The largest Plinian eruption of our era and the latest caldera-forming eruption in the Kurile-Kamchatka region occurred 1700-1800 14C yr BP from the Ksudach volcano. This catastrophic explosive eruption is similar in type and characteristics to the 1883 Krakatau eruption. The volume of pyroclastics ejected was 18-19 km3, including 15 km3 of tephra and 3-4 km of pyroclastic flows. The eruptive column reached 23 km height. A collapse caldera was 4 X 6,5 km in size with a cavity volume of 6.5-7 km3. Tephra was deposited to the north of the volcano to a distance of more than 1000 km. Pyroclastic flows accompanied by ash cloud pyroclastic surges were as long as 20 km. The eruption was first phreatomagmatic, then it became rhythmic, and each rhythm began with the pumiceous tephra eruption followed by the pyroclastic flow formation. Erupted products were rhyolite-dacite remaining invariable during the whole eruption. At the post-caldera stage when the Shtyubel cone started to form within the caldera the basaltic-andesite material began to come to the surface. The driving mechanism of the onset of the eruption is suggested to be an intrusion of magma of basic composition and its mixing with acid magma from a previously existed chamber. The eruption had substantial environmental impact and may have produced a large acidity peak in the Greenland glacial shield.

Based on a detailed seismological observations from 1962 through 1994, the deep structure, focal layer geometry, magmatic feeding system of the Shiveluch volcano are investigated. Beneath the Shiveluch volcano, a focal layer deepens at an angle of 70 at depths of 100—200 km. Based on the established interrelation between the seismicity at depths of 105—120 km and a phase of extrusive volcano eruption in 1980 through 1994, a conclusion has been made that melting of primary magmas, periodically feeding the crustal chamber, occurs at depths at least 100 km. Increase of extrusive-explosive activity of the volcano is preceded and accompanied by the increasing number and energy of volcanic earthquakes both beneath the dome and in the zones of crustal faults of northwestern trend. A description is given of the methods used in the successful short-term prediction of the eruption in April 1993 which has been the most powerful since 1964. A mean-term prediction of possible increase of the volcano activity in April — July of 1995 is made.

We consider a problem of disturbance of a continuous medium which surrounds a magma chamber or channel. This disturbance is caused by excess internal pressure in the magma chamber. We used Godunov's entropy criterion to estimate the value of the shear strain required. Relations were found to determine critical pressure in the chamber and the time needed to achieve the critical state. The zone of rocks which surrounds the magma chamber and which is located next the melting zone has to be fracturing-resistant.

The following new quantitative indices have been obtained for the Kamchatkan volcanic centers with duration of development of 350-700 ... 1500-2500 thousand years; total volume of volcanics, average productivity, duration of formation, etc. Balance of substance has been calculated for the first lime: absolute volumes of basic, mean, and acid rocks and their ralios in percents of the whole mass of the eruption products. Calculative average indices have been obtained for the Kamchatkan long-lived centers both in separate volcanic zones and Kamchatka region as a whole. Principal volcanic and magmatic centers with maximum summary productivity have been revealed. The decisive role of basic magmas in volcanic process and structure formation is shown.

Four Plinian eruptions from Ksudach volcano ha' been reconstructed and dated by the 14C method. Three collapse calderas formed as a resu of these eruptions: KSi and caldera V 1700-1800 yrs ago; KS2 + KS3 and caldera IV 6000- 6100 yrs ago; KS4 and caldera III 8700-8800 yrs ago. KSi was the most voluminous eruptio with 18-19 km of pyroclastics and column height reaching 23 km. The volume of produci of KS2 + KS3 was 10-11 km3 and that of KS4, at least 1.5-1.7 km3. Sizes of calderas wer as follows: V - 4 X 6.5 km, IV - 5x5 km, III - presumably 2-3 km across. The juveni pyroclastics were supplied during eruptions rather rhythmically. Each rhythm began wil tephra ejection and completed with the formation of pyroclastic flows. The composition < products varied from andesited to rhyodacites: KS2 and KS4 - andesites dominated, KS3 - dacites and rhyodacites, and KSi - rhyodacites. It is possible that "the mechanism triggering onsets of all caldera-forming eruptions was the intrusion of very hot fresh magma of basi composition and its mixing with less hotter acid magma in the magma chamber existe previously. The eruptions, in accordance with their scales, may have had an impact on clima and ozone layer of the Earth. It is likely that the large acidity peaks in Greenland ice cor* result from these eruptions.

For the first time, tracks from 5 catastrophic lahars associated with subsynchronously occurring rueptions of the Avachinsky (great explosions with a large volume ejections of juvenile pyroclastics) and Koryaksky (voluminous lateral lava effusions) volcanoes have been found, described and dated by the C-14 method. Their reconstruction has been conducted. The C-14 ages of lahars are from 3500 to 3200 yrs ago, the calendar ages are from 1900 to 1500 BP. These lahars were significantly greater in volume, degree of hazard, and intensity of impact on the natural environment than those triggered by historic (19th - 20th centuries) eruptions from the both volcanoes. In particular, they caused a considerable (1-3 km) SW migration of the Avacha river down-stream channel, the largest waterway in Southern Kamchatka.