Volcanic eruption in Iceland in 1783--New island thrown up--Lava
currents of Skaptar Jokul, in same year--their immense
volume--Eruption of Jorullo in Mexico--Humboldt's theory of the
convexity of the plain of Malpais--Eruption of Galongoon in
Java--Submarine volcanoes--Graham island, formed in 1831--Volcanic
archipelagoes--Submarine eruptions in mid-Atlantic--The
Canaries--Teneriffe--Cones thrown up in Lancerote, 1730-36--Santorin
and its contiguous isles--Barren island in the Bay of Bengal--Mud
volcanoes--Mineral composition of volcanic products.
_Volcanic eruptions in Iceland._--With the exception of Etna and
Vesuvius, the most complete chronological records of a series of
eruptions are those of Iceland, for their history reaches as far back as
the ninth century of our era; and, from the beginning of the twelfth
century, there is clear evidence that, during the whole period, there
has never been an interval of more than forty, and very rarely one of
twenty years, without either an eruption or a great earthquake. So
intense is the energy of the volcanic action in this region, that some
eruptions of Hecla have lasted six years without ceasing. Earthquakes
have often shaken the whole island at once, causing great changes in the
interior, such as the sinking down of hills, the rending of mountains,
the desertion by rivers of their channels, and the appearance of new
lakes.[583] New islands have often been thrown up near the coast, some
of which still exist; while others have disappeared, either by
subsidence or the action of the waves.
In the interval between eruptions, innumerable hot springs afford vent
to subterranean heat, and solfataras discharge copious streams of
inflammable matter. The volcanoes in different parts of this island are
observed, like those of the Phlegraean Fields, to be in activity by
turns, one vent often serving for a time as a safety-valve to the rest.
Many cones are often thrown up in one eruption, and in this case they
take a linear direction, running generally from northeast to southwest,
from the northeastern part of the island, where the volcano Krabla lies,
to the promontory Reykianas.
_New island thrown up in 1783._--The convulsions of the year 1783 appear
to have been more tremendous than any recorded in the modern annals of
Iceland; and the original Danish narrative of the catastrophe, drawn up
in great detail, has since been substantiated by several English
travellers, particularly in regard to the prodigious extent of country
laid waste, and the volume of lava produced.[584] About a month previous
to the eruption on the mainland, a submarine volcano burst forth in the
sea in lat. 63 degrees 25 minutes N., long. 23 degrees 44 minutes W., at
a distance of thirty miles in a southwest direction from Cape Reykianas,
and ejected so much pumice, that the ocean was covered with that
substance to the distance of 150 miles, and ships were considerably
impeded in their course. A new island was thrown up, consisting of high
cliffs, within which fire, smoke, and pumice were emitted from two or
three different points. This island was claimed by his Danish Majesty,
who denominated it Nyoe, or the New Island; but before a year had
elapsed, the sea resumed its ancient domain, and nothing was left but a
reef of rocks from five to thirty fathoms under water.
_Great eruption of Skaptar Jokul._--Earthquakes which had long been felt
in Iceland, became violent on the 11th of June, 1783, when Skaptar
Jokul, distant nearly 200 miles from Nyoe, threw out a torrent of lava
which flowed down into the river Skapta, and completely dried it up. The
channel of the river was between high rocks, in many places from four
hundred to six hundred feet in depth, and near two hundred in breadth.
Not only did the lava fill up this great defile to the brink, but it
overflowed the adjacent fields to a considerable extent. The burning
flood, on issuing from the confined rocky gorge, was then arrested for
some time by a deep lake, which formerly existed in the course of the
river, between Skaptardal and Aa, which it entirely filled. The current
then advanced again, and reaching some ancient lava full of
subterraneous caverns, penetrated and melted down part of it; and in
some places, where the steam could not gain vent, it blew up the rock,
throwing fragments to the height of more than 150 feet. On the 18th of
June another ejection of liquid lava rushed from the volcano, which
flowed down with amazing velocity over the surface of the first stream.
By the damming up of the mouths of some of the tributaries of the
Skapta, many villages were completely overflowed with water, and thus
great destruction of property was caused. The lava, after flowing for
several days, was precipitated down a tremendous cataract called
Stapafoss, where it filled a profound abyss, which that great waterfall
had been hollowing out for ages, and after this, the fiery current
again continued its course.
On the third of August, fresh floods of lava still pouring from the
volcano, a new branch was sent off in a different direction; for the
channel of the Skapta was now so entirely choked up, and every opening
to the west and north so obstructed, that the melted matter was forced
to take a new course, so that it ran in a southeast direction, and
discharged itself into the bed of the river Hverfisfliot, where a scene
of destruction scarcely inferior to the former was occasioned. These
Icelandic lavas (like the ancient streams which are met with in
Auvergne, and other provinces of Central France), are stated by
Stephenson to have accumulated to a prodigious depth in narrow rocky
gorges; but when they came to wide alluvial plains, they spread
themselves out into broad burning lakes, sometimes from twelve to
fifteen miles wide, and one hundred feet deep. When the "fiery lake"
which filled up the lower portion of the valley of the Skapta, had been
augmented by new supplies, the lava flowed up the course of the river to
the foot of the hills from whence the Skapta takes its rise. This
affords a parallel case to one which can be shown to have happened at a
remote era in the volcanic region of the Vivarais in France, where lava
issued from the cone of Thueyts, and while one branch ran down, another
more powerful stream flowed up the channel of the river Ardeche.
The sides of the valley of the Skapta present superb ranges of basaltic
columns of older lava, resembling those which are laid open in the
valleys descending from Mont Dor, in Auvergne, where more modern
lava-currents, on a scale very inferior in magnitude to those of
Iceland, have also usurped the beds of the existing rivers. The eruption
of Skaptar Jokul did not entirely cease till the end of two years; and
when Mr. Paulson visited the tract eleven years afterwards, in 1794, he
found columns of smoke still rising from parts of the lava, and several
rents filled with hot water.[585]
Although the population of Iceland was very much scattered, and did not
exceed fifty thousand, no less than twenty villages were destroyed,
besides those inundated by water; and more than nine thousand human
beings perished, together with an immense number of cattle, partly by
the depredations of the lava, partly by the noxious vapors which
impregnated the air, and, in part, by the famine caused by showers of
ashes throughout the island, and the desertion of the coasts by the
fish.
_Immense volume of the lava._--But the extraordinary volume of melted
matter produced in this eruption deserves the particular attention of
the geologist. Of the two branches, which flowed in nearly opposite
directions, the greatest was fifty, and the lesser forty miles in
length. The extreme breadth which the Skapta branch attained in the low
countries was from twelve to fifteen miles, that of the other about
seven. The ordinary height of both currents was one hundred feet, but in
narrow defiles it sometimes amounted to six hundred. Professor Bischoff
has calculated that the mass of lava brought up from the subterranean
regions by this single eruption "surpassed in magnitude the bulk of Mont
Blanc."[586] But a more distinct idea will be formed of the dimensions
of the two streams, if we consider how striking a feature they would now
form in the geology of England, had they been poured out on the bottom
of the sea after the deposition and before the elevation of our
secondary and tertiary rocks. The same causes which have excavated
valleys through parts of our marine strata, once continuous, might have
acted with equal force on the igneous rocks, leaving, at the same time,
a sufficient portion undestroyed to enable us to discover their former
extent. Let us, then, imagine the termination of the Skapta branch of
lava to rest on the escarpment of the inferior and middle oolite, where
it commands the vale of Gloucester. The great platform might be one
hundred feet thick, and from ten to fifteen miles broad, exceeding any
which can be found in Central France. We may also suppose great tabular
masses to occur at intervals, capping the summit of the Cotswold Hills
between Gloucester and Oxford, by Northleach, Burford, and other towns.
