Smith Rock

Central Oregon, USA

For what it’s worth, I’m also over at BlueSky, a short-form platform. I’m not really sure what I can do there that’s useful, so it might often just be announcements of the articles I do here on Substack. Check it out if you want to. It seems easy to create your own interest lists and (so far) is not full of ads, suggestions, and AI-generated crap. There are a lot of geology people and organizations there.

Most of the red-brown material in Smith Rock is tuff, solidified volcanic ash that erupted 29 million years ago from a caldera 25 kilometers (15 miles) across. The tuff is part of the John Day Formation, in places red and other bright colors because of variable oxidation of iron in the deposit. The volcanism 29 million years ago was part of the subduction of a complex terrane composed mostly of oceanic seamounts and related older volcanoes. This volcanism isn’t part of the later Columbia River Basalt Flows. See Miller, 2014, Roadside Geology of Oregon, for more information.

The flat, black basalt at left center in the top photo flowed from the Newberry volcano about 400,000 years ago, traveling about 65 km (40 miles) to get here. In the photo, the back edge of the basalt is its actual physical limit, where it was stopped by the existing older pile of tuff. That flow is probably the oldest from Newberry Volcano, where most of the volcanic rocks are quite a bit younger, even as recent as just 1,300 years old.  

The Crooked River Caldera that erupted the tuff at Smith Rock 29 million years ago was huge, erupting more than half the volume of material that came from the most recent Yellowstone caldera eruption 630,000 years ago — 580 cubic kilometers for Crooked River vs. 1,000 for the Lava Creek Tuff at Yellowstone (McClaughry and others, 2009, Field trip guide to the Oligocene Crooked River caldera: Central Oregon’s Supervolcano, Crook, Deschutes, and Jefferson Counties, Oregon: Oregon Geology 69:1)

Even more recently, the Crooked River cut through both the older tuff and younger basalt, producing the modern topography which is popular with rock climbers.

Photos from June 2019.