THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE
VOLCANIC MOUNTAINS OF EXUDATION
=Prevalent misconceptions about volcanoes.=—The more or less common
impression that a volcano is a “burning mountain” or a “smoking
mountain” has been much fostered by the school texts in physical
geography in use during an earlier period. The best introduction
to a discussion of volcanoes is, therefore, a disillusionment from
this notion. Far from being burning or smoking, there is normally
no combustion whatever in connection with a volcanic eruption. The
unsophisticated tourist who, looking out from Naples, sees the steam
cap which overhangs the Vesuvian crater tinged with brown, easily
receives the impression that the material of the cloud is smoke. Even
more at night, when a bright glow is reflected to his eye and soon
fades away, only to again glow brightly after a few moments have
passed, is it difficult to remove the impression that one is watching
an intermittent combustion within the crater. The cloud which floats
away from the crest of the mountain is in reality composed of steam
with which is admixed a larger or smaller proportion of fine rock
powder which gives to the cloud its brownish tone. The glow observed at
night is only a reflection from molten lava within the crater, and the
variation of its brightness is explained by the alternating rise and
fall of the lava surface by a process presently to be explained.
Not only is there no combustion in connection with volcanic eruptions,
but so far as the volcano is a mountain it is a product of its own
action. The grandest of volcanic eruptions have produced no mountains
whatever, but only vast plains or plateaus of consolidated molten rock,
and every volcanic mountain at some time in its history has risen out
of a relatively level surface.
When the traditional notions about volcanoes grew up, it was supposed
that the solid earth was merely a “crust” enveloping still molten
material. As has already been pointed out in an earlier chapter, this
view is no longer tenable, for we now know that the condition of matter
within the earth’s interior, while perhaps not directly comparable to
any that is known, yet has properties most resembling known matter in
a solid state; it is much more rigid than the best tool steel. While
there must be reservoirs of molten rock beneath active volcanoes, it
is none the less clear that they are small, local, and temporary.
This is shown by the comparative study of volcanic outlets within any
circumscribed district.
It is perhaps not easy to frame a definition of a volcano, but its
essential part, instead of being a mountain, is rather a vent or
channel which opens up connection between a subsurface reservoir of
molten rock and the surface of the earth. An eruption occurs whenever
there is a rise of this material, together with more or less steam and
admixed gases, to the surface. Such molten rock arriving at the surface
is designated _lava_. The changes in pressure upon this material during
its elevation induce secondary phenomena as the surface is approached,
and these manifestations are often most awe inspiring. While often
locally destructive, the geological importance of such phenomena is by
reason of their terrifying aspect likely to be greatly exaggerated.
=Early views concerning volcanic mountains.=—As already pointed out,
a volcano at its birth is not a mountain at all, but only, so to
speak, a shaft or channel of communication between the surface and a
subterranean reservoir of molten rock. By bringing this melted rock to
the surface there is built up a local elevation which may be designated
a mountain, except where the volume of the material is so large and is
spread to such distances as to produce a plain (see fissure eruptions
below).
In the early history of geology it was the view of the great German
geologist von Buch and his friend and colleague von Humboldt, that a
volcanic mountain was produced in much the same manner as is a blister
upon the body. The fluids which push up the cuticle in the blister were
here replaced by fluid rock which elevated the sedimentary rock layers
at the surface into a dome or mound which was open at the top—the
so-called _crater_. This “elevation-crater” theory of volcanoes long
held the stage in geological science, although it ignored the very
patent fact that the layers on the flanks of volcanic cones are not
of sedimentary rock at all, but, on the contrary, of the volcanic
materials which are brought up to the surface during the eruption.
The observational phase of science was, however, dawning, and the
English geologists Scrope and Lyell were able to show by study of
volcanic mountains that the mound about the volcanic vent was due to
the accumulation of once molten rock which had been either exuded or
ejected. Making use of data derived from New Zealand, Scrope showed
that, instead of being elevated during the formation of a volcanic
mountain, the sedimentary strata of the vicinity may be depressed near
the volcanic vent (Fig. 87).
[Illustration:
FIG. 87.—Breached volcanic cone near Auckland, New Zealand, showing
the bending down of the sedimentary strata in the neighborhood of the
vent (after Heaphy and Scrope).]
