mainly or wholly of erupted materials), from the summit and often
also from the sides of which hot vapors issue, and ashes and streams
of molten rock are intermittently expelled. The term “volcanic”
designates all the phenomena essentially connected with one of these
channels of communication between the surface and the heated interior
of the globe. Yet there is good reason to believe that the active
volcanoes of the present day do not afford by any means a complete
type of volcanic action. The first effort in the formation of a new
volcano is to establish a fissure in the earth’s crust. A volcano is
only one vent or group of vents established along the line of such
a fissure. But in many parts of the earth, alike in the Old World
and the New, there have been periods in the earth’s history when
the crust was rent into innumerable fissures over areas thousands of
square miles in extent, and when the molten rock, instead of issuing,
as it does at a modern volcano, in narrow streams from a central
elevated cone, welled out from numerous small vents along the rents,
and flooded enormous tracts of country without forming any mountain
or conspicuous volcanic cone in the usual sense of these terms. Of
these “fissure-eruptions,” apart from central volcanic cones, no
examples appear to have occurred within the times of human history,
except in Iceland, where vast lava-floods issued from a fissure in
1783. They can best be studied from the remains of former convulsions.
The materials erupted from volcanic vents may be classed as (1) gases
and vapors, (2) water, (3) lava, (4) fragmentary substances.
Gases and vapors exist dissolved in the molten magma within the
earth’s crust. They play an important part in volcanic activity,
showing themselves in the earliest stages of a volcano’s history,
and continuing to appear for centuries after all other subterranean
action has ceased. By much the most abundant of them all is
water-gas, which, ultimately escaping as steam, has been estimated
to form 999-1000ths of the whole cloud that hangs over an active
volcano. In great eruptions, steam rises in prodigious quantities,
and is rapidly condensed into a heavy rainfall. M. Fouqué calculated
that, during 100 days, one of the parasitic cones on Etna had
ejected vapor enough to form, if condensed, 2,100,000 cubic metres
(462,000,000 gallons) of water. But even from volcanoes which, like
the Solfatara of Naples, have been dormant for centuries, steam
sometimes still rises without intermission and in considerable
volume. Jets of vapor rush out from clefts in the sides and bottom
of a crater with a noise like that made by the steam blown off by a
locomotive. The number of these funnels or “fumaroles” is often so
large, and the amount of vapor so abundant, that only now and then,
when the wind blows the dense cloud aside, can a momentary glimpse
be had of a part of the bottom of the crater; while at the same
time the rush and roar of the escaping steam remind one of the din
of some vast factory. Aqueous vapor rises likewise from rents on
the outside of the volcanic cone. It issues so copiously from some
flowing lavas that the stream of rock may be almost concealed from
view by the cloud; and it continues to escape from fissures of the
lava, far below the point of exit, for a long time after the rock has
solidified and come to rest.
Abundant discharges of water accompany some volcanic explosions.
Three sources of this water may be assigned: (1) from the melting of
snow by a rapid accession of temperature previous to or during an
eruption; this takes place from time to time on Etna, in Iceland,
and among the snowy ranges of the Andes, where the cone of Cotopaxi
is said to have been entirely divested of its snow in a single night
by the heating of the mountain; (2) from the condensation of the
vast clouds of steam which are discharged during an eruption; this
undoubtedly is the chief source of the destructive torrents so
frequently observed to form part of the phenomena of a great volcanic
explosion; and (3) from the disruption of reservoirs of water filling
subterranean cavities, or of lakes occupying crater-basins; this has
several times been observed among the South American volcanoes, where
immense quantities of dead fish, which inhabited the water, have
been swept down with the escaping torrents. The volcano of Agua in
Guatemala received its name from the disruption of a crater-lake at
its summit by an earthquake in 1540, whereby a vast and destructive
debacle of water was discharged down the slopes of the mountain. In
the beginning of the year 1817, an eruption took place at the large
crater of Idjèn, one of the volcanoes of Java, whereby a steaming
lake of hot acid water was discharged with frightful destruction down
the slopes of the mountain. After the explosion, the basin filled
again with water, but its temperature was no longer high.
