THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE
VOLCANIC MOUNTAINS OF EJECTED MATERIALS
=The mechanics of crater explosions.=—If we now turn from the lava
volcano to the active cinder cone, we encounter an entire change of
scene. In place of the quiet flow and convulsive movements of the
molten lava, we here meet with repeated explosions of greater or less
violence. If we are to profitably study the manner of the explosions,
considering the volcanic vent as a great experimental apparatus, it
would be well to select for our purpose a volcano which is in a not too
violent mood. The well-known cinder cone of Stromboli in the Eolian
group of islands north of Sicily has, with short and unimportant
interruptions, remained in a state of light explosive activity since
the beginning of the Christian era. Rising as it does some three
thousand feet directly out of the Mediterranean, and displaying by day
a white steam cap and an intermittent glow by night, its summit can be
seen for a distance of a hundred miles at sea and it has justly been
called the “Lighthouse of the Mediterranean.” The “flash” interval
of this beacon may vary from one to twenty minutes, and it may show,
furthermore, considerable variation of intensity.
For the reason that the crater of the mountain is located at one side
and at a considerable distance below the actual summit, the opportunity
here afforded of looking into the crater is most favorable whenever
the direction of the wind is such as to push aside the overhanging
steam cloud (Fig. 111). Long ago the Italian vulcanologist Spallanzani
undertook to make observations from above the crater, and many others
since his day have profited by his example.
Within the crater of the volcano there is seen a lava surface lightly
frozen over and traversed by many cracks from which vapor jets
are issuing. Here, as in the Kilauea crater, there are open pools
of boiling lava. From some of these, lava is seen welling out to
overflow the frozen surface; from others, steam is ejected in puffs
as though from the stack of a locomotive. Within others lava is seen
heaving up and down in violent ebullition, and at intervals a great
bubble of steam is ejected with explosive violence, carrying up with
it a considerable quantity of the still molten lava, together with
its scumlike surface, to fall outside the crater and rattle down
the mountain’s slope into the sea. Following this explosion the
lava surface in the pool is lowered and the agitation is renewed,
to culminate after the further lapse of a few minutes in a second
explosion of the same nature. The rise of the lava which precedes the
ejection appears at night as a brighter reflection or glow from the
overhanging steam cloud—the flash seen by the mariner from his vessel.
[Illustration:
FIG. 111.—The volcano of Stromboli, showing the excentric position of
the crater (after a sketch by Judd).]
What is going on within the crater of Stromboli we may perhaps best
illustrate by the boiling of a stiff porridge over a hot fire. Any one
who has made corn mush over a hot camp fire is fully aware that in
proportion as the mush becomes thicker by the addition of the meal, it
is necessary to stir the mass with redoubled vigor if anything is to be
retained within the kettle. The thickening of the mush increases its
viscosity to such an extent that the steam which is generated within it
is unable to make its escape unless aided by openings continually made
for it by the stirring spoon. If the stirring motion be stopped for a
moment, the steam expands to form great bubbles which soon eject the
pasty mass from the kettle.
For the crater of Stromboli this process is illustrated by the series
of diagrams in Fig. 112. As the lava rises toward the surface,
presumably as a result of convectional currents within the chimney of
the volcano, the contained steam is relieved from pressure, so that
at some depth below the surface it begins to separate out in minute
vesicles or bubbles, which, expanding as they rise, acquire a rapidly
accelerating velocity. Soon they flow together with a quite sudden
increase of their expansive energy, and now shooting upward with
further accelerated velocity, a layer of liquid lava with its cover of
scum is raised on the surface of a gigantic bubble and thrown high into
the air. Cooled during their flight, the quickly congealed lava masses
become the tuff or volcanic ash which is the material of the cinder
cone.
[Illustration: FIG. 112.—Diagrams to illustrate the nature of
eruptions within the crater of Stromboli.]
=Grander volcanic eruptions of cinder cones.=—Most cinder and
composite cones, in the intervals between their grander eruptions, if
not entirely quiescent, lapse into a period, of light activity during
which their crater eruptions appear to be in all essential respects
like the habitual explosions within the Strombolian crater. This phase
of activity is, therefore, described as _Strombolian_. By contrast,
the occasional grander eruptions which have punctuated the history
of all larger volcanoes are described in the language of Mercalli as
_Vulcanian_ eruptions, from the best studied example.
Just what it is that at intervals brings on the grander Vulcanian
outburst within a volcano is not known with certainty; but it is
important to note that there is an approach to periodicity in the
grander eruptions. It is generally possible to distinguish eruptions of
at least two orders of intensity greater than the Strombolian phase;
a grander one, the examples of which may be separated by centuries,
and one or more orders of relatively moderate intensity which recur
at intervals perhaps of decades, their time intervals subdividing the
larger periods marked off by the eruptions of the first order.
[Illustration:
FIG. 113.—Map of Volcano in the Eolian group of islands. The smaller
craters partially dissected by the waves belong to Vulcanello (after
Judd).]
=The eruption of Volcano in 1888.=—In the Eolian Islands to the north
of Sicily was located the mythical forge of Vulcan. From this locality
has come our word “volcano”, and both the island and the mountain bear
no other name to-day (Fig. 113). There is in the structure of the
island the record of a somewhat complex volcanic history, but the form
of the large central cinder cone was, according to Scrope, acquired
during the eruption of 1786, at which time the crater is reported to
have vomited ash for a period of fifteen days. Passing after this
eruption into the solfatara condition, with the exception of a light
eruption in 1873, the volcano remained quiet until 1886. So active
had been the fumeroles within the crater during the latter part of
this period that an extensive plant had been established there for
the collection especially of boracic acid. In 1886 occurred a slight
eruption, sufficient to clear out the bottom of the crater, though
not seriously to disturb the English planter whose vineyards and fig
orchards were in the valley or _atrio_ near the point _d_ upon the map
(Fig. 113), nearly a mile from the crater rim. On the 3d of August,
1888, came the opening discharge of an eruption, which, while not of
the first order of magnitude, was yet the greatest in more than a
century of the mountain’s history, and may serve us to illustrate the
Vulcanian phase of activity within a cinder cone. During the day, to
the accompaniment of explosions of considerable violence, projectiles
fell outside the crater rim and rolled down the steep slopes toward the
_atrio_. These explosions were repeated at intervals of from twenty to
thirty minutes, each beginning in a great upward rush of steam and
ash, accompanied by a low rumbling sound. During the following night
the eruptions increased in violence, and the anxious planter remained
on watch in his villa a mile from the crater. Falling asleep toward
morning, he was rudely awakened by a rain of projectiles falling upon
his roof. Hastily snatching up his two children he ran toward the door
just as a red hot projectile, some two feet in diameter, descended
through the roof, ceiling, and floor of the drawing room, setting fire
to the building. A second projectile similar to the first was smashed
into fragments at his feet as he was emerging from the house, burning
one of the children. Making his escape to Vulcanello at the extremity
of the island, the remainder of the night and the following day, until
rescue came from Lipari, were spent just beyond the range of the
falling masses.