=The birth of volcanoes.=—To confirm the impression that the formation
of the volcanic mountain is in reality a secondary phenomenon
connected with eruptions, we may cite the observed birth of a number
of volcanoes. On the 20th of September, 1538, a new volcano, since
known as Monte Nuovo (new mountain), rose on the border of the ancient
Lake Lucrinus to the westward of Naples. This small mountain attained
a height of 440 feet, and is still to be seen on the shore of the bay
of Naples. From Mexico have been recorded the births of several new
volcanoes: Jorullo in 1759, Pochutla in 1870, and in 1881 a new volcano
in the Ajusco Mountains about midway between the Gulf of Mexico and the
Pacific Ocean. The latest of new volcanoes is that raised in Japan on
November 9, 1910, in connection with the eruption of Usu-san. This “New
Mountain” reached an elevation of 690 feet.
[Illustration:
FIG. 88.—View of the new Camiguin volcano from the sea. It was formed
in 1871 over a nearly level plain. The town of Catarman appears at the
right near the shore (after an unpublished photograph by Professor Dean
C. Worcester).]
As described by von Humboldt, Jorullo rose in the night of the 28th
of September, 1759, from a fissure which opened in a broad plain at
a point 35 miles distant from any then existing volcano. The most
remarkable of new volcanoes rose in 1871 on the island of Camiguin
northward from Mindanao in the Philippine archipelago. This mountain
was visited by the _Challenger_ expedition in 1875, and was first
ascended and studied thirty years later by a party under the leadership
of Professor Dean C. Worcester, the Secretary of the Interior of the
Philippine Islands, to whom the writer is indebted for this description
and the accompanying illustration of this largest and most interesting
of new-born volcanoes. As in the case of Jorullo, the eruption began
with the formation of a fissure in a level plain, some 400 yards
distant from the town of Catarman (Fig. 88). The eruption continued
for four years, at the end of which time the height of the summit was
estimated by the _Challenger_ expedition to be 1900 feet. At the time
of the first ascent in 1905, the height was determined by aneroid as
1750 feet, with sharp rock pinnacles projecting some 50 or 75 feet
higher.
=Active and extinct volcanoes.=—The terms “active” and “extinct”
have come into more or less common use to describe respectively those
volcanoes which show signs of eruptive activity, and those which are
not at the time active. The term “dormant” is applied to volcanoes
recently active and supposed to be in a doubtfully extinct condition.
From a well-known volcano in the vicinity of Naples, volcanoes which no
longer erupt lava or cinder, but show gaseous emanations (_fumeroles_)
are said to be in the _solfatara_ condition, or to show _solfataric_
activity.
Experience shows that the term “extinct”, while useful, must always be
interpreted to mean apparently extinct. This may be illustrated by the
history of Mount Vesuvius, which before the Christian era was forested
in the crater and showed no signs of activity; and in fact it is
known that for several centuries no eruption of the volcano had taken
place. Following a premonitory earthquake felt in the year 63, the
mountain burst out in grand explosive eruption in 79 A.D. This eruption
profoundly altered the aspect of the mountain and buried the cities
of Pompeii, Stabeii, and Herculaneum from sight. Once more, this time
during the middle ages, for nearly five centuries (1139 to 1631) there
was complete inactivity, if we except a light ash eruption in the year
1500. During this period of rest the crater was again forested, but the
repose was suddenly terminated by one of the grandest eruptions in the
mountain’s history.
[Illustration: FIG. 89.—Map showing the location of the belts of
active volcanoes.]
=The earth’s volcano belts.=—The distribution of volcanoes is not
uniform, but, on the contrary, volcanic vents appear in definite zones
or belts, either upon the margins of the continents or included within
the oceanic areas (Fig. 89). The most important of these belts girdles
the Pacific Ocean, and is represented either by chains or by more
widely spaced volcanic mountains throughout the Cordilleran Mountain
system of South and Central America and Mexico, by the volcanoes of the
Coast and Cascade ranges of North America, the festooned volcanic chain
of the Aleutian Islands, and the similar island arcs off the eastern
coast of the Eurasian continent. The belt is further continued through
the islands of Malaysia to New Zealand, and on the Pacific’s southern
margin are found the volcanoes of Victoria Land, King Edward Land, and
West Antarctica.
