remarkable orifices of eruption may be noticed here, to which the
names of _mud-volcanoes_, _salses_, _air-volcanoes_, and _maccalubas_
have been applied (Sicily, the Apennines, Caucasus, Kertch, Tamar).
These are conical hills formed by the accumulation of fine and
usually saline mud, which, with various gases, is continuously or
intermittently given out from the orifice or crater in the centre.
They occur in groups, each hillock being sometimes less than a yard
in height, but ranging up to elevations of 100 feet or more. Like
true volcanoes, they have their periods of repose, when either no
discharge takes place at all, or mud oozes out tranquilly from the
crater, and their epochs of activity, when large volumes of gas,
and sometimes columns of flame, rush out with considerable violence
and explosion, and throw up mud and stones to a height of several
hundred feet. The gases play much the same part, therefore, in these
phenomena that steam does in those of true volcanoes. They consist
of marsh-gas and other hydrocarbons, carbon-dioxide, sulphureted
hydrogen, and nitrogen, with petroleum vapors. The mud is usually
cold. In the water occur various saline ingredients, among which
common salt generally appears; hence the names _Salses_. Naphtha is
likewise frequently present. Large pieces of stone, differing from
those in the neighborhood, have been observed among the ejections,
indicative doubtless of a somewhat deeper source than in ordinary
cases. Heavy rains may wash down the minor mud-cones and spread out
the material over the ground; but gas-bubbles again appear through
the sheet of mud, and by degrees a new series of mounds is once more
thrown up.
The second class of mud-volcano presents itself in true volcanic
regions, and is due to the escape of hot water and steam through
beds of tuff or some other friable kind of rock. The mud is kept
in ebullition by the rise of steam through it. As it becomes more
pasty and the steam meets with greater resistance, large bubbles are
formed which burst, and the more liquid mud from below oozes out from
the vent. In this way, small cones are built up, many of which have
perfect craters atop. In the Geyser tracts of the Yellowstone region,
there are instructive examples of such active and extinct mud-vents.
Some of the extinct cones there are not more than a foot high, and
might be carefully removed as museum specimens.
Mud-volcanoes occur in Iceland, Sicily (Maccaluba), in many districts
of northern Italy, at Tamar and Kertch, at Baku on the Caspian, near
the mouth of the Indus, and in other parts of the globe.
It is not only on the surface of the land that volcanic action shows
itself. It takes place likewise under the sea, and as the geological
records of the earth’s past history are chiefly marine formations,
the characteristics of submarine volcanic action have no small
interest for the geologist. In a few instances, the actual outbreak
of a submarine eruption has been witnessed. Thus, in the early summer
of 1783, a volcanic eruption took place about thirty miles from Cape
Reykjanaes on the west coast of Iceland. An island was built up, from
which fire and smoke continued to issue, but in less than a year the
waves had washed the loose pumice away, leaving a submerged reef
from five to thirty fathoms below sea-level. About a month after
this eruption, the frightful outbreak of Skaptar-Jökull began, the
distance of this mountain from the submarine vent being nearly 200
miles. A century afterward, viz., in July, 1884, another volcanic
island is said to have been thrown up near the same spot, having at
first the form of a flattened cone, but soon yielding to the power
of the breakers. Many submarine eruptions have taken place within
historic times in the Mediterranean. The most noted of these occurred
in the year 1831, when a new volcanic island (Graham’s Island, Ile
Julia) was thrown up, with abundant discharge of steam and showers of
scoriæ, between Sicily and the coast of Africa. It reached an extreme
height of 200 feet or more above the sea-level (800 feet above
sea-bottom), with a circumference of 3 miles, but on the cessation of
the eruptions was attacked by the waves and soon demolished, leaving
only a shoal to mark its site. In the year 1811, another island
was formed by submarine eruption of the coast off St. Michael’s in
the Azores. Consisting, like the Mediterranean example, of loose
cinders, it rose to a height of about three hundred feet, with a
circumference of about a mile, but subsequently disappeared. In
the year 1796 the island of Johanna Bogoslawa, in Alaska, appeared
above the water, and in four years had grown into a large volcanic
cone, the summit of which was 3,000 feet above sea-level.