The wide valley of the Oxford clay would then occasion an interruption
for many miles; but the same rocks might recur on the summit of Cumnor
and Shotover Hills, and all the other oolitic eminences of that
district. On the chalk of Berkshire, extensive plateaus, six or seven
miles wide, would again be formed; and lastly, crowning the highest
sands of Highgate and Hampstead, we might behold some remnants of the
current five or six hundred feet in thickness, causing those hills to
rival, or even to surpass, in height, Salisbury Craigs and Arthur's
Seat.
The distance between the extreme points here indicated would not exceed
ninety miles in a direct line; and we might then add, at the distance of
nearly two hundred miles from London, along the coast of Dorsetshire and
Devonshire, for example, a great mass of igneous rocks, to represent
those of contemporary origin, which were produced beneath the level of
the sea, where the island of Nyoe rose up.
_Volume of ancient and modern flows of lava compared._--Yet, gigantic as
must appear the scale of these modern volcanic operations, we must be
content to regard them as perfectly insignificant in comparison to
currents of the primeval ages, if we embrace the theoretical views of
many geologists, which were not inaccurately expressed by the late
Professor Alexander Brongniart, when he declared that "aux apoques
gaognostiques anciennes, tous les phanomenes gaologiques se passoient
dans des dimensions _centuples_ de celles qu'ils prasentent
aujourd'hui."[587] Had Skaptar Jokul, therefore, been a volcano of the
olden time, it would have poured forth lavas at a single eruption a
hundred times more voluminous than those which were witnessed by the
present generation in 1783. But it may, on the contrary, be affirmed
that, among the older formations, no igneous rock of such colossal
magnitude has yet been met with; nay, it would be most difficult to
point out a mass of ancient date (distinctly referable to a single
eruption) which would even rival in volume the matter poured out from
Skaptar Jokul in 1783.
_Eruption of Jorullo in 1759._--As another example of the stupendous
scale of modern volcanic eruptions, I may mention that of Jorullo in
Mexico, in 1759. The great region to which this mountain belongs has
already been described. The plain of Malpais forms part of an elevated
platform, between two and three thousand feet above the level of the
sea, and is bounded by hills composed of basalt, trachyte, and volcanic
tuff, clearly indicating that the country had previously, though
probably at a remote period, been the theatre of igneous action. From
the era of the discovery of the New World to the middle of the last
century, the district had remained undisturbed, and the space, now the
site of the volcano, which is thirty-six leagues distant from the
nearest sea, was occupied by fertile fields of sugar-cane and indigo,
and watered by the two brooks Cuitimba and San Pedro. In the month of
June, 1759, hollow sounds of an alarming nature were heard, and
earthquakes succeeded each other for two months, until, at the end of
September, flames issued from the ground, and fragments of burning rocks
were thrown to prodigious heights. Six volcanic cones, composed of
scoriae and fragmentary lava, were formed on the line of a chasm which
ran in the direction from N. N. E. to S. S. W. The least of these cones
was 300 feet in height; and Jorullo, the central volcano, was elevated
1600 feet above the level of the plain. It sent forth great streams of
basaltic lava, containing included fragments of granitic rocks, and its
ejections did not cease till the month of February, 1760.[588]
[Illustration: Fig. 57.
_a_, Summit of Jorullo. _b_, _c_, Inclined plane sloping at an angle of
6 degrees from the base of the cones.]
Humboldt visited the country more than forty years after this
occurrence, and was informed by the Indians, that when they returned,
long after the catastrophe, to the plain, they found the ground
uninhabitable from the excessive heat. When he himself visited the
place, there appeared, around the base of the cones, and spreading from
them, as from a centre, over an extent of four square miles, a mass of
matter of a convex form, about 550 feet high at its junction with the
cones, and gradually sloping from them in all directions towards the
plain. This mass was still in a heated state, the temperature in the
fissures being on the decrease from year to year, but in 1780 it was
still sufficient to light a cigar at the depth of a few inches. On this
slightly convex protuberance, the slope of which must form an angle of
about 6 degrees with the horizon, were thousands of flattish conical
mounds, from six to nine feet high, which, as well as large fissures
traversing the plain, acted as fumeroles, giving out clouds of
sulphurous acid and hot aqueous vapor. The two small rivers before
mentioned disappeared during the eruption, losing themselves below the
eastern extremity of the plain, and reappearing as hot springs at its
western limit.
_Cause of the convexity of the plain of Malpais._--Humboldt attributed
the convexity of the plain to inflation from below; supposing the
ground, for four square miles in extent, to have risen up in the shape
of a bladder to the elevation of 550 feet above the plain in the highest
part. But Mr. Scrope has suggested that the phenomena may be accounted
for far more naturally, by supposing that lava flowing simultaneously
from the different orifices, and principally from Jorullo, united into a
sort of pool or lake. As they were poured forth on a surface previously
flat, they would, if their liquidity was not very great, remain thickest
and deepest near their source, and diminish in bulk from thence towards
the limits of the space which they covered. Fresh supplies were probably
emitted successively during the course of an eruption which lasted more
than half a year; and some of these, resting on those first emitted,
might only spread to a small distance from the foot of the cone, where
they would necessarily accumulate to a great height. The average slope
of the great dome-shaped volcanoes of the Sandwich Islands, formed
almost exclusively of lava, with scarce any scoriae, is between 6
degrees 30 minutes and 7 degrees 46 minutes, so that the inclination of
the convex mass around Jorullo, if we adopt Mr. Scrope's explanation
(see fig. 57), is quite in accordance with the known laws which govern
the flow of lava.
The showers, also, of loose and pulverulent matter from the six craters,
and principally from Jorullo, would be composed of heavier and more
bulky particles near the cones, and would raise the ground at their
base, where, mixing with rain, they might have given rise to the stratum
of black clay, which is described as covering the lava. The small
conical mounds (called "hornitos," or little ovens) may resemble those
five or six small hillocks which existed in 1823 on the Vesuvian lava,
and sent forth columns of vapor, having been produced by the
disengagement of elastic fluids heaping up small dome-shaped masses of
lava. The fissures mentioned by Humboldt as of frequent occurrence, are
such as might naturally accompany the consolidation of a thick bed of
lava, contracting as it congeals; and the disappearance of rivers is the
usual result of the occupation of the lower part of a valley or plain by
lava, of which there are many beautiful examples in the old
lava-currents of Auvergne. The heat of the "hornitos" is stated to have
diminished from the first; and Mr. Bullock, who visited the spot many
years after Humboldt, found the temperature of the hot spring very
low,--a fact which seems clearly to indicate the gradual congelation of
a subjacent bed of lava, which from its immense thickness may have been
enabled to retain its heat for half a century. The reader may be
reminded, that when we thus suppose the lava near the volcano to have
been, together with the ejected ashes, more than five hundred feet in
depth, we merely assign a thickness which the current of Skaptar Jokul
attained in some places in 1783.
