Of lava, Yellowstone and the Owyhee

The fascinating geology of the Owyhee Canyonlands ranges from millions of years ago, when the land was part of the supervolcano that’s now Yellowstone, to just thousands of years ago when Jordan Craters became coated in lava. Here, Oregon State University-Cascades student Chloe Hallock interviews one of her instructors, Daniele McKay, about this diverse landscape. Professor McKay holds a doctorate in geology from University of Oregon and studies volcanoes.

Chloe: Why do you feel it is important to protect the Owhyee Canyonlands?

Daniele: It is one of the most isolated areas in the continental United States, which makes it incredibly unique. There are not many places that remote left in the U.S., and it is worth protecting simply for that reason alone. The geology of the Owhyee area is also special. Volcanic deposits from explosive calderas associated with the Yellowstone Hotspot have been eroded by the river, exposing layers of rock that were deposited millions of years ago. In areas where the hotspot has created more recent calderas, these layers are not yet exposed and they may not be for millions of years. The Owyhee area provides an excellent opportunity to study these rocks and piece together the history of the Yellowstone Hotspot. We are lucky to have access to this pristine landscape with such rich geologic history and diversity.

What is the Yellowstone “hotspot” and how we can see it “moving” across North America. Can you provide some insight into that?

Contrary to how it appears, the Yellowstone Hotspot is not actually moving, but the crust of the Earth above it is moving. The hotspot has created a series of large calderas located in southeastern Oregon, across southern Idaho through the Snake River Plain, and ending in Yellowstone. This chain of calderas makes it seem like the hotspot is moving toward the northeast. However, the hotspot, or plume of hot material coming up from deep within the Earth, is stationary and the North American plate is moving over the top of it. As the plate moves, the hotspot creates a chain of large calderas, the oldest of which is in southeastern Oregon at about 16 million years old and the youngest is in Yellowstone at about 600,000 years old. This means for the last 16 million years the hotspot has been in the same place and the North American plate has moved southwest over the top of it.

Tell us more about the geology of the Owyhee Canyonlands.

The geology of the Owyhee area is quite diverse and breathtaking. One of the most immediate and impressive features are the deep canyons cutting through the area. Much of the rock in these canyons is rhyolite, which was produced by huge caldera forming-eruptions, similar to the eruption that formed Crater Lake but much, much bigger. These very explosive eruptions left behind thick deposits of rhyolite ash and pumice, which cooled and consolidated to form a type of rock called welded tuff. Some of these deposits are hundreds of feet thick, so when the river cut through them it created the vertical walls we see in the Owyhee canyons today. Similar deposits are also responsible for the spectacular pinnacles and cliffs along the Little Owyhee River.

Other unique landscapes in the Owhyee area are the soft, chalk-like hillsides seen in some sections of the river, such as Lambert Rocks. These are a great contrast to the steep canyons we just spoke about. The range of colors and textures in these hillsides are the result of ash deposits, lake deposits and lava flows. The ash layers and lava flows came from various ancient volcanoes in the area, and the lake deposits are a result of lava flows periodically damming the river. This created temporary lakes where materials like clay and silt were deposited. Over time these sediments were eroded into the colorful landscapes we see today. The contrasting colors were made more dramatic by lava flows that were injected into the colorful, horizontal layers. The hot lava essentially baked the sedimentary deposits, like terra cotta, leaving behind darker colors and adding more texture.

Another interesting feature in the Owyhee is the honeycomb-like rock seen in the Leslie Gulch area. What causes this?

This honeycomb texture in rock is called tafoni. It results when rock weathers at different rates, creating a network of pockets and holes. The rock at Leslie Gulch is welded tuff, created by the explosive caldera eruptions we spoke about earlier. The tuff is densely compacted in some sections, and less compacted in others, which makes it erode at different rates. In areas where it is less compacted, it is more susceptible to weathering and holes can form, creating a honeycomb-like texture. Other processes of erosion, like water freezing and thawing in the rock, and sand blowing across the rock, can contribute to tafoni. Water also carries dissolved minerals, which are deposited on the rock. Areas where more minerals have been deposited often become harder and are less susceptible to weathering.

Where are the most recent lava flows in the Owyhee Canyonlands?

The most recent volcanic activity occurred at Jordan Craters, about 3,200 years ago. Although this sounds like a long time ago to us, geologically this is very recent and it indicates that the Owyhee area is still volcanically active.

Do you have a favorite geological area in Oregon?

That is a tough question! Oregon is such a great state because of its diverse climates and ecosystems, which is ultimately related to its diverse geology. There are excellent examples of many different kinds of landscapes in Oregon, again due to the wide range of geology. The Cascades are a textbook example of very recent volcanic activity caused by subduction. The Owyhee area preserves ancient, highly explosive caldera eruptions that have been eroded over time. Other parts of the state preserve different geologic stories, from ancient ocean sediments deposited during the time of the dinosaurs, to recent earthquakes along the Pacific Coast. Picking one part of Oregon’s geologic story as my favorite is impossible because there are so many special geologic features throughout the state.