[Illustration:
FIG. 114.—“Bread-crust” lava projectile from the eruption of Volcano
in 1888 (after Mercalli).]
When the writer visited the island some months later, the eruption was
still so vigorous that the crater could not be reached. The ruined
villa, smashed and charred, stood with its walls half buried in ash and
lapilli, among which were partly smashed pumiceous lava projectiles.
The entire _atrio_ about the mountain lay buried in cinder to the depth
of several feet and was strewn with projectiles which varied in size
from a man’s fist to several feet in diameter (Fig. 114). The larger of
these exhibited the peculiar “bread-crust” surface and had generally
been smashed by the force of their fall after the manner of a pumpkin
which has been thrown hard against the ground. One of these projectiles
fully three feet in diameter was found at the distance of a mile and
a half from the crater. Though diminished considerably in intensity,
the rhythmic explosions within the crater still recurred at intervals
varying from four minutes to half an hour, and were accompanied
by a dull roar easily heard at Lipari on a neighboring island six
miles away. Simultaneously, a dark cloud of “smoke”, the peculiar
“cauliflower cloud” or _pino_ mounted for a couple of miles above the
crater (Fig. 115), and the rise was succeeded by a rain of small lava
fragments or _lapilli_ outside the crater rim.
[Illustration:
FIG. 115.—Peculiar “cauliflower cloud” or _pino_ composed of steam and
ash, rising above the cinder cone of Volcano during the waning phases
of the explosive eruption of 1888 (after a photograph by B. Hobson).]
There seems to be no good reason to doubt that Vulcanian cinder
eruptions of this type differ chiefly in magnitude from the rhythmic
explosion within the crater of Stromboli, if we except the elevation of
a considerable quantity of accessory and older tuff which is derived
from the inner walls of the crater and carried upward into the air
together with the pasty cakes of fresh lava derived from the chimney.
It is this accessory material which gives to the _pino_ its dark or
even black appearance.
[Illustration:
FIG. 116.—Double explosive eruption of Taal volcano on the morning of
January 30, 1911.]
=The eruption of Taal volcano on January 30, 1911.=—The recent
eruption of the cinder cone known as Taal volcano is of interest,
not only because so fresh in mind, but because two neighboring vents
erupted simultaneously with explosions of nearly equal violence (Fig.
116). This Philippine volcano lies near the center of a lake some
fifteen miles in diameter and about fifty miles south of the city of
Manila. After a period of rest extending over one hundred and fifty
years, the symptoms of the coming eruption developed rapidly, and on
the morning of January 30 grand explosions of steam and ash occurred
simultaneously in the neighboring craters, and the condensed moisture
brought down the ash in an avalanche of scalding mud which buried the
entire island. Almost the entire population of the island, numbering
several hundreds, was literally buried in the blistering mud (Fig.
117); and the gases from the explosions carried to the distant shores
of the lake added to this number many hundred victims.
[Illustration:
FIG. 117.—The thick mud veneer upon the island of Taal (after a
photograph by Deniston).]
[Illustration: FIG. 118.—A pear-shaped lava projectile.]
The shocks which accompanied the explosions raised a great wave upon
the surface of the lake, which, advancing upon the shores, washed away
structures for a distance of nearly a half mile.
=The materials and the structure of cinder cones.=—Obviously the
materials which compose cinder cones are the cooled lava fragments of
various degrees of coarseness which have been ejected from the crater.
If larger than a finger joint, such fragments are referred to as
_volcanic projectiles_, or, incorrectly, as “volcanic bombs.” Of the
larger masses it is often true that the force of expulsion has not been
applied opposite the center of mass of the body. Thus it follows that
they undergo complex whirling motions during their flight, and being
still semiliquid, they develop curious pear-shaped or less regular
forms (Fig. 118). When crystals have already separated out in the lava
before its rise in the chimney of the volcano, the surrounding fluid
lava may be blown to finely divided volcanic dust which floats away
upon the wind, thus leaving the crystals intact to descend as a crystal
rain about the crater. Such a shower occurred in connection with the
eruption of Etna in 1669, and the black augite crystals may to-day be
gathered by the handful from the slopes of the Monti Rossi (Fig. 125,
p. 125).
[Illustration:
FIG. 119.—Artificial production of the structure of a cinder cone with
use of colored sands carried up in alternation by a current of air
(after G. Linck).]
The term _lapilli_, or sometimes _rapilli_, is applied to the ejected
lava fragments when of the average size of a finger joint. This is the
material which still partially covers the unexhumed portions of the
city of Pompeii. Volcanic _sand_, _ash_, and _dust_ are terms applied
in order to increasingly fine particles of the ejected lava. The
finest material, the volcanic dust, is often carried for hundreds and
sometimes even for thousands of miles from the crater in the high-level
currents of the atmosphere. Inasmuch as this material is deposited far
from the crater and in layers more or less horizontal, such material
plays a small rôle in the formation of the cinder cone. The coarser
sands and ash, on the other hand, are the materials from which the
cinder cone is largely constructed.
The manner of formation and the structure of cinder cones may be
illustrated by use of a simple laboratory apparatus (Fig. 119). Through
an opening in a board, first white and then colored sand is sent up in
a light current of air or gas supplied from suitable apparatus. The
alternating layers of the sand form in the attitudes shown; that is
to say, dipping inward or toward the chimney of the volcano at all
points within the crater rim, and outward or away from it at all points
outside (Fig. 119). If the experiment is carried so far that at its
termination sand slides down the crater walls into the chimney below,
the inward dipping layers will be truncated, or even removed entirely,
as shown in Fig. 119 _b_.
[Illustration:
FIG. 120.—Diagram to show the contrast between a lava dome and a
cinder cone. _AAA_, cinder cone; _BabC_, lava dome; _DE_, line of low
cinder cones above a fissure (after Thoroddsen).]
=The profile lines of cinder cones.=—The shapes of cinder cones are
notably different from those of lava mountains. While the latter are
domes, the mountains constructed of cinder are conical and have curves
of profile that are concave upward instead of convex (Fig. 120). In
the earlier stages of its growth the cinder cone has a crater which in
proportion to the height of the mountain is relatively broad (Fig. 99,
p. 104).
[Illustration:
FIG. 121.—Mayon volcano on the island of Luzon, P.I. A remarkably
perfect high cinder cone.]
Speaking broadly, the diameter of the crater is a measure of the
violence of the explosions within the chimney. A single series of short
and violent explosive eruptions builds a low and broad cinder cone.
