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Cooling Units and Composite Sheets in Relation to Caldera Structure

出版	Geological Society of America, 1979
URL	http://books.google.com.hk/books?id=Sjr8zgEACAAJ&hl=&source=gbs_api

註釋Many source areas for voluminous ash flows have histories of repeated catastrophic eruption and caldera-forming collapse within a few million years or less. Examples range in complexity from simple calderas related to eruption of a single cooling unit, through sequential collapses of the same caldera related to successive eruptions of separate cooling units, and nested calderas related to eruption of successively smaller cooling units between successively long time intervals, to complexes of overlapping calderas related to eruption of several cooling units from overlapping source areas within a large volcanic field. Some caldera complexes are related to collapse of adjacent, simultaneously active ring-fracture zones by immediately successive ash-flow eruptions to form composite sheets. Despite this wide range in complexity, a basically similar primary sequence of events in each area reflects the formation of a large volume of magma, at least part of which rose to form shallow epizonal magma chambers. In some instances, only a single high-level chamber formed and produced ash-flow eruption and collapse at one chamber may have triggered events in an adjacent one. Variations in the pattern of ash-flow-caldera relations reflect variations in relative volumes of magma in the chambers, of tectonic controls on intrusions and the timing of eruptions, and of the continuity of magma generation beneath a volcanic region. These diverse patterns represent variations on R.L. Smith's concept that voluminous ash-flow eruptions and related caldera formation are surficial expressions of the degassing, but not the total emptying, of large magma chambers at high levels in the Earth's crust. Study of these systems offers unique insights into the physical and chemical processes and evolution of silicic magmas.