A volcano is usually a conical mountain or hill, with an opening, or crater, through which various solid, molten, or gaseous materials are ejected. The essential part of the volcano is the opening, or vent, and some volcanoes consist of almost nothing else. The mountain, when present, is secondary and is formed by the materials which the volcano itself has piled up; it is thus the effect and in no sense the cause of the phenomena.
The geographical distribution of volcanic vents has greatly changed at different periods of the earth's history. While there are some large land areas, like most of the Mississippi Valley, which appear never to have been visited by volcanic activity, such areas are comparatively few in number. In most regions we find distinct traces of such action, though it may have died out ages ago and though at present no active vent may occur for very great distances in any direction. Such is the case with the valley of the Connecticut and northern New Jersey, Ireland, Great Britain, and very many other countries.
We cannot definitely determine the number of vents which are at present in activity in various regions of the earth, because a volcano may remain dormant for centuries, and then break out again. Almost all tradition of the volcanic nature of Vesuvius had died away among the inhabitants of Italy, until the dreadful eruption of the year 79 a.d. showed that it had only been slumbering. Many volcanic regions, such as the western part of North and South America, and the East Indian islands, have been known to civilized man for only a few centuries, and in such regions the distinction between dormant and extinct vents cannot always be made.
Furthermore, the number of vents is constantly changing, new openings forming, and old ones closing up, while some that had escaped observation are not infrequently discovered. Another distinction which is often arbitrary, is that between independent volcanoes and mere subsidiary vents connected with larger ones. Several submarine volcanoes have been observed, but it is altogether probable that many more exist which have escaped detection. Making these allowances, the number of volcanoes now active may be estimated at about 328, of which rather more than one-third are situated in the continents, and the remainder on islands.
The active volcanoes are not scattered haphazard over the surface of the globe, but are arranged in belts or lines, which bear a definite relation to the great topographical features of the earth as well as to the seismic belts described in the preceding chapter. Two of these belts together encircle the Pacific Ocean; one on the west coast of the Americas runs from Alaska to Cape Horn, the other, a very long and sinuous band, running from Kamschatka through the islands parallel to the east coast of Asia, the East Indian and south Pacific islands, to the Antarctic circle, where it joins the American band.
As in the case of the seismic belts, the volcanic bands are not continuous, but consist of a series of volcanic tracts' separated by others which have no volcanoes. The coincidence of the volcanic and earthquake belts which encircle the Pacific Ocean is very close and striking.
A third band occupies a ridge in the eastern bed of the Atlantic, from Iceland to beyond St. Helena, from which arise numerous volcanic islands and submarine vents. Included in this Atlantic band are Jan Mayen, Iceland, the Azores, Canary and Cape Verde Islands, Ascension, St. Helena, and, in the far south (38° S. lat.), Tristan d'Acunha. South of Iceland there are no known volcanoes for a great distance, until the Azores are reached, and on the east coast of the Americas are none at all.
A subsidiary belt, parallel to the Atlantic band, includes the volcanoes of east Africa, the Mascarene and Comores Islands, together with the extinct vents of Madagascar in the south and those of Armenia, Syria and Arabia in the north.
Other subsidiary volcanic belts are scattered along the great equatorial seismic zone mentioned in Chapter I. Thus, the volcanoes of southern Mexico and Central America have a general east-west arrangement and are in line with those of the West Indies. On another portion of the same zone are placed the Mediterranean vents. At the crossing of the equatorial and west Pacific zones are the volcanoes of the Philippines and Japan, and those of the former continue westward through Java, Sumatra, the Nicobar and Andaman Islands, to Burmah.
A very striking fact is the nearness of most active volcanoes to the sea; by far the greater number of vents are upon islands, and those of the continents are, with a few exceptions, not far from the coasts. Some of the volcanoes in Mexico and Ecuador are 150 miles from the ocean, and Kirunga in east Africa is more than 600 miles from the sea. Another relation which should be noted, is that between the volcanic bands and the mountain-chains, the bands running parallel to or coinciding with the mountains, as in the great volcanoes of the Andes. Not all coast-lines or all mountain chains have volcanoes associated with them, but where the mountains are near the seashore, volcanoes are usually, though not invariably, found. The seat of volcanic activity is frequently shifted, as we have learned, and it has been observed that this activity tends to die out of the older rocks and to make its appearance in those of a later date.
The relations of volcanoes to lines of fracture and faulting are much disputed. As we shall see, lava may force its way to the surface independently of such lines; nevertheless, "the great majority of recent and earlier eruptions are connected with fissures and zones of fracture in the earth's crust." (Kayser).