into larger or smaller vesicles, works its way upward through
the substance of the molten lava, and as the elasticity of this
compressed vapor overcomes the pressure of the overlying lava,
it escapes at the surface, and there the lava is thus kept in
ebullition. But this comparatively quiet operation, which may be
watched within the craters of many active volcanoes, does not produce
clouds of fine dust. The collision or friction of millions of stones
ascending and descending in the dark column above the crater must
doubtless cause much dust and sand. But the explosive action of steam
is probably also an immediate cause of much trituration. The aqueous
vapor or water-gas which is so largely dissolved in many lavas must
exist within the lava-column, under an enormous pressure, at a
temperature far above its critical point, even at a white heat, and
therefore possibly in a state of dissociation. The sudden ascent of
lava so constituted relieves the pressure rapidly without sensibly
affecting the temperature of the mass. Consequently, the white-hot
gases or vapors at length explode, and reduce the molten mass to the
finest powder, like water shot out of a gun.
As every shower of dust and sand adds to the height of the ground
on which it falls, thick volcanic accumulations may be formed far
beyond the base of the mountain. The volcano of Sangay, in Ecuador,
for instance, has buried the country around it to a depth of 4,000
feet under its ashes. In such loose deposits are entombed trees and
other kinds of vegetation, together with the bodies of animals, as
well as the works of man. In some cases, where the layer of volcanic
dust is thin, it may merely add to the height of the soil, without
sensibly interfering with the vegetation. But it has been observed at
Santorin that though this is true in dry weather, the fall of rain
with the dust at once acts detrimentally. On the 3d of June, 1866,
the vines were there withered up, as if they had been burned, along
the track of the smoke cloud. By the gradual accumulation of volcanic
ashes, new geological formations arise which, in their component
materials, not only bear witness to the volcanic eruptions that
produced them, but preserve a record of the land-surfaces over which
they spread. In the third place, besides the distance to which the
fragments may be hurled by volcanic explosions, or to which they may
be diffused by the ordinary aerial movements, we have to take into
account the vast spaces across which the finer dust is sometimes
borne by upper air-currents. In the instance already cited, ashes
from Coseguina fell 700 miles away, having been carried all that
long distance by a high counter-current of air, moving apparently at
the rate of about seven miles an hour in an opposite direction to
that of the wind which blew at the surface. By the Sumbawa eruption,
also referred to above, the sea west of Sumatra was covered with a
layer of ashes two feet thick. On several occasions ashes from the
Icelandic volcanoes have fallen so thickly between the Orkney and
Shetland Islands, that vessels passing there have had the unwonted
deposit shoveled off their decks in the morning. In the year 1783,
during the memorable eruption of Skaptar-Jökull, so vast an amount
of fine dust was ejected that the atmosphere over Iceland continued
loaded with it for months afterward. It fell in such quantities
over parts of Caithness--a distance of 600 miles--as to destroy
the crops; that year is still spoken of by the inhabitants as the
year of “the ashie.” Traces of the same deposit have been observed
in Norway, and even as far as Holland. Hence it is evident that
volcanic accumulations may take place in regions many hundreds
of miles distant from any active volcano. A single thin layer of
volcanic detritus in a group of sedimentary strata would not thus of
itself prove the existence of contemporaneous volcanic action in its
neighborhood.
At its exit from the side of a volcano, lava glows with a white heat,
and flows with a motion which has been compared to that of honey or
of melted iron. It soon becomes red, and like a coal fallen from a
hot fireplace rapidly grows dull as it moves along, until it assumes
a black, cindery aspect. At the same time the surface congeals, and
soon becomes solid enough to support a heavy block of stone. The
aspect of the stream varies with the composition and fluidity of
the lava, form of the ground, angle of slope, and rapidity of flow.
Viscous lavas, like those of Vesuvius, break up along the surface
into rough brown or black cinder-like slags and irregular ragged
cakes, bristling with jagged points, which, in their onward motion,
grind and grate against each other with a harsh, metallic sound,
sometimes rising into rugged mounds or becoming seamed with rents and
gashes, at the bottom of which the red-hot glowing lava may be seen.
