噴火現象の分類とメカニズム

概要

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Volcanic eruption may be defined as a kind of volcanic phenomena in which volcanic materials are ejected from the earth's interior to the earth's surface in a relatively rapid fashion. One school of classifying the kind of eruptions is to name each type after the name of a volcano in which such a type of eruptive activity occurs characteristically. Examples are Strombolian, Vulcanian, Hawaiian, etc. Such nomenclature is not satisfactory due to inadequate and conflicting definitions, and the basic scheme of classification should be followed by parallel description of various factors which govern the nature of eruptions. Ascent of magma may be caused by bouyancy, tectonic squeezing, and vesiculation. Migration of H_2O with other components toward the top of magma column during the period of hundreds of years is apparent but the mechanism of transport is not clear. Complexity of governing factors of the vesiculation and fragmentation and difficulties with models of explosive eruptions are pointed out. Subaqueous eruptions are characterized by rapid quenching of magma by water, suppression of vesiculation due to high ambient pressure exerted by water column, generation of hydrovolcanic explosion in shallow water, and characteristic mode of transportation and emplacement in water. Subaqueous volcanic products are much more fragmentary than the equivalent subaerial volcanic products and grades into non-volcanic sediments.
1975-12-25