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.

The paper continues the series of papers that describe activity of the volcanoes of Northern group of Kamchatka since 1935. Activity of Kluchevskoy, Bezymianny and Shiveluch volcanoes in 1990—1992 is described.

The results of determination of lead isotope composition in 5 samples of andesites of the Bezymyanny volcano, belonging to the Klyuchevskaya group of volcanoes, are given. Most of the andesites are identical in lead isotopy to basalts of this area and to tholeiites of the ocean ridges. Rocks of the upper mantle or their derivatives could be the source of andesite magmas of volcanoes belonging to the Klyuchevskaya group. At some ca¬ses contamination of andesite magma by the material of upper parts of the crust could occur but it did not play a significant role in the andesite genesis. Facies and mineralogical varieties of the andesites are mainly analogous in lead isotopy so a contamination of rich in water rocks of the crust could not be a cause of appearance of the hornblende andesites.