Fig.1

Fig.2

Fig.3

Fig.4

Tolmachev Dol is a lava plateau northeast of Opala volcano, divided in two parts, northern and southern, by a large sub-latitude depression enclosing Tolmacheva lake and Tolmacheva river.  Both parts of the plateau host numerous Late Pleistocene and Holocene basaltic cinder cones. The latest eruption, which formed a cinder cone and a large lava flow in the far northwestern part of the plateau, occurred only 1600-1700 years BP as suggested by stratigraphic position of its erupted products between OP (1500 years BP) and KS₁ (1800 years BP) marker ash layers (Oleg Dirksen, personal communication). 

The most unusual Holocene event at Tolmachev Dol was an eruption of about 1 km³ of rhyolitic tephra from a large Chasha ("Chalice") crater in the northern part of the plateau (Figs. 2, 3). The vent consists of two craters (Fig.3), the smaller and less deep of which was probably the first to form since it was later partly destroyed by the Chasha crater. The eruption took place about 4600 ¹⁴C years BP. Its ash layer, coded OPtr, is a good marker in the regions between Avachinsky volcano in the north and Mutnovsky volcano in the south. Earlier it was thought to originate from Opala volcano based on the resemblance of its mineralogical and chemical composition (biotite-bearing high-potassic rhyolite) to the Opala erupted products (Braitseva et al., 1997). Tephra OPtr at a distance of 4 km southwest of the source forms a sratified package about 60 cm thick (Fig.4).

Literature

Dirksen OV, Ponomareva VV, Sulerzhitsky LD (2002) Eruption from Chasha crater (South Kamchatka) - a unique large silicic eruption at a monogenetic basaltic lava field. Volcanology and Seismology, 5  (In Russian)

Braitseva OA, Ponomareva VV, Sulerzhitsky LD, Melekestsev IV, Bailey J (1997) Holocene key-marker tephra layers in Kamchatka, Russia. Quaternary Research 47/2: 125-139