Volcanology

Macdonald, G

A volcano is commonly defined as a place where molten rock (magma) and gases issue at the surface of the earth, or as the hill or mountain built up around that vent by escaping material. But volcanology, the science of volcanoes, goes beyond these surficial structures and the processes that take place at and around them and concerns itself also with the structures and processes within the earth that give rise to the surface phenomena. Indeed, volcanoes are one of our principal windows to the earth's interior; the composition and condition of materials issuing at volcanoes supply our best direct evidence as to the nature of the rocks and the processes that go on within the earth below the very thin rind that is known to us from direct observation. Less than 0.3 percent of the ratio of the earth is directly visible to geologists, even in the deepest drill holes and the most deeply eroded mountain belts. All the rest must be studied by indirect methods, such as seismology, and by inference from things observed in the visible surficial film and at volcanoes. There is today a tendency to include all magma chemistry within volcanology, perhaps partly because of the growing realization that many of the deep seated, 'plutonic' rocks traditionally considered to be igneous·that is, to have consolidated from a molten state·actually have never been molten. There can, however, be no question that the molten lavas poured out at volcanoes represent real magmas.

Volcanology

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