[Illustration: Ideal Landscape of the Carboniferous Period
Showing Lepidodendra and other Giant Ferns and Mosses whose remains
are found in the Coal-Measures]
Unfortunately, the phenomena of recent volcanic eruptions under the
sea are for the most part inaccessible. Here and there, as in the
Bay of Naples, at Etna, among the islands of the Greek Archipelago,
and at Tahiti, elevation of the sea-bed has taken place, and brought
to the surface beds of tuff or of lava, which have consolidated
under water. Both Vesuvius and Etna began their career as submarine
volcanoes. The Islands of Santorin and Therasia form the unsubmerged
portions of a great crater-rim rising round a crater which descends
1,278 feet below sea-level.
Confining attention to vents now active, of which the total number
may be about 300, the chief facts regarding their distribution over
the globe may be thus summarized. (1) Volcanoes occur along the
margins of the ocean-basins, particularly along lines of dominant
mountain ranges, which either form part of the mainland of the
continents or extend as adjacent lines of islands. The vast hollow
of the Pacific is girdled with a wide ring of volcanic foci. (2)
Volcanoes rise, as a striking feature, from the submarine ridges
that traverse the ocean basins. All the oceanic islands are either
volcanic or formed of coral, and the scattered coral-islands have
in all likelihood been built upon the tops of submarine volcanic
cones. (3) Volcanoes are situated not far from the sea. The only
exceptions to this rule are certain vents in Manchuria and in the
tract lying between Tibet and Siberia; but of the actual nature of
these vents very little is yet known. (4) The dominant arrangement
of volcanoes is in series along subterranean lines of weakness,
as in the chain of the Andes, the Aleutian Islands, and the Malay
Archipelago. A remarkable zone of volcanic vents girdles the globe
from Central America eastward by the Azores and Canary Islands to
the Mediterranean, thence to the Red Sea, and through the chains of
islands from the south of Asia to New Zealand and the heart of the
Pacific. (5) On a smaller scale the linear arrangement gives place to
one in groups, as in Italy, Iceland, and the volcanic islands of the
great oceans.
In the European area there are six active volcanoes--Vesuvius, Etna,
Stromboli, Volcano, Santorin, and Nisyros. Asia contains twenty-four,
Africa ten, North America twenty, Central America twenty-five, and
South America thirty-seven. By much the larger number, however, occur
on islands in the ocean. In the Arctic Ocean rises the solitary Jan
Mayen. On the ridge separating the Arctic and Atlantic basins, the
group of Icelandic volcanoes is found. Along the great central ridge
of the Atlantic bottom, numerous volcanic vents have risen above
the surface of the sea--the Azores, Canary Islands, and the extinct
degraded volcanoes of St. Helena, Ascension, and Tristan d’Acunha.
On the eastern border lie the volcanic vents of the islands off the
African coast, and to the west those of the West Indian Islands.
Still more remarkable is the development of volcanic energy in the
Pacific area. From the Aleutian Islands southward, a long line of
volcanoes, numbering upward of a hundred active vents, extends
through Kamtchatka and the Kurile Islands to Japan, whence another
numerous series carries the volcanic band far south toward the
Malay Archipelago, which must be regarded as the chief centre of
the present volcanic activity of our planet. In Sumatra, Java, and
adjoining islands, no fewer than fifty active vents occur. The chain
is continued through New Guinea and the groups of islands to New
Zealand. Even in the Antarctic regions, Mounts Erebus and Terror are
cited as active vents; while in the centre of the Pacific Ocean rise
the great lava cones of the Sandwich Islands. In the Indian Ocean,
the Red Sea, and off the east coast of Africa a few scattered vents
appear.
