SOME FORMS OF LAVA
In describing the wonders of volcanoes, we must not fail to say
something of the many remarkable forms that lava is capable of
assuming.
All volcanic lavas contain large quantities of an acid substance known
as _silica_, or what is known better as _quartz sand_. This material
exists in lava combined chemically with various substances called
bases, the principal of which are alumina, magnesia, lime, iron,
potash, and soda.
Although there are many kinds of lava, yet all lavas can be arranged
under three great classes according to the quantity of silica they
contain.
_Acid lavas_ are those in which the quantity of silica is greatest.
In these lavas the silica, which varies from 66 to 80%, is combined
with small quantities of lime or magnesia, and comparatively large
quantities of potash or soda. Some of the most important varieties of
acid lavas are known as _trachytes_, _andesites_, _rhyolites_, and
_obsidians_.
_Basic lavas_ are those containing from 45 to 55% of silica. They
are rich in lime and magnesia, but poor in soda or potash. Some of
the most important of basic lavas are the _dolerites_ and _basalts_.
Generally speaking, basic lavas are of a darker color than acid lavas,
and fuse at much lower temperatures.
_Intermediate lavas_ are those containing silica in the proportion of
from 55 to 66%.
While the temperature of liquid lava has not been very accurately
determined, yet, since we know that molten lava is able to melt silver
or copper, its temperature must be somewhere between 2,500° F. and
3,000° F., the melting point varying with the chemical composition.
According to Dana lavas can be divided into the following classes
according to their fusibility; i. e., _lavas of easy fusibility_, such
as _basalts_; these lavas fuse at about 2,250° F.; _lavas of medium
fusibility_, including andesites; these lavas fuse at about 2,520° F.;
_lavas of difficult fusibility_, such as trachytes; these lavas fuse
at about 2,700° F.
But what is, perhaps, most curious about lavas is that when the
surface of a freshly broken piece of cold lava is carefully examined,
it is found to contain a number of small crystals of such mineral
substances as quartz, feldspar, hornblende, mica, magnetite, etc.
The best way to study the different forms of lava crystals is to
prepare a thin transparent slice of hardened lava and then examine it
with a good magnifying glass. It will be found that the slice consists
of a mass of a glass-like material through which the crystals are
irregularly distributed, not unlike the raisins and currants in a
slice of not over rich plumcake.
When examined by a more powerful glass, such as a microscope, cloudy
patches can be seen distributed irregularly through the glass-like
mass. When these patches are examined by a higher power of the
microscope they are seen to consist of small solid particles of
definite forms known as _microliths_ and _crystallites_. It has been
shown by a careful study of these minute objects that they form the
exceedingly small particles of which crystals are built up.
If we fuse a small quantity of lava and then let it slowly cool, the
glassy mass will be found to contain numerous crystallites. On the
other hand, when fused lava is permitted to cool quickly, it takes on
the form of a black, glass-like mass known as _obsidian_ or _volcanic_
glass, a very common form of lava in some parts of the world.
In some lavas there are found larger crystals that appear to have been
separated from the glassy mass, under the great pressure that exists
in the subterranean reservoirs at great depths below the volcanic
crater, and then floated to the surface surrounded by the glass-like
material. Now when we examine these crystals with a higher power of
the microscope, we frequently find in them minute cavities containing
a small quantity of liquid and a bubble of gas, thus causing them to
resemble small spirit levels. The liquid in such cavities has been
examined chemically and in most cases has been found to consist of
water containing several salts in solution. Sometimes, however, the
liquid consists of liquefied carbonic acid gas. These wonderful things
will be discussed at greater length in the Wonder Book of Light.
When the mass of molten rock or lava that comes out of the crater
of a volcano is thrown upwards in the air the condition it assumes
by the time it falls back again to the earth depends on the height
it reaches. If this height is great the lava chills or hardens
before reaching the earth, and assumes various forms according to
the size of the fragments. The largest of these fragments are called
_cinders_; the finer particles _volcanic dust_; while most of those
of intermediate particles are known among other things as _volcanic
ashes_.
We have already seen that when an explosive volcanic eruption occurs
there is suddenly thrown out of the crater of the volcano a huge
column of various substances that rises sometimes as high as 30,000
feet or even more. The smaller fragments of lava are quickly cooled
and form volcanic ashes, sand, cinders, or dust. These are rapidly
spread out by the wind in the form of a black cloud, that not only
covers the mountain but reaches out over the surrounding country,
completely shutting off the light of the sun. From this cloud
particles of red hot ashes, cinders, sand, etc., begin to fall, the
largest particles near the crater of the volcano, and the smaller
particles at much greater distances. In very powerful explosive
volcanic eruptions such as Krakatoa, the finer dust may be carried to
practically all parts of the world.