[Illustration: FIG. 90.—A portion of the “fire girdle” of the Pacific,
showing the relation of the chains of volcanic mountains to the deeps
of the neighboring ocean floor.]
This volcano girdle is by no means a perfect one, for in addition
to the principal festoons of the western border there are many
secondary ones, and still other arcs are found well toward the center
of the oceanic area. Another broad belt of volcanoes borders the
Mediterranean Sea, and is extended westward into the Atlantic Ocean.
Narrower belts are found in both the northern and southern portions
of the Atlantic Ocean, on the margins of the Caribbean Sea, etc. The
fact of greatest significance in the distribution seems to be that
bands of active volcanoes are to be found wherever mountain ranges are
paralleled by deeps on the neighboring ocean floor (Fig. 90). As has
been already pointed out in the chapter upon earthquakes, it is just
such places as these which are the seat of earthquakes; these are zones
of the earth’s crust which are undergoing the most rapid changes of
level at the present time. Thus the rise of the land in mountains is
proceeding simultaneously with the sinking of the sea floor to form the
neighboring deeps.
[Illustration: FIG. 91.—Volcanic cones formed in 1783 above the
Skaptár fissure in Iceland (after Helland).]
[Illustration:
FIG. 92.—Diagrams to illustrate the location of volcanic vents upon
fissure lines, _a_, openings caused by lateral movement of fissure
walls; _b_, openings formed at fissure intersections.]
=Arrangement of volcanic vents along fissures and especially at their
intersections.=—Within those districts in which volcanoes are widely
separated from their neighbors, the law of their arrangement is
difficult to decipher, but the view that volcanic vents are aligned
over fissures is now supported by so much evidence that illustrations
may be supplied from many regions. An exceptionally perfect line of
small cones is found along the Skaptár cleft in Iceland, upon which
stands the large volcano of Laki. This fissure reopened in 1783, and
great volumes of lava were exuded. Over the cleft there was left a long
line of volcanic cones (Fig. 91). There are in Iceland two dominating
series of parallel fissures of the same character which take their
directions respectively northeast-southwest and north-south. Many such
fissures are traceable at the surface as deep and nearly straight
clefts or _gjás_, usually a few yards in width, but extending for many
miles. The Eldgjá has a length of more than 18 English miles and a
depth varying from 400 to 600 feet. On some of these fissures no lava
has risen to the surface, whereas others have at numerous points exuded
molten rock. Sometimes one end only of a fissure, the more widely
gaping portion, has supplied the conduits for the molten lava. This
is well illustrated by the cratered monticules raised by the common
ant over the cracks which separate the blocks of cement sidewalk, the
hillocks being located where the most favorable channel was found for
the elevation of the materials.
[Illustration:
FIG. 93.—Outline map of the eastern portion of the island of Java,
displaying the arrangement of volcanic vents in alignment upon fissures
with the larger mountains at fissure intersections (after Verbeek).]
Those places upon fissures which become lava conduits appear to be the
ones where the cleft gapes widest so as to furnish the widest channel.
Wherever a differential lateral movement of the walls has occurred,
openings will be found in the neighborhood of each minor variation from
a straight line (Fig. 92_a_). Wherever there are two or more series
of fissures, and this would appear to be the normal condition, places
favorable for lava conduits occur at fissure intersections. Within such
veritable volcano gardens as are to be found in Malaysia, the law of
volcano distribution became apparent so soon as accurate maps had been
prepared. Thus the outline map of a portion of the island of Java (Fig.
93) shows us that while the volcanoes of the island present at first
sight a more or less irregular band or zone, there are a number of
fissures intersecting in a network, and that the volcanoes are aligned
upon the fissures with the larger cones located at the intersections.
So also in Iceland, the great eruption of Askja in 1875 occurred at
the intersection of two lines of fissure.
