A new approach to definition of the term "active volcano" has been worked out on the basis of detailed reconstruction of the eruptive activity of Kamchatka volcanoes. We suggest to consider active those poligenous volcanoes whose at least a single eruption has been ascertained and dated over the last 3000-3500. Two sub-groups of active volcanoes have been distinguished: sub-group of active volcanoes with data available on the historically documented eruptions or fumarolic activity and the one of potentially active volcanoes without such data but whose eruptions have been revealed over the last 3500 years. Using similar criteria the potentially active fields of the areal basalt volcanism, regional zones of cinder cones and concentrated manifestation of the multivent extrusive volcanism are also distinguished. We propose to use data obtained for the new catalogue of active Kamchatka volcanoes and for the long-term prediction of volcanic activity and associated hazard.

We have identified, and describe in this paper, a phase of multifactor natural catastrophism that has been the greatest during our era in Kamchatka, to be dated 0-650 A. D. Its chief components were. The last catastrophic eruptions to have occurred (a caldera-generating one at about 240 A. D., the pyroclastics volume being 18-19 km3 and a subcaldera one around 600 A. D. with the volume of lava and pyroclastics 9.5-10.5 km3) which were followed by irreversible relief changes over areas of hundreds of square kilometers and have affected rather injuriously many other environmental components. An exceptionally intensive activity of the other volcanoes (at least 75-80% of all active and potebtially active Kamchatkan volcanoes were erupting, tens of large and catastrophic eruptions occurred). Regional catastrophic and large ashfalls. A sharp, large-amplitude (between 1.5-2 and 12-15 m) tectonic uplift of various blocks in Kamchatka. Large earthquakes accompanied by large-volume rockfalls, landslides, large and frequent tsunamis. The catastrophic events of that time are argued to have been part of a worldwide phase of natural catastrophism that we hypothesize to have occurred at the beginning of our era.

Theories are presented about the time of origin, dynamics and formation mechanism of active volcanoes in the Kuril-Kamchatka zone during the Late Pleistocene-Holocene period. The study was based on more than 500 radiocarbon dates and the results of geological, geomorphological, tephrochronological and isotopic geochronological research. It is noted that there are different rhythms in the activity of the volcanoes and in the mechanisms and rates of formation of the volcanic structures. Periods of synchronous activity of the volcanoes on Kamchatka and the Kuril islands, and apparently, in other volcanic areas are identified, suggesting their global nature. The concept of "active volcano' is discussed. -P.Cooke

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.

В работе впервые рассмотрен широкий круг вопросов, связанных с историей развития рельефа Курило-Камчатской области. Проводится анализ истории образования различных типов морфоструктур в новейшее время. Особое внимание уделено анализу динамики образования морфоструктур под действием тектонических процессов и специфике формирования рельефа вулканических областей. Выделены основные тектонические эпохи.

В работе впервые рассмотрен широкий круг вопросов, связанных с историей развития рельефа Курило-Камчатской области. Проводится анализ истории образования различных типов морфоструктур в новейшее время. Особое внимание уделено анализу динамики образо­вания морфоструктур под действием тектонических процессов и специфике формирования рельефа вулканических областей. Выделены основные тектонические эпохи.

This study is concerned with the structure of Nemo III caldera which came into being 24.500-25.000 B.P. in the north of Onekotan Island, Northern Kurils. We describe the pyroclastics of the caldera- generating eruption sequence, and estimate its volume (9-11 km3) and weight (12-14) * 10^9 t. The composition of juvenile products was found to vary in an antidromous manner during the eruptions from dacytes to basaltic andesites. The pre-caldera rocks and the pyroclastics of the caldera-generating eruption belong to the tholeiitic series, while the post-caldera rocks to the calc-alkalic. Evidence was found in the pumice discharged by the terminal eruption to indicate mixed magmas of dacyte and basaltic andesite (basaltic?) composition. The caldera-generating eruption sequence is supposed to have been caused by a "fresh" basic magma injected into a pre-existing acid magma chamber produced by fractionation of the magma which had been supplied to the pre-caldera volcanic structures.