_Hollow sound of the plain when struck._--Another argument adduced in
support of the theory of inflation from below, was, the hollow sound
made by the steps of a horse upon the plain; which, however, proves
nothing more than that the materials of which the convex mass is
composed are light and porous. The sound called "rimbombo" by the
Italians is very commonly returned by _made ground_ when struck sharply;
and has been observed not only on the sides of Vesuvius and other
volcanic cones where there is a cavity below, but in such regions as the
Campagna di Roma, composed in a great measure of tuff and porous
volcanic rocks. The reverberation, however, may perhaps be assisted by
grottoes and caverns, for these may be as numerous in the lavas of
Jorullo as in many of those of Etna; but their existence would lend no
countenance to the hypothesis of a great arched cavity, four square
miles in extent, and in the centre 550 feet high.[589]
_No recent eruptions of Jorullo._--In a former edition I stated that I
had been informed by Captain Vetch, that in 1819 a tower at Guadalaxara
was thrown down by an earthquake, and that ashes, supposed to have come
from Jorullo, fell at the same time at Guanaxuato, a town situated 140
English miles from the volcano. But Mr. Burkhardt, a German director of
mines, who examined Jorullo in 1827, ascertained that there had been no
eruption there since Humboldt's visit in 1803. He went to the bottom of
the crater, and observed a slight evolution of sulphurous acid vapors,
but the "hornitos" had entirely ceased to send forth steam. During the
twenty-four years intervening between his visit and that of Humboldt,
vegetation had made great progress on the flanks of the new hills; the
rich soil of the surrounding country was once more covered with
luxuriant crops of sugar-cane and indigo, and there was an abundant
growth of natural underwood on all the uncultivated tracts.[590]
_Galongoon, Java_, 1822.--The mountain of Galongoon (or Galung Gung) was
in 1822 covered by a dense forest, and situated in a fruitful and
thickly-peopled part of Java. There was a circular hollow at its summit,
but no tradition existed of any former eruption. In July, 1822, the
waters of the river Kunir, one of those which flowed from its flanks,
became for a time hot and turbid. On the 8th of October following a loud
explosion was heard, the earth shook, and immense columns of hot water
and boiling mud, mixed with burning brimstone, ashes, and lapilli, of
the size of nuts, were projected from the mountain like a waterspout,
with such prodigious violence that large quantities fell beyond the
river Tandoi, which is forty miles distant. Every valley within the
range of this eruption became filled with a burning torrent, and the
rivers, swollen with hot water and mud, overflowed their banks, and
carried away great numbers of the people, who were endeavoring to
escape, and the bodies of cattle, wild beasts, and birds. A space of
twenty-four miles between the mountain and the river Tandoi was covered
to such a depth with bluish mud that people were buried in their houses,
and not a trace of the numerous villages and plantations throughout that
extent was visible. Within this space the bodies of those who perished
were buried in mud and concealed, but near the limits of the volcanic
action they were exposed, and strewed over the ground in great numbers,
partly boiled and partly burnt.
It was remarked, that the boiling mud and cinders were projected with
such violence from the mountain, that while many remote villages were
utterly destroyed and buried, others much nearer the volcano were
scarcely injured.
The first eruption lasted nearly five hours, and on the following days
the rain fell in torrents, and the rivers, densely charged with mud,
deluged the country far and wide. At the end of four days (October 12th)
a second eruption occurred more violent than the first, in which hot
water and mud were again vomited, and great blocks of basalt were thrown
to the distance of seven miles from the volcano. There was at the same
time a violent earthquake, and in one account it is stated that the face
of the mountain was utterly changed, its summits broken down, and one
side, which had been covered with trees, became an enormous gulf in the
form of a semicircle. This cavity was about midway between the summit
and the plain, and surrounded by steep rocks, said to be newly heaped up
during the eruption. New hills and valleys are said to have been formed,
and the rivers Banjarang and Wulan changed their course, and in one
night (October 12th) 2000 persons were killed.
The first intimation which the inhabitants of Bandong received of this
calamity on the 8th of October, was the news that the river Wulna was
bearing down into the sea the dead bodies of men, and the carcasses of
stags, rhinoceroses, tigers, and other animals. The Dutch painter Payen
determined to travel from thence to the volcano, and he found that the
quantity of the ashes diminished as he approached the base of the
mountain. He alludes to the altered form of the mountain after the 12th,
but does not describe the new semicircular gulf on its side.
The official accounts state that 114 villages were destroyed, and above
4000 persons killed.[591]
_Submarine volcanoes._--Although we have every reason to believe that
volcanic eruptions as well as earthquakes are common in the bed of the
sea, it was not to be expected that many opportunities would occur to
scientific observers of witnessing the phenomena. The crews of vessels
have sometimes reported that they have seen in different places
sulphurous smoke, flame, jets of water, and steam, rising up from the
sea, or they have observed the waters greatly discolored, and in a state
of violent agitation as if boiling. New shoals have also been
encountered, or a reef of rocks just emerging above the surface, where
previously there was always supposed to have been deep water. On some
few occasions the gradual formation of an island by a submarine eruption
has been observed, as that of Sabrina, in the year 1811, off St.
Michael's in the Azores. The throwing up of ashes in that case, and the
formation of a cone about three hundred feet in height, with a crater in
the centre, closely resembled the phenomena usually accompanying a
volcanic eruption on land. Sabrina was soon washed away by the waves.
Previous eruptions in the same part of the sea were recorded to have
happened in 1691 and 1720. The rise of Nyoe, also, a small island off
the coast of Iceland, in 1783, has already been alluded to; and another
volcanic isle was produced by an eruption near Reikiavig, on the same
coast, in June, 1830.[592]
_Graham Island_[593], 1831.--We have still more recent and minute
information respecting the appearance, in 1831, of a new volcanic island
in the Mediterranean, between the S. W. coast of Sicily and that
projecting part of the African coast where ancient Carthage stood. The
site of the island was not any part of the great shoal, or bank, called
"Nerita," as was first asserted, but a spot where Captain W. H. Smyth
had found, in his survey a few years before, a depth of more than one
hundred fathoms water.[594]
[Illustration: Fig. 58. Form of the cliffs of Graham Island, as seen
from S. S. E., distant one mile, 7th August, 1831.[596]]
[Illustration: Fig. 59. View of the interior of Graham Island, 29th
Sept., 1831.]
[Illustration: Fig. 60. Graham Island, 29th Sept., 1831.[597]]
The position of the island (lat. 37 degrees 8 minutes 30" N., long. 12
degrees 42 minutes 15" E.) was about thirty miles S. W. of Sciacca, in
Sicily, and thirty-three miles N. E. of Pantellaria.[595] On the 28th of
June, about a fortnight before the eruption was visible, Sir Pulteney
Malcolm, in passing over the spot in his ship, felt the shocks of an
earthquake, as if he had struck on a sand-bank; and the same shocks were
felt on the west coast of Sicily, in a direction from S. W. to N. E.
About the 10th of July, John Corrao, the captain of a Sicilian vessel,
reported that, as he passed near the place, he saw a column of water
like a water-spout, sixty feet high, and 800 yards in circumference,
rising from the sea, and soon afterwards a dense steam in its place,
which ascended to the height of 1800 feet. The same Corrao, on his
return from Girgenti, on the 18th of July, found a small island, twelve
feet high with a crater in its centre, ejecting volcanic matter, and
immense columns of vapor; the sea around being covered with floating
cinders and dead fish. The scoriae were of a chocolate color, and the
water which boiled in the circular basin was of a dingy red. The
eruption continued with great violence to the end of the same month; at
which time the island was visited by several persons, and among others
by Capt. Swinburne, R. N., and M. Hoffmann, the Prussian geologist. It
was then from fifty to ninety feet in height, and three-quarters of a
mile in circumference. By the 4th of August it became, according to some
accounts, above 200 feet high, and three miles in circumference; after
which it began to diminish in size by the action of the waves, and it
was only two miles round on the 25th of August; and on the 3d of
September, when it was carefully examined by Captain Wodehouse, only
three-fifths of a mile in circumference; its greatest height being then
107 feet. At this time the crater was about 780 feet in circumference.
On the 29th of September, when it was visited by Mons. C. Prevost, its
circumference was reduced to about 700 yards. It was composed entirely
of incoherent ejected matter, scoriae, pumice, and lapilli, forming
regular strata, some of which are described as having been parallel to
the steep inward slope of the crater, while the rest were inclined
outwards, like those of Vesuvius.[598] When the arrangement of the
ejected materials has been determined by their falling continually on
two steep slopes, that of the external cone and that of the crater,
which is always a hollow inverted cone, a transverse section would
probably resemble that given in the annexed figure (61). But when I
visited Vesuvius, in 1828, I saw no beds of scoriae inclined towards the
axis of the cone. (See fig. 45, p. 381.) Such may have once existed; but
the explosions or subsidences, or whatever causes produced the great
crater of 1822, had possibly destroyed them.
