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Caldera Chronicles

Caldera Chronicles is a weekly article written by U.S. Geological Survey Yellowstone Volcano Observatory scientists and colleagues.

May 13, 2019 Article Link

Feeling petrified! Evidence of Yellowstone's distant volcanic past found in rocks and trees

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Annie Carlson, Research Coordinator at the Yellowstone Center for Resources, Yellowstone National Park.

Yellowstone is renowned as one of the largest volcanoes in the world, generating intense interest from both the scientific community and the public. While much is known about volcanism occurring here within the past 2 million years, many people are not aware that this landscape was also profoundly shaped by much older volcanism. Fifty million years ago, the Absaroka Volcanic Field dominated the region that is now Yellowstone National Park. Today, you can explore extensive fossil forests resulting from this explosive past.

Amphitheater Mountain viewed from the northeast entrance road. NPS photo by Jacob Frank. (Click image to view full size.)
Amphitheater Mountain and Soda Butte Creek viewed from the northeast entrance road. National Park Service photo by Jacob Frank, June 29, 2017.

Imagine the scene 50 million years ago during the Eocene epoch. The climate was much warmer. Dense forests of subtropical and temperate trees covered the lowlands. Out of these forests emerged several magnificent stratovolcanoes resembling Mount Rainier or Mount Hood in the Cascade Range. Moving higher in elevation up the flanks of these volcanoes, the forest composition changed to coniferous trees and then to stunted alpine vegetation. At the summits, vegetation gave way to thick fields of snow and ice.

Now imagine that the Yellowstone Volcano Observatory (YVO) was actively monitoring these Absaroka volcanoes. (we know, we know, there weren't people here 50 million years ago. Please, humor us.) Eocene volcanoes followed the same laws of physics and chemistry as modern volcanoes. YVO scientists would have monitored ground deformation, seismic activity, gas emissions, and other signals much as they do today. Occasionally, YVO scientists would have noted dramatic spikes in these signals and witnessed several truly impressive eruptions rivaling the 1980 eruption of Mount St. Helens.

Upright petrified tree in Yellowstone National Park. NPS photo by Jacob Frank. (Click image to view full size.)
Upright petrified tree in Yellowstone National Park. National Park Service photo by Jacob Frank, July 4, 2017.

For roughly 8 million years, the mighty Absaroka volcanoes transformed the landscape that we now know as Yellowstone. During eruptions, lava and ash flows gushed from the volcanic summits. The heat melted snow and ice, creating lahars—thick flows of muddy debris that rushed down the mountainsides like wet cement. Similar to the 1980 Mount St. Helens eruption, millions of trees were leveled by blasts and buried under lahars. Layer after layer of lava, ash, and lahar flows accumulated to thicknesses of thousands of feet, eventually hardening into the rocks that make up today's Absaroka Mountain Range. This expansive mountain range covers the northern and eastern Yellowstone region. Not surprisingly, it contains vast deposits of petrified wood—remnants of the ancient buried forests.

The petrified trees of Yellowstone are remarkable for several reasons. First is the sheer volume—it's one of the largest deposits of petrified wood found anywhere in the world. Second is the diversity of tree species, including spruce, pine, cedar, oak, cypress, ash, dogwood, magnolia, and more. This diversity represents the array of ancient trees that inhabited the lowlands and up the flanks of the Absaroka volcanoes. Third is the fact that many of these petrified trees are upright rather than lying on the ground. Some trees were buried upright where they grew while other trees were transported downslope in debris flows and eventually settled vertically with their root wad beneath them.

Logs Floating in Spirit Lake, Mount St. Helens in the background. (Click image to view full size.)
Logs float in Spirit Lake near the Spirit Lake gaging station and outflow tunnel. View is to the south with Mount St. Helens in the background.

Today, park visitors can observe ample evidence of the Absaroka volcanoes. A fantastic example of an upright tree is found just west of Tower Junction—follow the sign for Petrified Tree. If you continue on the road toward the northeast entrance, you will drive by scenic mountains like Barronette Peak and Amphitheater Mountain. The stratified reddish-brown rocks of these peaks are the hardened layers deposited by multiple eruptions of the Absaroka volcanoes. And if you hike up Mount Washburn, you will stand atop the eroded remains of one of the actual volcanoes!

It is thrilling to have pieces of the geologic past laid out for discovery on the modern landscape. Recall once again the 1980 Mount St. Helens eruption. Now imagine scientists from the Cascades Volcano Observatory 50 million years in the future. (Remember, you're humoring us). Perhaps they are studying an impressive array of petrified wood resulting from geologic forces at work today!