The volcanic feature commonly called the Island Park Caldera in the states of Idaho and Wyoming, U.S., is actually two calderas, one nested inside the other. The Island Park Caldera is the older and much larger caldera, with approximate dimensions of 58 by 40 mi (93 by 64 km). Its ashfall is the source of the Huckleberry Ridge Tuff that is found from southern California to the Mississippi River near St. Louis. This super eruption, approximately 2,500 km3 (600 cu mi) occurred 2.1 Ma (million years ago) and produced 2,500 times as much ash as the 1980 Mount St. Helens eruption.
The caldera clearly visible today is the later Henry's Fork Caldera that is the source of the Mesa Falls Tuff. It was formed in an eruption of more than 280 km3 (67 cu mi) 1.3 Ma. The two nested calderas share the same rim on their western sides, but the older Island Park Caldera is much larger and more oval and extends well into Yellowstone Park. The Island Park Caldera is sometimes referred to as the First Phase Yellowstone Caldera or the Huckleberry Ridge Caldera.
To the southwest of the caldera lies the Snake River Plain, which was formed by a succession of older calderas marking the path of the Yellowstone hotspot. The Plain is a depression, sinking under the weight of the volcanic rocks that formed it, through which the Snake River winds. Other observable volcanic features in the Plain include: the Menan Buttes, the Big Southern Butte, Craters of the Moon, the Wapi Lava Field and Hell's Half Acre.
These calderas are in an area called Island Park that is known for beautiful forests, large springs, clear streams, waterfalls, lakes, ponds, marshes, wildlife, and fishing. Harriman State Park is located in the caldera. Snowmobiling, fishing, and Nordic skiing, and wildlife viewing are popular activities in the area. The peaks of the Teton Range to the southeast are visible from places in the caldera.
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