The two Islands of Ryponkich and Yankich are parts of the single volcano Ushishir, located 17 km south-west of Rasshua Island. Ushishir volcano is a heavily destroyed caldera with a diameter of up to 1.6 km with a slope height of 20 m in the north to 130 m in the south. The southern part of the caldera wall is torn, and the waters of the Pacific Ocean fill its bottom, forming a caldera bay with a diameter of 1 km and a depth of up to 58 m. The width of the entrance to the bay barely reaches 300 m, the width of the eastern lintel between the crater bay and the open ocean is 200 - 300 m. Inside the caldera are the remains of four extrusive domes. Two older domes in the southern part of the crater are connected by a low sand bar with the caldera. Approximately in the center of the caldera bay, two flat, well-preserved extrusion domes measuring 100 x 200 m at a height of 12 m and 200 x 300 m at a height of 32 m rise up. All four domes are composed of fairly uniform pyroxene-hornblende andesites. Somme rocks from the eastern part of the caldera are hornblende dacites, augite andesites, andesite basalts, two-pyroxene andesites.
Despite the lack of information on eruptions, it can be assumed that the formation of two northern extrusive domes occurred after 1769, when the Island was visited by Cherny. He described the Island in great detail and even noted individual rocks and stones off the coast of the Island. In his description there is mention only of two domes in the bay; there are currently four domes in the caldera. Obviously, two more recent domes appeared after 1769.
References
Горшков Г.С. Вулканизм Курильской островной дуги / Отв. ред. Рудич К.Н. 1967. 288 с. Горшков Г.С. Каталог действующих вулканов Курильских островов // Бюллетень вулканологических станций. 1957. № 25. С. 96-178. |