A long-continued succession of moderately violent explosions, on the
other hand, builds a high cone with crater diameter small if compared
with the mountain’s altitude, and the profile afforded is a remarkably
beautiful sweeping curve (Fig. 121). Toward the summit of such a cone
the loose materials of which it is composed are at as steep an angle as
they can lie, the so-called angle of repose of the material; whereas
lower down the flatter slopes have been determined by the distribution
of the cinder during its fall from the air. When one makes the ascent
of such a mountain, he encounters continually steeper grades, with the
most difficult slope just below the crest.
[Illustration:
FIG. 122.—A series of breached cinder cones where the place of
eruption has migrated along the underlying fissure. The Puys Noir,
Solas, and La Vache in the Mont Dore Province of central France (after
Scrope).]
=The composite cone.=—The life histories of volcanoes are generally
so varied that lava domes and the pure types of cinder cones are less
common than volcanoes in which paroxysmal eruptions have alternated
with explosions, and where, therefore, the structure of the mountain
represents a composite of lava and cinder. Such composite cones possess
a skeleton of solid rock upon which have been built up alternate
sloping layers of cinder and lava. In most respects such cones stand in
an intermediate position between lava domes and cinder cones.
[Illustration:
FIG. 123.—The _bocca_ or mouth upon the inner cone of Mount Vesuvius
from which flowed the lava stream of 1872. This lava stream appears in
the foreground with its characteristic “ropy” surface.]
Regarded as a retaining wall for the lava which mounts in the chimney,
the cinder cone is obviously the weakest of all. Should lava rise in a
cinder cone without an explosion occurring, the cone is at once broken
through upon one side by the outwelling of the lava near the base. Thus
arises the characteristic _breached_ cone of horseshoe form (Fig. 122).
[Illustration:
FIG. 124.—A row of parasitic cones raised above a fissure which was
opened upon the flanks of Mount Etna during the eruption of 1892 (after
De Lorenzo).]
Quite in contrast with the weak cinder cone is the lava dome with
its rock walls and relatively flat slopes. Considered as a retaining
wall for lava it is much the strongest type of volcanic mountain, and
it is likely that the hydrostatic pressure of the lava within the
crater would seldom suffice to rupture the walls, were it not that
the molten rock first fuses its way into old stream tunnels buried
under the mountain slopes (see _ante_, p. 112). Composite cones have
a strength as retaining walls for lava which is intermediate between
that of the other types. Their Vulcanian eruptions of the convulsive
type are initiated by the formation of a rent or fissure upon the
mountain flanks at elevations well above the base, the opening of the
fissure being generally accompanied by a local earthquake of greater or
less violence.
From one or more such fissures the lava issues usually with sufficient
violence at the place of outflow to build up over it either an enlarged
type of driblet cone, referred to as a “mouth”, or _bocca_[1] (Fig.
123), or one or more cinder cones which from their position upon the
flanks of the larger volcano are referred to as _parasitic cones_ (Fig.
124). The lava of Vesuvius more frequently yields _bocchi_ at the place
of outflow, whereas the flanks of Etna are pimpled with great numbers
of parasitic cinder cones, each the monument to some earlier eruption
(Fig. 125).
[Illustration:
FIG. 125.—View looking toward the summit of Etna from a position upon
the southern flank near the village of Nicolosi. The two breached
parasitic cones seen behind this village are the Monti Rossi which were
thrown up in 1669 and from which flowed the lava which overran Catania
(after a photograph by Sommer).]
It is generally the case that a single eruption makes but a relatively
small contribution to the bulk of the mountain. From each new cone or
_bocca_ there proceeds a stream of lava spread in a relatively narrow
stream extending down the slopes (Fig. 126).
[Illustration:
FIG. 126.—Sketch map of Etna, showing the individual surface lava
streams (in black) and the tuff covered surface (stippled).]
=The caldera of composite cones.=—Because of the varied episodes
in the history of composite cones, they lack the regular lines
characteristic of the two simpler types. The larger number of the more
important composite cones have been built up within an outer crater of
relatively large diameter, the Somma cone or _caldera_, which surrounds
them like a gigantic ruff or collar. This caldera is clearly in most
cases at least the relic of an earlier explosive crater, after which
successive eruptions of lesser violence have built a more sharply
conical structure. This can only be interpreted to mean that most
larger and long-active volcanoes have been born in the grandest throes
of their life history, and that a larger or smaller lateral migration
of the vent has been responsible for the partial destruction of the
explosion crater. Upon Vesuvius we find the crescent-like rim of Monte
Somma; on Etna it is the Val del Bove, etc. It is this caldera of
composite cones which gave rise to the theory of the “elevation crater”
of von Buch (see _ante_, p. 95, and Fig. 127).
[Illustration:
FIG. 127.—Panum crater, showing the caldera and the later interior
cones (after Russell).]
=The eruption of Vesuvius in 1906.=—The volcano Vesuvius rises on the
shores of the beautiful bay of Naples only about ten miles distant
from the city of Naples. The mountain consists of the remnant of an
earlier broad-mouthed explosion crater, the Monte Somma, and an inner,
more conical elevation, the Monte Vesuvio. Before the eruption of 1906
this central cone was sharply conical and rose to a height of about
4300 feet above the surface of the bay, or above the highest point of
the ancient caldera. The base of this inner cone is at an elevation of
something less than half that of the entire mass, and is separated from
the encircling ring wall of the old crater by the _atrio_, to which
corresponds in height a perceptible shelf or _piano_ upon the slope
toward the bay of Naples (Fig. 128).
[Illustration:
FIG. 128.—View of Mount Vesuvius as it appeared from the Bay of Naples
shortly before the eruption of 1906. The horn to the left is Monte
Somma.]
An active composite cone like that of Vesuvius is for the greater part
of the time in the Strombolian condition; that is to say, light crater
explosions continue with varying intensity and interval, except when
the mountain has been excited to the periodic Vulcanian outbreaks with
which its history has been punctuated. The Strombolian explosions
have sufficient violence to eject small fragments of hot lava, which,
falling about the crater, slowly built up a rather sharp cone. The
period of Strombolian activity has, therefore, been called the
_cone-producing period_. Just before each new outbreak of the Vulcanian
type, the altitude of the mountain has, therefore, reached a maximum,
and since the larger explosive eruptions remove portions of this cone
at the same time that they increase the dimensions of the crater,
the Vulcanian stage in contrast to the other has been called the
_crater-producing period_. In this period, then, the material ejected
during the explosions does not consist solely of fresh lava cakes,
but in part of the older débris derived from the crater walls, whence
it is avalanched upon the chimney after each larger explosion. The
overhanging cloud, which during the Strombolian period has consisted
largely of steam and is noticeably white, now assumes a darker tone,
the “smoke” which characterizes the Vulcanian eruption.