The term lava is applied generally to all the molten rocks of
volcanoes. The use of the word in this broad sense is of great
convenience in geological descriptions, by directing attention to the
leading character of the rocks as molten products of volcanic action,
and obviating the confusion and errors which are apt to arise from an
ill-defined or incorrect lithological terminology.
While still flowing or not yet cooled, lavas differ from each other
in the extent to which they are impregnated with gases and vapors.
Some appear to be saturated, others contain a much smaller gaseous
impregnation; and hence arise important distinctions in their
behavior. After solidification, lavas present some noticeable
characters, then easily ascertainable. (1) Their average specific
gravity may be taken as ranging between 2.37 and 3.22. (2) The
heavier varieties contain much magnetic or titaniferous iron,
with augite and olivine, their composition being basic, and their
proportion of silica averaging about 45 to 55 per cent. (3) Lavas
differ much in structure and texture. (4) Lavas vary greatly in
color and general external aspect. The heavy basic kinds are usually
dark gray, or almost black, though, on exposure to the weather,
they acquire a brown tint from the oxidation and hydration of their
iron. Their surface is commonly rough and ragged, until it has been
sufficiently decomposed by the atmosphere to crumble into soil which,
under favorable circumstances, supports a luxuriant vegetation. The
less dense lavas, such as phonolites and trachytes, are frequently
paler in color, sometimes yellow or buff, and decompose into light
soils; but the obsidians present rugged black sheets of rock,
roughened with ridges and heaps of gray froth-like pumice. Some of
the most brilliant surfaces of color in any rock-scenery on the globe
are to be found among volcanic rocks. The walls of active craters
glow with endless hues of red and yellow. The Grand Cañon of the
Yellowstone River has been dug out of the most marvelously tinted
lavas and tuffs.
Volcanic action may be either constant or periodic. Stromboli, in the
Mediterranean, so far as we know, has been uninterruptedly emitting
hot stones and steam, from a basin of molten lava, since the earliest
period of history. Among the Moluccas, the volcano Sioa, and in the
Friendly Islands, that of Tofua, have never ceased to be in eruption
since their first discovery. The lofty cone of Sangay, among the
Andes of Quito, is always giving off hot vapors; Cotopaxi, too, is
ever constantly active. But, though examples of unceasing action may
thus be cited from widely different quarters of the globe, they are
nevertheless exceptional. The general rule is that a volcano breaks
out from time to time with varying vigor, and after longer or shorter
intervals of quiescence.
It is usual to class volcanoes as _active_, _dormant_, and _extinct_.
This arrangement, however, often presents considerable difficulty in
its application. An active volcano can not of course be mistaken, for
even when not in eruption, it shows by its discharge of steam and
hot vapors that it might break out into activity at any moment. But
in many cases it is impossible to decide whether a volcano should
be called extinct or only dormant. The volcanoes of Silurian age
in Wales, of Carboniferous age in Ireland, of Permian age in the
Harz, of Miocene age in the Hebrides, of younger Tertiary age in
the Western States and Territories of North America, are certainly
all extinct. But the older Tertiary volcanoes of Iceland are still
represented there by Skaptar-Jökull, Hecla, and their neighbors.
Somma, in the First Century of the Christian era, would have been
naturally regarded as an extinct volcano. Its fires had never been
known to have been kindled; its vast crater was a wilderness of wild
vines and brushwood, haunted, no doubt, by wolf and wild boar. Yet in
a few days, during the autumn of the year 79, the half of the crater
walls was blown out by a terrific series of explosions, the present
Vesuvius was then formed within the limits of the earlier crater, and
since that time volcanic action has been intermittently exhibited up
to the present day. Some of the intervals of quietude, however, have
been so considerable that the mountain might then again have been
claimed as an extinct volcano. Thus, in the 131 years between 1500
and 1631, so completely had eruptions ceased that the crater had once
more become choked with copse-wood. A few pools and springs of very
salt and hot water remained as memorials of the former condition of
the mountain. But this period of quiescence closed with the eruption
of 1631--the most powerful of all the known explosions of Vesuvius,
except the great one of 79.
