Mount Iwate consists of a younger eastern symmetrical stratovolcano (Higashi-Iwate, East Iwate) overlapping an older western stratovolcano (Nishi-Iwate, West Iwate) which has collapsed to form a caldera. Nishi-Iwate was formed approximately 700,000 years ago, and forms two-thirds of the mountain body. Nishi-Iwate was a later (300,000 years ago) parasitic volcano, which now forms the summit of the mountain. The oval-shaped, 1.8 x 3 km caldera of Nishi-Iwate has a 0.5 km wide crater, partially filled by a crater lake called Lake Onawashiro. Several somma including Yakushidake, the largest, surround the more recent Higashi Iwate crater rim.