Geohistory - Rocks from Volcanic Islands Period
Rhyolite is one of the representative rocks of Dōgo Island. Much of the western coast as well as the mountains are made up of this rock, which formed from volcanic activity around 5.5 million years ago. The flowing striped pattern (flow structure) seen in the often white-colored rock face is a characteristic feature of rhyolite. In the Oki Islands, it is also characterized by its high content of alkali feldspar, which is rich in potassium and sodium, etc. Rhyolite with this high content of alkali feldspar is known as trachyte. In the Japanese archipelago, many volcanoes are composed of basalt, a rock rich in colored minerals, or andesite, a rock with intermediate components; there are not many volcanoes composed of rhyolite or trachyte. It can be considered a rare rock type in Japan.
Rhyolitic magma has a high viscosity (stickiness). When it flows from volcanoes, it tends to make high and steep mountains. This is one of the reasons for the steep topography of the Oki Islands.
