Smith Rock

For a Google Earth .kmz file to explore Smith Rock, scroll to the very bottom of the page.

Smith Rock is a large monolith of rhyolite found just to the east of the tiny town of Terrebonne, Oregon. It’s well known in the outdoor community for being one of the greatest climbing spots in Oregon and a great place for hiking. Smith Rock has a very interesting history of study in comparison to the other field trips that are discussed on this website because knowledge about this particular site has really only just been discovered. When I first learned about Smith Rock in high school the prevailing theory of how it formed was through a local rhyolitic volcanic vent and that Smith Rock is all that remained after the flanks of the volcano had been eroded away. It was not until 2006 and 2007, after the area surrounding Smith Rock was mapped by geologists, that a large caldera was identified. Smith Rock turns out to be on northwestern edge of what is referred to as the Crooked River Caldera. The discovery of this caldera makes a few other Oregon geologic mysteries much more clear such as where the voluminous tephra in the John Day Formation originated from and why there was a huge rhyolitic rock seemingly with no source at Smith Rock. Also, it turns out based on the current mapping, the Crooked River Caldera is HUGE. If we compare the eruption of the Crooked River Caldera with another famous caldera, Yellowstone Caldera, the Crooked River’s eruption expelled a little more than half of the amount of tephra (>580 km3) than the eruption of Yellowstone Caldera 600,000 years ago (1000 km3) (McClaughry et al., 2009). A better way to think about this volume of magma is through comparison; for instance, the volume of the Crooked River Caldera deposit is equal to over 30 Crater Lakes. That is a huge eruption.

The Crooked River Caldera is approximately 25 mi by 17 mi (41 km by 17 km) stretching east-west between about 6 miles west of Prineville and Redmond, Oregon. It also stretches north-south between Grizzly Mountain and Prineville Reservoir State Park. The eruption that formed this caldera must have been huge to have created the resulting depression and also to have deposited the very viscous rhyolitic rocks that are associated with the caldera. The rocks that have been sampled and dated from the Crooked River Caldera correlate with the late Eocene (55.8 to 33.9 million years ago) to the early Miocene (23.03 to 5.332 million years ago) (McClaughry and Ferns, 2006, McClaughry et al., 2009). Specifically, Smith Rock has been dated at approximately 30 million years old using argon isotopes to radiometrically date samples (McClaugherty et al., 2009). Much of the Crooked River Caldera’s eruptive products are mainly rhyolite, other pyroclastic materials like tuff that have been deposited around the entire caldera area, and even some basalt. This type of volcanism is referred to as bimodal volcanism and can be seen in quite a few continental calderas, one great example is the Yellowstone Caldera (Seligman et al., 2014). Bimodal volcanism is just a fancy way of saying there are two distinct compositions of volcanic rocks, both mafic and felsic.

In the case of bimodal volcanism there are a couple ways to form the two different compositions of magma in the same volcanic system. One way would be through the cooling of basaltic magma in a magma chamber. As minerals form in the chamber they separate from the liquid causing the magma to become more felsic over time. This can take a huge amount of basaltic magma and over a LONG period of time make a small amount of rhyolite. This type of magma evolution is referred to as fractional crystallization. Another way to create a bimodal system would be to emplace a very hot, 1200°C basaltic magma into the crust. This basaltic magma is hot enough to melt some of the crust creating what is referred to as a partial melt. A partial melt is sort of the opposite of fractional crystallization and is driven by heat being added to a magma chamber as opposed to the cooling of a magma chamber. Adding heat by way of magma being injected into a magma chamber can initiate an eruption and so it’s more likely that a partial melt is the culprit for the Crooked River Caldera’s bimodal behavior.

Smith Rock is in the very western bit of the Crooked River Caldera. It is an exposure of tan-ish colored tuffs that that were erupted inside the caldera and extend up to 650 ft (200 m) in height (McClaugherty et al., 2009). A tuff is a rocks that is composed of lithified volcanic ash. These tuffs are mainly massive, meaning that they don’t show a lot of layering (maybe some very locally) and contain pumice. The presence and composition of tuff suggest eruption was most likely violent and explosive. Other evidence for an explosive type eruption is that the tuff found at Smith Rock correlates in age with the Member G tuffs exposed in the John Day Formation exposed north of Crooked River Caldera (McClaugherty et al., 2009). The age of these two tuffs are important because it is suggests that the Crooked River Caldera is a possible source for the tuffs found in the John Day Formation. The John Day Formation has a lot of tuff, the source of which has eluded geologists for many years. The Smith Rock tuff is also locally intruded by rhyolite dikes that break up the colors of the tuff showing a more reddish color. There are also exposures of green-tinted rocks that are thought to be hydrothermally altered tuff. After Smith Rock was emplaced, the Crooked River cut down through Smith Rock, leaving the sheer cliffs that are used by climbers. The Crooked River was later pinned by in its location by encroaching lava flows erupted from Newberry Volcano and flowed north 45 mi (75 km). These basalt flows create the flat buttes that can be seen surrounding Smith Rock State Park.