Diamond Peak forms part of the central arc of the Cascade Volcanoes, and it joins several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades. Formed towards the end of the Pleistocene epoch, these mountains are underlain by more ancient volcanoes that subsided due to parallel north-south faulting in the surrounding region. The High Cascades consist mostly of Pleistocene volcanoes constructed on top of upper Cenozoic lava flows and volcanic vents (produced within the last 15 million years), with compositions varying from homogenous, mafic lava flows (rich in magnesium and iron) and pyroclastic rocks to lava more enriched with basalt and rhyodacite.