Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

VOLCANOES.

Number of volcanoes.—It is impracticable to state exactly the number of volcanoes that are active at the present time, because most volcanoes are periodic, and become active at more or less distant periods, and it is impossible to say whether a given volcano that may be now quiescent has really become extinct or is only enjoying its customary period of rest. It is quite safe to include at least 300 in the active list, and the number may reach 350 or more. The numbers that have been active so recently that their cones have not been entirely worn away is several times as great.

Distribution of Volcanoes.

1. In time.—In the earliest known ages igneous action appears to have been very general, if not practically universal. No area of the earliest (Archean) rocks is now known which is not formed chiefly of rocks that appear to have been either intruded or extruded. Rocks which can reasonably be assigned to the hypothetical molten globe, if there be such, are not here included. It is probable that the surface of the early earth was as thickly occupied with points of extrusion as the surface of the moon appears to be. In the ages between the Archean and the present, the distribution of volcanic action over the surface seems to have been in a general way much what it is to-day; that is, certain areas were volcanically active at times, while other and larger areas were measurably free from any outward expressions of igneous action. This is not equally true of all ages, as will be seen in the historical studies that follow. There were periods when volcanic activity seems to have been widespread and energetic, and others when it was limited both in amount and distribution. The known facts do not indicate a steady decline in volcanic activity, but rather a periodicity; at least this is so for the portion of the globe that is now well enough known geologically to warrant conclusions. One of the greatest of the volcanic periods falls within the Cenozoic era, just preceding the present geological period, and the volcanic activity of the present is perhaps but a declining phase of that time.

2. Relative to land and sea.—At present the active volcanoes are chiefly distributed about the borders of the continents, and, less notably, within the great oceanic basins. On this account the sea has often been supposed to have some connection with volcanic action, and the presence of chlorine in the volcanic emanations has been cited in support of this position. When critically examined, however, the argument from distribution is not very strong; for the volcanoes are not distributed equally or proportionately about the several oceans, as if dependent on them. Volcanoes are especially numerous around and within the Pacific, the greatest of the oceans, and this might seem a favorable instance, but they are also numerous around and within the Mediterranean, a relatively small body of water. Volcanoes are not especially abundant in or about the margins of the Atlantic.

Fig. 460.—Volcanoes in the Pacific. Jones Relief Globe. (Photo. by R. T. Chamberlin.)

If volcanoes were dependent upon proximity to the sea, the relation should be close in the past as well as in the present, but this does not seem to be true. There has recently been much volcanic activity in the plateau region of western America at long distances from the Pacific basin. Even on the plains east of the Rocky Mountains notable volcanic action took place. There were also volcanoes in the interior of Asia and of Africa.

3. Relative to crustal deformations.—The distribution of present and recent volcanoes is much more suggestively associated with those portions of the crust that have undergone notable changes in position in comparatively recent times. The great “world-ridge” stretching from Cape Horn to Alaska and thence onwards along the east coast of Asia is a striking instance, for it is dotted throughout with active and recently extinct volcanoes. The tortuous zone of mountainous wrinkles that borders the Mediterranean and stretches thence eastward to the Polynesian Islands is another notable volcanic tract. These two belts include the greater number of existing and recent volcanoes on the land, while the great basins associated with them embrace the chief oceanic volcanoes.