Higher on the volcano, basaltic lava is mostly absent, andesite dominates, and rhyolite and small volumes of dacite are present, the latter mainly near the 7 x 12 kilometer Medicine Lake caldera . At the heart of Yellowstone's past, present, and future lies volcanism. Catastrophic eruptions occurred here about 2 million years ago, then 1.2 million years ago, and then 600,000 years ago. The latest eruption spewed out nearly 240 cubic miles of debris. The magmatic heat powering those eruptions still powers the park's famous geysers, hot springs, fumaroles, and mud pots.
Rugged mountains flank the park's volcanic plateau, rewarding both eye and spirit.