THOUGHTS ABOUT KRAKATOA
--SIR ROBERT S. BALL
Midway between Sumatra and Java lies a group of small islands,
which, prior to 1883, were beautified by the dense forests and
glorious vegetation of the tropics. Of these islands Krakatoa was
the chief, though even of it but little was known. Its appearance
from the sea must, indeed, have been familiar to the crews of the
many vessels that navigated the Straits of Sunda, but it was not
regularly inhabited. Glowing with tropical verdure, such an island
seemed an unlikely theatre for the display of an unparalleled effect
of plutonic energy, but yet there were certain circumstances which
may tend to lessen our surprise at the outbreak. In the first place,
as Professor Judd has so clearly pointed out, not only is Krakatoa
situated in a region famous, or perhaps infamous, for volcanoes and
earthquakes, but it actually happens to lie at the intersection of
two main lines, along which volcanic phenomena are, in some degree,
perennial. In the second place, history records that there have been
previous eruptions at Krakatoa. The last of these appears to have
occurred in May, 1680, but unfortunately only imperfect accounts
of it have been preserved. It seems, however, to have annihilated
the forests of the island, and to have ejected vast quantities of
pumice, which cumbered the seas around. Krakatoa then remained active
for a year and a half, after which the mighty fires subsided. The
irrepressible tropical vegetation again resumed possession. The
desolated islet again became clothed with beauty, and for a couple of
centuries reposed in peace.
It was one o’clock in the afternoon of Sunday, August 26, 1883, when
Krakatoa commenced a series of gigantic volcanic efforts. Detonations
were heard which succeeded each other at intervals of about ten
minutes. These were loud enough to penetrate as far as Batavia and
Buitenzorg, distant 96 and 100 miles respectively from the volcano.
A vast column of steam, smoke, and ashes ascended to a prodigious
elevation. It was measured at two P. M. from a ship 76 miles away,
and was then judged to be 17 miles high--that is, three times the
height of the loftiest mountain in the world. As the Sunday afternoon
wore on, the volcanic manifestations became ever fiercer. At 3 P. M.
the sounds were loudly heard in a town 150 miles away. At 5 P. M.
every ear in the island of Java was engaged in listening to volcanic
explosions, which were considered to be of quite unusual intensity
even in that part of the world. These phenomena were, however, only
introductory. Krakatoa was gathering strength. Between 5 and 6 P.
M. the British ship _Charles Bal_, commanded by Captain Watson,
was about ten miles south of the volcano. The ship had to shorten
sail in the darkness, and a rain of pumice, in large pieces and
quite warm, fell upon her decks. At 7 P. M. the mighty column of
smoke is described as having the shape of a pine tree, and as being
brilliantly illuminated by electric flashes. The sulphurous air is
laden with fine dust, while the lead dropped from a ship in its
anxious navigation astounds the leadsman by coming up hot from the
bottom of the sea. From sunset on Sunday till midnight the tremendous
detonations followed each other so quickly that a continuous roar
may be said to have issued from the island. The full terrors of the
eruption were now approaching. The distance of 96 miles between
Krakatoa and Batavia was not sufficient to permit the inhabitants of
the town to enjoy their night’s sleep. All night long the thunders of
the volcano sounded like the discharges of artillery at their very
doors, while the windows rattled with aerial vibrations.
On Monday morning, August 27, the eruption culminated in four
terrific explosions, of which the third, shortly after 10 A. M.
Krakatoa time, was by far the most violent. The quantity of material
ejected was now so great that darkness prevailed even as far as
Batavia soon after 11 A. M., and there was a rain of dust until
three in the afternoon. The explosions continued with more or less
intensity all the afternoon of Monday and throughout Monday night.
They finally ceased at about 2:30 A. M. on Tuesday, August 28. The
entire series of grand phenomena thus occupied a little more than
thirty-six hours.
It seems to be certain that if all the materials poured forth from
Krakatoa during the critical period could be collected together,
the mass they would form would be considerably over a cubic mile
in volume. It is in the other standards of comparison that the
importance of the explosion of Krakatoa is to be sought. The
intensity of this outbreak in its last throes was such that mighty
sounds were heard and mighty waves arose in the sea for which we can
find no parallel. Every part of our globe’s surface felt the pulse of
the air-waves, and beautiful optical phenomena made the circuit of
the globe even more than once or twice. In these last respects the
eruption of Krakatoa is unique.
It appears to me that the most remarkable incident connected with
the eruption of Krakatoa was the production of the great air-wave by
that particular explosion that occurred at ten o’clock on the morning
of Monday, August 27. The great air-wave was truly of cosmical
importance, affecting as it did every particle of the atmosphere on
our globe.