[Illustration: Fig. 61.]
Few of the pieces of stone thrown out from Graham Island exceeded a foot
in diameter. Some fragments of dolomitic limestone were intermixed; but
these were the only non-volcanic substances. During the month of August,
there occurred on the S. W. side of the new island a violent ebullition
and agitation of the sea, accompanied by the constant ascension of a
column of dense white steam, indicating the existence of a second vent
at no great depth from the surface. Towards the close of October, no
vestige of the crater remained, and the island was nearly levelled with
the surface of the ocean, with the exception, at one point, of a small
monticule of sand and scoriae. It was reported that, at the commencement
of the year following (1832), there was a depth of 150 feet where the
island had been: but this account was quite erroneous; for in the early
part of that year Captain Swinburne found a shoal and discolored water
there, and towards the end of 1833 a dangerous reef existed of an oval
figure, about three-fifths of a mile in extent. In the centre was a
black rock, of the diameter of about twenty-six fathoms, from nine to
eleven feet under water; and round this rock are banks of black volcanic
stones and loose sand. At the distance of sixty fathoms from this
central mass, the depth increased rapidly. There was also a second shoal
at the distance of 450 feet S. W. of the great reef, with fifteen feet
water over it, also composed of rock, surrounded by deep sea. We can
scarcely doubt that the rock in the middle of the larger reef is solid
lava, which rose up in the principal crater, and that the second shoal
marks the site of the submarine eruption observed in August, 1831, to
the S. W. of the island.
From the whole of the facts above detailed, it appears that a hill eight
hundred feet or more in height was formed by a submarine volcanic vent,
of which the upper part (only about two hundred feet high) emerged above
the waters, so as to form an island. This cone must have been equal in
size to one of the largest of the lateral volcanoes on the flanks of
Etna, and about half the height of the mountain Jorullo in Mexico, which
was formed in the course of nine months, in 1759. In the centre of the
new volcano a large cavity was kept open by gaseous discharges, which
threw out scoriae; and fluid lava probably rose up in this cavity. It is
not uncommon for small subsidiary craters to open near the summit of a
cone, and one of these may have been formed in the case of Graham
Island; a vent, perhaps, connected with the main channel of discharge
which gave passage in that direction to elastic fluids, scoriae, and
melted lava. It does not appear that, either from this duct, or from the
principal vent, there was any overflowing of lava; but melted rock may
have flowed from the flanks or base of the cone (a common occurrence on
land), and may have spread in a broad sheet over the bottom of the sea.
[Illustration: Fig. 62.
Supposed section of Graham Island. (C. Maclaren.[599])]
The dotted lines in the annexed figure are an imaginary restoration of
the upper part of the cone, now removed by the waves: the strong lines
represent the part of the volcano which is still under water: in the
centre is a great column, or dike, of solid lava, two hundred feet in
diameter, supposed to fill the space by which the gaseous fluids rose;
and on each side of the dike is a stratified mass of scoriae and
fragmentary lava. The solid nucleus of the reef, where the black rock is
now found, withstands the movements of the sea; while the surrounding
loose tuffs are cut away to a somewhat lower level. In this manner the
lava, which was the lowest part of the island, or, to speak more
correctly, which scarcely ever rose above the level of the sea when the
island existed, has now become the highest point in the reef.
No appearances observed, either during the eruption or since the island
disappeared, gave the least support to the opinion promulgated by some
writers, that part of the ancient bed of the sea had been lifted up
bodily.
The solid products, says Dr. John Davy, whether they consisted of sand,
light cinders, or vesicular lava, differed more in form than in
composition. The lava contained augite; and the specific gravity was
2.07 and 2.70. When the light spongy cinder, which floated on the sea,
was reduced to fine powder by trituration, and the greater part of the
entangled air got rid of, it was found to be of the specific gravity
2.64; and that of some of the sand which fell in the eruption was
2.75;[600] so that the materials equalled ordinary granites in weight
and solidity. The only gas evolved in any considerable quantity was
carbonic acid.[601]
_Submarine eruptions in mid-Atlantic._--In the Nautical Magazine for
1835, p. 642, and for 1838, p. 361, and in the Comptes Rendus, April,
1838, accounts are given of a series of volcanic phenomena, earthquakes,
troubled water, floating scoriae and columns of smoke, which have been
observed at intervals since the middle of the last century, in a space
of open sea between longitudes 20 degrees and 22 degrees west, about
half a degree south of the equator. These facts, says Mr. Darwin, seem
to show, that an island or an archipelago is in process of formation in
the middle of the Atlantic; a line joining St. Helena and Ascension
would, if prolonged, intersect this slowly nascent focus of volcanic
action.[602] Should land be eventually formed here, it will not be the
first that has been produced by igneous action in this ocean since it
was inhabited by the existing species of testacea. At Porto Praya in St.
Jago, one of the Azores, a horizontal, calcareous stratum occurs,
containing shells of recent marine species, covered by a great sheet of
basalt eighty feet thick.[603] It would be difficult to estimate too
highly the commercial and political importance which a group of islands
might acquire, if in the next two or three thousand years they should
rise in mid-ocean between St. Helena and Ascension.
CANARY ISLANDS.
_Eruption in Lancerote, 1730 to 1736._--The effects of an eruption which
happened in Lancerote, one of the Canary Islands, between the years 1730
and 1736, were very remarkable; and a detailed description has been
published by Von Buch, who had an opportunity, when he visited that
island in 1815, of comparing the accounts transmitted to us of the
event, with the present state and geological appearances of the
country.[604] On the 1st of September, 1730, the earth split open on a
sudden two leagues from Yaira. In one night a considerable hill of
ejected matter was thrown up; and, a few days later, another vent
opened, and gave out a lava-stream, which overran Chinanfaya and other
villages. It flowed first rapidly, like water, but became afterwards
heavy and slow, like honey. On the 7th of September an immense rock was
protruded from the bottom of the lava with a noise like thunder, and the
stream was forced to change its course from N. to N. W., so that St.
Catalina and other villages were overflowed.
Whether this mass was protruded by an earthquake, or was a mass of
ancient lava, blown up like that before mentioned in 1783 in Iceland, is
not explained.
On the 11th of September more lava flowed out, and covered the village
of Maso entirely, and for the space of eight days precipitated itself
with a horrible roar into the sea. Dead fish floated on the waters in
indescribable multitudes, or were thrown dying on the shore. After a
brief interval of repose, three new openings broke forth immediately
from the site of the consumed St. Catalina, and sent out an enormous
quantity of lapilli, sand, and ashes. On the 28th of October the cattle
throughout the whole country dropped lifeless to the ground, suffocated
by putrid vapors, which condensed and fell down in drops. On the 1st of
December a lava-stream reached the sea, and formed an island, round
which dead fish were strewed.
_Number of cones thrown up._--It is unnecessary here to give the details
of the overwhelming of other places by fiery torrents, or of a storm
which was equally new and terrifying to the inhabitants, as they had
never known one in their country before. On the 10th of January, 1731, a
high hill was thrown up, which, on the same day, precipitated itself
back again into its own crater; fiery brooks of lava flowed from it to
the sea. On the 3d of February a new cone arose. Others were thrown up
in March, and poured forth lava-streams. Numerous other volcanic cones
were subsequently formed in succession, till at last their number
amounted to about thirty. In June, 1731, during a renewal of the
eruptions, all the banks and shores in the western part of the island
were covered with dying fish, of different species, some of which had
never before been seen. Smoke and flame arose from the sea, with loud
detonations. These dreadful commotions lasted without interruption for
_five successive years_, so that a great emigration of the inhabitants
became necessary.