In lavas possessing somewhat greater fluidity, the surface presents
froth-like, curving lines, as in the scum of a slowly flowing river,
or is arranged in curious ropy folds, as the layers have successively
flowed over each other and congealed. A large area which has been
flooded with lava is perhaps the most hideous and appalling scene of
desolation anywhere to be found on the surface of the globe.
A lava-stream usually spreads out as it descends from its point
of escape, and moves more slowly. Its sides look like huge
embankments, or like some of the long mounds of “clinkers” in a great
manufacturing district. The advancing end is often much steeper,
creeping onward like a great wall or rampart, down the face of which
the rough blocks of hardened lava are ever rattling.
In a lofty volcano, lava occasionally rises to the lip of the crater
and flows out there; but more frequently it escapes from some fissure
or orifice in a weak part of the cone. In minor volcanoes, on the
other hand, where the explosions are less violent, and where the
thickness of the cone in proportion to the diameter of the funnel is
often greater, the lava very commonly rises into the crater. Should
the crater-walls be too weak to resist the pressure of the molten
mass, they give way, and the lava rushes out from the breach. This is
seen to have happened in several of the puys of Auvergne. But if the
crater be massive enough to withstand the pressure, the lava may at
last flow out from the lowest part of the rim.
As soon as the molten rock reaches the surface, the superheated
water-vapor or gas dissolved within its mass escapes copiously,
and hangs as a dense white cloud over the moving current. The
lava-streams of Vesuvius sometimes appear with as dense a steam-cloud
at their lower ends as that which escapes at the same time from the
main crater. Even after the molten mass has flowed several miles,
steam continues to rise abundantly both from its end and from
numerous points along its surface, and continues to do so for many
weeks, months, or it may be for several years.
Should the point of escape of a lava-stream lie well down on the
cone, far below the summit of the lava-column in the funnel, the
molten rock, on its first escape, driven by hydrostatic pressure,
will sometimes spout up high into the air--a fountain of molten rock.
This was observed in 1794 on Vesuvius, and in 1832 on Etna. In the
eruption of 1852 at Mauna Loa, an unbroken fountain of lava, from 200
to 700 feet in height and 1,000 feet broad, burst out at the base of
the cone. Similar “geysers” of molten rock have subsequently been
noticed in the same region. Thus in March and April, 1868, four fiery
fountains, throwing lava to heights varying from 500 to 1,000 feet,
continued to play for several weeks. According to Mr. Coan, such
outbursts take place from the bottom of a column of lava 3,000 feet
high. The volcano of Mauna Loa strikingly illustrates another feature
of volcanic dynamics in the position and outflow of lava. It bears
upon its flanks at a distance of 20 miles, but 10,000 feet lower,
the huge crater Kilauea. As Dana has pointed out, these orifices
form part of one mountain, yet the column of lava stands 10,000 feet
higher in one conduit than in the other. On a far smaller scale the
same independence occurs among the several pipes of some of the
geysers in the Yellowstone region of North America.
The rate of movement is regulated by the fluidity of the lava, by
its volume, and by the form and inclination of the ground. Hence, as
a rule, a lava-stream moves faster at first than afterward, because
it has not had time to stiffen, and its slope of descent is usually
steeper than further down the mountain. One of the most fluid and
swiftly flowing lava-streams ever observed on Vesuvius was that
erupted on 12th August, 1805. It is said to have rushed down a space
of 3 Italian (3⅔ English) miles in the first four minutes, but to
have widened out and moved more slowly as it descended, yet finally
to have reached Torre del Greco in three hours. A lava erupted by
Mauna Loa in 1852 went as fast as an ordinary stage-coach, or fifteen
miles in two hours; but some of the lavas from that mountain have in
parts of their course moved with double that rapidity.
In some cases, lava escaping from craters or fissures comes to rest
before reaching the base of the slopes, like the obsidian current
which has congealed on the side of the little volcanic island of
Volcano. In other instances, the molten rock not only reaches the
plains, but flows for many miles away from the point of eruption.