Outside these closely packed volcanic regions, similar though less
marked networks are indicated; as, for example, in and near the Gulf of
Guinea. If now, instead of reducing the scale of our volcano maps, we
increase it, the same law of distribution is no less clearly brought
out. The monticules or small volcanic cones which form upon the flanks
of larger volcanic mountains are likewise built up over fissures which
on numerous occasions have been observed to open and the cones to form
upon them.
[Illustration:
FIG. 94.—Map of the Puy Pariou in the Auvergne of central France. The
seat of eruption has migrated along the fissure upon which the earlier
cone had been built up (after Scrope).]
Still further reducing now the area of our studies and considering for
the moment the “frozen” surface of the boiling lava within the caldron
of Kilauea, this when observed at night reveals in great perfection
the sudden formation of fissures in the crust with the appearance
of miniature volcanoes rising successively at more or less regular
intervals along them.
It not infrequently happens that after a volcanic vent has become
established above some conduit in a fissure, the conduit migrates along
the fissure, thus establishing a new cone with more or less complete
destruction of the old one (Fig. 94).
=The so-called fissure eruptions.=—The grandest of all volcanic
eruptions have been those in which the entire length and breadth of
the fissures have been the passageway for the upwelling lava. Such
grander eruptions have been for the most part prehistoric, and in later
geologic history have occurred chiefly in India, in Abyssinia, in
northwestern Europe, and in the northwestern United States. In western
India the singularly horizontal plateaus of basaltic lava, the Dekkan
traps, cover some 200,000 square miles and are more than a mile in
depth. The underlying basement where it appears about the margins of
the basalt is in many places intersected by dikes or fissure fillings
of the same material. No cones or definite vents have been found.
[Illustration:
FIG. 95.—Basaltic plateau of the northwestern United States due to
fissure eruptions of lava.]
The larger portion of the northwestern British Isles would appear to
have been at one time similarly blanketed by nearly horizontal beds of
basaltic lava, which beds extended northwestward across the sea through
the Orkney and Faroe islands to Iceland. Remnants of this vast plateau
are to-day found in all the island groups as well as in large areas of
northeastern Ireland, and fissure fillings of the same material occur
throughout large areas of the British Isles. In many cases these dikes
represent once molten rock which may never have communicated with the
surface at the time of the lava outpouring, yet they well illustrate
what we might expect to find if the basalt sheets of Iceland or Ireland
were to be removed.
The floods of basaltic lava which in the northwestern United States
have yielded the barren plateau of the Cascade Mountains (Fig. 95)
would appear to offer another example of fissure eruption, though cones
appear upon the surface and perhaps indicate the position of lava
outlets during the later phases of the eruptive period. The barrenness
and desolation of these lava plains is suggested by Fig. 96.
[Illustration: FIG. 96.—Lava plains about the Snake River in Idaho.]
Though the greater effusions of lava have occurred in prehistoric
times, and the manner of extrusion has necessarily been largely
inferred from the immense volume of the exuded materials and the
existence of basaltic dikes in neighboring regions, yet in Iceland
we are able to observe the connection between the dikes and the lava
outflows. Professor Thoroddsen has stated that in the great basaltic
plateau of Iceland, lava has welled out quietly from the whole length
of fissures and often on both sides without giving rise to the
formation of cones. At three wider portions of the great Eld cleft,
lava welled out quietly without the formation of cones, though here in
the southern prolongation of the fissure, where it was narrower, a row
of low slag cones appeared. Where the lava outwellings occurred, an
area of 270 square miles was flooded.
[Illustration: FIG. 97.—Characteristic profiles of lava volcanoes. 1,
basaltic lava mountain; 2, mountain of siliceous lava (after Judd).]
=The composition and the properties of lava.=—In our study of igneous
rocks (Chapter IV) it was learned that they are composed for the most
part of silicate minerals, and that in their chemical composition they
represent various proportions of silica, alumina, iron, magnesia,
lime, potash, and soda. The more abundant of these constituents is
silica, which varies from 35 to 70 per cent of the whole. Whenever the
content of silica is relatively low,—basic or basaltic lava,—the
cooled rock is dark in color and relatively heavy. It melts at a
relatively low temperature, and is in consequence relatively fluid
at the temperatures which lavas usually have on reaching the earth’s
surface. Furthermore, from being more fluid, the water which is nearly
always present in large quantity within the lava more readily makes its
escape upon reaching the surface. Eruptions of such lava are for this
reason without the violent aspects which belong to extrusions of more
siliceous (more “acidic”) lavas. For the same reason, also, basaltic
lava flows more freely and can spread much farther before it has
cooled sufficiently to consolidate. This is equivalent to saying that
its surface will assume a flatter angle of slope, which in the case
of basaltic lava seldom exceeds ten degrees and may be less than one
degree (Fig. 97).
