Ketoi volcano forms a 10 km wide island 19 km NE of Simushir Island across Diana Strait in the Kurile Islands. The first historical eruption of Pallas Peak, during 1843-46, was its largest. Strong fumarolic activity was observed on the northern flank of Pallas Peak in the 1980's.

The volcano's structure is very complex and includes a mostly buried 5-km-wide Pleistocene caldera exposed only on the NE side. A younger 1172-m-high stratovolcano forms the NW part of the island, but cut by a horst-and-graben structure containing 2 fumarole fields.

A 1.5-km-wide freshwater lake (Lake Malakhitovoye) fills an explosion crater in the center of the island.

Pallas Peak> is a large andesitic cone in the NE part of the caldera and truncated by a 550-m-wide crater containing a brilliantly colored turquoise crater lake. Lava flows from Pallas Peak overtop the caldera rim and descend nearly 5 km to the SE coast.

Source: Smithsonian GVP Ketoi volcano information