Geology & History
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.
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 cycles of high-volume eruptions each climaxed with an immense explosive phase, which produced pyroclastic density currents that deposited thick ignimbrite, followed by collapse of an area centered above the evacuated magma chamber, which formed a caldera. Today, the three nested calderas have been partially filled in by massive rhyolite lava flows that spread up to 30 km (20 mi) from their source vents to thicknesses in excess of 100 m (330 ft). 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. Yellowstone National Park, encompassing parts of Wyoming, Montana, and Idaho, is the location for the most recent volcanic activity.
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. Pyroclastic density currents that were produced during the explosive caldera-forming eruptions spread outward from the eruption site and filled valleys and deposited ignimbrite tuffs in the surrounding mountains.