Stratigraphy and Eruptive History of Nodake Volcano, Unzen, Kyushu, Japan

Abstract

Nodake volcano is a small edifice, located at the south of the central part of Unzen Volcano, Kyushu, Japan. It is composed mainly of thick lavas and pyroclastics, and is divided into five geologic units in ascending order, i.e., Fukhoshi lava, Yugawachi pyroclastic flow deposits, Tawaraishi debris avalanche deposit, Nodake lava and Nodake pyroclastic flow deposit. The Fukkoshi lava, which has two lithofaces of dark gray and light gray in color, erupted about 100 to 110 ka. The Yugawachi pyroclastic flow deposits are divided into four units intercalated with weathered volcanic ash beds, and spread over the central and the southern flanks of Nodake volcano. The Tawaraishi debris avalanche deposit contains light gray dacitic block derived from Fukkoshi lava. The Nodake lava forms an andesitic lava dome on the summit area of Nodake volcano. The Nodake pyroclastic flow deposits are block and ash flows generated at about 74 ka upon collapses of Nodake lava dome.