Lava
Lava is molten rock that a volcano expels during an eruption. Due to its high temperature, lava can be quite fluid when first exuded from a volcanic vent, but eventually solidifies into rock. However, the lava may flow many miles before solidification. While still below the earth's surface, the molten rock is termed magma. Solidified lava is known as igneous rock, although the term lava flow refers to the hardened formation.The word 'lava' is derived from the Latin verb lavare which means "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.
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The three main forms of lava are 'a'a, pahoehoe, and pillow lava.
'A'a (from Hawaiian ‘a‘a, rough lava) is one of three basic types of flow lava. Aa is characterized by a rough or rubbly surface composed of broken lava blocks called clinker. The incredibly spiny surface of a recently solidified aa flow makes walking very difficult and slow. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an aa flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an aa flow. Accretionary lava balls as large as 3 m (10 ft) are common on aa flows. Aa is usually of higher viscosity than pahoehoe. Pahoehoe can turn into aa if it becomes turbulent due to meeting impediments or steep slopes.
Pahoehoe is a Hawaiian term for basaltic lava that has a smooth, hummocky, or ropy surface. A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust. The surface texture of pahoehoe flows varies widely, displaying all kinds of bizarre shapes often referred to as lava sculpture.
Pillow lava (NOAA)
Pillow lava is a variety of rock formed when lava emerges from a volcanic vent that is under water. The liquid lava gains a thick solid crust immediately on contact with the water, which cracks and oozes additional "pillows" as more lava emerges from the vent behind it. Since the majority of Earth's surface is covered by water, and most volcanoes are situated under it, pillow lava is actually very common.
Lava, being much more viscous than water, tends not to flow as rapidly. Nevertheless, where the slope of the ground is considerable, it can advance 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!
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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.
Lava types
'A'a
Pahoehoe
Toes of a pahoehoe advance
across a road in Kalapana
on the east rift zone
of Kilauea Volcano in Hawai'i.
''Pillow lava
Lava formations
Cascades and Jets of Lava
Different Sorts of Lava
Images
 Lavacicles in Valentine Cave in Lava Beds National Monument |  Dripstone in Skull Cave |
 Lava tree mold near Black Crater |  Lava tube |
Lava is also a cossack military formation.External Links