[Illustration: FIG. 98.—A driblet cone (after J. D. Dana).]
Siliceous lavas, on the other hand, are, when consolidated,
relatively light both in color and weight and melt at relatively high
temperatures. They are, therefore, usually but partly fused and of a
viscous consistency when they arrive at the earth’s surface. Because
of this viscosity they offer much resistance to the liberation of the
contained water, which therefore is released only to the accompaniment
of more or less violent explosions. The lava is blown into the air
and usually falls as consolidated fragments of various degrees of
coarseness.
[Illustration:
FIG. 99.—View of Leffingwell crater, a cinder cone in the Owens
valley, California (after an unpublished photograph by W. D. Johnson).]
It must not, however, be assumed that the temperature of lava is always
the same when it arrives at the surface, and hence it may happen that
a siliceous lava is exuded at so high a temperature that it behaves
like a normal basaltic lava. On the other hand, basaltic lavas may be
extruded at unusually low temperatures, in which case their behavior
may resemble that of the normal siliceous lavas. If, however, as is
generally the case, the energy of explosion of a basaltic lava is
relatively small, any ejected portions of the liquid lava travel to
a moderate height only in the air, so that on falling they are still
sufficiently pasty to adhere to rock surfaces and thus build up the
remarkably steep cones and spines known as “spatter cones” or “driblet
cones” (Fig. 98). When, on the other hand, the energy of explosion is
great, as is normally the case with siliceous lavas, the portions of
ejected lava have been fully consolidated before their fall to the
surface, so that they build up the same type of accumulation as would
sand falling in the same manner. The structures which they form are
known as tuff, cinder, or ash cones (Fig. 99).
Whenever the contained water passes off from siliceous lavas without
violent explosions, the lava may flow from the vent, but in contrast to
basaltic lavas it travels a short distance only before consolidating.
The resulting mountain is in consequence proportionately high and steep
(Fig. 97). Eruptions characterized by violent explosions accompanied by
a fall of cinder are described as _explosive_ eruptions. Those which
are relatively quiet, and in which the chief product is in the form of
streams of flowing lava, are spoken of as _convulsive_ eruptions.
=The three main types of volcanic mountain.=—If the eruptions at a
volcanic vent are exclusively of the explosive type, the material
of the mountain which results is throughout tuff or cinder, and the
volcano is described as a _cinder cone_. If, on the other hand, the
vent at every eruption exudes lava, a mountain of solid rock results
which is a _lava dome_. It is, however, the exception for a volcano
which has a long history to manifest but a single kind of eruption. At
one time exuding lava comparatively quietly, at another the violence
with which the steam is liberated yields only cinder, and the mountain
is a composite of the two materials and is known as a _composite
volcanic cone_.
=The lava dome.=—When successive lava flows come from a crater, the
structure which results has the form of a more or less perfect dome. If
the lava be of the basaltic or fluid type, the slopes are flat, seldom
making an angle of as much as ten degrees with the horizon and flatter
toward the summit (Fig. 101, p. 106). If of siliceous or viscous lava,
on the other hand, the slopes are correspondingly steep and in some
cases precipitous. To this latter class belong some of the _Kuppen_ of
Germany, the _puys_ of central France, and the _mamelons_ of the Island
of Bourbon.
[Illustration:
FIG. 100.—Map of Hawaii and the lava volcanoes of Mokuaweoweo (Mauna
Loa) and Kilauea (after the government map by Alexander).]
