Smith Rock State Park –great geology at the edge of Oregon’s largest caldera
The view from outside the small visitor center at Smith Rock State Park offers a landscape of contrasts. The parking lot, and nearby camping and picnic areas, are flat, underlain by the edge of a basaltic lava flow that drops off in a series of steps to a narrow canyon, some 120 feet (37 m) below. The Crooked River, which rises about 100 miles (162 km) away in the High Lava Plains, fills much of the canyon bottom. Across the canyon, tan cliffs and spires of tuff, another volcanic rock, soar overhead. Smith Rock itself forms a peninsula of this rock, enclosed by a hairpin bend of the Crooked River. The tuff erupted 29.5 million years ago in the largest volcanic eruption to occur entirely within Oregon.
The eruption created the gigantic Crooked River Caldera, which stretches like an ellipse southeastward from Smith Rock more than 20 miles (32 km) and traversed along its length by the Crooked River. Around its perimeter are rhyolite bodies that intruded after the eruption along ring fractures surrounding the caldera. These rhyolites include Gray Butte, just north of the park, Grizzly Mountain, to the northeast, and Powell Buttes, some 15 miles (24 km) to the southeast. The imposing red-colored tower, just downstream from the hikers’ bridge in the state park, is a rhyolite dike that also intruded after caldera collapse.
Some of the finer-grained, airborne material likely accumulated 75 miles to the east to form much of the green-colored Turtle Cove Member of the John Day Formation, which has the same age. All told, the eruption produced more than 140 cubic miles (580 km3) of material. The eruption size as well as its caldera, however, eluded detection until about 2006 when Jason McClaughry and Mark Ferns of DOGAMI completed detailed mapping of the region. Now, some researchers even consider the eruption to be an early phase of the Yellowstone hot spot!
Hiking any of the state park’s numerous trails gives you plenty of chances to see the rock close-up. It’s pretty much all tuff, the volcanic rock made mostly of consolidated ash and pumice. Many tuffs become welded into hard rocks because of the high temperatures as they compact, but most of these tuffs are not welded and so relatively soft. Some exceptions include the red-colored welded tuffs at the crest of the Misery Ridge trail.
You might also notice that parts of the cliffs display layering inclined roughly southward. Up-close, you can see that some layering is defined by subtle changes in the proportions of fine to coarse ash to suggest variations in the ferocity of the eruption. Many layers contain small rock fragments, the rocks being older material incorporated into the eruption. Most of these rocks are volcanic, derived from the underlying Clarno Formation. A tiny fraction, however, are pieces of Permian-age limestone. These limestones are significant as they came from the basement rock, accreted to North America during the later Jurassic or early Cretaceous periods.
Numerous pocket-like holes dot many of the cliff faces. The outer edges of these features typically consist of more durable rock whereas the insides typically consist of softer, more easily erodible material. The durable crusts form because minerals in the tuff dissolve in pore water and re-precipitate on the surface as the pore water evaporates. At Smith Rock, the responsible minerals are mostly zeolites, a group of minerals with wide-ranging compositions but containing silica, aluminum, and water. Along with the enhanced weathering and erosion along fractures, this case-hardening results in an endless variety interesting shapes and pinnacles.
The case hardening also creates outstanding surfaces for climbing, so on sunny weekends, the state park draws hundreds of rock climbers of all abilities. Considered one of Oregon’s top climbing areas, Smith Rock offers more than 1500 routes. Moreover, the cliffs are recognized as the birthplace of American sport climbing, which unlike traditional climbing, benefits from anchors placed permanently in the rock
The basaltic lava flows at the state park have their own story, as they originated from vents on the north side of Newberry Volcano some 400 thousand years ago. If you look closely at the basalt, you’ll see that it’s uniformly very finely crystalline, but in some places full of air bubbles called vesicles. Because gases tend to rise through lava flows, the vesicles tend to become larger and more abundant towards the top of individual flows–as shown in the photo on the left –although you really need to click on it to see the larger version. The photo is 4 feet high from bottom to top.
For the most part, the lavas covered the older Smith Rock Tuff. They blanket the southwestern part of the caldera and sit on top the tuff in many places within the canyon. However, the cliffs of Smith Rock soar far above, so the lavas had to flow around them on their way to the Deschutes Canyon, some ten miles to the northwest. The interface between the high-standing tuff and the basaltic lava must have been highly irregular to make it ripe for erosion by the Crooked River, which now meanders through a lovely gorge.
And this post! It’s a draft of an entry I’m planning for my upcoming book, “Oregon Rocks!” –to be published by Mountain Press, probably sometime in 2021. I’m only allowed a few photos per entry, so here are a few more! You can click on each to see them at a larger size.
Some useful references:
Bishop, E.M., 2003, In search of ancient Oregon: Timber Press, Portland, 288p.
McClaughry, J.D., Ferns, M.J., Gordon, C.L., and Patridge, K.A., 2009A, Field trip guide to the Oligocene Crooked River caldera: Central Oregon’s Supervolcano, Crook, Deschutes, and Jefferson Counties, Oregon, Oregon Geology, v. 69, p. 25-44.
McClaughry, J.D., Ferns, M.L., Streck, M.J., Patridge, K.A., and Gordon, C.L., 2009B, Paleogene calderas of central and eastern Oregon: Eruptive sources of widespread tuffs in the John Day and Clarno Formations, in O’Connor, J.E., Dorsey, R.J., and Madin, I.P., eds., Volcanoes to Vineyards: Geologic Field Trips through the dynamic landscape of the Pacific Northwest: Geological Society of America Field Guide 15, p. 407-434.
Miller, M.B., 2014, Roadside Geology of Oregon: Mountain Press, Missoula, 386p.
Robinson, P.T., and Stensland, D.H., 1979, Geologic Map of the Smith Rock Area, Jefferson, Deschutes, and Crook Counties, Oregon. US Geological Survey Miscellaneous Investigations Map I-1142. 1:48,000.
Walker, G.W., and MacLeod, 1991, Geologic Map of Oregon. US Geological Survey. 1:500,000.
And some links:
Smith Rock State Park: https://oregonstateparks.org/index.cfm?do=parkPage.dsp_parkPage&parkId=36
Climbing article: https://www.climbing.com/places/thanks-volcanoes-climbing-in-oregon/