The conical Etorofu-Atosanupuri stratovolcano forms a prominent peninsula joined to the SW side of Iturup (Etorofu) Island by a low isthmus only 30 m high. A somma volcano whose caldera rim is exposed only at about 900 m elevation on the SE side was constructed during the late Pleistocene or early Holocene, forming an island up to about 1.5 km high that was later connected to the main island by erosional material. A fault with large displacement offsets the NW side of the somma. The 2-km-wide caldera was subsequently largely overtopped by a central cone that forms the present summit. Strombolian eruptions have dominated the history of this basaltic volcano; few lava flows are exposed. Only two historical eruptions are known, during 1812 and 1932.