Gray Butte is a bimodal, basalt and rhyolite, volcanic center in north central Oregon. The basalt at Gray Butte is about 18 million years old. The rhyolite at Gray Butte, shown in the photo, is about 12 million years old. In this photo Gray Butte is the high peak on the left. The cliffs to the right are Smith Rock, also made of rhyolite.
Gray Butte (peak in background) is made of rhyolite tuff that was deposited as an ash flow. The vent for the eruption has not been located. The rock outcrop in the bottom left is a rhyolite dike that was probably a vent for some of the Gray Butte volcanic rocks.
Basaltic lava flows from Newberry volcano flowed down the canyon of the Crooked River about 1.2 million years ago. These flows (middle left of photo) now form low cliffs above the Crooked River in Smith Rock State Park. The steep vertical fin of rock on the right side of the photo is a dike.Steep rocks in the background are the Smith Rock Tuff.
Photos courtesy of the Oregon Department of Geology and Mineral Industries. Special thanks to Rosemary Kenney.
Sources of Information:
Morris-Bishop, E., Smith, G.A., 1990, A field guide to the geology of Cove Palisades State park and the Deschutes Basin in central Oregon: Oregon Geology, v. 52, no. 1, p. 3-16.
Morris-Bishop, E., 1989, Smith Rock and Gray Butte complex: Oregon Geology, v. 51, no. 4, p. 75-80.
Obermiller, W.A., Geologic, structural, and geochemical features of basaltic and rhyolitic volcanic rocks of the Smith Rock/Gray Butte area, central Oregon: Eugene, Oregon, masters thesis, University of Oregon, 189 p.