[Illustration:
FIG. 129.—A series of consecutive sketches of the summit of the
Vesuvian cone, showing the modifications in its outline (after Sir
William Hamilton).]
On several historical occasions the cone of Vesuvius has been lowered
by several hundred feet, the greatest of relatively recent truncations
having occurred in 1822 and in 1906. Between Vulcanian eruptions the
Strombolian activity is by no means uniform, and so the upward growth
of the cone is subject to lesser interruptions and truncations (Fig.
129).
The Vesuvian eruption of 1906 has been selected as a type of the
larger Vulcanian eruption of composite cones, because it combined the
explosive and paroxysmal elements, and because it has been observed and
studied with greater thoroughness than any other. The latest previous
eruption of the Vulcanian order had occurred in 1872. Some two years
later the period of active cone building began and proceeded with
such rapidity that by 1880 the new cone began to appear above the rim
of the crater of 1872. From this time on occasional light eruptions
interrupted the upbuilding process, and as the repairs were not in
all cases completed before a new interruption, a nest of cones, each
smaller than the last, arose in series like the outdrawn sections of
an old-time spyglass. At one time no less than five concentric craters
were to be seen.
For a brief period in the fall of 1904 Vesuvius had been in almost
absolute repose, but soon thereafter the Strombolian crater explosions
were resumed. On May 25, 1905, a small stream of lava began to issue
from a fissure high up upon the central cone, and from this time on
the lava continued to flow down to the valley or _atrio_, separating
the inner cone from the caldera remnant of Monte Somma. Seen in the
night, this stream of lava appeared from Naples like a red hot wire
laid against the mountain’s side (Fig. 130). With gradual augmentation
of Strombolian explosions and increase in volume of the flowing lava
stream, the same condition continued until the first days of April in
1906. The flowing lava had then overrun the tracks of the mountain
railway and accumulated in considerable quantity within the _atrio_
(Fig. 131).
[Illustration:
FIG. 130.—Night view of Vesuvius from Naples before the outbreak of
1906. A small lava stream is seen descending from a high point upon the
central cone (after Mercalli).]
On the morning of April 4, a preliminary stage of the eruption was
inaugurated by the opening of a new radial fissure about 500 feet
below the summit of the cone (Fig. 132 _a_), and by early afternoon
the cone-destroying stage began with the rise of a dark “cauliflower
cloud” or _pino_ to replace the lighter colored steam cloud. The
cone was beginning to fall into the crater, and old lava débris was
mingled in the ejections with the lava clots blown from the still fluid
material within the chimney. From now on short and snappy lightning
flashes played about the black cloud, giving out a sharp staccato
“tack-a-tack.” The volume and density of the cloud and the intensity
of the crater explosions continued to increase until the culmination
on April 7. On April 5 at midnight a new lava mouth appeared upon
the same fissure which had opened near the summit, but now some 300
feet lower (Fig. 132 _b_). The lava now welled out in larger volume
corresponding to its greater head, and the stream which for ten months
had been flowing from the highest outlet upon the cone now ceased to
flow. The next morning, April 6, at about 8 o’clock, lava broke out at
several points some distance east of the opening _b_, and evidently
upon another fissure transverse to the first (Fig. 132 _c_). The lava
surface within the chimney must still have remained near its old
level,—effective draining had not yet begun,—since early upon the
following morning a small outflow began nearly at the top of the cone
upon the opposite side and at least a thousand feet higher.
[Illustration: FIG. 131.—Scoriaceous lava encroaching upon the tracks
of the Vesuvian railway (after a photograph by Sommer).]
[Illustration:
FIG. 132.—Map of Vesuvius, showing the position and order of formation
of the lava mouths upon its flanks during the eruption of 1906 (after
Johnston-Lavis).]
The culmination of the eruption came in the evening of April 7, when,
to the accompaniment of light earthquakes felt as far as Naples, lava
issued for the first time in great volume from a mouth more than
halfway down the mountain side (Fig. 132 _f_), and thus began the
drainage of the chimney. At about the same time with loud detonations
a huge black cloud rose above the crater in connection with heavy
explosions, and a rain of cinder was general in the region about the
mountain but especially within the northeast quadrant. Those who were
so fortunate as to be in Pompeii had a clear view of the mountain’s
summit where red hot masses of lava were thrown far into the air. The
direction of these projections was reported to have been not directly
upward, but inclined toward the northeast quadrant of the mountain; but
since with a northeast surface wind the heaviest deposit of ash and
dust should have been upon the southwestern quadrant of the mountain,
it is evident that the material was carried upward until it reached the
contrary upper currents of the atmosphere, to be by them distributed.
[Illustration: FIG. 133.—The ash curtain which had overhung Vesuvius
lifting and disclosing the outlines of the mountain on April 10, 1911
(after De Lorenzo).]
[Illustration:
FIG. 134.—The central cone of Vesuvius as it appeared after the
eruption of 1906, but with the earlier profile indicated. The
truncation represents a lowering of the summit by some five hundred
feet, with corresponding increase in the diameter of the crater (after
Johnston-Lavis).]
When the heavy curtain of ash, which now for a number of succeeding
days overhung all the circum-Vesuvian country, began to lift (Fig.
133), it was seen that the summit of the cone had been truncated
an average of some 500 feet (Fig. 134). All the slopes and much of
the surrounding country had the aspect of being buried beneath a
cocoa-colored snow of a depth to the northeastward of several feet,
where it had drifted into all the hollow ways so as almost to efface
them (Fig. 135). More than thrice as heavy as water, the weak roof
timbers of the houses at the base of the mountain gave way beneath the
added load upon them, thus making many victims. Inasmuch, however, as
the ash-fall partakes of the same general characters as in eruptions
from cinder cones, we may here give our attention especially to the
streams of lava which issued upon the opposite flank of the mountain
(Fig. 136).
[Illustration:
FIG. 135.—A sunken road filled with indrifted cocoa-colored ash from
the Vesuvian eruption of 1906.]
[Illustration:
FIG. 136.—View of Vesuvius taken from the southwest during the waning
stages of the eruption of 1906. In the middle distance may be discerned
the several lava mouths aligned upon a fissure, and the courses of the
streams which descend from them. In the foreground is the main lava
stream with scoriaceous surface (after W. Prinz).]
The main lava stream descended the first steep slopes with the velocity
of a mile in twenty-five minutes, about the strolling speed of a
pedestrian, but this rate was gradually reduced as the stream advanced
farther from the mouth. Taking advantage of each depression of the
surface, the black stream advanced slowly but relentlessly toward
the cities at the southwest base of the mountain. With a motion not
unlike that of a heap of coal falling over itself down a slope, the
block lava advances without burning the objects in its path (Fig.