Volcanic ashes consist of a fine, light, gray powder. These particles
take the name ashes from their resemblance to the ashes left after the
burning of pieces of wood or coal in an open fire. The name, however,
as Geicke points out, is unfortunate, since it is apt to lead one to
suppose that volcanic ashes consist of some burned material. Such an
idea is erroneous, however, since ashes do not consist of anything
that is left after burning, but merely of fine particles of molten
rock that have hardened by cooling. When in the shape of what is known
as volcanic dust these particles are so exceedingly small that they
can readily make their way through the smallest openings in a closed
room just as does the finest dust in the rooms of our houses when
they are shut up. There are cases on record where people have been
suffocated by the entrance of volcanic dust in closed rooms to which
they have fled for safety during volcanic eruptions.
_Volcanic sand_ consists of the coarser particles of chilled lava that
are partly round and partly angular. They are of various sizes up to
that of an ordinary pea. Volcanic sand is formed by the breaking up of
the lava by the explosion of the vapors as they escape from the lava
on relief from pressure. Volcanic dust when examined by the microscope
is found to consist of very small particles that are more or less
crystalline.
But besides the above there are larger fragments known as _lapilli_,
consisting of rounded or angular bits of lava varying in size from
that of a pea to an ordinary black walnut. These sometimes consist of
solid fragments, but are usually porous, sometimes so much so that
they readily float on water.
A curious form sometimes assumed by lava consists of what are called
_volcanic bombs_. These are formed during explosive eruptions,
when masses of liquid lava are hurled high up into the air. During
their flight they take on a rotary motion, which tends to make them
globular, so that cooling, while still revolving, they assume the
form of a more or less spherical mass. At times, however, they are
still sufficiently soft when they strike the earth to be flattened out
in the form of flat cakes. When of a spherical form these are very
properly called volcanic bombs.
That volcanic bombs have actually been subjected to a spinning motion
while in the air can sometimes be shown by the fact that masses of
scoriæ are frequently found in the interior with air cells largest at
the centre of the bomb.
Volcanic bombs are sometimes thrown from the crater to great
distances. During one of its recent eruptions, Cotopaxi threw out
bombs that fell at a distance of nine miles from the crater.
According to Dana another form of lava bombs is sometimes found on the
slopes of the active volcanoes of Hawaii, where masses of lava acquire
a ball-like shape while rolling down an inclination.
What are sometimes called volcanic bombs, but which are more properly
_volcanic vesicles_, are produced by small fragments of lava which
are thrown up in the air for only a moderate height and, on cooling,
assume pear-like forms. Fig. 25 represents the appearance of volcanic
vesicles. The direction in which these vesicles moved through the air
while in a molten state is indicated by their shape, the blunt end
being the end towards which the particles were projected.
[Illustration: FIG. 25. VOLCANIC VESICLES _From Dana's Manual
of Geology_]
But by far the greater portion of the hardened lava; i. e., the
coarser, heavier particles, fall back on the mountain, and collecting
around the crater build up volcanic cones, as already described in the
case of mountains of the Vesuvian type.
There are two different ways in which the melted lava is broken up
into fine particles when it is thrown upwards from the crater of the
volcano. Nearly all lava contains large quantities of steam that are
shut up, or occluded in the mass, being prevented from escaping by
reason of the pressure to which the lava is subjected. The lava is
released from this pressure as it is thrown out of the crater. The
steam or gases escape explosively and thus break the lava into fine
liquid spray, which rapidly hardens.
There is another way in which small particles of lava are formed.
Sometimes large pieces of hardened lava are shot upwards into the air
with a velocity as great as that with which a heavy projectile leaves
the muzzle of a large gun. These heavy particles striking against
one another, either while rising or falling, are broken into smaller
fragments. Sometimes, indeed, these fragments fall back again into
the crater from which they are again violently thrown out, and are
again broken into smaller fragments either while rising or falling.
You will, probably, remember several instances of volcanic eruptions
where masses of rock were thrown violently up into the air out of
the crater. These larger masses are known as _volcanic blocks_. They
probably consist of masses of hardened lava that have collected in
the tube of the volcano during some of its periods of inactivity.
Sometimes, however, they consist of fragments of rocks that are not
of volcanic origin. Cases are on record where volcanic blocks have
been thrown out of the craters in so great quantities as to cover the
surface of many square miles of land with fragments hundreds of feet
deep.
