The Chetco River flows through the ancient Klamath Mountain terrane, which is between 400 and 100 million years old, the oldest rocks in Oregon. The Klamath microcontinent was originally located beneath the ocean near southern California before separating hundreds of millions of years ago. Plate tectonics pushed the microcontinent north, and bits of granite, sea floor sediment, subduction zones, and coral reefs gradually accreted into small islands. Between 212 and 170 million years ago, a massive volcanic arc erupted on the Klamath microcontinent, binding the islands together in a single block. The Klamath microcontinent went through a period of intense tectonic activity known as the Siskiyou orogeny roughly 170 to 165 million years ago. The process was strong enough to force sedimentary rocks deep into the Earth's crust, melting them into large plutons of granite, which rose slowly to the surface. Shortly after, a large portion of sea floor was thrust over the older Klamath terranes; much of it is still visible atop Vulcan and Chetco peaks. This region is known as the Josephine Ophiolite, and contains a rare type of rock called peridotite, originating from the Earth's mantle.