_Their linear direction._--As to the height of the new cones, Von Buch
was assured that the formerly great and flourishing St. Catalina lay
buried under hills 400 feet in height; and he observes that the most
elevated cone of the series rose 600 feet above its base, and 1378 feet
above the sea, and that several others were nearly as high. The new
vents were all arranged _in one line_, about two geographical miles
long, and in a direction nearly east and west. If we admit the
probability of Von Buch's conjecture, that these vents opened along the
line of a cleft, it seems necessary to suppose that this subterranean
fissure was only prolonged upwards to the surface by degrees, and that
the rent was narrow at first, as is usually the case with fissures
caused by earthquakes. Lava and elastic fluids might escape from some
point on the rent where there was least resistance, till, the first
aperture becoming obstructed by ejections and the consolidation of lava,
other orifices burst open in succession along the line of the original
fissure. Von Buch found that each crater was lowest on that side on
which lava had issued; but some craters were not breached, and were
without any lava streams. In one of these were open fissures, out of
which hot vapors rose, which in 1815 raised the thermometer to 145 degrees
Fahrenheit, and was probably at the boiling point lower down. The
exhalations seemed to consist of aqueous vapor; yet they could not be
pure steam, for the crevices were incrusted on either side by siliceous
sinter (an opal-like hydrate of silica of a white color), which extended
almost to the middle. This important fact attests the length of time
during which chemical processes continue after eruptions, and how open
fissures may be filled up laterally by mineral matter, sublimed from
volcanic exhalations. The lavas of this eruption covered nearly a third
of the whole island, often forming on slightly inclined planes great
horizontal sheets several square leagues in area, resembling very much
the basaltic platforms of Auvergne.
_Pretended distinction between ancient and modern lavas._--One of the
new lavas was observed to contain masses of olivine of an olive-green
color, resembling those which occur in one of the lavas of the Vivarais.
Von Buch supposes the great crystals of olivine to have been derived
from a previously existing basalt melted up by the new volcanoes; but we
have scarcely sufficient data to bear out such a conjecture. The older
rocks of the island consist, in a great measure, of that kind of
basaltic lava called dolerite, sometimes columnar, and partly of common
basalt and amygdaloid. Some recent lavas assumed, on entering the sea, a
prismatic form, and so much resembled the older lavas of the Canaries,
that the only geological distinction which Von Buch appears to have been
able to draw between them was, that they did not alternate with
conglomerates, like the ancient basalts. Some modern writers have
endeavored to discover, in the abundance of these conglomerates, a proof
of the dissimilarity of the volcanic action in ancient and modern times;
but this character is more probably attributable to the difference
between submarine operations and those on the land. All the blocks and
imperfectly rounded fragments of lava, transported during the intervals
of eruption, by rivers and torrents, into the adjoining sea, or torn by
the continued action of the waves from cliffs which are undermined, must
accumulate in stratified breccias and conglomerates, and be covered
again and again by other lavas. This is now taking place on the shores
of Sicily, between Catania and Trezza, where the sea breaks down and
covers the shore with blocks and pebbles of the modern lavas of Etna;
and on parts of the coast of Ischia, where numerous currents of trachyte
are in like manner undermined in lofty precipices. So often, then, as an
island is raised in a volcanic archipelago by earthquakes from the deep,
the fundamental and (relatively to all above) the oldest lava will often
be distinguishable from those formed by subsequent eruptions on dry
land, by their alternation with beds of sandstone and fragmentary rocks.
The supposed want of identity, then, between the volcanic phenomena of
different epochs resolves itself partly at least into the marked
difference between the operations simultaneously in progress, above and
below the waters. Such, indeed, is the source, as was before stated in
the First Book (Chap. V.), of many of our strongest theoretical
prejudices in geology. No sooner do we study and endeavor to explain
submarine appearances, than we feel, to use a common expression, out of
our element; and unwilling to concede that our extreme ignorance of
processes now continually going on can be the cause of our perplexity,
we take refuge in a "pre-existent order of nature."
_Recent formation of oolitic travertin in Lancerote._--Throughout a
considerable part of Lancerote, the old lavas are covered by a thin
stratum of limestone, from an inch to two feet in thickness. It is of a
hard stalactitic nature, sometimes oolitic, like the Jura limestone, and
contains fragments of lava and terrestrial shells, chiefly helices and
spiral bulimi. It sometimes rises to the height of 800 feet above the
level of the sea. Von Buch imagines that this remarkable superstratum
has been produced by the furious northwest storms, which in winter drive
the spray of the sea in clouds over the whole island; from whence
calcareous particles may be deposited stalactitically. Mr. Darwin
informs me that he found a limestone in St. Helena, the harder parts of
which correspond precisely to the stone of Lancerote. He attributes the
origin of this rock in St. Helena not to the spray of the sea, but to
drifting by violent winds of the finer particles of shells from the
sea-beach. Some parts of this drift are subsequently dissolved by
atmospheric moisture, and redeposited, so as to convert calcareous sand
into oolite.
_Recent eruption in Lancerote._--From the year 1736 to 1815, when Von
Buch visited Lancerote, there had been no eruption; but, in August,
1824, a crater opened near the port of Rescif, and formed by its
ejections, in the space of twenty-four hours, a considerable hill.
Violent earthquakes preceded and accompanied this eruption.[605]
_Teneriffe._--The Peak of Teneriffe is about 12,000 feet high, and
stands, says Von Buch, like a tower encircled by its fosse and bastion.
The bastion consists, like the semicircular escarpment of Somma turned
towards Vesuvius, of precipitous cliffs, composed of trachyte, basalt,
coarse conglomerates, and tuffs, traversed by volcanic dikes, mostly
vertical, and of basalt. These cliffs vary in height from 1000 to 1800
feet, and are supposed by Von Buch to have been heaved up into their
present position by a force exerted from below, in accordance with the
theory proposed by the same author for the origin of the cones of
Vesuvius and Etna. According to the observations of M. Deville in
1839[606], the trachytes are often granitoid in their aspect, and
contain instead of glassy felspar the allied mineral called oligoclase,
which had been previously considered as characteristic of more ancient
igneous rocks. The same traveller supposes, although he found no
limestone or trace of fossils in any of the rocks of Teneriffe, that the
alternating trachytes and trachytic conglomerates originated beneath the
sea. If this opinion be correct, and it is at least very probable,
geologists may still speculate on two modes in which the mass of the
island acquired its present form and elevation above the sea. 1st, The
advocates of Von Buch's crater-of-elevation hypothesis may imagine that
a succession of horizontally superimposed beds were upheaved by a sudden
movement, and tilted so as to dip in all directions outwards from the
centre of a new dome-shaped eminence, in the middle of which a large
opening or bowl-shaped cavity was produced. 2dly, Or according to the
theory which to me appears preferable, a submarine hill in the form of a
flattened dome may have gradually accumulated, partly below the waters
and partly above by the continued outpourings of sheets of lava and the
ejection of ashes from a central orifice. In this case the dikes would
represent the fissures, which were filled during successive eruptions,
and the original inclination of the beds may have been increased by the
distension and upheaval of the mass during reiterated convulsions,
acting most forcibly at or near the channel of discharge, which would
become partially sealed up with lava from time to time, and then be
burst open again during eruptions. At length the whole island may have
been raised bodily out of the sea by a gradual upward movement.
Whatever theory we adopt, we must always explain the abrupt termination
of the dikes and layers of trachyte and basalt in the steep walls of the
escarpments surrounding the great crater by supposing the removal of
part of the materials once prolonged farther inward towards the centre.