=The basaltic lava domes of Hawaii.=—At the “crossroads of the
Pacific” rises a double line of lava volcanoes which reach from 20,000
to 30,000 feet above the floor of the ocean, some of them among
the grandest volcanic mountains that are known. More than half the
height and a much larger proportion of the bulk of the largest of
these are hidden beneath the ocean’s surface. The two great active
vents are Mokuaweoweo (on Mauna Loa) and Kilauea, distinct volcanoes
notwithstanding the fact that their lava extravasations have been
merged in a single mass. The rim of the crater of Mauna Loa is at an
elevation of 13,675 feet above the sea, whereas that of Kilauea is
less than 4000 feet and appears to rest upon the flank of the larger
mountain (Figs. 100 and 101). Although one crater is but 20 miles
distant from the other and nearly 10,000 feet lower, their eruptions
have apparently been unsympathetic. Nowhere have still active lava
mountains been subjected to such frequent observations extending
throughout a long period, and the dynamics of their eruptions are
fairly well understood. To put this before the reader, it will be
best to consider both mountains, for though they have much in common,
the observations from one are strangely complementary to those of the
other. The lower crater being easily accessible, Kilauea has been often
visited, and there exists a long series of more or less consecutive
observations upon it, which have been assembled and studied by Dana and
Hitchcock. The place of outflow of the Kilauea lavas has not generally
been visible, whereas Mokuaweoweo has slopes rising nearly 14,000 feet
above the sea and displays the records of outflow of many eruptions,
some of which were accompanied by the grandest of volcanic phenomena.
[Illustration: FIG. 101.—Section through Mauna Loa and Kilauea.]
=Lava movements within the caldron of Kilauea.=—The craters of these
mountains are the largest of active ones, each being in excess of
seven miles in circumference. In shape they are irregularly elliptical
and consist of a series of steps or terraces descending to a pit at the
bottom, in which are open lakes of boiling lava. Enough is known of the
history of Kilauea to state that the steep cliffs bounding the terraces
are fault walls produced by inbreak of a frozen lava surface. The cliff
below the so-called “black ledge” was produced by the falling in of
the frozen lava surface at the time of the outflow of 1840, the lava
issuing upon the eastern flank of the mountain and pouring into the sea
near Nanawale. Since that date the floor of the pit below the level of
this ledge has been essentially a movable platform of frozen lava of
unknown and doubtless variable thickness which has risen and descended
like the floor of an elevator car between its guiding ways (Fig. 102).
The floor has, however, never been complete, for one or more open lakes
are always to be seen, that of Halemaumau located near the southwestern
margin having been much the most persistent. Within the open lakes the
boiling lava is apparently white hot at the depth of but a few inches
below the surface, and in the overturnings of the mass these hotter
portions are brought to the surface and appear as white streaks marking
the redder surface portions. From time to time the surface freezes
over, then cracks open and erupt at favored points along the fissures,
sending up jets and fountains of lava, the material of which falls in
pasty fragments that build up driblet cones. Small fluid clots are
shot out, carrying a threadlike line of lava glass behind them, the
well-known “Pelé’s hair.” Sometimes the open lakes build up congealed
walls, rising above the general level of the pit, and from their rim
the lava spills over in cascades to spread out upon the frozen floor,
thus increasing its thickness from above (Fig. 103). At other times
a great dome of lava has been pushed up from the pit of Halemaumau
under a frozen shell, the molten lava shining red through cracks in
its surface and exuding so as to heal each widely opened fissure as it
forms.
[Illustration:
FIG. 102.—Schematic diagram to illustrate the moving platform of
frozen lava which rises and falls in the crater of Kilauea.]
At intervals of from a few years to nine or ten years the crater has
been periodically drained, at which times the moving platform of
frozen lava has sunk more or less rapidly to levels far below the black
ledge and from 900 to 1700 feet below the crater rim. Following this
descent a slow progressive rise is inaugurated, which has sometimes
gone on at a rate of more than a hundred feet per year, though it is
usually much slower than this. When the platform has reached a height
varying from 700 to 350 feet below the crater rim, another sudden
settlement occurs which again carries the pit floor downward a distance
of from 300 to 700 feet.
[Illustration:
FIG. 103.—View of the open lava lake of Halemaumau within the crater
of Kilauea, the molten lava shown cascading over the raised lava walls
on to the floor of the pit (after Pavlow).]
