Five major explosive eruptions discharging pyroclastic volumes of more than 1 km^3 have been identified in the Holocene eruptive history of Avacha Volcano. All of these were Plinian-type eruptions, three (of andesitic composition) occurring 7150, 5500 and 4500 14C B.P. Two eruptions (3500 and 3280 14C B.P.) that supplied andesitic basalts preceded or started the formation of Young Cone on Avacha Volcano. We have reconstructed the dynamics and parameters of the eruptions, assessed their geologic, geomorphic and ecologic effects, and provided a long-term forecast.

Приведены 34 14С датировки для плейстоценовых отложений Центральной Камчатской депрессии в диапазоне от "запредельных" значений (>55000 лет) до 10000 лет. Даты получены по древесине и торфу из межледниковых аллювиальных отложений, а также по погребенным почвам и остаткам млекопитающих из лессовидных покровных супесей. Ледниковые отложения II стадии позднеплей-стоценового оледенения датированы возрастом 21000 лет по найденным в них остаткам мамонта. В верхненеоплейстоценовых отложениях депрессии фиксируются многочисленные мощные прослои вулканических пеплов, связанные с крупнейшими извержениями в Срединном хребте и в пределах Восточной вулканической зоны Камчатки.

It is shown that the young andesite-basaltic cone of Avacha Volcano was more complex, of the "cone-in-cone" type, in the 18th and early 19th century than is now the case. The upper cone was then nested in a crater of diameter 350-400 m, its base being at absolute heights of 2720 m, while the summit crater had a diameter of 50-100 m at absolute heights of 2800-2850 m. Since the summit crater was small, S.P. Krasheninnikov was induced to call Avacha Volcano a "peaked" one in 1738. The major eruption of 27-29 June, 1827 nearly anni-hilated the nested upper cone, while Young Cone of Avacha Volcano acquired nearly the present-day outward shape.

It is shown in the present study, in contrast to what has been thought geretofore, that Sredyaya Mt in the Klyuchevskoi Volcanic Group, Kamchatka is not a volcanic formation of its own, but a giant (of volume about 1 km3) allochthon. The allochthon resulted from the collapse and displacement of an enormous monolithic block from the summit of Pra-blizhnyaya Sopka Volcano which came into being when a Hawai'ian type caldera formed there about 8600 14C B. P.

It is shown that the cardinal revision, attempted in 1988 and 1998, of the G.S. Gorshkov hypothesis as to the explosive origin of the debris avalanche (“directional explosion agglomerate”) caused by the 30 March, 1956 culminating phase of the catastrophic eruption on Bezymiannyi Volcano, Kamchatka does not rest on a safe foundation. The hypothesis of a different (avalanche-landslide) origin, similar to the May 18, 1980 Mount St. Helens, U.S. event, must be rejected owing to an absence of necessary evidence. New arguments are adduced to support the Gorshkov hypothesis, corroborating that the main factor that caused the debris avalanche was a giant directional explosion.