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For the first time in two years, Yellowstone National Park visitors will be able to drive from Tower to Canyon Junctions across Dunraven Pass, now that road construction is complete.  The fascinating geology of this part of the park spans from well before Yellowstone’s recent volcanism to the latest glaciation of the region.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

Columnar jointed lava flows from 1.3 million years ago at Narrows o...
The two distinct, columnar-jointed, lava flows seen here in the Narrows of the Grand Canyon of the Yellowstone River erupted about 1.3 million years ago at about the time of the second caldera-forming rhyolitic ash-flow eruption, which occurred west of Yellowstone National Park. Thick layers of stream-deposited gravels lie beneath and between the two lava flows visible in this photo. The cliffs eroded into pinnacles below the lower basalt are older volcanic rocks of the Absaroka volcanic field.

The road over Dunraven Pass between Tower and Canyon Junctions, in the northeast part of Yellowstone National Park, exposes an outstanding sequence of geological history, much of which significantly predates recent Yellowstone volcanism.  Construction work closed the road during the summers of 2020 and 2021, but the route is scheduled to open to park visitors on May 27.  The occasion provides an ideal opportunity to rediscover some of the most diverse geology and outstanding vistas in all of Yellowstone National Park!  Let’s have a look, traveling south from Tower Junction to Canyon Junction.

The geology of the eastern portion of Yellowstone National Park, outside the boundaries on Yellowstone Caldera, is dominated by the Absarokas—a volcanic range that was active from about 53 to 43 million years ago.  In contrast to the giant Yellowstone caldera now filled with hummocky hills of rhyolite lava flows, the Absaroka Range back then might have looked more like the Cascade Range does today—a chain of large, steep-sided stratovolcanoes that erupted lava and ash flows, and whose slopes failed on multiple occasions with large collapses that created mudflow events.  The Absaroka volcanoes have a different rock chemistry, a different origin and evolution, and are distinct from the younger Yellowstone volcanism.

Even though the volcanoes have long since eroded way—the Absaroka Rage today only exposes the “guts” of these long-dead eruptive centers—debris from their eruptions is scattered across the region.  In fact, petrified trees near Tower Junction were preserved because they were encased in ancient mudflow deposits from these volcanoes.  The area is actually one of the largest deposits of petrified trees found anywhere in the world!

The road from Tower Junction (near Roosevelt Lodge) to Tower Fall—a magnificent waterfall that formed within the Absaroka volcanic rocks—follows the path of the Yellowstone River and provides numerous vistas into its spectacular canyon.  The base of the canyon is made up of debris from the ancient Absaroka volcanic eruptions. 

image related to volcanoes. See description
View of the Yellowstone caldera from the Washburn Range. The caldera extends to the base of the Red Mountains in the upper right of the photo. The rim of the Grand Canyon of the Yellowstone is in the foreground. National Park Service photo by Jacob Frank, July 1, 2017.

About halfway up the canyon walls of the Yellowstone River, and easily visible across the river from the road (especially from the Calcite Springs overlook), are a pair of dark, horizontal layers with a well-defined columnar structure (formed due to slow cooling of hot material).  These are lava flows known as the basalt of the Narrows and that erupted about 1.3 million years ago—roughly the same time as the formation of second cycle caldera-forming eruption, the Henrys Fork caldera, southwest of Yellowstone National Park in eastern Idaho.  The two lava flows are separated by a thick accumulation of gravels, including from glacial deposits, and they are overlain by more glacial debris.  Similar flows of columnar basaltic lava are also present along the road.  Named the Junction Butte Basalt, this flow erupted about 2 million years ago, after the formation of, and outside, the Huckleberry Ridge caldera—the first major caldera-forming eruption in the Yellowstone region 2.1 million years ago.

A curious non-volcanic feature is also present in the same area—oil seeps!  Deep beneath the surface in this area are sediments from an inland sea that are tens to hundreds of millions of years old.  These sediments contain hydrocarbons formed from long-buried organic material.  Perhaps aided by heat from Yellowstone’s magmatic system and fractures formed by circulating hydrothermal fluids, the hydrocarbons have made it to the surface as oil seeps at Calcite Springs and a few other areas in this part of Yellowstone National Park.

Continuing south from Tower Fall, the road climbs through Absaroka volcanic rocks to Mount Washburn.  Roadcuts along the route expose mud flow deposits that dip away from the summit of Mount Washburn, a massive Absaroka volcano.  The dipping rock units follow the slope of the ancient volcano!

If you have the time, be sure to take in the view from Mount Washburn—either from the trailhead, the parking area off Chittenden Road, or from the lookout atop the peak itself.  Looking east, the rugged Absaroka Range is laid out before you.  To the southwest is the Grand Canyon of the Yellowstone and Yellowstone caldera, 631,000 years old and largely filled by lava flows and glacial debris.  In the distance is Yellowstone Lake and the southern rim of the Yellowstone Caldera at the base of Mount Sheridan and Flat Mountain.

image related to volcanoes. See description
This huge boulder was dropped by a retreating glacier on the north rim of the Grand Canyon of the Yellowstone in Yellowstone National Park -- a testament to Yellowstone's icy past.

As you descend from Dunraven Pass toward Canyon Junction, keep an eye out for Washburn Hot Springs to the east.  Within about three miles of Canyon Junction, you will finally leave the steep slopes of the Absaroka Range and its diverse forests and cross into the lodgepole pine stands that grow on the soils of rhyolite lava flows that erupted after the formation of Yellowstone Caldera.  After another mile or two, you have descended into the caldera itself, although much of its boundary is obscured by the rhyolite lava flows.

The geology of Canyon Junction is dominated by rhyolite lava flows that are several hundred thousand years old.  Atop these flows are lake sediments and glacial debris, including some impressive boulders, known as erratics, that were transported at least tens of miles by ice that covered the Yellowstone region as recently as about 15,000 years ago.

The short drive between Tower and Canyon Junctions showcases tens of millions of years of Earth history, from petrified trees encased in ancient mudflows, to relatively young glacial and lake deposits that sit atop thick rhyolite lava flows.  If you happen to be in Yellowstone this summer, be sure to take advantage of the opening of the road over Dunraven Pass to explore some of the most diverse geology in all of Yellowstone National Park!

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