BEZYMYANNYI ERUPTION ON MARCH 30, 1956 (KAMCHATKA): SEQUENCE OF EVENTS AND DEBRIS-AVALANCHE DEPOSITS

Abstract

A detailed reexamination of the deposits and comparison with the descriptions of the eruption revealed that on March 30, 1956, a collapse and a landslide 0.5 km3 in volume took place on the eastern slope of Bezymyannyi (Central Kamchatka). After a series of explosions, an old dome was slowly uplifted by rising magma, and a cryptodome intruded the eastern flank prior to a cataclysmic explosion. A rockslide changed to a cold (<100°C) debris avalanche which rushed down at a speed of more than 60 m/s and covered a distance of 10 km from the volcano. The avalanche split into three branches that flowed along the river valleys. The central flow covered the largest distance (22 km). The avalanche stripped and pushed the material at the volcano's foot (snow, soil, alluvium, and vegetation), which produced long mud flows. The landslide unroofed the cryptodome and triggering a devastating lateral blast followed by the eruption of pyroclastic flows.