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2 Cascades and Jets of Lava 3 Variations in its Consistency - Pumice 4 Different Sorts of Lava 5 Images |
The most prodigous product of some active volcanos is the streams
of lava poured forth — sometimes from vents in a summit crater — sometimes from vents lower down the slope. When it issues from the mountain its heat is intense and it glows like a furnace, so that, during the
night especially, these fiery rivers present a grand yet awful
spectacle. The streams spread themselves till they sometimes attain
a breadth of several miles, with a depth of several hundred feet,
and they flow onward till their length sometimes reaches fifty
miles.
Lava, not being so liquid as water, does not flow so rapidly:
nevertheless, when it is careering down the sides of a mountain, or
where the slope of the ground is considerable, it advances with
great speed. Even when at its hottest, it is somewhat viscid, like
treacle, and this viscidness increases as it cools. Hence on a
level plain, and at some distance from its source, the lava-stream
advances at a leisurely pace. In such circumstances the cooling
proceeds so quickly that a crust of considerable thickness is soon
formed on the top of the current, and persons who are bold enough
may cross the stream by means of this natural bridge. Even where
the current continues flowing rapidly, this crust may be formed on
its surface; and a man, whose curiosity exceeds his prudence, may
stand on the top of it, bore a hole through the crust, and see the
lava flowing underneath his feet!
Nothing can resist the progress of the lava-flood; trees, houses,
everything yields to its massive assault, The trees take fire
before its approach, and when it reaches them they emit a hissing
noise almost amounting to a shriek, and then plunging into the
molten flood are seen no more. Even the sea cannot withstand the
lava-stream, but retires on its approach; so that promontories
stretching to a considerable distance from the shore are formed in
this manner, when the molten matter hardens into stone.
The eruptions of lava are sometimes attended by peculiarities which
impart to them much additional grandeur. Instances have occurred in
which the fiery stream has plunged over a sheer precipice of
immense height, so as to produce a glowing cascade exceeding in
breadth and perpendicular descent the celebrated Falls of Niagara.
In other cases, the lava, instead of at once flowing down the sides
of the mountain, has been first thrown up into the air as a fiery
fountain several hundred feet in height. This happens when the
great crater at the summit of the cone is full of liquid lava but
does not overflow. Then, on the formation of an opening in the side
of the cone, a good way down, the lava issuing from it is projected
upwards to nearly the same height that it occupies in the interior
of the crater at the top of the cone. It is hardly possible for the
fancy to picture to itself anything so magnificent as such a
fountain of liquid fire must be. A simple jet of water of
considerable volume, thrown into the air to the height of a hundred
feet, is itself a beautiful spectacle. What then must be a huge jet
of glowing white lava projected to the height of several hundred
feet, and with what an awful thundering sound must it come tumbling
to the ground, thence to rush as a roaring torrent down the
mountain's side!
Lava, when congealed, differs in its consistency according as it is
near the top or near the bottom of the stream. When near the top it
is porous, owing to its rapid cooling; when near the bottom it is
dense, owing to its slow cooling and the great pressure to which it
is subjected. Lava can be ejected into the air as a froth, containing masses of air bubbles, during an explosive eruption. As the froth solidifies, the bubbles remain frozen in its structure and it forms a light porous rock called
pumice.
The lavas of different mountains, when cooled and hardened, differ
much in their appearance and composition.
If a rhyolite lava-stream comes into contact with water, for example entering the sea, it can quickly freeze into a black glassy substance called obsidian. This is particularly common in Iceland and Lipari.
It is used for ornamental purposes - it presents a different appearance according to the manner in which it is cut. When cut in one direction it is of a beautiful jetty black; when cut across that direction it is
glistering gray. In prehistoric times its hardness and conchoidal fracture properties meant it was was widely used to make knives.
The lavas of Vesuvius are generally of a brown
colour, and are also used in the arts. In them are found the
beautiful olive-green crystals of the mineral called olivine,
sometimes used by jewellers. But the most useful of all volcanic
productions is perhaps sulphur, in which Mount Etna has been very
prolific.
The three main forms of lava are aa, pillow lava, and pahoehoe.
Solidified lava is known as igneous rock.
The word is derived from the Latin verb lavare which meant "to clean". The first time it was used in connection with extruded magma was apparently in a short account written by Francesco Serao of the eruption of Vesuvius which took place between May 14 and June 4 1737. In this he described "a flow of fiery lava" in analogy to the flow of water and mud down the flanks of the volcano following heavy rain.
See also: Lava tube
Lava streams
Lava stream bed going away from Double Hole Crater lava pool
.
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Cascades and Jets of Lava
Variations in its Consistency - Pumice
Different Sorts of Lava
Images
Dripstone in Skull Cave
Lava tree mold near Black Crater
Lava is also a cossack military formation.