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.

The devestating explosive eruptions at Khangar (about 7000 14C BP), Khodutkinskiy "maar" (about 2800 14C BP), and Baraniy Amphitheater (about 1500 14C BP) are classified into a special type, subcaldera eruptions. They were analogues of caldera-forming eruptions by their dynamics, erupted volume (1.5-15 km^3), aspect, facies family, and the composition {from dacites to rhyolites) of the pyroclastics, but were not followed by the development of collapse calderas whose cavity volumes would fit the volume of discharge pyroclastics when converted to solid rock (magma). The discrepancy between a "caldera-like" aspect of the pyroclastics and the type of erupting vent can probably be explained by the greal depths of reservoirs of silicic magma which were "galvanized" when hot basaltic magma was injected into them. A subcaldera eruption usually began with a violent discharge of tephra, much greater in volume than the other volcanic products, to be followed by the formation of pyroclastic flows associated with pyrociastic surges. This sequence of events repeated itself several times during the eruption. No major explosion breccias were formed. Intensive ashfall involved areas of n * 10^4 ... n * 10^5 km^2, so that dated tephra beds have been excellent regional marker horizons. Subcaldera eruptions are hypothesized to have influenced the Earth's climate and are reflected as synchronous acid peaks in the Greenland glacier shield.

The old views on the style of the Avachinsky eruptions during 1737- 1909 have partially been revised based on new data obtained by the authors. We specified their types and geological-geomorphological effect and made an assessment of the related volcanic hazards. All the eruptions were merely explosive except for the effusive-explosive eruption of 1894-1895. The eruptions of 1737, 1779 and 1827 are referred to large for this historical eruptive stage, the rest (of 1772, 1851-1855, 1878, 1881, 1894-1895 and 1909) are regarded as small and moderate.

We have summed up and supplemented new data on the eruptions of 1926-1927, 1938 and 1945 plus added the description of the last eruption of the volcano in January 1991. Considering the dynamics of the eruptive activity of the Avachinsky volcano in 1737-1991 we have presented a classification for these eruptions. Also we made a forecast of the type and parameters of possible future eruption as well as estimated the volcanic hazard associated with it. Because of high degree of volcanic hazard in the SW and S sectors of the base of the volcano it is proposed to prohibit constructions of any kind in these areas.