The comprehensive series of phenomena wherein the atmosphere of the
entire globe participates in an organized vibration has, so far as
we know, only once been witnessed, and that was after the greatest
outbreak at Krakatoa, at ten o’clock on the morning of August 27. But
the ebb and the flow of these mighty undulations are not immediately
appreciable to the senses. The great wave, for instance, passed
and re-passed and passed again over London, and no inhabitant was
conscious of the fact. But the automatic records of the barometer at
Greenwich show that the vibration from Krakatoa to its antipodes, and
from the antipodes back to Krakatoa, was distinctly perceptible over
London not less than six or seven times.
From all parts of Europe, from Berlin to Palermo, from St.
Petersburg to Valencia, we obtain the same indications. Fortunately
self-recording barometric instruments are now to be found all over
the world. Almost all the instruments show distinctly the first great
wave from Krakatoa to its antipodes in Central America, and the
return wave from the antipodes to Krakatoa. They also all show the
second great wave which sped from Krakatoa, as well as the second
great wave which returned from the antipodes. Thus, the first four of
the oscillations are depicted on upward of forty of the barograms.
The fifth and sixth oscillations are also to be distinguished on
several of the curves, and even the seventh is certainly established
at some few places, of which Kew is one. Then the gradually
increasing faintness of the indications renders them unrecognizable,
from which we conclude that after seven pulsations our atmosphere
had sensibly regained its former condition ere it was disturbed by
Krakatoa.
In the whole annals of noise there is nothing which can be compared
to the records. Lloyd’s agent at Batavia, 94 miles distant, says
that on the morning of August 27 the reports and concussions were
deafening. At Carimon, Java Island, reports were heard which led to
the belief that some vessel offshore was making signals of distress,
and boats were accordingly put out to render succor, but no vessel
was found, as the reports were from Krakatoa, at a distance of 355
miles. At Macassar, in Celebes, explosions were heard all over the
province. Two steamers were sent out to discover the cause, for
the authorities did not then know that what they heard came from
Krakatoa, 969 miles away. But mere hundreds of miles will not suffice
to exemplify the range of this stupendous siren. In St. Lucia Bay,
in Borneo, a number of natives, who had been guilty of murder,
thought they heard the sounds of vengeance in the approach of an
attacking force. They fled from their village, little fancying that
what alarmed them really came from Krakatoa, 1,116 miles distant.
All over the island of Timor alarming sounds were heard, and so
urgent did the situation appear that the government was aroused, and
sent off a steamer to ascertain the cause. The sounds had, however,
come 1,351 miles, all the way from Krakatoa. In the Victoria Plains
of West Australia the inhabitants were startled by the discharge
of artillery--an unwonted noise in that peaceful district--but the
artillery was at Krakatoa, 1,700 miles distant. The inhabitants of
Daly Waters, in South Australia, were rudely awakened at midnight on
Sunday, August 26, by an explosion resembling the blasting of a rock,
which lasted for a few minutes. The time and other circumstances
show that here again was Krakatoa heard, this time at the monstrous
distance of 2,023 miles. But there is undoubted testimony that to
distances even greater than 2,023 miles the waves of sound conveyed
tidings of the mighty convulsion. Diego Garcia, in the Chagos
Islands, is 2,267 miles from Krakatoa, but the thunders traversed
even this distance, and created the belief that there must be some
ship in distress, for which a diligent but necessarily ineffectual
search was made. To pass at once to the most remarkable case of all,
we have a report from Mr. James Wallis, chief of police in Rodriguez,
that “several times during the night of August 26-27, 1883, reports
were heard coming from the eastward, like the distant roar of heavy
guns. These reports continued at intervals of between three and four
hours.” We have thus the astounding fact that almost across the
whole wide extent of the Indian Ocean, that is, to a distance of
nearly 3,000 miles (2,968), the sound of the throes of Krakatoa was
propagated.
I shall content myself with the mention of three facts in
illustration of the great sea waves which accompanied the eruption
of Krakatoa. Of these, probably the most unusual is the magnitude
of the area over which the undulations were perceived. Thus, to
mention but a single instance, and that not by any means an extreme
one, we find that the tide gauge at Table Bay reveals waves which,
notwithstanding that they have traveled 5,100 miles from Krakatoa,
have still a range of eighteen inches when they arrive at the
southern coast of Africa. The second fact that I mention illustrates
the magnitude of the seismic waves by the extraordinary inundations
that they produced on the shores of the Straits of Sunda. Captain
Wharton shows that the waves, as they deluged the land, must have
been fifty feet, or, in one well authenticated case, seventy-two
feet high. It was, of course, these vast floods which caused the
fearful loss of life. The third illustrative fact concerns the fate
of a man-of-war, the _Berouw_. This unhappy vessel was borne from
its normal element and left high and dry in Sumatra, a mile and
three-quarters inland, and thirty feet above the level of the sea.
