History of eruptive activity of Avachinsky volcano over the last 10 mln years has been reconstructed; age, frequency and parameters of the past eruptions, character and scale of potentially hazardous volcanic phenomena have been determined. Assessment of volcanic hazard has been given associated with the activity of Molodoi cone which began its formation 38000 years ago. Specific features of its eruption are used as the basis for the map of volcanic hazard for adjacent territories.

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.

This paper discusses two catastrophic Plinian-type eruptions which occurred close enough in time (IIAV1 -3500 and IIAV3 - 3280 14C yrs B.P.) on Avacha Volcano and initiated the activity of its Young Cone. We studied the stratigraphy of the ejecta, reconstructed their chronology and parameters, assessed their environmental impact. The ejecta of both of these eruptions were dominated by tephra whose volume is >3 and >1.1 km3, respectively. The eruptive columns rose as high as 21-28 km. The IIAV1 ash layer can be followed for 300 km northeast of the volcano, the ashfall area enclosed within the 1 cm isopach being about 50000 km2. Both eruptions were accompanied by pyroclastic flows, surges, and catastro3phic lahars. The juvenile3pyroclastics is basaltic andesite. By the values of total discharge volume (>3.6 km3 for IIAV1 and >1.21 km3 for IIAV3), these eruptions must be among the largest to have occurred during the eruptive history of the Young Cone.

В работе представлены результаты комплексного изучения моногенного вулканизма в МалкоПетропавловской зоне поперечных дислокаций (МПЗ), высказывается предположение о формировании моногенного вулканизма на границах разновозрастных слэбов, на продолжении Авачинского трансформного разлома. В районе МПЗ проживает около 75 % населения (~250 тыс. чел.) Камчатки, поэтому изучение вулканоопасности актуально.

Studies of melt inclusions in minerals of some Kamchatka volcanoes are reported. We studies andesite basalts and andesites in volcanoes situated in the Central Kamchatka Depression (Shiveluch and Bezymyannyi) and in the East Kamchatka Volcanic Belt (Avacha and Karymskii). In addition, we studied the basalts of the 1996 eruption in the Karymskii Volcanic Center and the dacites on Dikiy Greben Volcano, southern Kamchatka. We used homogenization of melt inclusions and the analysis of chilled glass in these inclusions with electron and ion microlog sondes. More than 200 melt inclusions have been studied in minerals from 25 volcanic rock samples. It was found that the compositions of melt inclusions in andesite phenocrysts strongly vary in basicity; the content of SiO 2 varies between 56 and 80% wt, increasing silica percentage going along with decreasing concentrations of Al2O3, FeO, MgO, CaO and increasing concentrations of Na2O and K2O. Most (about 80%) of glass in the inclusions have dacite and rhyolite compositions. However, the compositions of acid melts (SiO2 > 65% wt) which produce the andesites are significantly different from those which produce the dacites and rhyolites as to the content of TiO2, FeO, MgO, CaO and K2O. Comparison between the compositions of melt inclusions from andesite plagioclase on the four volcanoes showed considerable differences. The basic andesites (56-59% SiO2) of Bezymyannyi Volcano exhibit a large scatter in inclusion composition (SiO2 = = 56-81%). The more acid andesites (SiO2 = 61%) of Shiveluch Volcano show a lower scatter in inclusion composition (SiO2 = 67-79%). The melt inclusions in the Karymskii andesites are amore basic. They are enriched with Fe, Ti, Ca, Mg, P, Na and are considerably depleted in K. The melts that produce the Karymskii andesites are probably less differentiated. On the volcanoes under study we also found high-potassium melts (K 2O = 3.8-6.8% wt), independent of the concentrations of SiO2 in them (the range here is between 51.4 and 77.2% wt). The melts differ considerably between volcanoes as to the concentration of volatiles. The greatest concentrations of H2O are found in the Shiveluch melts (3.0 to 7.2% wt with the mean equal to 4.7% wt) and on Avacha (4.7-4.8% wt), lower concentrations being in the melts of Dikiy Greben (0.4-1.8% wt) and Bezymyanyi (<1% wt). Using fluid inclusions of CO2 found in andesite plagioclase of Shiveluch Volcano, we inferred a pressure of 350-1600 bars, which corresponds to the depth of the magma chamber equal to 1.5-6 km. We found the concentrations of 17 admixture elements in glass of melt inclusions sampled from the plagioclase of four volcanoes (Avacha, Bezymyannyi, Dikiy Greben, Shiveluch). Judging by the concentrations of these elements, the melts here studied are similar to the typical island arc magmas.