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A 30-minute drive along U.S. Route 20 in Idaho between Ashton and Island Park will take you through the history of the first two caldera systems that formed in the Yellowstone region, 2.1 and 1.3 million years ago.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mark Stelten, research geologist with the U.S. Geological Survey.

When people think of Yellowstone, they typically think of Yellowstone National Park, complete with its spectacular geysers and hot springs, abundant wildlife, and stunning volcanic features that attract millions of visitors each year. Volcanic features within the park largely reflect the recent geologic history of Yellowstone (well, recent in a geologic sense) over the past 631,000 years. But the beauty and outstanding geologic history of the Yellowstone Plateau volcanic field is not confined to Yellowstone National Park. Indeed, a stretch of U.S. Route 20 in Idaho between Ashton and Island Park, just west of Yellowstone National Park, provides a unique opportunity to view 2.1 million years of Yellowstone’s volcanic history in an easy 30-minute drive.

So, ready to take a road trip?

Digital elevation model of Yellowstone National Park and vicinity
Digital elevation model of Yellowstone National Park and vicinity, showing the location of the calderas formed during each of Yellowstone’s three most recent volcanic cycles. The youngest caldera-forming eruption produced Yellowstone Caldera (green line), located within Yellowstone National Park. Henrys Fork Caldera (blue line), was formed as a result of Yellowstone’s second caldera-forming eruption, approximately 1.3 million years ago, and has since been filled in with basaltic lava flows that cause the flat, low-relief topography in that region. Figure modified from Christiansen et al. (2007).
View north along U.S. Route 20 from near Ashton, ID, at the margin of the Huckleberry Ridge Tuff in the distance
Google maps photo taken just north of Ashton, ID, along U.S. Route 20. The photo was taken looking north towards Island Park, ID. The forested ridge in the distance marks the margin of a caldera that formed 2.08 million years ago, when the Huckleberry Ridge Tuff erupted. The hill labeled as the Rhyolite of Snake Butte is the only known pre-caldera rhyolite lava flow, at 2.14 million years old.

Leaving Ashton

Aside from being the world’s largest seed potato producing area, Ashton is also a gateway to Yellowstone for many visitors. The city of Ashton lies just south of the calderas produced during Yellowstone’s first and second major eruptions at 2.08 and 1.3 million years ago, respectively. Driving north from Ashton along U.S. Route 20 provides an excellent view of some of the oldest volcanic features associated with Yellowstone. As you leave Ashton, U.S. Route 20 ascends a forested ridge—the margin of a large caldera that was formed 2.08 million years ago, when >2,400 cubic kilometers (>575 cubic miles) of magma erupted. This deposit is known as the Huckleberry Ridge Tuff.

This caldera-forming eruption is one of the earliest eruptions associated with the Yellowstone Plateau volcanic field, but it is not the oldest. Keep your eyes open as you drive north from Ashton along U.S. Route 20. At the east side of the caldera margin, you will notice a dome-shaped feature. This is the Rhyolite of Snake River Butte that erupted as a lava flow 2.14 million years ago and is the only known rhyolite eruption to precede the Huckleberry Ridge Tuff.

Up the caldera margin

Roadcut exposure of Huckleberry Ridge Tuff and Mesa Falls Tuff along U.S. Route 20 between Ashton and Island Park, Idaho
Photograph showing roadcut exposure of Huckleberry Ridge Tuff and Mesa Falls Tuff along U.S. Route 20 between Ashton and Island Park, Idaho. This exposure is approximately 60 feet high. From bottom-to-top, the section is composed of 1, Huckleberry Ridge Tuff: 2, unconsolidated loess (wind-blown sediment) separating the Huckleberry Ridge Tuff from the Mesa Falls Tuff; 3 and 4, base of Mesa Falls Tuff composed of non-welded white and well-sorted crystal-rich ash; and 5, Mesa Falls Tuff pyroclastic density current deposits. Figure from Vazquez et al. (2017)—

As you travel along U.S. Route 20, north of Ashton, you will leave flat ground and begin to climb up the margin of Yellowstone’s first-cycle caldera. There will be many rock outcrops exposed on both the east and west side of the road. The rocks you see along this part of the road are deposits from Yellowstone’s first two caldera-forming eruptions, the Huckleberry Ridge Tuff that erupted 2.08 million years ago, and the Mesa Falls Tuff that erupted 1.3 million years ago. These are two of the three major caldera-forming eruptions that the Yellowstone volcanic system has produced over its lifetime. Along the road, the Mesa Falls Tuff appears generally as a pink to white rock overlying the Huckleberry Ridge Tuff, which is a dense, grey to brown rock. Within the Mesa Falls Tuff deposit you can see both ash fall layers and deposits from pyroclastic density currents.

Into the caldera

Once you have gone over the caldera margin you are now within a composite caldera structure produced by the major eruptions of the Mesa Falls Tuff and Huckleberry Ridge Tuff. This portion of the caldera provides a chance to view the volcanic history of this region from 1.45 million years ago to present day. An excellent spot to view features within this region is from a pullout located at the intersection of U.S. Route 20 and Old Hwy 47 (the Mesa Falls scenic byway pullout). The landscape within the caldera is strikingly flat and the caldera margin is the forested ridge in the distance. Several lava flows and domes can be seen near the western margin of the caldera, appearing as forested hills. These rhyolites erupted shortly prior to and following the Mesa Falls Tuff. For example, Moonshine Mountain and Silver Lake dome are located within the caldera, whereas the Bishop Mountain flow and Green Canyon flow are not found within the caldera but instead are only exposed on and beyond the caldera margin.

Although less topographically striking, the flat landscape that characterizes this area is also a volcanic feature that represents what may be the last stage of volcanism in this region. Once the rhyolite magma chambers that existed underneath this area shut off and solidified, deeply sourced basalt magmas were able to ascend to the surface and erupt as lava flows. These basalts, similar to those that erupt in Hawaiʻi, are less viscous than rhyolite lava flows, so they fill in any topographic lows. This process has been occurring for the last several hundreds of thousands of years in the area and produced the flat landscape we have in the caldera today.

Looking west from the intersection of U.S. Route 20 and Old Hwy 47, in Idaho, at lava flows associated with the Henrys Fork caldera
Google Maps photo taken at the intersection of U.S. Route 20 and Old Hwy 47 in Idaho, looking west. The tops of Moonshine Mountain and Silver Lake dome, rhyolite lava flows located inside the caldera, are indicated with a dashed black line. Green Canyon flow and Bishop Mountain flow are located on and outside the caldera margin, behind Moonshine Mountain and Silver Lake dome.

End of the journey

After leaving the Mesa Falls scenic byway pullout, continue along U.S. Route 20 until you reach Island Park, ID. At this point you are now near the northern caldera margin and have reached the end of our journey through this part of Yellowstone’s geologic history. In just a short 30-minute drive (not counting the stops you will make to examine all of the rocks, of course!), you can get a glimpse of Yellowstone’s long and complicated volcanic history in this region, and the ways in which volcanism can shape the landscape.

If you find yourself in Yellowstone, consider taking a brief journey through geologic time by making this drive!


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