Lava
Lava is molten rock which a volcano expels during an eruption: due to its high temperature, it is quite liquid when it is first exuded from the volcano, but soon solidifies into rock. While it is still below the earth's surface it is called magma.
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2 Cascades and Jets of Lava 3 Variations in its Consistency - Pumice 4 Different Sorts of Lava |
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. This
lava consists of melted rock. 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. When the lighter superficial lava is brought suddenly
into contact with water, as when a lava-stream enters the sea, it
becomes still lighter and more porous--forming the well-known
substance called pumice, so much used for polishing. It may be
regarded as the solidified froth of lava, and is so light that it
floats on the surface of water.
The lavas of different mountains, when cooled and hardened, differ
much in their appearance and composition. Among those of Iceland is
found the beautiful black volcanic glass named obsidian. It is a
good deal used for ornamental purposes; for it possesses the
peculiar property of presenting 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. 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 native sulphur, in which Mount Etna has been very
prolific. It is to this mountain chiefly, therefore, that we are
indebted for our beautiful fire-works--our squibs, crackers, Roman
candles, serpents, Catherine-wheels, and sky-rockets. Would it had
produced nothing more harmful than these! But it has also supplied
one of the ingredients of that villainous gunpowder, which has been
the means of thrusting so many of our fellow-creatures prematurely
out of the world. Etna, however, can hardly be held responsible for
this unfortunate misuse of the valuable substance which it affords; while
even gunpowder itself has, on the whole, been of vast benefit to
mankind. Could we only refrain from shooting each other with it, we
might regard it as an almost unmixed good; for it has helped us
greatly in forming our roads, railways, and tunnels, and in working
our quarries and mines.
The three main forms of lava are aa, pillow lava, and pahoehoe.
Solidified lava is known as igneous rock.
See also: Lava lamp
Lava streams
Cascades and Jets of Lava
Variations in its Consistency - Pumice
Different Sorts of Lava
Lava is also a cossack military formation.
