Blackfoot Volcanic Field - Rhyolite Domes

The Blackfoot Volcanic Field is one of the largest regions of young, bimodal (basalt-rhyolite) volcanism in the interior U.S.A. In this view we are looking west to northwest from Highway XX about XX miles north of Soda Springs, ID. The prominent hills are three young (~50 ka) rhyolite domes. The domes consist of very similar looking rhyolite and occur along a notheast trending line, suggesting the are connected at depth along a northeast trending dike. They are located near the center of a youthful field of basaltic volcanoes. In contrast to the rhyolite domes, many of these basaltic volcanoes (mostly relatively small cinder and agglutinate cones) are aligned along northwest-trending systems of faults. Recent movement along the faults have produced a shallow, youthful graben near the center of the XXX valley.

China Hat is the largest of the domes (## km3). It consists of a central domical region, and an annular stubby lava flow, slightly elongated towards the southwest. Small quarries on the north side of the dome (just out of sight on the right side of the dome on this photo) reveal pyroclastic deposits (following photos). These are part of a small tephra cone formed during opening stages of rhyolite magma extrusion, immediately prior to formation of the dome. Rhyolite lithics are poorly vesiculated, and are commonly mixed with basaltic clasts, suggesting that the opening eruptions were at least partly hydrovolcanic (i.e. phreatomagmatic).