"Narugo (also known as Naruko) is a poorly defined, 5.5 x 7 km caldera NW of the city of Sendai. The caldera, formed during eruptions about 73,000 and 45,000 years ago, contains a cluster of four dacitic lava domes of Holocene age surrounding the 400-m-wide lake-filled Katanuma crater. The highest of the low, but steep-sided lava domes, Oga-dake, lies SE of the lake and reaches only 470 m above sea level. Oga-dake and Kurumiga-dake domes are capped by conglomerates and mudstones uplifted during dome growth. The only known eruption at Narugo in historical time occurred in 837 AD. Water vapor and sulfur-dioxide gas emission occurs from Shurado crater on Toyagamori lava dome west of the lake and from the bottom of the crater lake, which is one of the most acidic in Japan, with a pH of 1.6. Solfataras are found on the western shore of Katanuma, and sulfur is mined from sediments on the lake floor."  -Smithsonian Volcano Archive
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