Lava

Lava, a general term for all those rocks which flow in a molten condition from volcanoes (q.v.). They differ very much in the extent to which they are saturated with steam and other gases, and also in composition, texture, density, and colour. Some are acid, containing 60 to 80 per cent. of silica, often pale buff in colour, and with a specific gravity between 2-3 and 2-7. Such are the liparites, trachytes, -obsidian, pitchstone, and most pumice. Others, such as the basalts, are basic, with 45 to 50

per cent. of silica,- much oxide of iron and such minerals as augite and olivine, and a density between 27 and 33. These are mostly dark grey or black, though the iron rusts on exposure. Lavas may be entirely glassy, as are obsidian and pitchstone; vesicular, like pumica; semi-crystalline, as.in the bulk of volcanic rock; or even wholly crystalline, as are some liparites and basalts. The expansion of the contained vapours causes the lava to rise in the crater; and, though it may reach the rim and overflow it, its great density, exerting a pressure of about 120 lbs. per square inch, or 7 or 8 tons per square foot, for each 100 feet of the height of the column, generally forces it out as veins and dykes, Assuring the sides of the cone, and even rising sometimes in a fountain of molten rock. It glows with a white heat, being at first considerably more than 2,000° Fahr., steam and other vapours (especially that of salt) escaping from it at jets, known as fumaroles, and long hanging over it as a cloud. The motion of the stream has been compared to that of honey; but its rate varies not only with the slope and with the volume of the stream, but also with its fluidity. A stream from Vesuvius in 1805 travelled nearly four miles in four minutes, and one from Mauna Loa, in 1852, fifteen miles in two hours. It cools from white to red, and from red to black, with a slaggy, cindery surface, widening out at the foot of a slope and ending in a slowly-advancing wall like a mound of clinkers. A lava-flow may be many years before it has completely cooled; but, contrary to the opinion once held, it may consolidate on a surface inclined 35° or even 40°. In volume, the largest recorded outpouring was that of Skaptar Jokull, in Iceland, in 1783, when two streams flowed forty miles and fifty miles respectively, in opposite directions, with a width of from twelve miles to fifteen miles and a depth of 100 feet, deepening in river-gorges to 600 feet. The upper part of the lava stream is generally pumiceous and glassy; but even in obsidian there are small crystals, and the bulk of the flow i in consolidating has usually become at least semi-crystalline or micro-crystalline: