During or shortly after major movement on the Hoback fault, and perhaps
related to it, there was a complex series of volcanic eruptions west and
north of the town of Jackson, along the south boundary of the park. In
rapid succession, lavas of many types, with a combined thickness of more
than 1,000 feet, were extruded and volcanic plugs intruded into the
near-surface sedimentary rocks. These volcanic rocks can be seen on the
East and West Gros Ventre Buttes.
There are no active volcanoes in the Teton region today and no
postglacial lava flows or cinder cones. Five miles north of Grand Teton
National Park are boiling springs (Flagg Ranch hot springs) that are
associated with the youngest (late Quaternary) lavas in southern
Yellowstone Park. Elsewhere in Jackson Hole are a number of lukewarm
springs but their relation to volcanic rocks has not been determined.
What happened to the vast thicknesses of volcanic debris? We know they
existed because sections of them have been measured on the eroded edges
of uptilted folds and fault blocks. Many cubic miles of these rocks are
now buried beneath the floor of Jackson Hole, but a much greater volume
was carried completely out of the region by water, ice, and wind during
the final chapter of geologic history.