137).
[Illustration:
FIG. 137.—The main lava stream of 1906 advancing upon the village of
Boscotrecase.]
[Illustration:
FIG. 138.—An Italian pine snapped off by the lava and carried forward
upon its surface as a passenger (after Haug).]
The beautiful pines are merely charred where snapped off and are
carried forward upon the surface of the stream (Fig. 138). When a real
obstruction, such as a bridge or a villa, is encountered, the stream is
at first halted, but the rear crowding upon the van, unless a passage
is found at the side, the lava front rises higher and higher until by
its weight the obstruction is forced to give way (Figs. 139 and 140).
[Illustration:
FIG. 139.—Lava front both pushing over and running around a wall which
lies athwart its course (after Johnston-Lavis).]
[Illustration:
FIG. 140.—One of the villas in Boscotrecase which was ruined by the
Vesuvian lava flow of 1906. The fragments of masonry from the ruined
walls traveled upon the lava current, where they sometimes became
incased in lava.]
=The sequence of events within the chimney.=—The thorough study of
this Vesuvian eruption has placed us in a position to infer with
some confidence in our conclusions the sequence of events within the
chimney and crater of the volcano, both before and during the eruption.
Anticipating some conclusions derived from the observed dissection of
volcanoes, which will be discussed below, it may be stated that what
might be termed the core of the composite cone—the chimney—is a more
or less cylindrical plug of cooled lava which during the active period
of the vent has an interior bore of probably variable caliber. This
plug in its lower section appears in solid black in all the diagrams
of Fig. 141. During the cone-building period (Fig. 141 _a_ and _b_)
the plug is obviously built upward along with the cone, for lava often
flows out at a level a few hundred feet only below the crater rim. By
what process this chimney building goes on is not well understood,
though some light is thrown upon it by the post-eruption stage of Mont
Pelé in 1902-1903 (see below).
[Illustration:
FIG. 141.—Three diagrams to illustrate the sequence of events
within the crater of a composite cone during the cone-building and
crater-producing periods. _a_ and _b_, two successive stages of the
cone building or Strombolian period; _c_, enlargement of the crater,
truncation of the cone, and destruction of the upper chimney during the
relatively brief crater-producing or Vulcanian period.]
Both the older and newer sections of this plug or chimney are furnished
some support against the outward pressure of the contained lava by
the surrounding wall of tuff; and they are, therefore, in a condition
not unlike that of the inner barrel of a great gun over which sleeves
of metal have been shrunk so as to give support against bursting
pressures. On the other hand, when not sustaining the hydrostatic
pressure of the liquid lava within, the chimney would tend to be
crushed in by the pressure of the surrounding tuff. Its strength to
withstand bursting pressures is dependent not alone upon the thickness
of its rock walls, but also upon its internal diameter or caliber. A
steam cylinder of given thickness of wall, as is well known, can resist
bursting pressures in proportion as its internal diameter is small. So
in the volcanic chimney, any tendency to remelt from within the chimney
walls must weaken them in a twofold ratio.
We are yet without accurate temperature observations upon the lava in
volcanic chimneys, but it seems almost certain that these temperatures
rise as the Vulcanian stage is approaching, and such elevation of
temperature must be followed by a greater or less re-fusion of the
chimney walls. The sequence of events during the late Vesuvian eruption
is, then, naturally explained by progressive re-fusion and consequent
weakening of the chimney walls, thus permitting a radial fissure to
open near the top and gradually extend downwards. Thus at first small
and high outlets were opened insufficient to drain the chimney, but
later, on April 7, after this fissure had been much extended and a new
and larger one had opened at a lower level, the draining began and the
surface of lava commenced rapidly to sink.
[Illustration: FIG. 142.—The spine of Pelé rising above the chimney of
the volcano after the eruption of 1902 (after Hovey).]
When the rapid sinking of the lava surface occurred, the lower lava
layers were almost immediately relieved of pressure, thus causing a
sudden expansion of the contained steam and resulting in grand crater
explosions. The partially refused and fissured upper chimney, now
unable to withstand the inward pressure of the surrounding tuff walls,
since outward pressures no longer existed, crushed in and contributed
its materials and those of the surrounding tuff to the fragments of
fresh lava rising in volume in the grand explosions (Fig. 141 _c_). In
outline, then, these seem to be the conditions which are indicated by
the sequence of observed events in connection with the late Vesuvian
outbreak.
[Illustration:
FIG. 143.—Outlines of the Pelé spine upon successive dates. The full
line represents its outline on December 26, 1902; the dotted-dashed
line is a profile of January 3, 1902; while the dotted line is that of
January 9, 1903. The dark line is a fissure (after E. O. Hovey).]
=The spine of Pelé.=—The disastrous eruption of Mont Pelé upon
Martinique in the year 1902 is of importance in connection with the
interesting problem of the upward growth of volcanic chimneys during
the cone-building period of a volcano. After the conclusion of this
great Vulcanian eruption, a spine of lava grew upward from the
chimney of the main crater until it had reached an elevation of more
then a thousand feet above its base, a figure of the same order of
magnitude as the probable height of the upper section of the Vesuvian
chimney previous to the eruption of 1906. The Pelé spine (Fig. 142)
did not grow at a uniform rate, but was subject to smaller or larger
truncations, but for a period of 18 days the upward growth was at the
rate of about 41 feet per day. Later, the mass split upon a vertical
plane revealing a concave inner surface, and was somewhat rapidly
reduced in altitude to 600 feet (Fig. 143), only to rise again to its
full height of about 1000 feet some three months later.
While apparently unique as an observed phenomenon, and not free from
uncertainty as to its interpretation, the growth of this obelisk has
at least shown us that a mass of rock can push its way up above the
chimney of an active volcano even when there are no walls of tuff about
it to sustain its outward pressures.
[Illustration:
FIG. 144.—Corrugated surface of the Vesuvian cone after the mud flows
which followed the eruption in 1906 (after Johnston-Lavis).]
=The aftermath of mud flows.=—When the late Vulcanian explosions of
Vesuvius had come to an end, all slopes of the mountain, but especially
the higher ones, were buried in thick deposits of the cocoa-colored
ash, included in which were larger and smaller projectiles. As this
material is extremely porous, it greedily sucks up the water which
falls during the first succeeding rains. When nearly saturated, it
begins to descend the slopes of the mountain and soon develops a
velocity quite in contrast with that of the slow-moving lava. The upper
slopes are thus denuded, while the fields and even the houses about the
base are invaded by these torrents of mud (_lava d’acqua_). Inasmuch as
these mud flows are the inevitable aftermath of all grander explosive
eruptions, the Italian government has of late spent large sums of
money in the construction of dikes intended to arrest their progress
in the future. It was streams of this sort that buried the city of
Herculaneum after the explosive eruption of 79 A.D.
