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Geology & History

The Black Rock Desert volcanic field is divided into five “subfields,” each with their own history. These are, from south to north: 1) Cove Fort, active from 1.3 to 0.3 Ma; 2) Twin Peaks, active from 2.7 to 2.1 Ma; 3) Beaver Ridge, active from 1.5 to 0.4 Ma; 4) Ice Springs, active in the last 100,000 years and still considered active today; and, 5) Fumarole Butte, active sporadically from 6.1 to 0.3 Ma. The Black Rock Desert volcanic field differs from other similar-age volcanic fields in that the basalts are high in iron (tholeiitic), and it contains significant amounts of rhyolite.

The Cove Fort subfield contains several rhyolite lava domes and associated pyroclastic deposits (tuffs) formed along the crest of the Mineral Mountains between 0.8 and 0.5 Ma. All basalt to andesite eruptions occurred in the valley east of the Mineral Mountains, beginning with fissure-fed pahoehoe flows at 1.3 Ma. Between 1 and 0.5 Ma two cinder cones, a shield volcano, and associated flows formed. The final eruption, a high-silica andesite, created a cinder cone and flow at 0.3 Ma.

Research indicates that the Twin Peaks subfield represents a rhyolitic caldera and associated eruptions. Early eruptions (2.7 Ma) formed an arcuate-shaped field of rhyolite lava domes that were then followed by the eruption of an ash-flow tuff that produced about 100 m (330 ft) of ground subsidence over an 120 km2 (45 mi2) area. That caldera was filled with a lake and associated sediments, which formed a thick limestone deposit. Eruptions of both basalt flows and rhyolite lava domes filled the rest of the caldera, culminating in the formation of the Burnt Mountain cinder cone and flows at 2.1 Ma.

The Beaver Ridge subfield was formed at the same time as the Cove Fort field, from 1.5 to 0.4 Ma. It contains several basalt flows, cinder cones that were highly eroded by Lake Bonneville, and dacite and andesite lava domes. Beaver Ridge contains the youngest known rhyolite in Utah, the White Mountain lava dome, which formed at 0.4 Ma. It is also unique for containing dacite and andesite that are highly iron-enriched, along with basalt flows that contain large, up to 3 cm (1.2 in), sunstone crystals (labradorite).

The Fumarole Butte subfield was active sporadically from 6.1 to 0.3 Ma. At 6.1 Ma. This subfield includes the basalt of North Butte and the rhyolite lava domes of Topaz Mountain. Topaz Mountain contains abundant topaz crystals that are often collected by gem enthusiasts. The youngest and most visible volcanoes in Fumarole Butte include the 0.9 Ma Fumarole Butte shield volcano and the 0.3 Ma Smelter Knolls basalt flow and maar crater.

Ice Springs subfield, the youngest in the Black Rock Desert volcanic field, is the most visible. It has been active for the last 100,000 years and has produced almost exclusively basalt. Eruptions from Ice Springs have helped geologists to constrain the timing of Lake Bonneville flooding in the region. Early eruptions at Ice Springs pre-date Lake Bonneville and were highly eroded by its arrival. The most prominent features of Ice Springs, however, were formed while Lake Bonneville was in the area, including the 15 ka Pahvant Butte and 14 ka Tabernacle Hill tuff cones. Tabernacle Hill is not as prominent as the much larger Pahvant Butte, although it has several of its own interesting features, including many lava tubes and a central crater that once contained a lava lake. The youngest eruption from Ice Springs volcano, occurred 720 years ago and represents the youngest volcanism in the state of Utah. This volcano consists of four cinder cones and associated flows. Scoria from the Ice Springs cinder cones is currently being mined for road material.