Andesite is an extrusive igneous rock that is intermediate in composition between basalt and rhyolite. It is typically gray to dark gray in color and has a fine-grained texture. Andesite lava flows are typically viscous and tend to form steep-sided cones.

Mount Usu is a stratovolcano, which means it is built up by layers of lava flows, ash, and other volcanic materials. The andesitic lava flows from Mount Usu are responsible for the volcano's distinctive cone shape.