There is sometimes formed on the surface of a pool of lava as it
collects in the craters of such volcanoes as Mt. Loa or Kilauea, when
the volcanoes are not in eruption, a material resembling froth or
scum. The same thing sometimes occurs on the surface of some kinds
of lava as it runs down the side of the mountain. In this way a very
light variety of highly cellular lava known as _pumice stone_ is
produced. The action which thus takes place is not unlike that which
occurs during the raising of a lot of the dough from which bread is
made, where the carbonic acid gas which is formed during the raising
of the dough expands, and produces the well-known open cellular
structure of well-raised bread. In the case of pumice stone, however,
this raising goes on to such an extent that the mass consists often of
less than 2% of solid matter, the remainder being a tangled mass of
air.
[Illustration: THE LAVA FLOW OF THE CRATER OF KILAUEA, HAWAIIAN
ISLANDS _From a Stereograph, Copyright, by Underwood & Underwood_]
Fragments of lava that possess a cellular structure form what are
known as _scoriæ_. The lightest of all kinds of scoriæ is what is
known as _thread-lace scoriæ_. Here the thin walls consist of mere
threads. Figs. 26 and 27 represent the appearance of thread-lace
scoriæ from Kilauea. The separate threads are very fine, being only
from one-thirtieth to one-fortieth of an inch in thickness. As can be
seen, this form of scoriæ have six-sided or hexagonal shapes. You can
form some idea of the great lightness of such scoriæ when you learn
that they contain only 1.7% of rocky material. Indeed, they contain
so little solid material that a layer of volcanic glass only one inch
thick, if blown out into scoriæ, would be able to produce a layer
sixty inches thick.
[Illustration: FIG. 26. THREAD-LACE SCORIÆ FROM KILAUEA _From
Dana's Manual of Geology_]
Another curious form sometimes assumed by lava, especially in the
case of Kilauea, is where the lava is spun out in the form of long
silk-like hairs. This is called by the natives _Pele's hair_, after
the name of their goddess. Inasmuch as the origin of this form of lava
was at one time generally attributed to the action of the wind in
drawing out thread-like pieces from the jets of lava thrown upwards
from the pool, it will be interesting if its true cause is explained.
[Illustration: FIG. 27. THREAD-LACE SCORIÆ FROM KILAUEA _From
Dana's Manual of Geology_]
Dutton, in his report on the Hawaiian volcanoes, refers to the
formation of Pele's hair as follows:
"The phenomenon of Pele's hair is often spoken of in the
school books, and receives its name from this locality.
It has generally been explained as the result of the
action of the wind upon minute threads of lava drawn out
by the spurting up of boiling lava. Nothing of the sort
was seen here, and yet Pele's hair was seen forming in
great abundance. Whenever the surface of the liquid lava
was exposed during the break-up the air above the lake was
filled with these cobwebs, but there was no spurting or
apparent boiling on the exposed surface. The explanation of
the phenomenon which I would offer is as follows: Liquid
lava coming up from the depths always contains more or
less water, which it gives off slowly and by degrees, in
much the same way as champagne gives off carbonic acid
when the bottle is uncorked. Water-vapor is held in the
liquid lava by some affinity similar to chemical affinity,
and though it escapes ultimately, yet it is surrendered
by the lava with reluctance so long as the lava remains
liquid. But when the lava solidifies the water is expelled
much more energetically, and the water-vapor separates
in the form of minute vesicles. Since the congelation of
all siliceous compounds is a passage free from a liquid
condition through an intermediate state of viscosity to
final solidity, the walls of these vesicles are capable of
being drawn out as in the case of glass. The commotion set
up by the descending crust produces eddies and numberless
currents in the surface of the lava. These vesicles are
drawn out on the surface of the current with exceeding
tenuity, producing myriads of minute filaments, and the
air, agitated by the intense heat at the surface of the
pool, readily lifts them and wafts them away. It forms
almost wholly at the time of the break-up. The air is then
full of it. Yet I saw no spouting or sputtering, but only
the eddying of the lava like water in the wake of a ship.
The country to the leeward of Kilauea shows an abundance
of Pele's hair, and it may be gathered by the barrelful. A
bunch of it is much like finely shredded asbestos."
You have probably often seen the beautiful frost pictures that collect
on the panes of glass in a room where the ventilation has been
neglected. These pictures consist of groupings of ice crystals that
collect on the surface of the windows, when the moist vapor-laden
air in the room is chilled by contact with their cold surfaces. Now
the crystals formed in cooling lavas are sometimes grouped in forms
closely resembling frost pictures. A few of such forms are represented
in Figs. 28 and 29 in lava from Mt. Loa and Mt. Kea.
