This whitish rock is commonly seen on Dōgo Island. It is distributed widely around the southern, western and northern coastlines.
Rhyolitic rocks are light igneous rock composed of over 70% colourless silicate minerals quartz and feldspar. These are high in the minerals quartz and feldspar. Within this category, alkali rhyolite rocks have a greater proportion of quartz and alkaline feldspar (if there is a greater quantity of alkali feldspar the rock becomes trachyte). Volcanoes that produce rhyolite are not that common. Only around 10% of active volcanoes in Japan are rhyolitic. The majority are basalt, largely composed of coloured minerals, or andesite which has both colourless and coloured minerals.
There is a reason that volcanic rocks are categorized according to their mineral composition. This is because the compositional differences in magma tell us about its origin and how it erupted. This information can tell us when volcanoes erupted, the reason for the volcanic activity, the types of rocks underground, and so on.
In the Oki Islands, the volcanic glass obsidian is high in quartz which has the same chemical composition as glass. Obsidian forms when rhyolitic lava cools rapidly with minimal crystal growth.
Obsidan has the same qualities as glass. It shatters easily and the shards are very sharp making them useful as cutting tools. For this reason, obsidian has been a very important asset for groups of people since ancient times.