The island of Onekotan lies to the south-west of Kamchatka, near the northern end of the Kuril island arc. The uninhabited island (excepting several Russian border guards), c45km x 7-15km across, contains two, at-present quiescent, active volcanoes, with Mt Krenitsyn in the south, the highest point at 1324m and Nemo Peak – 1019m – in the north. Both peaks are central volcanoes associated with large calderas, the peaks being developed within the caldera remnants.
The Kuril arc, part of the circum-pacific volcanic belt, began development to the west and north-west of the Kuril Trench during the Cretaceous Period (c90myrs BP) where the mobile Pacific plate subducts beneath the Okhotsk plate. In common with such locations, explosive andesitic volcanism is dominant. The Pacific plate has an approximate north-westerly progression rate (at present) of c8mm/yr, the subduction shear being at a depth of about 120km below Onekotan; with the island arc/trench distance at around 300-350km. The crustal thickness beneath Onekotan has been estimated at 20km.
The pre-glacial form of the northern eruptive centre of Onekotan comprises an andesitic composite stratovolcano complex of two semi-concentric but overlapping calderas, the largest of 10km diameter. A third caldera, from about 25000years ago, destroyed a cone built within these calderas, with the present twin-peaked Nemo Peak developing within this latest structure, along with the crescentic Chernoe Lake.
Tephro-chronological dates from earlier historic and pre-historic central vent eruptions have been listed (from GVP) at (approximately) 9550, 9050, 7550, 5050, 3850, 2500 years BP and 750 and 1350AD.
Within historic times, moderate explosive eruptions have occurred in 1710, 1906, ?1932, and 1938, with subsequent fumarole/thermal activity.
The southern part of the island is dominated by the spectacular combination of the almost circular Tao-Rusyr caldera with the subsequently developed symetrical cone, Krenitsyn, within the 7km caldera-lake, Kal’tsevoe.
The 7.5km diameter caldera was formed c7500 years ago (from radio-carbon dating) by chamber collapse following a VEI-6 eruption; one of three VEI-6 eruptions in the Kuril arc (here classified as producing 10-100km^3 total ejecta) in the period 9400-7500 years BP. In addition there have been, since 1712 when the area was first exploited, ten VEI-4 eruptions in Kuril arc.
The strato-volcanic cone of Krenitsyn peak, 1325m high and 4km in diameter, is composed of andesite-basalt. Krenitsyn has a 350m wide-100m deep summit crater, and a subsidiary crater on the north-east flank.
At present it has been quiescent since the emplacement of a small lava-dome from a lateral conduit in the cone at lake-level in 1952, a short time after a M9 earthquake along the subduction fault, since when only thermal/fumarolic activity has been observed.
P.S. Keep the questions coming for the Q&A on GPS:s in Iceland. D.S.