VOLCANO (ItaL vokano.; from Lat. Vtucaosus, the god of fire; originally the vol cano of Etna, the tabled abode of the god), an opening in the crust of the earth from which proceeds heated gases, volumes of steam, eruptions of ashes mixed with scoria! and large stones and molten rock called lava. The phe nomenon is chiefly limited to certain regions in different parts of the earth, known as volcanic districts; and in these districts established and permanent vents may continue constantly send ing forth steam and other vapors, like Strom boli on one of the Lipari Islands in the Medi terranean; or eruptions of more severe char acter may take place at irregular intervals. The matter thrown out from volcanoes generally accumulates around the openings or craters till they build up a hill, or even a mountain several thousand feet high; but the vent may continue for a long time at a low level, and is even formed beneath the sea, sometimes without rising above the surface. Instances have oc curred of a volcanic eruption suddenly form ing an island in the midst of the sea. Other volcanoes that have been suddenly raised up have remained permanently in the form of mountains. Such is the volcano of Jorullo (q.v.) in Mexico. The greatest volcanic moun tains, as Etna, Hecla and Vesuvius, are pro duced by accumulations of volcanic matters, as beds of lava, ashes and scorize, sometimes alter nating with beds deposited beneath the sea charged with the vestiges of marine animals, the collection of which must have occupied long periods of time. The form of the cone depends on the nature and material of the eruption. Ash forms steep slopes; fluid lava, low flat cones. Volcanoes sometimes remain inactive so long as to lose their peculiar character; but they may at any time break forth again. Vesuvius was not lcnown to the ancients as a volcano, though-, it was apparent from the form of the moun tain and the materials of which it was com posed that such must have been its character at some former period. Volcanoes which show outbursts of more or less frequency are called active. Those known to have been active in historic times, but long quiescent, are called dorntant Ancient volcamc activity took place in Hungary, central France, eastern United States, the eastern Rocky Mountains, the British Isles and other places, of whose erup tions no record exists. Such are termed ex tinct volcanoes, but many so-called extinct vents become active again. The Mississippi Valley is almost without evidence of volcanism since very early geologic times. Probably the most general understanding of a volcano is of a ccmical hill or mountain. There is, how ever, no limitation as to height, some compara tively low vokanoes. such as Pelee and Sou friere, manifesting violent and disastrous eruptions, while, on the other hand, such lofty peaks as Kilimanjaro, Cotopaxi and Popocata pet' are of a volcanic character.
As generally considered, a volcano is divided into three fundamental parts—the base, or ele vation proper ; the cone, rising steeply from the base; and the crater, the depression occurring at the summit of the cone. These parts are not invariabby present in distinct form in all volca noes; izz many, eruption takes place without any crater. Nor do eruptions always proceed from
the crater when present; the crater being lo Cated at die summit of the cone, while activity is in many Ca9e8 exhibited independent of the crater by supplementary craters in additional cones, broken out on the slopes of the moun tain. These are known as parasitic cones and craters. Craters of course vary greatly in size and sustain no proportional relation to the mountain or elevation. Haleakla (island of Maui, Hawaii), 10,000 feet, has a crater of 20 miles' circumference; but the crater of Orizaba (Mexico, on the bounda7 between the states of Vera Cruz and Puebla , three and one-half miles high, has a crater with a diameter of less than 1,900 feet The causes of volcanic action are still much obscured, though vanous explanatory theories have been advanced. In early times, the rising cloud of dust attending an eruption, lit by the glow of the lava, came readily to be regarded as mingled smoke and flame, and blacksmith gods were thought to have their forges in the subterranean regions. At the present time many believe that volcanic lavas are made up of still uncooled portions of a once molten earth squeezed out by compression. Many geologists, however, no longer believe that the earth was ever a molten mass. These latter ac count for lava in various ways. Some believe that the rocks are hot enough to melt, if not prevented from doing so by pressure, and hold that whenever folding produces an arch or anticline the pressure is removed from the roc.ks below and they at once become fluid. Others hold that heat is generated by friction during great crustal movements, causing the rocks to melt. Other and more complicated views are held, but they are all little more than speculation, and the problem is still unsolved. Volcanoes are, in general, to be found in areas where the earth's crust is of an inferior strength. These areas are naturally ocean basins, the territory bordering on such basins, or the ranges of mountains marlcing or flanIc ing the outlines of the continents. With the possible exception of Wrangel (Alaska) no active volcano exists far inland save in a dis trict which is either the scene of a compara tively recent displacement or is marked by an instability in the earth's crust. One of the chief lines of distribution surrounds the Pacific Ocean, running along the western coast of South, Central and North America; the Aleutian Islands, Kamchatka and the Kurile group; Japan, Formosa, the Philippines, the Moluccas, the North Hebrides, New Zealand and South Victoria Land. Included within this line are numerous other volcanoes, such as Kil auea, Mauna Loa and Mauna Kea (Hawaii) and those of the Polynesian Islands. In the Atlantic section are the Antilles, the Canaries, the Azores. the Cape Verdes, Iceland, Madeira, etc. The European line follows the Mediter ranean and is continued into eastern Asia, about the Caspian. It includes the Lipari Islands, the ./Egean Islands, Etna, Vesuvius, Ararat and Demavend. J. W. Judd ((Vol canoes) 1881) estimated that there are from 300 to 350 vents of customary activity. There are perhaps about an eqtral number whose latest activity was at a date comparatively re cent. Submarine volcanoes are known at many points.