Abstract
The greatest advance in paleovolcanology in the 20th century has been the elucidation of the origin of ignimbrites; following the realization that most of the great rhyolite and dacite sheets of the world are of pyroclastic origin, ignimbrites have been recognized in every continent and have been found in geologic series ranging in age from Precambrian to Recent. Meanwhile, detailed investigations of floodbasalt flows, which reveal that they occur as thin sheets that may be thousands of square miles in extent, have brought the question of their mechanism of emplacement into sharper focus. Detailed mapping of volcanic regions continues to raise troublesome questions of the sources of volcanic magmas, the extent and nature of their alteration before extrusion, and the relation, if any, of magma type to structural environment. Of fundamental importance to paleovolcanological studies is the answer, not yet agreed upon, to the question of how far the present can or should be relied upon as the key to the past.
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