Sartorius von Waltershausen computed the lava emitted by Etna in 1865
at 92 millions of cubic metres, that of 1852 at 420 millions, that
of 1669 at 980 millions, and that of a prehistoric lava-stream near
Randazzo at more than 1,000 millions. The most stupendous outpouring
of lava on record was that which took place in Iceland in the year
1783. Successive streams issued from a fissure about 12 miles long,
filling up river gorges which were sometimes 600 feet deep and 200
feet broad, and advancing into the alluvial plains in lakes of
molten rock 12 to 15 miles wide and 100 feet deep. Two currents of
lava which, filling up the valley of the Skapta, escaped in nearly
opposite directions, extended for 45 and 50 miles respectively, their
usual thickness being 100 feet. Bischof estimated that the total
amount of lava poured forth during this single eruption “surpassed in
magnitude the bulk of Mont Blanc.”
The varying degrees of liquidity are manifested in a characteristic
way on the surface of lava. Thus, in the great lava-pools of Hawaii,
the rock exhibits a remarkable liquidity, throwing up fountains of
molten rock to a height of 300 feet or more. During its ebullition in
the crater-pools, jets and driblets a quarter of an inch in diameter
are tossed up, and, falling back on one another, make “a column of
hardened tears of lava,” one of which was found to have attained
a height of 40 feet, while in other places the jets thrown up and
blown aside by the wind give rise to long threads of glass which
lie thickly together like mown grass, and are known by the natives
under the name of “Pele’s Hair,” after one of their divinities. Yet,
although the ebullition is caused by the uprise and escape of highly
heated vapors, there is no cloud over the boiling lake itself, heavy
white vapor only escaping at different points along the edge.
EARTHQUAKES
--WILLIAM HUGHES
It appears, from the accurate records of such phenomena which have
been kept within recent periods, that earthquakes are of much more
frequent occurrence than is commonly supposed. Upward of three
thousand earthquakes are recorded as having occurred within the first
half of the Nineteenth Century--an average of more than one for every
week throughout the entire period. But not more than one in forty is
of considerable importance, by far the greater number consisting of
such slight shocks as are occasionally experienced in Great Britain
and other countries favored with a like immunity in this regard. An
important earthquake, however, in some part of the world or other,
appears, from the above average, to occur once in every eight months.
In Europe alone, where a more complete record of such occurrences is
obtainable than in other parts of the world, as many as 320 distinct
earthquakes are recorded to have occurred within a period of ten
years (1833-42)--an average of thirty-two annually, and of one such
shock for every ten days throughout the period.
[The _geographical area_ within which shocks of earthquakes
are experienced is a widely spread one, and does not appear to
undergo any material change (if, indeed, any change whatever)
as to its limits. At any rate, the regions in which violent
earthquakes are recorded to have occurred in former times are
those in which such disturbances are of most frequent recurrence
at the present day. One of the most striking evidences in favor
of the supposition that the volcanic eruption is due to the same
deeply seated cause which produces the shock of the earthquake,
is afforded by the fact, that all the volcanoes which have been
in eruption within the modern period of geology are found within
regions liable to earthquakes, and, for the most part, to violent
shocks.]
Regarding the earthquake and the volcanic eruption as the
manifestation, under different conditions, of the earth’s internal
fires, we readily mark out upon the globe the great regions of
geographical distribution in the case of such phenomena. The most
widely extended of these coincides with the circuit of the Pacific
Ocean. Along the entire western coast of the New World, from Tierra
del Fuego to the peninsula of Alaska and the neighborhood of the
Aleutian Islands, shocks of earthquakes are known to occur; and,
within a large portion of the space, vents of active eruption
are found. The subterranean igneous force is, indeed, much more
powerfully displayed in the southern than in the northern half of
the American continent, and the active volcanoes that occur within
the limits referred to are nearly all found amid the cordilleras of
the Andes, or upon the plateaus of the Mexican isthmus. One of the
Mexican volcanoes--Jorullo--is especially deserving of notice, from
the circumstance of its having first risen above the surrounding
plain by the accumulation of volcanic matter during an eruption in
the year 1759.