After the mud flows have occurred, the Vesuvian cone, like all
similar volcanic cones under the same conditions, is found with deep
radial corrugations (Fig. 144), such as were long ago described as
“barrancoes” and supposed to support the “elevation crater” theory of
volcano formation.
=The dissection of volcanoes.=—To the uninitiated it might appear a
hopeless undertaking to attempt to learn by observation the internal
structure of a volcano, and especially of a complex volcano of the
composite type. The earliest successful attempt appears to have been
made by Count Caspar von Sternberg in order to prove the correctness
of the theory of his friend, the poet Goethe. Goethe had claimed that
a little hill in the vicinity of Eger, on the borders of Bohemia, was
an extinct volcano, though the foremost geologist of the time the
famous Werner, had promulgated the doctrine that this hill, in common
with others of similar aspect, originated in the combustion of a bed
of coal. The elevation in question, which is known as the Kammerbühl,
consists mainly of cinder, and Goethe had maintained that if a tunnel
were to be driven horizontally into the mountain from one of its
slopes, a core or plug of lava would be encountered beneath the summit.
The excavations, which were completed in 1837, fully verified the
poet’s view, for a lava plug was found to occupy the center of the mass
and to connect with a small lava stream upon the side of the hill (Fig.
145).
[Illustration:
FIG. 145.—The Kammerbühl near Eger, showing the tunnel completed in
1837 which proved the volcanic nature of the mountain (after Judd).]
It is not, however, to such expensive projects that reference is here
made, but rather to processes which are continually going on in nature,
and on a far grander scale. The most important dissecting agent for our
purpose is running water, which is continually paring down the earth’s
surface and disclosing its buried structures. How much more convincing
than any results of artificial excavation, as evidence of the internal
structure of a volcano, is the monument represented in Fig. 146,
since here the lava plug stands in relief like a gigantic thumb still
surrounded by a remnant of cinder deposits. Such exposed chimneys of
former volcanoes are found in many regions, and have become known as
volcanic _necks_, _pipes_, or _plugs_.
[Illustration: FIG. 146.—Volcanic plug exposed by natural dissection
of a volcanic cone in Colorado (U. S. G. S.).]
[Illustration:
FIG. 147.—A dike cutting beds of tuff in a partly dissected volcano of
southwestern Colorado (after Howe, U. S. G. S.).]
Not infrequently the beds of tuff composing the flanks of the volcano,
upon dissection by the same process, bring to light walls of cooled
lava standing in relief (Fig. 147)—the filling of the fissure which
gave outlet to the flanks of the mountain at the time of the eruption.
Study of exposed dikes formed in connection with recent eruptions of
Vesuvius has shown that in many instances they are still hollow, the
lava having drained from them before complete consolidation.
Another agent which is effective in uncovering the buried structures
of volcanoes is the action of waves on shores. Always a relatively
vigorous erosive agency, the softer structures of volcanic cones are
removed with especial facility by this agent. On the shores of the
island of Volcano, the little cone of Vulcanello has been nearly half
carried away by the waves, so as to reveal with especial perfection the
structure of the cinder beds as well as the internal rock skeleton of
the mass. Here the characteristic dips of lava streams, intercalated as
they now are between tuff deposits and the lava which consolidated in
fissures, are both revealed.
[Illustration:
FIG. 148.—Map and general view of St. Paul’s Rocks, a volcanic cone
dissected by waves.]
In mid-Atlantic a quite perfect crater, the St. Paul’s Rocks, has been
cut nearly in half so as to produce a natural harbor (Fig. 148).
In still other instances we may thank the volcano itself for opening
up the interior of the mountain for our inspection. The eruption in
1888 of the Japanese volcano of Bandai-san, by removing a considerable
part of the ancient cone, has afforded us a section completely through
the mountain. The summit and one side of the small Bandai was carried
completely away, and there was substituted a yawning crater eccentric
to the former mountain and having its highest wall no less than 1500
feet in height (Fig. 149). In two hours from the first warning of the
explosion the catastrophe was complete and the eruption over.
[Illustration:
FIG. 149.—Dissection by explosion of Little Bandai-san in 1888 (after
Sekiya).]
The eruption of Krakatoa in 1883, probably the grandest observed
volcanic explosion in historic times, left a volcanic cone divided
almost in half and open to inspection (Fig. 150). Rakata, Danan,
and Perbuatan had before constituted a line of cones built up round
individual craters subsequent to the partial destruction of an
earlier caldera, portions of which were still existent in the islands
Verlaten and Lang. By the eruption of 1883 all the exposed parts and
considerable submerged portions of the two smaller cones were entirely
destroyed, and the larger one, known as Rakata, was divided just
outside the plug so as to leave a precipitous wall rising directly from
the sea and showing lava streams in alternation with somewhat thicker
tuff layers, the whole knit together by numerous lava dikes.
[Illustration:
FIG. 150.—The half-submerged volcano of Krakatoa in the Sunda Straits
before and after the eruption of 1883 (after Verbeek).]
In order to carry our dissecting process down to levels below the
base of the volcanic mountain, it is usually necessary to inspect the
results of erosion by running water. Here the plug or chimney, instead
of being surrounded by tuff, is inclosed by the country rock of the
region, which is commonly a sedimentary formation. Such exposed lower
sections of volcanic chimneys are numerous along the northwestern
shores of the British Isles. Where aligned upon a dislocation or
noteworthy fissure in the rocks, the group of plugs has been referred
to as a scar or _cicatrice_ (Fig. 151). Associated with the plugs of
the cicatrice are not infrequently dikes, or, it may be, sheets of lava
extended between layers of sediment and known as _sills_.
[Illustration: FIG. 151.—The cicatrice of the Banat (after Suess).]
If we are able to continue the dissection process to still greater
depths, we encounter at last igneous rock having a texture known as
granitic and indicating that the process of consolidation was not
only exceedingly slow but also uninterrupted. This rock is found in
masses of larger dimensions, and though generally of more or less
irregular form, no one dimension is of a different order of magnitude
from the others. Such masses are commonly described as _bosses_, or,
if especially large, as _batholites_ (Fig. 152). Wherever the rock
beds appear as though they had been forced up by the upward pressure
of the igneous mass, the latter takes the form of a mushroom and has
been described as a _laccolite_ (Figs. 479-481, pp. 441-442). Evidence
seems, however, to accumulate that in the greater number of cases the
molten rock has fused its way upward, in part assimilating and in part
inclosing the rock which it encountered. This process of upward fusion
has been likened to the progress of a red hot iron burning its way
through a board.
