Eruptions of the active Karymsky stratovolcano began about 5300 (6100 C-14) B.P. from within a pre-existing caldera which formed 7700 C-14 B.P. As indicated by 32 C-14 determinations on buried soils and charcoal, the volcano has gone through two major cycles of activity, separated by a 2300 year period of repose. The first cycle can be divided into two stages (6100-5100 and 4300-2800 B.P.). The earlier stage began with especially intense eruptions of basaltic andesite to dacite. The later stage was characterized by moderate-strength eruptions of andesite. The second cycle, which is characterized by weak to moderate intermittent eruptions of andesite, started 500 B.P. and continues to the present. Eruptive patterns suggest that this cycle may continue for at least another 200 years with an eruptive character similar to that of the recent past.

Gas samples have been collected at the place of magma effusion during the 1988 flank eruption of Klyuchevskoy, for the first time in the course of studies at this volcano. The high-temperature gases (1000–1100°C) are rich in water and halogens but depleted in sulphur. Their molar composition is close to chemical equilibrium at the collection temperature, while their oxidation state corresponds to redox conditions between FMO and NNO buffers. The isotopic composition of the water (δD = −71 to −44‰; δ18O = +6.3 to +8.4‰, versus SMOW) plots within the field of “primary magmatic” waters. The isotopic composition of H2 (δD = −187‰ to −160‰) is consistent with isotopic equilibrium between H2 and H2O in the conditions of emission. Both the chemistry of the gases and the low δ13C of carbon dioxide (−11.6‰, PDB) suggest extensive magma outgassing occurred during the course of the eruption.