmful to the life of
the earth, they are in a large way beneficent.