=The formation of lava reservoirs.=—The discarding of the earlier
notion that the earth has a liquid interior makes it proper in
discussing the subject of volcanoes to at least touch upon the origin
of the molten rock material. As already pointed out, such reservoirs as
exist must be local and temporary, or it would be difficult to see how
the existing condition of earth rigidity could be maintained. From the
rate at which rock temperatures rise, at increasing depths below the
surface, it is clear that all rocks would be melted at very moderate
depths only, if they were not kept in a solid state by the prodigious
loads which they sustain. Any relief from this load should at once
result in fusion of the rock.
[Illustration:
FIG. 152.—Diagram to illustrate a probable cause of formation of lava
reservoirs, and to show the connection between such reservoirs and the
volcanoes at the surface.]
Now the restriction of active volcanoes to those zones of the earth’s
surface within which mountains are rising, and where in consequence
earthquakes are felt, has furnished us at least a clew to the origin
of the lava. Regarded as a structure capable of sustaining a load,
the competency of an arch is something quite remarkable, so that the
arching up of strong rock formations into anticlines within the upper
layers of the zone of flow, or of combined fracture and flow, would
be sufficient to remove the load from relatively weak underlying beds,
which in consequence would be fused and form local reservoirs of lava
(Figs. 152 and 153).
It has been further quite generally observed that lines of volcanoes,
in so far as they betray any relation in position to neighboring
mountain ranges, tend to appear upon the rear or flatter limb of
unsymmetrical arches, or where local tension would favor the opening of
channels toward the surface. Moreover, wherever recent block movements
of surface portions of the earth’s shell have been disclosed in the
neighborhood of volcanoes, the latter appear to be connected with
downthrown blocks, as though the lava had, so to speak, been squeezed
out from beneath the depressed block or blocks.
[Illustration:
FIG. 153.—Result of experiment with layers of composition to
illustrate the effect of relief of load upon rocks by arching of
competent formation (after Willis).]
We must not, however, forget that the igneous rocks are greatly
restricted in the range of their chemical composition. No igneous
rock type is known which could be formed by the fusion of any of
the carbonate rocks such as limestone or dolomite, or of the more
siliceous rocks, such as sandstone or quartzite. There remains only
the argillaceous class of sediments, the shales and slates, and so
soon as we examine the composition of these rocks we are struck by
the remarkable resemblance to that of the class of igneous rocks. For
purposes of comparison there is given below the composite or average
constitution of igneous rocks in parallel column, with the average
attained by combining the analyses of 56 slates and shales, the latter
recalculated with water excluded:
══════════════╤══════════════════════════════════╤════════════════
│ AVERAGE IGNEOUS ROCK │
├——————————————————┬———————————————┤ AVERAGE SHALE
│ (Clark) │ (Washington) │
——————————————┼——————————————————┼———————————————┼————————————————
│ │ │
SiO_2 │ 61.25 │ 61.69 │ 63.34
Al_{2}O_3 │ 15.81 │ 15.94 │ 16.56
Fe_{2}O_3 │ 2.70} 6.31 │ 1.88} 4.53 │ 4.41} 7.89
FeO │ 3.61} │ 2.65} │ 3.48}
MgO │ 4.47 │ 4.90 │ 3.54
CaO │ 5.03 │ 5.02 │ 3.33
Na_{2}O │ 3.64 │ 4.09 │ 1.29
K_{2}O │ 2.87 │ 3.35 │ 3.52
TiO_2 │ .62 │ .48 │ .53
│ —————— │ —————— │ ——————
│ 100.00 │ 100.00 │ 100.00
══════════════╧══════════════════╧═══════════════╧════════════════
This close resemblance is probably of deep significance, for the reason
that shales and slates are structurally the weakest of all rocks and
for the further reason that they rather generally directly underlie
the carbonate rocks, which are by contrast the strongest (see _ante_,
p. 37). For these reasons shales and slates are the only rocks which
are likely to be fused by relief from load through the formation of
anticlinal arches within the earth’s zone of flow. If this view is
well founded, lavas and other igneous rocks are in large part fused
argillaceous sediments formed in connection with the process of
folding, or are refused rocks of igneous origin and similar composition.
=Character profiles.=—The character profiles of features connected in
their origin with volcanoes are particularly easy to recognize, and in
a few cases in which they might be confused with others of a different
origin, an examination of the materials of the features should lead to
a definitive judgment.
The lava plains which result from massive outflows of basalt might
perhaps strictly be regarded as lack of feature, so great may be their
continuous extent. Wherever definite vents exist, a broad flat dome is
the usual result of the extravasation of a basaltic lava. The puys of
France and many of the Kuppen of Germany, being formed from less fluid
lava, have afforded profiles with relatively small radius of curvature.
In its youthful stage, the cinder cone usually presents a broad summit
sag and relatively short side slopes, whereas the cone of later stages
is apt to present long sweeping and upwardly concave curves with both
the gradient and the radius of curvature increasing rapidly toward
the summit. In contrast, too, with the earlier stage, the crest is
relatively small. A marked reduction in the high symmetry of such
profiles is noted wherever a breaching by lava outflow has occurred
(Fig. 154).
With the composite cone, complexity and corresponding lack of symmetry
is introduced, especially in the partially ruined caldera, and by
the more or less accidental distribution of parasitic cones, as well
as by migrations of the central cone. Peculiarly similar acuminated
profiles result from spatter-cone formation, from the formation of
a superchimney spine, and by the uncovering of the chimney through
denudational processes—the volcanic neck.
[Illustration: FIG. 154.—Character profiles connected with volcanoes.]
Another important feature resulting from denudation is the Mesa or
table mountain with its protecting basalt cap above softer rocks. Its
profile most resembles that of table mountains due to differential
erosion of alternately strong and weak horizontally bedded rocks, such
as compose the upper portion of the section in the Grand Cañon of the
Colorado. Here, however, in place of a single unusually strong top
layer there are found several strong layers in alternation with weaker
ones so as to produce additional steps in the profile.
READING REFERENCES TO CHAPTERS IX AND X
General works:—
PAULETT SCROPE. The Geology of the Extinct Volcanoes of Central
France. John Murray, London, 1858, pp. 258. (An epoch-making work of
early date which, like the following reference, may be studied to
advantage to-day.)
SIR CHARLES LYELL. Principles of Geology, vol. 1, Chapters xxiii-xxv.
MELCHIOR NEUMAYR. Erdgeschichte, vol. 1, Allgemeine Geologie, revised
edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable
information clearly presented).
J. D. DANA. Characteristics of Volcanoes, with Contributions of Facts
and Principles from the Hawaiian Islands. Dodd, Mead, and Company, New
York, 1890, pp. 397.
TEMPEST ANDERSON. Volcanic Studies in Many Lands, being reproductions
of photographs by the author with explanatory notes. John Murray,
London, 1903, pp. 200, pls. 105.
