Osorezan, on the Shimokita Peninsula in northernmost Honshu, contains a 5-km-wide caldera partly occupied by a shallow lake that drains through the N caldera wall. Dissected lava domes ring the caldera floor. The andesitic-to-dacitic volcano is largely Pleistocene in age, with activity dating back about 800,000 years. The last identified distal tephra deposit was erupted during the late Pleistocene. The only report of historical activity was in a Tohoku district travel document published in 1787, which stated that Yakeyama in Mutsu country (Osoreyama) had erupted. Older residents reported that large fires and clouds were sometimes seen from the summit, accounting for the volcano's local name, Yakeyama, or "Burning Mountain" (Murayama, 1987). Earthquake swarms have been documented during the second half of the 20th century, and magma bodies have been detected beneath the volcano.