[Illustration: FIG. 28. FROST-LIKE LAVA CRYSTALS _From Dana's
Manual of Geology_]
[Illustration: FIG. 29. FROST-LIKE LAVA CRYSTALS _From Dana's
Manual of Geology_]
Certain varieties of lava, especially that which is found in dikes,
form cool, beautiful columns called basaltic columns. They are due to
the contraction that occurs on the cooling of the material. Instances
of basaltic columns are seen in the Giant's Causeway, on the northern
coast of Ireland, as well as in the Isle of Cyclops on the coast of
Italy. The general appearance of the latter is represented in Fig. 30.
[Illustration: FIG. 30. BASALTIC COLUMNS, ISLE OF CYCLOPS,
ITALY]
It is a curious fact that the entire mass of basalt does not generally
take the columnous form but only certain layers which terminate
suddenly above and below at structureless masses of basalt, as shown
in Fig. 31. These columns, however, are always found at right angles
to the cooling surfaces as seen in the figures. They may, therefore,
be inclined at all angles to the horizon.
[Illustration: FIG. 31. COLUMNAR AND NON-COLUMNAR BASALT]
When molten lava is only thrown up a short distance into the air
from a crater it is still partially molten when on falling it again
reaches the earth, and therefore clings to any surface on which it
falls. There are thus built up curious cones known as _driblet cones_,
in which the separate drops covering the sides of the cone can be
distinctly traced. Driblet cones are represented in Figs. 32 and 33.
Here, as can be seen, the separate drops can be readily traced as they
run down a short distance before cooling.
[Illustration: FIGS. 32, 33. DRIBLET CONES _From Dana's
Manual of Geology_]
We have already referred briefly to the _lava caves_ or _grottoes_,
that are formed in some of the lava streams issuing from Vesuvius,
Etna, or Hawaii. These caves consist either of a number of
communicating huge bubbles, or of the tunnels that are formed in the
lava by the hardening of the outside of the lava streams as they flow
down the sides of the mountain, and towards the close of the eruption
are afterwards emptied by the molten lava within continuing to flow to
a lower level before solidifying. Now, in the interior of these caves,
there are often found on the walls, as well as on the portions of the
floors of the caves, immediately below them, curious pendants, like
icicles, or, more correctly, like the _stalactites of limestone_ that
are seen hanging to the walls of caves in limestone districts, where
they are formed as follows: as the rain water sinks through limestone
strata it dissolves some of the lime, when, slowly falling, drop after
drop, from the roofs of the caverns, small particles of lime are
deposited on the roof, and in this manner a pendant of limestone is
formed. The water that falls to the floor of the causeway immediately
below, also builds up a dome-like hillock called a stalagmite. In due
time the pillar reaches downwards, and the opposite hillock upwards
until the two meet, thus forming great natural pillars that appear
to hold up the roof of the vast cave in which they have been slowly
formed. A number of _lava stalactites_ are represented in Fig. 34.
[Illustration: FIG. 34. LAVA STALACTITES _From Dana's Manual
of Geology_]
Now, in a similar manner these lava stalactites, formed in the lava
caves or grottoes, are caused by the stream as it escapes from the
walls of the caves depositing on them stalactites of various lava
minerals it has dissolved as it slowly passed through them.
But the most important of all volcanic products is _volcanic dust_.
This, as we have seen, is so light that it remains longest in the air,
and is often carried by the winds to great distances from the volcano
from which it escaped. It may interest you to know that some of the
most fruitful of the great wheat fields of the western parts of the
United States owe their extraordinary fertility to immense deposits
of volcanic dust that have been thrown out from some of the great
volcanoes of the geological past, now found in an extinct condition in
these parts of the United States.
According to Russell, immense deposits of volcanic dust are spread
over vast areas in Montana, Southern Dakota, Nebraska, and Kansas, as
well as over parts of Oregon, and Washington, and, indeed, over large
areas of southwestern Canada and Alaska.
It is practically certain that many of the eruptions producing this
dust occurred within historic times. There must, therefore, have been
many times in these parts of our country when the dense ash clouds
hiding the sun turned the day into night and destroyed the forests and
other vegetation by showers of red hot ashes. There were produced,
too, the same great dread, and possibly loss of life as common during
historical eruptions. It is pleasing, however, to think that while
these great catastrophes brought suffering and dread to the people who
then lived on the earth, they were, nevertheless, but the forerunners
of those fruitful fields that at a much later age were to bless the
people who afterwards lived on them.