Page:South African Geology - Schwarz - 1912.djvu/92

 indeed most volcanoes lie along fissures which have been enlarged to volcanic vents at certain points. The fissure may gape sufficiently to let the lava stream come pouring out with but little preparatory explosions; then only a few small spitting cones are formed, and the whole country for miles round is flooded with the issuing lava stream. If several fissures occur in the same area, whole regions may be inundated with lava. Such an area, as large as France, occurs in Idaho and neighbouring states, and the Mawi plateau, in British Central Africa, is of the same nature. It is possible, however, that the lava of these plateaux is of extra terrestrial origin, that is, a very large meteorite may have fallen upon the earth, have melted itself and the rocks it fell upon, and have thus produced these lava seas. The Maria or dark patches of the moon are similar lava seas.

Caldera. — Volcanoes, again, may have chimneys so large that the usual manifestations of those with narrow throats are not represented. These volcanoes have, instead of craters, caldera, which are simply rounded holes in the earth's crust, up to 2 ml. in diameter, up which the lava comes, but does not overflow, forming a gigantic lava lake. When the molten rock cools, the lava solidifies as a crust. The great Kilauea, in the Hawaiian Islands, is the most famous of these caldera; it rises 30,000 ft. from the sea floor, and Brun has recently observed in an eruption that the surplus lava flows down subterranean channels, and must find exit beneath the sea. Similar caldera exist in the South Sea Islands, and extinct volcanoes of this type are found on the northern flanks of the Alps, at Hegau and the Ries. Such large orifices are not probably filled entirely