In short, no essential distinction can be drawn between dormant and
extinct volcanoes. Volcanic action is apt to show itself again and
again, even at vast intervals, within the same regions and over the
same sites. The dormant or waning condition of a volcano, when only
steam and various gases and sublimates are given off, is sometimes
called the Solfatara phase, from the well-known dormant crater of
that name near Naples.
The interval between two eruptions of an active volcano shows a
gradual augmentation of energy. The crater, emptied by the last
discharge, has its floor slowly upraised by the expansive force
of the lava-column underneath. Vapors rise in constant outflow,
accompanied sometimes by discharges of dust or stones. Through
rents in the crater-floor red-hot lava may be seen only a few feet
down. Where the lava is maintained at or above its fusion-point and
possesses great liquidity, it may form boiling lakes, as in the
great crater of Kilauea, where acres of seething lava may be watched
throwing up fountains of molten rock, surging against the walls
and re-fusing large masses that fall into the burning flood. The
lava-column inside the pipe of a volcano is all this time gradually
rising, until some weak part of the wall allows it to escape, or
until the pressure of the accumulated vapors becomes great enough to
burst through the hardened crust of the crater-floor and give rise to
the phenomena of an eruption.
Kluge has sought to trace a connection between the years of maximum
and minimum sun-spots and those of greatest and feeblest volcanic
activity, and has constructed lists to show that years which
have been specially characterized by terrestrial eruptions have
coincided with those marked by few sun-spots and diminished magnetic
disturbance. Such a connection can not be regarded as having yet
been satisfactorily established. Again, the same author has called
attention to the frequency and vigor of volcanic explosions at or
near the time of the August meteoric shower. But in this case,
likewise, the cited examples can hardly yet be looked upon as more
than coincidences.
At many volcanic vents the eruptive energy manifests itself with
more or less regularity. At Stromboli, which is constantly in an
active state, the explosions occur at intervals varying from three or
four to ten minutes and upward. A similar rhythmical movement has
been often observed during the eruptions at other vents which are
not constantly active. Volcano, for example, during its eruption of
September, 1873, displayed a succession of explosions which followed
each other at intervals of from twenty to thirty minutes. At Etna and
Vesuvius a similar rhythmical series of convulsive efforts has often
been observed during the course of an eruption. Among the volcanoes
of the Andes a periodic discharge of steam has been observed; Mr.
Whymper noticed outrushes of steam to proceed at intervals of from
twenty to thirty minutes from the summit of Sangai, while during his
inspection of the great crater of Cotopaxi, this volcano was seen to
blow off steam at intervals of about half an hour. At the eruption of
the Japanese volcano, Oshima, in 1877, Mr. Milne observed that the
explosions occurred nearly every two seconds, with occasional pauses
of 15 or 20 seconds. Kilauea, in Hawaii, seems to show a regular
system of grand eruptive periods. Dana has pointed out that outbreaks
of lava have taken place from that volcano at intervals of from eight
to nine years, this being the time required to fill the crater up to
the point of outbreak, or to a depth of 400 or 500 feet.
The approach of an eruption is not always indicated by any
premonitory symptoms, for many tremendous explosions are recorded to
have taken place in different parts of the world without perceptible
warning. Much in this respect would appear to depend upon the
condition of liquidity of the lava, and the amount of resistance
offered by it to the passage of the escaping vapors through its mass.
In Hawaii, where the lavas are remarkably liquid, vast outpourings of
them have taken place quietly without earthquakes during the present
century. But even there the great eruption of 1868 was accompanied by
violent earthquakes.
The eruptions of Vesuvius are often preceded by failure or diminution
of wells and springs. But more frequent indications of an approaching
outburst are conveyed by sympathetic movements of the ground.
Subterranean rumblings and groanings are heard; slight tremors
succeed, increasing in frequency and violence till they become
distinct earthquake shocks. The vapors from the crater grow more
abundant as the lava-column in the pipe or funnel of the volcano
ascends, forced upward and kept in perpetual agitation by the passage
of elastic vapors through its mass. After a long previous interval of
quiescence, there may be much solidified lava toward the top of the
funnel, which will restrain the ascent of the still molten portion
underneath. A vast pressure is thus exercised on the sides of the
cone, which, if too weak to resist, will open in one or more rents,
and the liquid lava will issue from the outer slope of the mountain;
or the energies of the volcano will be directed toward clearing the
obstruction in the chief throat, until with tremendous explosions,
and the rise of a vast cloud of dust and fragments, the bottom and
sides of the crater are finally blown out, and the top of the cone
disappears. The lava may now escape from the lowest part of the lip
of the crater, while, at the same time, immense numbers of red-hot
bombs, scoriæ, and stones are shot up into the air. The lava at first
rushes down like one or more rivers of melted iron, but, as it cools,
its rate of motion lessens. Clouds of steam rise from its surface, as
well as from the central crater. Indeed, every successive paroxysmal
convulsion of the mountain is marked, even at a distance, by the rise
of huge ball-like wreaths or clouds of steam, mixed with dust and
stones, forming a column which towers sometimes a couple of miles
or more above the summit of the cone. By degrees these eructations
diminish in frequency and intensity. The lava ceases to issue, the
showers of stones and dust decrease, and after a time, which may
vary from hours to days or months, even in the _régime_ of the same
mountain, the volcano becomes once more tranquil.
The convulsions which culminate in the formation of a volcano usually
split open the terrestrial crust by a more or less nearly rectilinear
fissure, or by a system of fissures. In the subsequent progress of
the mountain, the ground at and around the focus of action is liable
to be again and again rent open by other fissures. These tend to
diverge from the focus; but around the vent where the rocks have been
most exposed to concussion, the fissures sometimes intersect each
other in all directions. In the great eruption of Etna, in the year
1669, a series of six parallel fissures opened on the side of the
mountain. One of these, with a width of two yards, ran for a distance
of 12 miles, in a somewhat winding course, to within a mile of the
top of the cone.
In the deeper portions of a volcanic vent the convulsive efforts of
the lava-column to force its way upward must often produce lateral as
well as vertical rifts, and into these the molten material will rush,
exerting as it goes an enormous upward pressure on the mass of rock
overlying it. At a modern volcano these subterranean manifestations
can not be seen, but among the volcanoes of Tertiary and older times
they have been revealed by the progress of denudation.
Though lava very commonly issues from the lateral fissures on a
volcanic cone, it may sometimes approach the surface in them without
actually flowing out. The great fissure on Etna in 1669, for example,
was visible even from a distance, by the long line of vivid light
which rose from the incandescent lava within. Again, it frequently
happens that minor volcanic cones are thrown up on the line of a
fissure, either from the congelation of the lava round the point
of emission, or from the accumulation of ejected scoriæ round the
fissure-vent. One of the most remarkable examples of this kind is
that of the Laki fissure in Iceland, the whole length of which (12
miles) bristles with small cones and craters almost touching each
other.
Apart from the appearance of visible fissures, volcanic energy may
be, as it were, concentrated on a given point, which will usually be
the weakest in the structure of that part of the terrestrial crust,
and from which the solid rock, shattered into pieces, is hurled into
the air by the enormous expansive energy of the volcanic vapors.
The history of the cone of Vesuvius brings before us a long series
of such explosions, beginning with that of A. D. 79, and coming
down to the present day. Even now, in spite of all the lava and
ashes poured out during the last eighteen centuries, it is easy to
see how stupendous must have been that earliest explosion by which
the southern half of the ancient crater was blown out. At every
successive important eruption, a similar but minor operation takes
place within the present cone. The hardened cake of lava forming the
floor is burst open, and with it there usually disappears much of the
upper part of the cone, and sometimes, as in 1872, a large segment
of the crater-wall. The islands of Santorin bring before us evidence
of a prehistoric catastrophe of a similar nature, by which a large
volcanic cone was blown up. The existing outer islands are a chain of
fragments of the periphery of the cone, the centre of which is now
occupied by the sea. In the year 1538 a new volcano, Monte Nuovo, was
formed in twenty-four hours on the margin of the Bay of Naples. An
opening was drilled by successive explosions, and such quantities of
stones, scoriæ, and ashes were thrown out from it as to form a hill
that rose 440 English feet above the sea-level, and was more than a
mile and a half in circumference.
