Volcanic Tuff in Central Utah

January 24, 2019

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Photographer: Patti Weeks 
Summary Author: Patti Weeks 

January 2019 Viewer's ChoiceFor several miles east of Interstate 15, along Clear Creek Canyon Road in central Utah’s Sevier County, there’s clear evidence of complex volcanic activity. Approximately 19 million years ago Mount Belknap of the Tushar mountain range and the Marysvale volcanic field — active 27-19 million years ago — exploded in a violent eruption. Tens of cubic kilometers of ash-flow tuff erupted to form what’s now called the Joe Lott Tuff Member of the Mount Belknap Volcanics. Volcanic rocks, along with ancient Fremont Indian petroglyphs, are featured at the Fremont Indian State Park and Museum and are seen for several miles east of the park on Clear Creek Canyon Road, a service road parallel to the east-west running Interstate 70.

The mushroom-shaped feature seen in the top photo is part of the Joe Lott Tuff Member and is comprised of moderately-welded alkali rhyolite ash-flow tuff with a small percentage of phenocrysts of quartz and other minerals. The bottom photo reveals what appears to be large pumice-like vesicles created by gases attempting to escape as the rock quickly cooled. A later volcanic flow appears to be atop this feature. The Joe Lott Tuff Member is named after Joseph Lott, a pioneer who raised a family and homesteaded 160 acres of farmland in this area in the mid-19th century. Photos taken November 6, 2018.

Photo Details: Top - Camera: SONY DSC-HX400V; Exposure Time 0.0006s (1/1600); Aperture: ƒ/3.5; ISO equivalent: 80; Focal Length: 10.9mm. Bottom - same except: Exposure Time: 0.0020s (1/500); Aperture: ƒ/5.0; Focal Length: 41.3mm.