The volcano on Oshima, Japan is a stratovolcano, which is the most common. A stratovolcano, or a composite volcano, is made of alternating layers of lava and pyroclastic material. Lava is molten rock above the surface, and pyroclastic material is molten lava that hardens while soaring through the air. There was a landslide, and the part that slid in the ocean could have caused the tsunami. The volcano is now horseshoe shaped. The summit collapsed, and was caused by a lot of ash on the volcano.
The Oshima Boom
The volcano on Oshima, Japan erupted on 1741, and due to the eruption, a tsunami occurred. The stratovolcano, which is the most common,is now dormant. During the 1741 eruption, it revealed two types of ash.
The Kinds of Ash
On January 22 to May 18,1742, the volcano erupted. The volcano was an explosive volcano, and black and grayish white ash fell down on Matsume,with about 10 cm in thickness; the Oshima Peninsula, the Tsugara district, which both also received about 10 cm of ash; and fell on the Oshima Peninsula once again on May 18 with 20-30 cm of ash. The black ash was basaltic, and the light gray ash was andestic ash.
There was an eruption in August 29, 1741 simultaneously a earthquake (m>7.5) is estimated to have occurred in the sea bottom near Ōshima, which generated a destructive tsunami and triggered a sector collapse of the volcanic body of Nishiyama.