A communication having been opened, either by fissuring or explosion,
between the heated interior and the surface, fragmentary materials
are commonly ejected from it, consisting at first mainly of the rocks
through which the orifice has been opened, afterward of volcanic
substances. In a great eruption, vast numbers of red-hot stones
are shot up into the air, and fall back partly into the crater
and partly on the outer slopes of the cone. According to Sir W.
Hamilton, cinders were thrown by Vesuvius, during the eruption of
1779, to a height of 10,000 feet. Instances are known where large
stones, ejected obliquely, have described huge parabolic curves
in the air, and fallen at a great distance. Stones eight pounds
in weight occur among the ashes which buried Pompeii. The volcano
of Antuco in Chili is said to send stones flying to a distance of
thirty-six miles, Cotopaxi is reported to have hurled a 200-ton block
nine miles, and the Japanese volcano, Asama, is said to have ejected
many blocks of stone measuring from 40 to more than 100 feet in
diameter.
But in many great eruptions, besides a constant shower of stones
and scoriæ, a vast column of exceedingly fine dust rises out of the
crater, sometimes to a height of several miles, and then spreads
outward like a sheet of cloud. The remarkable fineness of this dust
may be understood from the fact that during great volcanic explosions
no boxes, watches, or close-fitting joints have been found to be able
to exclude it. Mr. Whymper collected some dust that fell sixty-five
miles away from Cotopaxi, and which was so fine that from 4,000 to
25,000 particles were required to weigh a grain. So dense is the
dust-cloud as to obscure the sun, and for days together the darkness
of night may reign for miles around the volcano. The eruption of
Cotopaxi, on 26th June, 1877, began by an explosion that sent up
a column of fine ashes to a prodigious height into the air, where
it rapidly spread out and formed so dense a canopy as to throw the
region below it into total darkness. So quickly did it diffuse
itself, that in an hour and a half a previously bright morning became
at Quito, thirty-three miles distant, a dim twilight, which in the
afternoon passed into such darkness that the hand placed before
the eye could not be seen. At Guayaquil, on the coast, 150 miles
distant, the shower of ashes continued till the 1st of July. Dr.
Wolf collected the ashes daily, and estimated that at that place
there fell 315 kilogrammes on every square kilometre during the first
thirty hours, and on the 30th of June, 209 kilogrammes in twelve
hours.
One of the most stupendous outpourings of volcanic ashes on record
took place, after a quiescence of twenty-six years, from the volcano
Coseguina, in Nicaragua, during the early part of the year 1835. On
that occasion, utter darkness prevailed over a circle of thirty-five
miles radius, the ashes falling so thickly that, even eight leagues
from the mountain, they covered the ground to a depth of about ten
feet. It was estimated that the rain of dust and sand fell over an
area at least 270 geographical miles in diameter. Some of the finer
materials, thrown so high as to come within the influence of an upper
air-current, were borne away eastward, and fell, four days afterward,
at Kingston, in Jamaica--a distance of 700 miles. During the great
eruption of Sumbawa, in 1815, the dust and stones fell over an area
of nearly one million square miles, and were estimated by Zollinger
to amount to fully fifty cubic miles of material, and by Junghuhn
to be equal to one hundred and eighty-five mountains like Vesuvius.
Toward the end of the Eighteenth Century, during a time of great
disturbance among the Japanese volcanoes, one of them, Sakurajima,
threw out so much pumiceous material that it was possible to walk a
distance of twenty-three miles upon the floating débris in the sea.
The varying degree of liquidity or viscosity of the lava probably
modifies the force of explosions, owing to the different amounts
of resistance offered to the upward passage of the absorbed gases
and vapors. Thus explosions and accompanying scoriæ are abundant at
Vesuvius, where the lavas are comparatively viscid; they are almost
unknown at Kilauea, where the lava is remarkably liquid.