one that is currently clearly visible. The Henry's 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 by . 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 by wide. Non-explosive eruptions of lava and less-violent explosive eruptions have occurred in and near the Yellowstone Caldera since the last super eruption. 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 the Heise and Yellowstone volcanic fields produced a series of caldera-forming eruptions characterised by magmas with so-called "normal" oxygen isotope signatures (with heavy oxygen-18 isotopes) and a series of predominantly post-caldera magmas with so-called "light" oxygen isotope signatures (characterised as low in heavy oxygen-18 isotopes). 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 stage, but the volcano might still exit with a climactic fourth caldera event analogous to the fourth and final caldera-forming eruption of Heise (the Kilgore Tuff) – which was also made up of so-called "light" magmas. The appearance of "light" magmas would seem to indicate that the uppermost portion of the continental crust has largely been consumed by the earlier caldera- forming events, exhausting the melting potential of the crust above the mantle plume. In