If, according to the elevation-crater hypothesis, a series of sheets of
lava and ashes originally spread over a level and even surface have been
violently broken and uplifted, why do not the opposite walls of the
chasm correspond in such a manner as to imply by their present outline
that they were formerly united? It is evident that the precipices on
opposite sides of the crateriform hollow would not fit if brought
together, there being no projecting masses in one wall to enter into
indentations in the other, as would happen with the sides of many
mineral veins, trap-dikes, and faults, could we extract the intrusive
matter now separating them, and reunite the rocks which have been
fractured and disjoined.
The highest crater of the peak has merely disengaged sulphureous vapors
ever since it has been known to Europeans; but an eruption happened in
June, 1798, not far from the summit, and others are recorded, which
poured out streams of lava from great heights, besides many which have
broken out nearer the level of the sea. All these, however, seem to be
dependent on one great centre of eruption, or on that open channel
communicating between the interior of the earth and the atmosphere,
which terminates in the highest crater of the peak.
We may consider Teneriffe, then, as having been from a remote period the
principal and habitual vent of the volcanic archipelago of the Canaries.
The discharges which have taken place in the contiguous isles of Palma,
Lancerote, and the rest, may be of a subsidiary kind, and have probably
been most frequent and violent when the greater crater has been
partially sealed up, just as the violent eruptions of Ischia or that of
Monte Nuovo coincided with the dormant state of Vesuvius.
SANTORIN.
The Gulf of Santorin, in the Grecian Archipelago, has been for two
thousand years a scene of active volcanic operations. The largest of the
three outer islands of the group (to which the general name of Santorin
is given) is called Thera (or sometimes Santorin), and forms more than
two-thirds of the circuit of the gulf (see Map, fig. 63, p. 442). The
length of the exterior coast-line of this and the other two islands
named Therasia and Aspronisi, taken together, amounts to about thirty
miles, and that of the inner coast-line of the same islands to about
eighteen miles. In the middle of the gulf are three other islands,
called the Little, the New, and the Old "Kaimenis," or "Burnt Islands."
The accompanying map has been reduced from a recent survey executed in
1848 by Captain Graves, R. N., and shortly to be published by the
Admiralty.
[Illustration: Fig. 63.
Map of Santorin in the Grecian Archipelago, from a Survey in 1848, by
Captain Graves, R. N.
_The soundings are given in fathoms._
A, Shoal formed by submarine volcanic eruption in 1650.
B, Northern entrance.
C, Mansell's Rock.
D, Mount St. Elias, 1887 feet high.
]
[Illustration: Fig. 64.
Section of Santorin, in a N. E. and S. W. direction, from Thera through
the Kaimenia to Aspronisi.]
[Illustration: Fig. 65.
Part of the section, fig. 64, enlarged.]
Pliny informs us that the year 186, B. C., gave birth to the Old
Kaimeni, also called Hiera, or the "Sacred Isle," and in the year 19 of
our era "Thia" (the Divine) made its appearance above water, and was
soon joined by subsequent eruptions to the older island, from which it
was only 250 paces distant. The Old Kaimeni also increased successively
in size in 726 and in 1427. A century and a half later, in 1573, another
eruption produced the cone and crater called Micra-Kaimeni, or "the
Small Burnt Island." The next great event which we find recorded
occurred in 1650, when a submarine outbreak violently agitated the sea,
at a point three and a half miles to the N. E. of Thera, and which gave
rise to a shoal (see A in the map) carefully examined during the late
survey in 1848 by Captain Graves, and found to have ten fathoms water
over it, the sea deepening around it in all directions. This eruption
lasted three months, covering the sea with floating pumice. At the same
time an earthquake destroyed many houses in Thera, while the sea broke
upon the coast and overthrew two churches, exposing to view two
villages, one on each side of the mountain of St. Stephen, both of
which must have been overwhelmed by showers of volcanic matter during
some previous eruptions of unknown date.[607] The accompanying evolution
of sulphur and hydrogen issuing from the sea killed more than fifty
persons, and above 1000 domestic animals. A wave, also, 50 feet high,
broke upon the rocks of the Isle of Nia, about four leagues distant, and
advanced 450 yards into the interior of the Island of Sikino. Lastly, in
1707 and 1709, Nea-Kaimeni, or the New Burnt Island, was formed between
the two others, Palaia and Micra, the Old and Little isles. This isle
was composed originally of two distinct parts; the first which rose was
called the White Island, composed of a mass of pumice, extremely porous.
Gorce, the Jesuit, who was then in Santorin, says that the rock "cut
like bread," and that, when the inhabitants landed on it, they found a
multitude of full-grown fresh oysters adhering to it, which they
ate.[608] This mass was afterwards covered, in great part, by the matter
ejected from the crater of a twin-island formed simultaneously, and
called Black Island, consisting of brown trachyte. The trachytic lava
which rose on this spot appears to have been a long time in an
intumescent state, for the New Kaimeni was sometimes lowered on one side
while it gained height on the other, and rocks rose up in the sea at
different distances from the shore and then disappeared again. The
eruption was renewed at intervals during the years 1711 and 1712, and at
length a cone was piled up to the height of 330 feet above the level of
the sea, its exterior slope forming an angle of 33 degrees with the
horizon, and the crater on its summit being 80 yards in diameter. In
addition to the two points of subaerial eruption on the New and Little
Kaimenis, two other cones, indicating the sites of submarine outbursts
of unknown date, were discovered under water near the Kaimenis during
the late survey.
In regard to the "White Island," which was described and visited by
Gorce in 1707, we are indebted to Mr. Edward Forbes for having, in 1842,
carefully investigated the layer of pumiceous ash of which it is
constituted. He obtained from it many shells of marine genera,
Pectunculus, Arca, Cardita, Trochus, and others, both univalve and
bivalve, all of recent Mediterranean species. They were in a fine state
of preservation, the bivalves with the epidermis remaining, and valves
closed, showing that they had been suddenly destroyed. Mr. Forbes, from
his study of the habits of the mollusca living at different depths in
the Mediterranean, was able to decide that such an assemblage of species
could not have lived at a less depth than 220 feet, so that a bodily
upheaval of the mass to that amount must have taken place in order to
bring up this bed of ashes and shells to the level of the sea, and they
now rise five or six feet above that level.[609]
We may compare this partial elevation of solid matter to the rise of a
hardened crust of scoriae, such as is usually formed on the surface of
lava-currents, even while they are in motion, and which, although stony
and capable of supporting heavy weights, may be upraised without
bursting by the intumescence of the melted matter below. That the
upheaval was merely local is proved by the fact that the neighboring
Kaimenis did not participate in the movement, still less the three more
distant or outer islands before mentioned. The history, therefore, of
the Kaimenis shows that they have been the result of intermittent
action, and it lends no support to the hypothesis of the sudden
distension of horizontal beds blown up like a bladder by a single
paroxysmal effort of expansive gases.
It will be seen by the accompanying map and sections, that the Kaimenis
are arranged in a linear direction, running N. E. and S. W., in a manner
different from that represented in the older charts. In their longest
diameter they form at their base a ridge nearly bisecting the gulf or
crater (see sections, figs. 64, 65).
