Fig.1

Fig.2

Fig.3

Fig.4

Ichinsky is a large Somma-Vesuvius like edifice: its main Late Pleistocene cone bears a large summit crater (caldera) enclosing snow-and ice-covered extrusive domes (Figs.1, 2). Alpine glaciers descend down most of the valleys. The volcano was considered active based on fumarolic activity and rather fresh appearance of some of its lava flows as well as of the two cinder cones with lava flows (North Cherpuk and South Cherpuk) located southwest of the volcano. 

The detailed geological and tephrochronological studies have allowed us to document several large Holocene eruptions of Ichinsky. The earliest volcanic deposits, overlying Late Pleistocene moraine, are presented by thick pumice fall and flow units, presumably associated with the summit caldera. The stratigraphic position of these units suggests their age of about 10,000-15,000 years. Ten voluminous dacitic lava flows, issued from the vents at the summit caldera rim, overlie caldera deposits but are covered by a soil-pyroclastic sequence with a marker ash layer from Khangar volcano with an age of about 6900 ¹⁴C yrs. Therefore, we estimate the age of these lava flows at 8000-10,000 years.

The most interesting eruption of Ichinsky occurred about 6500 ¹⁴C yrs BP. It started from the formation of a large cinder cone (South Cherpuk) with its extensive lava flow 21 km southwest of Ichinsky volcano. Products of this eruption are basalt-basaltic andesite with a total volume of about 2.5 km³. The eruption proceeded from the next vent (North Cherpuk) (Fig.3) located 11 km SSW from Ichinsky, where a large cinder cone and a 30-200 m thick andesitic lava flow were formed. Total volume of this eruption products is estimated at about 2 km³. At the third stage, a huge andesite-dacite extrusive dome started to form at the southwest slope of Ichinsky itself (Fig. 2). Its growth was accompanied by block-and-ash pyroclastic flows, associated surges and lahars. Total volume of the dome and associated products is estimated at 2-3 km³. Tephra units of all these three eruptions form distinct layers directly overlying each other (Fig.4). Even in case these layers were measured in peat bogs with high rate of peat accumulation, no interval was documented between them. We think that all the three eruptions were caused by a basaltic dyke, which came to the surface to form S.Cherpuk cone, but also intruded two silicic chambers that triggered eruptions from N.Cherpuk and Ichinsky itself. Total volume of products of these eruptions is estimated at a minimum 6-7 km³.

Later eruptions of Ichinsky occurred 4000-4200 and 2400-2600 ¹⁴C yrs BP. The most recent eruption took place within the last 1800 years (possibly even few hundreds of years ago) and produced a voluminous lava flow (Fig. 2). Thus, we consider Ichinsky a dormant volcano able to resume its activity. 

Ichinsky Holocene erupted products (including Cherpuk cones) form a continuous trend from basalt to rhyodacite. They have high-K₂O contents, silicic varieties contain hornblende and biotite. 

Maria Pevzner

Literature

Volynets ON, Patoka MG, Melekestsev IV, Zubin MI. (1991) Ichinsky volcano. In: Fedotov SA and Masurenkov YuP (Eds) Active volcanoes of Kamchatka. V. 1. "Nauka Publishers", Moscow, pp 280-294 [in Russian and English] 

Pevzner MM, Melekestsev IV, Volynets ON, Melkii VA (2000) South Cherpuk and North Cherpuk - the largest Holocene monogenetic volcanoes on the Sredinnyi Range of Kamchatka. Volcanology and Seismology. Vol.21, pp.667-681 [while the  description of the erupted products in this paper is OK, the ages of the eruptions are not correct and are refined in this text]

Pevzner MM, VV Ponomareva, LI Bazanova (2002) New data on the Kamchatka back-arc volcanism during the Holocene time. Abstracts of the 3rd Biennal Workshop on subduction processes, Fairbanks, Alaska, June 2002.

Churikova TG, Dorendorf F, Wörner G. (2001) Sources and fluids in the mantle wedge below Kamchatka, evidence from across-arc geochemical variation. Journal of Petrology, 42/8: 1567-1593