Mt Usu (有珠山) volcano is Hokkaido is one of Japan's most well-known volcanoes. The summit of Usu consists of several lava domes that formed in historic time.
The last eruption was in March 2000 - September 2001, when Usu erupted for the first time in 22 years and forced the evacuation of 13,000 residents.
Usu is a small stratovolcano located astride the southern topographic rim of the 110,000-year-old Toya caldera. The center of the 10-km-wide, lake-filled caldera contains Naka-jima, a group of forested andesitic lava domes. The summit of the basaltic-to-andesitic edifice of Usu is cut by a somma formed about 7-8,000 years ago when collapse of the volcano produced a debris avalanche that reached the sea. Dacitic domes erupted along two NW-SE-trending lines fill and flank the summit caldera. Three of these domes, O-Usu, Ko-Usu and Showa-Shinzan, along with seven crypto-domes, were erupted during historical time. The 1663 eruption of Usu was one of the largest in Hokkaido during historical time. The war-time growth of Showa-Shinzan from 1943-45 was painstakingly documented by the local postmaster, who created the first detailed record of growth of a lava dome.
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