The Yellowstone Plateau Volcanic Field forms the high continental divide between the northern and middle Rocky Mountains.
The Yellowstone Plateau Volcanic Field forms the high continental divide between the northern and middle Rocky Mountains. The average elevation of the plateau is about 2,400 m (7,900 ft) and is surrounded on all sides but the southwest by mountainous terrain with peaks that reach 3,000-4,000 m (10,000 - 13,000 ft). The eastern Snake River Plain extends to the southeast as a structural depression that is about 350 km (220 mi) long. Yellowstone National Park, encompassing parts of Wyoming, Montana, and Idaho, is the location for the most recent volcanic activity.
Over the past 2.2 million years, the 17,000 km2 (6,500 mi2) Yellowstone Plateau has been shaped by explosive eruptions and profound collapse of the ground, enormously thick lava flows, uplift and extensive faulting, and the erosive power of flowing water and ice. Three eruption cycles each included an explosive caldera-forming eruption that excavated enormous volumes of magma from the volcano. Massive pyroclastic density currents deposited thick ignimbrite (tuff), and the area centered avove the evacuated magma chamber collapsed to form a caldera. Today, the three nested calderas have been partially filled in by the world's largest rhyolite lava flows, which spread up to 30 km (20 mi) from their source vents to thicknesses in excess of 100 m (330 ft). Smaller eruptions around the margins of the volcanic field have poured out basalt lava flows. Large earthquakes occur just off the plateau along the nearby Teton and Hebgen Lake faults, the latter of which ruptured in 1959 (Ms = 7.5), causing considerable damage to the region.
Prior to eruptive activity in the Yellowstone Plateau, the surrounding mountains and valleys were present in, essentially, their current structural configurations but with lower relief. The pyroclastic density currents that were produced during the explosive caldera-forming eruptions spread outward, filled valleys, and deposited ignimbrite tuffs in the surrounding mountains.