抄録
The Shirataki obsidian-rhyolite field (Shirataki Geopark, Hokkaido) contains many outcrops of densely compact obsidian layers of excellent quality. The Shirataki obsidian lavas (SiO2=76.7-77.4 wt.%) were erupted at ca. 2.2 Ma and formed a monogenetic volcano comprising 10 obsidian-rhyolite lava units. The thickness of the units ranges from 50 to ＞ 150m, and each unit comprises a surface clastic zone, an upper dense obsidian zone, an upper banded obsidian zone, a central thick rhyolite zone, a lower banded obsidian zone, a lower dense obsidian zone, and a lower clastic zone. The dense obsidian is ＞ 98% glass with microlites of mainly magnetite and plagioclase, and rare plagioclase phenocrysts. Obsidian and rhyolite within single lava flows have similar bulk-rock compositions and number density of microlites, although the rhyolite contains glass with perlitic cracks and a large amount of crystalline material (spherulites and lithophysae), while the dense obsidian contains 0.4-0.8 wt.% H2O. These geological and petrological features indicate that the formation of obsidian and rhyolite layers in the lava units was controlled mainly by the timing of the vesiculation and degassing of magmas, in addition to the cooling effect.