On considering these facts we are naturally led to compare the smaller
and newer islands in the centre of the gulf to the modern cone of
Vesuvius, surrounded by the older semicircular escarpment of Somma, or
to liken them to the Peak of Teneriffe before described, as surrounded
by its "fosse and bastion." This idea will appear to be still more fully
confirmed when we study the soundings taken during the late
hydrographical survey. Thera, which constitutes alone more than
two-thirds of the outer circuit, presents everywhere towards the gulf,
high and steep precipices composed of rocks of volcanic origin. In all
places near the base of its cliffs, a depth of from 800 to 1000 feet of
water was found, and Lieut. Leycester informs us[610] that if the gulf,
which is six miles in diameter, could be drained, a bowl-shaped cavity
would appear with walls 2449 feet high in some places, and even on the
southwest side, where it is lowest, nowhere less than 1200 feet high;
while the Kaimenis would be seen to form in the centre a huge mountain
five and a half miles in circumference at its base, with three principal
summits (the Old, the New, and the Little Burnt Islands) rising
severally to the heights of 1251, 1629, and 1158 feet above the bottom
of the abyss. The rim of the great caldron thus exposed would be
observed to be in all parts perfect and unbroken, except at one point
where there is a deep and long chasm or channel, known by mariners as
"the northern entrance" (B, fig. 63) between Thera and Therasia, and
called by Lieut. Leycester "the door into the crater." It is no less
than 1170 feet deep, and constitutes, as will appear by the soundings
(see map), a remarkable feature in the bed of the sea. There is no
corresponding channel passing out from the gulf into the Mediterranean
at any other point in the circuit between the outer islands, the
greatest depth there ranging from 7 to 66 feet.
We may conceive, therefore, if at some former time the whole mass of
Santorin stood at a higher level by 1200 feet, that this single ravine
or narrow valley now forming "the northern entrance," was the passage by
which the sea entered a circular bay and swept out in the form of mud
and pebbles, the materials derived by denudation from wasting cliffs. In
this manner the original crater may have been slowly widened and
deepened, after which the whole archipelago may have been partially
submerged to its present depth.
That such oscillations of level may in the course of ages have taken
place, will be the more readily admitted when we state that part of
Thera has actually sunk down in modern times, as, for example, during
the great earthquake before alluded to, which happened in 1650. The
subsidence alluded to is proved not only by tradition, but by the fact
that a road which formerly led between two places on the east coast of
Thera is now twelve fathoms under water.
MM. Boblaye and Virlet mention,[611] that the waves are constantly
undermining and encroaching on the cliffs of Therasia and Aspronisi, and
shoals or submarine ledges were found, during the late survey, to occur
round a great part of these islands, attesting the recent progress of
denudation. M. Virlet also remarks, in regard to the separation of the
three islands forming the walls of the crater, that the channels between
them are all to the W. and N. W., the quarter most exposed to the waves
and currents.
Mr. Darwin, in his work on volcanic islands, has shown that in the
Mauritius and in Santiago, there is an external circle of basaltic rocks
of vast diameter, in the interior of which more modern eruptions have
taken place, the older rocks dipping away from the central space in
every direction, as in the outer islands of Santorin. He refers the
numerous breaches, some of them very wide in the external ramparts of
those islands, to the denuding action of the sea. Every geologist,
therefore, will be prepared to call in the aid of the same powerful
cause, to account for the removal of a large part of the rocks which
must once have occupied the interior space, in the same manner as they
attribute the abstraction of matter from elliptical "valleys of
elevation," such as those of Woolhope and the Wealden in England, to the
waves and currents of the sea.
Thera, Therasia, and Aspronisi are all composed of volcanic matter,
except the southern part of Thera, where Mount St. Elias rises to three
times the height of the loftiest of the igneous rocks, reaching an
elevation of 1887 feet above the sea.[612] This mountain is formed of
granular limestone and argillaceous schist, and must have been
originally a submarine eminence in the bed of the Mediterranean, before
the volcanic cone, one side of the base of which now abuts against it,
was formed. The inclination, strike, and fractures of the calcareous and
argillaceous strata of St. Elias have no relation to the great cone,
but, according to M. Bory St. Vincent, have the same direction as those
of the other isles of the Grecian Archipelago, namely, from N. N. W. to
S. S. E. Each of the three islands, Thera, Therasia, and Aspronisi, is
capped by an enormous mass of white tufaceous conglomerate, from forty
to fifty feet thick, beneath which are beds of trachytic lava and tuff,
having a gentle inclination of only 3 degrees or 4 degrees. Each bed is
usually very narrow and discontinuous, the successive layers being
moulded or dove-tailed, as M. Virlet expresses it, into the inequalities
of the previously existing surface, on which showers of cinders or
streams of melted matter have been poured. Nothing, therefore, seems
more evident than that we have in Santorin the basal remains of a great
ruined cone, or flattened dome; and the absence of dikes in the cliffs
surrounding the gulf would indicate that the eruptions took place
originally, as they have done in the last two thousand years, not near
the margin but in the centre of the space now occupied by the gulf. The
central portions of the dome have since been removed by engulfment, or
denudation, or by both these causes.
An important fact is adduced by M. Virlet, to show that the gentle dip
of the lava-streams in the three outer islands towards all points of the
compass, away from the centre of the gulf, has not been due to the
upheaval of horizontal beds, as conjectured by Von Buch, who had not
visited Santorin.[613] The French geologist found that the vesicles or
pores of the trachytic masses were lengthened out in the several
directions in which they would have flowed if they had descended from
the axis of a cone once occupying the centre of the crater. For it is
well known that the bubbles of confined gas in a fluid in motion assume
an oval form, and the direction of their longer axis coincides always
with that of the stream.
On a review, therefore, of all the facts now brought to light respecting
Santorin, I attribute the moderate slope of the beds in Thera and the
other external islands to their having originally descended the inclined
flanks of a large volcanic cone, the principal orifice or vents of
eruption having been always situated where they are now, in or near the
centre of the space occupied by the gulf or crater--in other words,
where the outburst of the Kaimenis has been witnessed in historical
times. The single long and deep opening into the crater is a feature
common to all those remnants of ancient volcanoes, the central portions
of which have been removed, and is probably connected with aqueous
denudation. This denuding process has been the work of ages when the sea
was admitted into an original crater, and has taken place during the
gradual emergence of the island from the sea, or during various
oscillations in its level.
The volcanic island of St. Paul in the midst of the Indian Ocean, lat.
38 degrees 44 minutes S., long. 77 degrees 37 minutes E., surveyed by
Capt. Blackwood in 1842, seems to exemplify the first stage in the
formation of such an archipelago as that of Santorin. We have there a
crater one mile in diameter, surrounded by steep and lofty cliffs on
every side save one, where the sea enters by a single passage nearly dry
at low water. In the interior of the small circular bay or crater there
is a depth of 30 fathoms, or 180 feet. The surface of the island slopes
away on all sides from the crest of the rocks encircling the
crater.[614]
[Illustration: Fig. 66.
Cone and crater of Barren Island, in the Bay of Bengal. Height of the
central cone (according to Capt Miller, in 1834), 500 feet.]
_Barren Island._--There is great analogy between the structure of Barren
Island in the Bay of Bengal, lat. 12 degrees 15 minutes, and that of
Santorin last described. When seen from the ocean, this island presents,
on almost all sides, a surface of bare rocks, rising, with a moderate
acclivity, towards the interior; but at one point there is a cleft by
which we can penetrate into the centre, and there discover that it is
occupied by a great circular basin, filled by the waters of the sea, and
bordered all around by steep rocks, in the midst of which rises a
volcanic cone, very frequently in eruption. The summit of this cone is
about 500 feet in height, corresponding to that of the circular border
which incloses the basin; so that it can be seen from the sea only
through the ravine. It is most probable that the exterior inclosure of
Barren Island (_c_, _d_, fig. 67) is nothing more than the remains of a
truncated cone _c_, _a_, _b_, _d_, a great portion of which has been
removed by engulfment, explosion, or denudation, which may have preceded
the formation of the new interior cone, _f_, _e_, _g_.[615]
[Illustration: Fig. 67.
Supposed section of Barren Island, in the Bay of Bengal.]
MUD VOLCANOES.
