Origin of the giant eruption cloud of Pinatubo, June 15, 1991 |
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| Authors: | Takehiro Koyaguchi Masami Tokuno |
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| Abstract: | The series of eruptions of June 15, 1991 at Mt. Pinatubo, Philippines were observed hourly by satellite. A giant discshaped cloud covering an area of 60,000 km2 appeared in the satellite images at 14:40, Philippine time. The cloud expanded radially against wind of 20 m/s and spread to an area of more than 120,000 km2 within an hour. According to eyewitness accounts there was heavy fine-ash fall after 14:00, intermittent lapilli fall started at about 14:20, and heavy and continuous lapilli fall widely started at about 15:00. The occurrence of the giant cloud roughly corresponded to the initiation of the intermittent lapilli fall.The air-fall deposits of the major eruption are widely distributed, including upwind from the vent. They are composed of 3 units; a silt-size fine-ash layer (Layer B), a lapilli layer commonly including pumice grains of > 1 cm in diameter (Layer C), a lapilli bearing volcanic sand layer (Layer D). Judging from its wide distribution and depletion of coarse, grains, most of the fine ash of Layer B is not distal deposits of a small eruption, but is originated from a large co-ignimbrite cloud. It is suggested that the major eruption started with the generation of a pyroclastic flow, which was subsequently followed by a plinian eruption resulting in the formation of the giant cloud and the lapilli fall.The results of calculations on the dynamics of eruption cloud indicate that the dimension and dynamics of the giant eruption cloud is accounted for by a plinian eruption with a magma discharge rate of the order of 109 kg/s. |
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