=The draining of the lava caldrons.=—The changes which go on within
the crater of Mokuaweoweo, though less studied than those of Kilauea,
appear to be in some respects different. Here every eruption seems
to be preceded by a more or less rapid influx of melted lava to the
pit of the crater, this phenomenon being observed from a distance as
a brilliant light above the crater—the reflection of the glow from
overhanging vapor clouds. The uprising of the lava has often been
accompanied by the formation of high lava fountains upon the surface,
and the molten lava sometimes appears in fissures near the crater rim
at levels well above the lava surface within the pit.
Although in many cases the lava which has thus flooded the crater has
suddenly drained away without again becoming visible, it is probable
that in such cases an outlet has been found to some submarine exit,
since under-ocean discharge effects have been observed in connection
with eruptions of each of the volcanoes.
[Illustration:
FIG. 104.—Map showing the manner of outflow of lava from Kilauea
during the eruption of 1840. The outflowing lava made its appearance
successively at the points _A_, _B_, _C_, _m_, _n_, and finally at a
point below _n_, from whence it issued in volume and flowed down to the
sea at Nanawale (after J. D. Dana).]
Inasmuch as no earthquakes are felt in connection with such outflows
as have been described, it is probable that the hot lava fuses a
passageway for itself into some open channel underneath the flanks
of the mountain. Such a course is well illustrated by the outflow
of Kilauea in 1840, when, it will be remembered, occurred the great
down-plunge of the crater that yielded the pit below the black ledge.
At this time the lava first made its appearance upon the flanks of the
mountain at the bottom of a small pit or inbreak crater which opened
five miles southeast of the main crater of Kilauea (Fig. 104). Within
this new crater the lava rose, and small ejections soon followed from
fissures formed in its neighborhood. Some time after, the lava sank
in the first new crater, only to reappear successively at other small
openings (Fig. 104, _B_, _C_, _m_, _n_) and finally to issue in volume
at a point eleven miles from the shore and flow thereafter _upon
the surface_ of the mountain until it had reached the sea. Only the
slightest earth tremors were felt, and as no rumblings were heard, it
is evident that the lava fused its way along a buried channel largely
open at the time (see below, p. 112).
In a majority of the eruptions of Mokuaweoweo, when the outflowing
lavas have become visible, the molten rock has apparently fused its way
out to the surface of the mountain at points from 1000 to 3000 feet
below the bottom of the crater, and this discharge has corresponded
in time to the lowering of the lava surface within the crater. There
are, however, three instances upon record in which the lava issued
from definite rents which were formed upon the mountain flanks at
comparatively low levels. In contrast to the formation of fused
outlets, these ruptures of a portion of the mountain’s flank were
always accompanied by vigorous local earthquakes of short duration. In
one instance (the eruption of 1851) such a rent appeared under the same
conditions but at an elevation of 12,500 feet, or near the level of the
lava in the crater.
=The outflow of the lava floods.=—In order to properly comprehend
these and many otherwise puzzling phenomena connected with
volcanoes, it is necessary to keep ever in mind the quite remarkable
heat-insulating property of congealed lava. So soon as a thin crust
has formed upon the surface of molten rock, the heat of the underlying
fluid mass is given off with extreme slowness, so that lava streams no
longer connected with their internal lava reservoirs may remain molten
for decades.
[Illustration:
FIG. 105.—Lava of Matavanu upon the Island of Savaii flowing down to
the sea during the eruption of 1906. The course may be followed by the
jets of steam escaping from the surface down to the great steam cloud
which rises where the fluid lava discharges into the sea (after H. I.
Jensen).]
We have seen that for Mokuaweoweo and Kilauea, lava either quietly
melts its way to the surface at the time of outflow, or else produces a
rent for its egress to the accompaniment of vigorous local earthquakes.
In either case if the lava issues at a point far below the crater,
gigantic lava fountains arise at the point of outflow, the fluid rock
shooting up to heights which range from 250 to 600 or more feet above
the surface. A certain proportion of this fluid lava is sufficiently
cooled to consolidate while traveling in the air, and falling, it
builds up a cinder cone which is left as a location monument for
the place of discharge. From this outlet the molten lava begins its
journey down the slope of the mountain, and quickly freezes over to
produce a tunnel, beneath the roof of which the fluid lava flows with
comparatively slow further loss of heat. Save for occasional steam jets
issuing from its surface, it may give little indication of its presence
until it has reached the sea (Fig. 105).