_Iceland._--Mr. R. Bunsen, in his account of the pseudo-volcanic
phenomena of Iceland, describes many valleys where sulphurous and
aqueous vapors burst forth with a hissing sound, from the hot soil
formed of volcanic tuff. In such spots a pool of boiling water is seen,
in which a bluish-black argillaceous paste rises in huge bubbles. These
bubbles on bursting throw the boiling mud to a height of fifteen feet
and upwards, accumulating it in ledges round the crater or basin of the
spring.
_Baku on the Caspian._--The formation of a new mud volcano was witnessed
on the 27th of November, 1827, at Tokmali, on the peninsula of
Abscheron, east of Baku. Flames blazed up to an extraordinary height for
a space of three hours, and continued for twenty hours to rise about
three feet above a crater, from which mud was ejected. At another point
in the same district where flames issued, fragments of rock of large
size were hurled up into the air, and scattered around.[616]
[Illustration: Fig. 68.
Mud cones and craters of Hinglaj near Beila, district of Lus, 120 miles
northwest of mouth of Indus. From original drawing by Capt. Robertson.
(See Map, p. 460.)]
_Sicily._--At a place called Macaluba, near Girgenti in Sicily, are
several conical mounds from ten to thirty feet in height, with small
craters at their summits, from which cold water, mixed with mud and
bitumen, is cast out. Bubbles of carbonic acid and carburetted hydrogen
gas are also disengaged from these springs, and at certain periods with
such violence, as to throw the mud to the height of 200 feet. These "air
volcanoes," as they are sometimes termed, are known to have been in the
same state of activity for the last fifteen centuries; and Dr. Daubeny
imagines that the gases which escape may be generated by the slow
combustion of beds of sulphur, which is actually in progress in the blue
clay, out of which the springs rise.[617] But as the gases are similar
to those disengaged in volcanic eruptions, and as they have continued to
stream out for so long a period, they may perhaps be derived from a more
deep-seated source.
_Beila in India._--In the district of Luss or Lus, south of Beila, about
120 miles N. W. of Cutch and the mouths of the Indus (see Map, fig. 71,
p. 460), numerous mud volcanoes are scattered over an area of probably
not less than 1000 square miles. Some of these have been well described
by Captain Hart, and subsequently by Captain Robertson, who has paid a
visit to that region, and made sketches of them, which he has kindly
placed at my disposal. From one of these the annexed view has been
selected. These conical hills occur to the westward of the Hara
mountains and the river Hubb. (See Map, p. 460.) One of the cones is 400
feet high, composed of light-colored earth, and having at its summit a
crater thirty yards in diameter. The liquid mud which fills the crater
is continually disturbed by air-bubbles, and here and there is cast up
in small jets.[618]
_Mineral composition of volcanic products._--The mineral called felspar
forms in general more than half of the mass of modern lavas. When it is
in great excess, lavas are called trachytic: they consist generally of a
base of compact felspar, in which crystals of glassy felspar are
disseminated.[619] When augite (or pyroxene) predominates, lavas are
termed basaltic. They contain about 50 per cent. of silica, or much less
than the trachytes, in which there is usually about 75 per cent. of that
mineral. They also contain about 11 per cent. of protoxide of iron, and
as much of lime, both of which are wanting, or only in insignificant
quantities in the trachytic rocks.[620] But lavas occur of an
intermediate composition between the trachytic and basaltic, which from
their color have been called graystones. The abundance of quartz,
forming distinct crystals or concretions, characterizes the granitic and
other ancient rocks, now generally considered by geologists as of
igneous origin; whereas that mineral is rarely exhibited in a separate
form in recent lavas, although silica enters so largely into their
composition. Hornblende, so common in hypogene rocks, or those commonly
called "primary," is rare in modern lava; nor does it enter largely into
rocks of any age in which augite abounds. It should, however, be stated,
that the experiments of Mr. Gustav Rose have made it very questionable,
whether the minerals called hornblende and augite can be separated as
distinct species, as their different varieties seem to pass into each
other, whether we consider the characters derived from their angles of
crystallization, their chemical composition, or their specific gravity.
The difference in form of the two substances may be explained by the
different circumstances under which they have been produced, the form
of hornblende being the result of slower cooling. Crystals of augite
have been met with in the scoriae of furnaces, but never those of
hornblende; and crystals of augite have been obtained by melting
hornblende in a platina crucible; but hornblende itself has not been
formed artificially.[621] Mica occurs plentifully in some recent
trachytes, but is rarely present where augite is in excess.
_Frequency of eruptions, and nature of subterranean igneous
rocks._--When we speak of the igneous rocks of our own times, we mean
that small portion which, in violent eruptions, is forced up by elastic
fluids to the surface of the earth,--the sand, scoriae, and lava, which
cool in the open air. But we cannot obtain access to that which is
congealed far beneath the surface under great pressure, equal to that of
many hundred, or many thousand atmospheres.
During the last century, about fifty eruptions are recorded of the five
European volcanic districts, of Vesuvius, Etna, Volcano, Santorin, and
Iceland; but many beneath the sea in the Grecian archipelago and near
Iceland may doubtless have passed unnoticed. If some of them produced no
lava, others, on the contrary, like that of Skaptar Jokul, in 1783,
poured out melted matter for five or six years consecutively; which
cases, being reckoned as single eruptions, will compensate for those of
inferior strength. Now, if we consider the active volcanoes of Europe to
constitute about a fortieth part of those already known on the globe,
and calculate that, one with another, they are about equal in activity
to the burning mountains in other districts, we may then compute that
there happen on the earth about 2000 eruptions in the course of a
century, or about twenty every year.
However inconsiderable, therefore, may be the superficial rocks which
the operations of fire produce on the surface, we must suppose the
subterranean changes now constantly in progress to be on the grandest
scale. The loftiest volcanic cones must be as insignificant, when
contrasted to the products of fire in the nether regions, as are the
deposits formed in shallow estuaries when compared to submarine
formations accumulating in the abysses of the ocean. In regard to the
characters of these volcanic rocks, formed in our own times in the
bowels of the earth, whether in rents and caverns, or by the cooling of
lakes of melted lava, we may safely infer that the rocks are heavier and
less porous than ordinary lavas, and more crystalline, although composed
of the same mineral ingredients. As the hardest crystals produced
artificially in the laboratory require the longest time for their
formation, so we must suppose that where the cooling down of melted
matter takes place by insensible degrees, in the course of ages, a
variety of minerals will be produced far harder than any formed by
natural processes within the short period of human observation.
These subterranean volcanic rocks, moreover, cannot be stratified in the
same manner as sedimentary deposits from water, although it is evident
that when great masses consolidate from a state of fusion, they may
separate into natural divisions; for this is seen to be the case in
many lava-currents. We may also expect that the rocks in question will
often be rent by earthquakes, since these are common in volcanic
regions; and the fissures will be often injected with similar matter, so
that dikes of crystalline rock will traverse masses of similar
composition. It is also clear, that no organic remains can be included
in such masses, as also that these deep-seated igneous formations
considered in mass must underlie all the strata containing organic
remains, because the heat proceeds from below upwards, and the intensity
required to reduce the mineral ingredients to a fluid state must destroy
all organic bodies in rocks included in the midst of them.
If by a continued series of elevatory movements, such masses shall
hereafter be brought up to the surface, in the same manner as
sedimentary marine strata have, in the course of ages, been upheaved to
the summit of the loftiest mountains, it is not difficult to foresee
what perplexing problems may be presented to the geologist. He may then,
perhaps, study in some mountain-chain the very rocks produced at the
depth of several miles beneath the Andes, Iceland, or Java, in the time
of Leibnitz, and draw from them the same conclusion which that
philosopher derived from certain igneous products of high antiquity; for
he conceived our globe to have been, for an indefinite period, in the
state of a comet, without an ocean, and uninhabitable alike by aquatic
or terrestrial animals.