Structural variation and the development of thick rhyolite lava: A case study of the Sanukayama rhyolite lava on Kozushima Island, Japan

Geological studies of ancient rhyolite lavas are required for understanding flow dynamics because rhyolite lava eruption is an infrequent event. The Pleistocene Sanukayama rhyolite lava is about 130 m thick and located on Kozushima Island, Japan. Vertical sections of the lava are well exposed in the sea cliff and in a dissected valley, providing a good opportunity for the elucidation of the internal structural variations. The vertical lithofacies variations are classified into three zones of pumiceous, obsidian, and crystalline from the top to the base, respectively. The exposed area is considered to correspond to the upper half of the lava. In the boundaries between each zone, the lithofacies are mingled rather than exhibiting gradual change. This indicates that rheological contrasts are arisen within the lava due to differences of vesicularity and/or crystallinity. Various heterogeneous structures are recognized throughout the lava such as obsidian and reddish layers in the pumiceous zone, strongly deformed white-colored bands and a clastic layer in the obsidian zone, and dark-colored bands in the crystalline zone. We demonstrate on the basis of geological evidences and magnetic studies that these heterogeneous structures originated mostly from shear-induced fracturing within the advancing lava. This study indicates that brittle lava fracturing during ductile-brittle transition likely governs the acceleration of rhyolite lava heterogeneity.