During the crisis on August 26-27, the volume of material blown into
the air was sufficiently dense to obscure the coasts of Sumatra
to such a degree that at 10 A. M. the darkness there is stated to
have been more intense than it is even in the blackest of nights.
The fire-dust ascended to an elevation which, as we have already
mentioned, is estimated to have been as much as seventeen miles.
Borne aloft into these higher regions of our atmosphere, the clouds
of dust at once became the sport of the winds and the currents which
may be found there. If we had not previously known the prevailing
tendency of the winds at these elevations and in these latitudes, the
journey of the Krakatoa dust would have taught us.
It seems certain that, having attained their lofty elevation, the
mighty clouds of dust were seized by easterly winds, and were swept
along with a velocity which may not improbably be normal at a height
of twenty miles above the earth’s surface.
It appears that this cloud of dust started immediately from Krakatoa
for a series of voyages round the world. The highway which it at
first pursued may, for our present purpose, be sufficiently defined
by the Tropic of Cancer and the Tropic of Capricorn, though it hardly
approached these margins at first. Westward the dust of Krakatoa
takes its way. In three days it had crossed the Indian Ocean and
was rapidly flying over the heart of Equatorial Africa; for another
couple of days it was making a transatlantic journey; and then it
might be found, for still a couple of days more, over the forests of
Brazil ere it commenced the great Pacific voyage which brought it
back to the East Indies. The dust of Krakatoa had put a girdle round
the earth in thirteen days! The shape of the cloud appears to have
been elongated, so that it took two or three days to complete the
passage over any stated place.
It remains to give some brief account of the optical phenomena due
to the presence of dust, unusual both in quantity and in character,
in the upper atmosphere. Beautiful pictures show the twilight and
after-glow effects as seen by Mr. W. Ascroft on the bank of the
Thames a little west of London, on the evening of November 26, 1883.
Analogous phenomena were seen almost universally during November and
December in the same year. Who is there that does not remember the
wondrous loveliness of the twilights and the after-glows during that
remarkable winter! These appearances at sunrise and sunset are only
the more generally recognized of a whole system of strange optical
phenomena. One of the most striking indications of the presence of
the dust-stream in its first voyage round the earth was given by the
strange blue hue it imparted to the sun. The dust-stream was also
visible in its rapid voyages as a lofty haze or extensive cloud of
cirro-stratus. Then, too, strange halos were often seen, there were
occasional blue or green moons, and the sun was sometimes glorified
by a corona that had its origin in our atmosphere. Everywhere in the
world there were remarkable features in the sky that winter: from
Tierra del Fuego to Lake Superior; from China to the Gulf of Guinea;
from Panama to Australia. Wherever on land there were inhabitants
with sufficient intelligence to note the unusual, wherever on the sea
there were mariners who kept a careful log, from all such observers
we learn that in the autumn and winter months following the great
eruption of Krakatoa, there were extraordinary manifestations
witnessed in the heavens.
VOLCANOES
--SIR ARCHIBALD GEIKIE
The term volcanic action (volcanism or volcanicity) embraces all the
phenomena connected with the expulsion of heated materials from the
interior of the earth to the surface. Among these phenomena, some
possess an evanescent character, while others leave permanent proofs
of their existence. It is naturally to the latter that the geologist
gives chief attention, for it is by their means that he can trace
former phases of volcanic activity in regions where, for many ages,
there have been no volcanic eruptions. In the operations of existing
volcanoes, he can observe only superficial manifestations of volcanic
action. But examining the rocks of the earth’s crust, he discovers
that amid the many terrestrial revolutions which geology reveals,
the very roots of former volcanoes have been laid bare, displaying
subterranean phases of volcanism which could not be studied in any
modern volcano. Hence an acquaintance only with active volcanoes
will not afford a complete knowledge of volcanic action. It must
be supplemented and enlarged by an investigation of the traces of
ancient volcanoes preserved in the crust of the earth.