The Aleutian Islands connect the volcanic region of the eastern
Pacific with that which extends along its western shores. In the
latter case, however, it is upon the peninsular regions, or in the
chains of islands that adjoin the mainland, that the igneous force
is displayed. Kamtchatka, the Kurile Islands, Yesso, the Japanese
group, and the entire region of the Malay Archipelago, exhibit the
presence of igneous force below the ground. Seven active volcanoes
occur in Kamtchatka. The Japanese Archipelago is said to contain at
least twenty-seven active volcanoes, eight of them upon Yesso and
the adjacent islets. Between Japan and the Loo-choo group is Sulphur
Island, an insular volcano, from which smoke is constantly emitted.
The Philippine Islands, in which earthquakes are of frequent
occurrence, prolong the volcanic chain to the southward. Thence it
is traced, at intervals, along the northern shores of New Guinea,
and through the prolonged chains of the Solomon Islands, and the
New Hebrides, to the North Island of New Zealand. Slight shocks
of earthquake have also been experienced within the southern and
eastwardly portions of the Australian mainland.
The numerous volcanoes of the Malay Archipelago, the whole area of
which is liable to frequent earthquake shocks, often of the most
destructive violence, belong to the eastern portion of this region,
and display the agency of subterranean heat on the grandest scale.
The island of Java alone contains forty-three active volcanoes,
ranging in a linear direction throughout its length. The volcanic
chain of Java is prolonged to the eastward through the Lesser Sunda
Islands (Sumbawa, etc.), in which direction it is united with
that which borders the Pacific waters. There are active volcanoes
on an island in the Gulf of Siam, besides the well-known crater
of Barren Island, in the Bay of Bengal. The region adjoining the
last-named body of water, together with the whole of northern India,
is of frequent liability to earthquakes, some of them (as that of
Cutch, in 1819) of the most destructive violence. The volcanic
island of Mayotta (Comoro group), the active Piton of Réunion or
Bourbon Island, and the hot springs and extinct craters of St. Paul
and Amsterdam Islands, in a high southern latitude of the Indian
Ocean, constitute points which indicate, at distant intervals, the
continuity of the volcanic chain.
The southwestern portion of Asia, the southern shores of Europe, and
the northwestwardly portion of the African mainland, fall within
this region on the one side, as the islands of the West Indies do
upon the other. The entire breadth of the Atlantic Ocean, as well
as the circuit of the Mediterranean, is thus included within its
limits. To the northward, the numerous volcanoes of Iceland, and
the more distant cone of Jan Mayen Island, lying within the Arctic
circle, must be regarded as within its area; together with, in an
opposite direction, the still-burning peak of the Cameroon Mountains,
adjoining the upper extremity of the Gulf of Guinea. The volcanic
peaks found within the widely detached groups of the Azores and the
Cape Verde Islands, with Tenerife, in the Canary group, are among its
outlying members.
Throughout the wide region thus indicated, earthquakes are of
frequent occurrence. There are fewer active vents of eruption than
in the case of the Pacific circuit. But the cones of Etna and
Vesuvius, with the island of Santorin, in the Mediterranean, and
the numerous volcanoes of Iceland, attest the destructive violence
of the subterranean fires. Western Asia, from the Caspian to the
shores of the Archipelago (including Armenia, Syria, and the Lesser
Asia), Greece, southern Italy, the Spanish peninsula, and the region
of Mount Atlas, in Northwestern Africa, are all liable to the
frequent repetition of such convulsions. The only portion of the
Mediterranean coasts exempt from such disturbing phenomena is on its
southern shores, embracing that part of the North African coast which
stretches from the Lesser Syrtis to the valley of the Nile. We have
no record of the experience of any shocks of earthquake in Egypt.
Had it been otherwise, perhaps the pyramids of that land of wonders
might have proved less enduring monuments of the past.
