Geological Composition of Dōgo
The island of Dōgo formed approximately 500,000 years after the islands of Dōzen, and its topography is more varied. The tall, rugged cliffs bordering the sea on the island’s western side are predominantly composed of rhyolite. This rock appears white when weathered, but a newly split section reveals patterns of lavender, gray, and black. Rhyolite lava is extremely viscous, and many areas along Dōgo’s coast have intricately layered patterns formed by the hardening lava as it flowed over the earth’s surface. The layering effect is caused by the different weights and melting points of the rock’s mineral components. As the lava cooled, different components solidified and sank faster, causing them to separate.
Another striking feature of the coastal rhyolite is the large, spherical cavities in the rock. These are the result of the development of spherulites, typically spherical crystalline structures that form in rocks that are high in silica. The relatively high silica content of rhyolite is also responsible for the plentiful obsidian deposits that are found on the islands. Obsidian is formed when rhyolite cools rapidly, causing the silica inside to harden and become glassy and brittle. Deposits of obsidian can be seen in the rhyolite lava flows.
[labels for two circles, left to right]
weathered surface
newly fractured surface