T. G. BONNEY. Volcanoes, their Structure and Significance. John
Murray, London, 1899, pp. 331.
I. C. RUSSELL. Volcanoes of North America. Macmillan, New York, 1897,
pp. 346.
ELISÉE RÉCLUS. Les volcans de la terre, Belgian Society of Astronomy,
Meteorology, and Physics of the Globe, 1906-1910 (a valuable
descriptive geographical and bibliographical work of reference).
G. MERCALLI. I vulcani attivi della terre. Hoepli, Milan, 1907, pp.
421. (A most valuable work, beautifully illustrated, but in the
Italian language.)
Arrangement of volcanic vents:—
TH. THORODDSEN. Die Bruchlinien und ihre Beziehungen zu den Vulkanen,
Pet. Mitt., vol. 51, 1905, pp. 1-5, pl. 5.
R. D. M. VERBEEK. Various volumes and atlases of maps covering the
Dutch East Indies and fully cited in the following reference (p. 21).
WILLIAM H. HOBBS. The Evolution and the Outlook of Seismic Geology,
Proc. Am. Phil. Soc., vol. 48, 1909, pp. 17-27.
Birth of volcanoes:—
F. OMORI. The Usu-san Eruption and Earthquake and Elevation Phenomena,
Bull. Earthq. Inv. Com., Japan, vol. 5, No. 1, 1911, pp. 1-37, pls.
1-13.
Fissure eruptions:—
TH. THORODDSEN. Island, IV, Vulkane, Pet. Mitt., Ergänzungsh. 153,
1906, pp. 108-111.
A. GEIKIE. Text-book of Geology, 4th ed., pp. 342-346.
Lava domes of Hawaii:—
J. D. DANA. Characteristics of Volcanoes (as above).
C. H. HITCHCOCK. Hawaii and Its Volcanoes. Honolulu, 1909, pp. 314.
Eruption of Matavanu volcano in 1906:—
KARL SAPPER. Der Matavanu-Ausbruch auf Savaii, 1905-1906, Zeit. d.
Gesell. f. Erdk. z. Berlin, vol. 19, 1906, pp. 686-709, 4 pls.
H. J. JENSEN. The Geology of Samoa, and the Eruptions in Savaii, Proc.
Linn. Soc., New South Wales, vol. 31, 1906, pp. 641-672, pls. 54-64.
TEMPEST ANDERSON. The Volcano of Matavanu in Savaii, Quart. Jour.
Geol. Soc., London, vol. 66, 1910, pp. 621-639, pls. 45-52.
Eruption of Volcano in 1888:—
H. J. JOHNSTON-LAVIS. The South Italian Volcanoes. Naples, 1891, pp.
342, pls. 16.
Eruption of Taal volcano in 1911:—
W. E. PRATT. The Eruption of Taal Volcano, January 30, 1911, Phil.
Jour. Sci., vol. 6, No. 2, Sec. A, 1911, pp. 63-86, pls. 1-14.
F. H. NOBLE. Taal Volcano, album of views of 1911 eruption, Manila,
1911, pp. 1-48.
The volcano of Etna:—
G. VOM RATH. Der Aetna. Bonn, 1872, pp. 1-33. (A beautiful piece of
descriptive writing from both the geological and scenic standpoints.)
SARTORIUS VON WALTERSHAUSEN. Der Aetna. Leipzig, 1880, 2 quarto vols.,
pp. 371 and 548.
The eruption of Vesuvius in 1906:—
H. J. JOHNSTON-LAVIS. Geological Map of Monte Somma and Vesuvius, with
a short and concise account, etc. Geo. Philip & Son, London, 1891.
H. J. JOHNSTON-LAVIS. The Eruption of Vesuvius in April, 1906, Trans.
Roy. Dublin Soc., vol. 9, 1909, Pt. VIII, pp. 139-200, pls. 3-23 (the
most authoritative work upon the subject).
T. A. JAGGAR, JR. The Volcano Vesuvius in 1906, Tech. Quart., vol. 19,
1906, pp. 105-115.
W. PRINZ. L’éruption du Vesuv d’avril, 1906, Ciel et Terre, 27e Année,
1906, pp. 1-49.
FRANK A. PERRET. Notes on the Electrical Phenomena of the Vesuvian
Eruption, April, 1906, Sci. Bull., Brooklyn Inst. Arts and Sci., vol.
1, No. 11, pp. 307-312; Vesuvius, Characteristics and Phenomena of the
Present Repose Period, Am. Jour. Sci., vol. 28, 1909, pp. 413-430.
WILLIAM H. HOBBS. The Grand Eruption of Vesuvius in 1906, Jour. Geol.,
vol. 14, 1906, pp. 636-655.
The spine of Pelée:—
E. O. HOVEY. The New Cone of Mont Pelée and the Gorge of the Rivière
Blanche, Martinique, Am. Jour. Sci., vol. 16, 1903, pp. 269-281, pls.
11-14.
A. HEILPRIN. The Tower of Pelée. Philadelphia, 1904, pp. 62, pls. 22.
A. LACROIX. La montagne Pelée et ses éruptions, Acad. des Sciences,
Paris, 1904, Chapter iii.
KARL SAPPER. In den Vulkangebieten Mittelamerikas und Westindiens,
Stuttgart, 1905, pp. 172-178.
A. C. LANE. Absorbed Gases of Vulcanism, Science, N.S., vol. 18, 1903,
p. 760.
G. K. GILBERT. The Mechanism of the Mont Pelée Spine, _ibid._, vol.
19, 1904, pp. 927-928.
I. C. RUSSELL. Pelée Obelisk once More, _ibid._, vol. 21, 1905, pp.
924-931.
The dissection of volcanoes:—
J. W. JUDD. Volcanoes, Chapter v.
S. SEKYA and Y. KIKUCHI. The Eruption of Bandai-San, Trans. Seis.
Soc., Japan, vol. 13, Pt. 2, 1890, pp. 140-222, pls. 1-9.
R. D. M. VERBEEK. Krakatau. Batavia, 1885, pp. 557, pls. 25.
ROYAL SOCIETY. The Eruption of Krakatoa and Subsequent Phenomena.
London, 1888, pp. 494.
G. K. GILBERT. Report on the Geology of the Henry Mountains, U.S.
Geogr. and Geol. Surv., Rocky Mt. Region, Washington, 1877, pp. 22-60.
SIR A. GEIKIE. Ancient Volcanoes of Great Britain, vol. 2 especially.
D. W. JOHNSON. Volcanic Necks of the Mount Taylor Region, New Mexico,
Bull. Geol. Soc. Am., vol. 18, 1907, pp. 303-324, pls. 25-30.