The movement imparted to the ground during an earthquake may be
either horizontal or vertical. In the former case, the phenomenon
consists in an undulating, wave-like movement; in the latter, in
an upheaval or subsidence of land. The vertical shock affects most
the relative levels of adjacent objects, and produces the most
striking permanent changes in the natural aspect of the region in
which it is experienced. But the undulatory movement is attended
by more serious consequences to man, since it at once shakes the
foundations of the strongest edifices, and may overthrow in the
space of a few seconds the accumulated labors of prior ages. Whole
tracts of land, with their cities or villages, may be elevated or
depressed with comparatively little injury to life; but nothing can
withstand the force of a motion which rocks the solid strata of the
earth itself. The most solidly constructed buildings are not proof
against the earthquake any more than the weakest. Indeed, it has in
many instances been observed that those erections which displayed
the strongest masonry have suffered more from the effects of an
earthquake than buildings of slighter structure. The cracking of
walls, the falling-in of roofs, and the crash of tumbling houses on
every side, burying their inmates beneath the ruins, are among the
characteristics of the earthquake in its most violent and frightful
form.
It has been asserted that a third kind of movement--viz., in a
rotatory direction--sometimes occurs, and certain phenomena by
which earthquakes have been attended have favored this belief.
Thus, isolated columns or statues have been found, after such
an occurrence, to face a different quarter from that which they
previously did. This, however, would be sufficiently accounted for
by a vibratory movement, acting upon a column which was _unequally_
attached to its base; _i.e._, the fastening of which was of unequal
strength relatively to the central point of junction. During the
Chilian earthquake of 1835, vessels moored alongside of one another
in the harbor of Concepcion were afterward found with their cables
twisted together.
The duration of any single earthquake shock is seldom more than a few
seconds, though the terror which it inspires naturally tends to make
it seem of longer continuance; but in the case of the more violent
movements, even a few moments serve to destroy the work of ages. In
the Chilian earthquake of 1835, the great shock which destroyed the
city of Concepcion was preceded by several tremulous movements of
minor intensity. During the first half-minute, many persons remained
in their houses; but the convulsive motion of the earth then became
so strong that all rushed into the open streets for safety. The
horrid motion (writes an eye-witness of the scene) increased; people
could hardly stand; buildings waved and tottered; suddenly an awful
and overpowering shock caused universal destruction. In less than six
seconds the city was in ruins!
The earthquake is propagated to enormous distances from the region
in which the shock originates, the rate at which the motion travels
varying not merely with the violence of the originating impulse, but
also with the nature of the formations through which it passes. Rocks
of solid and homogeneous texture, as granite, favor the transmission
of the shock; while formations of loose texture, such as sand, most
retard its speed. The well-known Lisbon earthquake of 1755, by which
sixty thousand persons are said to have perished within the brief
space of six minutes, was felt in the British Islands, as well as
upon the coast of Barbary, and even among the islands of the West
Indies, on the opposite side of the Atlantic.
MOUNTAINS
--A. KEITH JOHNSTON
The number and altitude of the mountains of the globe are so great
that they form almost everywhere prominent objects, and operate to a
large extent in modifying the climatic conditions of every country
in the world. Yet the amount of solid material so raised above the
ordinary level of the land is not so much as might be expected.
Remembering that elevated plateaus of great extent occur in several
regions, and that the general surface of the earth is considerably
higher than the sea-level, it has been estimated that were the whole
dry land reduced to a uniform level, it would form a plain having an
elevation of 1,800 feet above the sea. And were these solid materials
scattered over the whole surface of the globe, so as to fill up the
bed of the ocean, the resulting level would be considerably below
the present surface of the sea, inasmuch as the main height of the
dry land most probably does not exceed one-fifteenth of the mean
depth of the bed of the ocean.
Mountains, and especially mountain-chains, subserve important uses in
the economy of nature, especially in connection with the water system
of the world. They are at once the great collectors and distributers
of water. In the passage of moisture-charged winds across them the
moisture is precipitated as rain or snow. When mountain ranges
intersect the course of constant winds by thus abstracting the
moisture, they produce a moist country on the windward side, and a
comparatively dry and arid one on the leeward. This is exemplified in
the Andes, the precipitous western surface of which has a different
aspect from the sloping eastern plains; and so also the greater
supply of moisture on the southern sides of the Himalayas brings the
snow-line 5,000 feet lower than on the northern side.