噴火における水の役割
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概要
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Eruptions of pyroclastic fall, pyroclastic flow, and lava (lava flow, lava dome, lava spine) often take place together in a single cycle of volcanic eruption. Probable genetical relations between these three kinds of eruptions are schematically shown in Fig. 1. An intense vesiculation of magma within the vent would throw pyroclastic materials high above the crater causing a pyroclastic fall eruption. A less intense vesiculation within the vent would generate an eruption of pyroclastic flow. Probably the water content of magma is smaller in this latter case than in the preceding one. A delay of vesiculation of a highly viscous magma may cause a nuee ardente eruption in the strict sense. A magma, still poorer in water, may cause no explosive vesiculation either within the vent or after leaving the crater. It will only cause a calm extrusion of lava. The water content of magma in the gravitational field is greater in the higher level of the magma column. Magmas erupted in earlier phases of an eruption woule represent the higher level of the column. Hence they will cause pyroclastic fall eruptions. In later phases of the same eruption, the water content of magma would decrease. Hence, pyroclastic flows and lava flows would be formed successively. The magmas may become poor in water in later phases, partly because of the esecape of water during earlier phases of the eruption. The order of successive events in seventeen historic and prehistoric eruptions manifested by fifteen volcanoes of the world is given in Table 1. The actual order of eruption is in harmony with the above consideration. In many calderas the volume of mass collapsed is found to be greater than that of the erupted materials. This question has never been furnished with a reasonable explanation. The writer proposes a hypothesis that the volume difference between the collapsed mass and the erupted materials corresponds to the volume of water dissolved in the magma prior to the eruption.
- 特定非営利活動法人日本火山学会の論文
- 1959-02-20