During the historical period, Mount Iwate erupted in 1686-1687 with a pyroclastic surge; however, the eruption of 1732 was much larger and resulted in a substantial lava flow on the northeast slopes of the mountain. This four-kilometre long lava flow (Yakebashiri Lava Flow) has been designated a Natural Monument by the Japanese government. Mount Iwate has largely been quiet since 1732, with a small emission of steam and ash in 1919 and a series of volcanic earthquakes from 1998 to 2003.