Volcanic Impact Crater-like Topography to the West of Usu Volcano, Southwestern Hokkaido, Japan

Paleotopographies Found at the Irie Shell Midden National Historical Monument

Abstract

Usu Volcano, one of the most active volcanoes in Japan, has erupted many times in the past and produced abundant ash and pyroclastic flows, including some that have been dated. Even though Usu Volcano has been investigated many times, no studies have been done on paleotopographic features resulting from ejecta that impacted in the vicinity of the volcano. However, we have discovered such features during excavation of the Irie Shell Midden National Historical Monument, located 5km west of Usu Volcano. The small craters or impact features caused by ejecta have the following characteristics: (1) They were covered by tephra Us-b in the year 1663. (2) Loamy, silty debris derived from basement Pleistocene deposits was splattered around the impact features. (3) The bottoms of the craters usually contain angular to subrounded gravel whose grain sizes are proportional to the diameter of the impact features. (4) The small impact craters are not artifacts of another process, because the boundaries between the craters and basement strata are indefinite to irregular. (5) The gravels are petrographically the same as lithologies that characterize Mt. Kousu, a lava dome within Usu Volcano.
Based on these criteria, we infer that these small impact features are paleotopographies formed by Usu Volcano ejecta in 1663.