Lava (original) (raw)

Lava is molten rock which a volcano expels during an eruption: due to its high temperature, it can be 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.

Lava streams

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 stream bed going away from Double Hole Crater lava pool
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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.

Cascades and Jets of Lava

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!

Variations in its Consistency - Pumice

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 calledpumice.

Different Sorts of Lava

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.

Images

Lava tree mold near Black Crater

Lava is also a cossack military formation.