Although it may have once been a towering stratovolcano, Goat Rocks has undergone extensive erosion since its extinction. Once eruptions ceased, the ongoing forces of glacial erosion, as well as hydrothermal alteration of the volcano's interior, stripped away layers of volcanic ash deposits and lava flows, uncovering the rocky lava spines where magma had cooled and hardened within the conduits of the volcano. Nearly all the remaining volcanic debris has been partially covered by a younger layer of rock.