[Illustration: FIG. 106.—Lava stream discharging into the sea from
beneath the frozen roof of a lava tunnel. Eruption of Matavanu on
Savaii in 1906 (after Sapper).]
If sufficient in volume and the shore be not too distant, the stream
of lava arrives at the sea, where, discharging from the mouth of its
tunnel, it throws up vast volumes of steam and induces ebullition of
the water over a wide area (Fig. 106). Professor Dana, who visited
Hawaii a few months only after the great outflow of 1840, states that
the lava, upon reaching the ocean, was shivered like melted glass and
thrown up in millions of particles which darkened the sky and fell like
hail over the surrounding country. The light was so bright that at a
distance of forty miles fine print could be read at midnight.
[Illustration:
FIG. 107.—Diagrammatic representation of the structure of the flanks
of lava volcanoes as a result of the draining of frozen lava streams.]
Protected from any extensive consolidation by its congealed cover, the
lava within a stream may all drain away, leaving behind an empty lava
tunnel, which in the case of the Hawaiian volcanoes sometimes has its
roof hung with beautiful lava stalactites and its floor studded with
thin lava spines. Later lava outflows over the same or neighboring
courses bury such tunnels beneath others of similar nature, giving
to the mountain flanks an elongated cellular structure illustrated
schematically in Fig. 107. These buried channels may in the future be
again utilized for outflows similar in character to that of Kilauea in
1840.
[Illustration:
FIG. 108.—Diagram to show the manner of formation of mesas or table
mountains by the outflow of lava in valleys and the subsequent more
rapid erosion of the intervening ridges. _R_, earlier river valley;
_R’R’_, later valleys.]
While the formation of lava stalactites of such perfection and beauty
is peculiar to the Hawaiian lava tunnels, the formation of the tunnel
in connection with lava outflow is the rule wherever a dissipation at
the end has permitted of drainage. A few hours only after the flow has
begun, the frozen surface has usually a thickness of a few inches, and
this cover may be walked over with the lava still molten below. At
first in part supported by the molten lava, the tunnel roof sometimes
caves in so soon as drainage has occurred.
[Illustration: FIG. 109.—Surface of lava of the Pahoehoe type.]
Wherever basaltic lava has spread out in valleys on the surface of
more easily eroded material, either cinder or sedimentary formations,
the softer intervening ridges are first carried away by the eroding
agencies, leaving the lava as cappings upon residual elevations.
Thus are derived a type of table mountain or _mesa_ of the sort well
illustrated upon the western slopes of the Sierra Nevadas in California
(Fig. 108).
[Illustration:
FIG. 110.—Three successive views to illustrate the growth of the
Island of Savaii from the outflow of lava at Matavanu in the year 1906.
_a_, near the beginning of the outflow; _b_, some weeks later than _a_;
_c_, some weeks later than _b_ (after H. I. Jensen).]
The surface which flowing lava assumes, while subject to considerable
variation, may yet be classified into two rather distinct types. On
the one hand there is the billowy surface in which ellipsoidal or
kidney-shaped masses, each with dimensions of from one to several feet,
lie merged in one another, not unlike an irregular collection of sofa
pillows. This type of lava has become known as the _Pahoehoe_, from
the Hawaiian occurrence (Fig. 109). A variation from this type is the
“corded” or “ropy” lava, the surface of which much resembles rope as it
is coiled along the deck of a vessel, the coils being here the lines of
scum or scoriæ arranged in this manner by the currents at the surface
of the stream (Fig. 123, p. 124). A quite different type is the block
lava (_Aa_ type) which usually has a ragged scoriaceous surface and
consists of more or less separate fragments of cooled lava (Fig. 131,
p. 130).
Wherever lava flows into the sea in quantity, it extends the margin of
the shore, often by considerable areas. The outflow of Kilauea in 1840
extended the shore of Hawaii outward for the distance of a quarter of a
mile, and a more recent illustration of such extension of land masses
is furnished by Fig. 110.