File:Cougar Point Welded Ash-Flow Tuff (Miocene, 10.5-12.5 Ma; near Grasmere, Idaho, USA) 1.jpg

English: Rhyolitic welded tuff from the Tertiary of Idaho, USA. (public display, Ashfall Fossil Beds State Historical Park, Nebraska, USA)

Volcanic tuff is a clastic-textured, extrusive igneous rock. Such rocks form from explosive volcanic eruptions. Tuff is composed of fine-grained particles - it's basically a solidified volcanic ash deposit. The original ash may have been deposited after falling from the sky (ash fall tuff) or after rushing across the landscape (ash flow tuff). Sometimes, the ash deposit is so hot that it self-lithifies before burial - it becomes a welded tuff. Occasionally, the heat is sufficient to melt the ash particles; with cooling, this results in glass.

Seen here is a welded ash flow tuff sample from a Miocene-aged volcanic center in southwestern Idaho called the Bruneau-Jarbidge Caldera. The volcano is part of the Yellowstone Hotspot track in southern Idaho. Volcanoes in this chain have large, explosive eruptions. Lava flows from effusive eruptive activity cover parts of the hotspot track in southern Idaho's Snake River Plain.

The Bruneau-Jarbidge Caldera has been identified as the source volcano for a famous fossiliferous ash deposit in northeastern Nebraska - Ashfall Fossil Beds. Numerous skeletons of Miocene vertebrates are preserved in the ash.

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From exhibit signage:

FROM THE CRATER

This rock solidified from a cloud of glowing hot ash ejected from a huge volcano in Idaho.

Rock name: Cougar Point Welded Ash-Flow Tuff

Composition: Rhyolitic - very rich in silica (Equivalent to granite, but much finer grained.)

Location: Southwestern Rim of Bruneau-Jarbidge Caldera, 8 miles southwest of Grasmere, Idaho on State Highway 51.

Age: 10.5 to 12.5 million years. This deposit was named by Bill Bonnichsen of the Idaho Bureau of Mines. He first identified the Bruneau-Jarbidge Caldera as the source of the Ashfall ash in 1982.

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Locality: exposure along Route 51, about 8 miles southwest of the town of Grasmere, southwestern Idaho, USA