Abstract

Volcanic rocks are present in slightly to highly eroded extrusive centers west and north of Great Salt Lake. These rocks, which were dated by 23 new K-Ar analyses, range from 0.44 to 5 million years old. Eruptive products are mostly basalt, but rhyolite, rhyodacite, and andesite crop out as well. K-Ar ages of three slightly eroded shield volcanoes along the east side of Curlew Valley decrease southward from about 1.2 million to 440,000 years. The youngest eruptive product we have found is a basaltic ash that once blanketed the area northeast of the lake. The ash layer lies within Lake Bonneville deposits, and is about 26,500 years old.

The youthful ages for volcanic rocks in the Great Salt Lake area suggest a chance for future eruptions. These young eruptive centers coincide with modern seismicity, strengthening the suggestion that volcanic activity is part of the modern tectonics of the area. Although the most common eruptive product is basaltic lava, the presence of basaltic ash in Pliocene terrestrial deposits and Quaternary lacustrine deposits of the area indicates that explosive eruptions also took place. If the southward progression in ages of basalt in Curlew Valley is extrapolated, future eruptions would take place within Great Salt Lake, a likely setting for a hydromagmatic eruption. Even a small explosive eruption could impact the people and infrastructure of the Wasatch Front. Although the data point to a possible threat from volcanism, we have observed no Holocene ground deformation or high-temperature water sources that might reflect shallow magma.