Yellowstone Hotspot - Yellowstone Plateau

Yellowstone Plateau

The Yellowstone Plateau volcanic field is composed of three adjacent calderas. The Henry's Fork Caldera in Idaho was formed in an eruption of more than 280 km3 1.3 million years ago, and is the source of the Mesa Falls Tuff. The Henry's Fork Caldera is nested inside of the Island Park Caldera and the calderas share a rim on the western side. The earlier Island Park Caldera is much larger and more oval and extends well into Yellowstone Park. Although much smaller than the Island Park Caldera, the Henry's Fork Caldera is still sizeable at 18 miles (29 km) long and 23 miles (37 km) wide and its curved rim is plainly visible from many locations in the Island Park area.

Of the many calderas formed by the Yellowstone Hotspot, including the later Yellowstone Caldera, the Henry's Fork Caldera is the only one that is currently clearly visible. The Henrys Fork of the Snake River flows through the Henry's Fork Caldera and drops out at Upper and Lower Mesa Falls. The caldera is bounded by the Ashton Hill on the south, Big Bend Ridge and Bishop Mountain on the west, by Thurburn Ridge on the North and by Black Mountain and the Madison Plateau on the east. The Henry's Fork caldera is in an area called Island Park. Harriman State Park is situated in the caldera.

The Island Park Caldera is older and much larger than the Henry's Fork Caldera with approximate dimensions of 58 miles (93 km) by 40 miles (64 km). It is the source of the Huckleberry Ridge Tuff that is found from southern California to the Mississippi River near St. Louis. This supereruption occurred 2.1 million years BP and produced 2500 km3 of ash. The Island Park Caldera is sometimes referred to as the First Phase Yellowstone Caldera or the Huckleberry Ridge Caldera. The youngest of the hotspot calderas, the Yellowstone Caldera, formed 640,000 years ago and is about 34 miles (55 km) by 45 miles (72 km) wide. Non-explosive eruptions of lava and less-violent explosive eruptions have occurred in and near the Yellowstone Caldera since the last supereruption.

The most recent lava flow occurred about 70,000 years ago, while the largest violent eruption excavated the West Thumb of Lake Yellowstone around 150,000 years ago. Smaller steam explosions occur as well - an explosion 13,800 years ago left a 5 kilometer diameter crater at Mary Bay on the edge of Yellowstone Lake. Both Heise and Yellowstone produced a series of normal caldera-forming magmas and a series of light post-caldera magmas. The final stage of volcanism at Heise was marked by light magma eruptions. If Heise is any indication, this could mean that the Yellowstone Caldera has entered its final act, but the volcano could exit with a climactic fourth caldera event analogous to the fourth and final caldera-forming light magma of Heise (the Kilgore Tuff). The appearance of light magmas indicates that the uppermost portion of the continental crust has been consumed, exhausting the melting potential of the crust above the mantle plume. In this case Yellowstone could be expiring. It could be another 1-2 million years before a new supervolcano is born to the northeast, and the Yellowstone Plateau volcanic field joins the ranks of its deceased ancestors in the Snake River Plain.

The Yellowstone hotspot is one of the few volcanic hotspots underlying North America; others include the Anahim and Raton hotspots.