utilization of geothermal energy as a source of heat and electrical power. Compared with many other fields of science, volcanology is in its infancy; yet substantial progress is being made both in understanding volcanic processes and in developing methods of forecasting eruptions.

This paper (1) reviews the relations that volcanoes bear to the tectonic belts of the Earth, (2) summarizes the major kinds of volcanic activity, (3) reviews principal methods that scientists have used to study and forecast volcanic activity, and (4) discusses the ways that people react to volcanic activity.

VOLCANISM IN THE CONTEXT OF TECTONICS

The distribution of volcanoes throughout the world broadly parallels the major tectonic belts, although they do not precisely coincide (Figure 15.1). Epicenters of major earthquakes are much more widely scattered than are volcanoes, and large segments of active tectonic belts have no volcanoes at all. Nevertheless, all but a few of the world’s active volcanoes lie close enough to the major zones of active earth movement to have long provoked speculation and discussion on the nature of and connection between earthquakes and volcanoes. The current theory of plate tectonics provides a unifying framework explaining the association.

Most volcanoes lie on or near two of the three principal types of boundaries between the moving crustal plates: (1) spreading boundaries, where plates move away from each other, and (2) compressive boundaries, where plates move toward each other and one overrides the other. The third type of boundary, transform, along which the plates slide laterally, is rarely associated with volcanism. But some volcanoes lie far from plate margins, and most of these are explained as a result of the plate moving across a stationary, magma-generating spot beneath the crust. Other intraplate volcanoes require a more elaborate explanation.

FIGURE 15.1 Map of the volcanoes of the world, showing the relation between volcanoes and tectonic belts. Solid circles are volcanoes with eruptions since 1880; open circles are volcanoes with dated eruptions prior to 1880; triangles are volcanoes with undated but geologically recent eruptions; small crosses are volcanoes with uncertain or solfataric activity. Light double lines are spreading boundaries; thin cross-hatched lines are compressive boundaries (adapted from Simkin and Siebert, 1984). Most spreading boundaries are sites of frequent volcanism, but most submarine eruptions go undetected (see text), hence few specific submarine volcanoes appear on this map.



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