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.

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Most visitors to Yellowstone are entranced by the spectacular thermal features and evidence of past volcanic eruptions that dot the landscape and that are a testament to the recent geological activity of the region.  But there are hundreds of millions of years of geologic history preserved in the rocks of Yellowstone National Park, telling a story of shallow seas, ancient volcanoes that left behind petrified wood, and glaciers that covered the landscape. 

One of the more interesting features is preserved at the top of Mount Everts and tells a story of missing geological time, where ash from the 2.1-million-year-old Huckleberry Ridge Tuff lies directly on top of 65-million-year-old sandstone. 

Mount Everts was named for Truman Everts, a member of the 1870 Washburn-Langford-Doane expedition to Yellowstone.  He became separated from his companions, horse, and all his supplies, and spent 37 days wandering in the wilderness, subsisting mostly off the roots of a thistle (which now bears his name) before he was discovered by a search party.

Mount Everts is composed of light-colored sediments that were deposited at the margins of a shallow sea that used to exist in the Yellowstone area.  The rocks are mostly shales and sandstones, and there are even some coal seams from plant matter that collected in coastal swamps.  The environment might have looked a little like the Mississippi delta does today.  These sediments accumulated during the Cretaceous period, about 120 to 65 million years ago.  But at the very top of the mountain, on its eastern side, is a red-brown layer that seems to cut at an angle across the grayish sediments.  This rock is the Huckleberry Ridge Tuff, which erupted from the Yellowstone system 2.1 million years ago.

The contact between the sediments and the tuff is an angular unconformity—a boundary where one rock unit cuts across the layering of another.  This indicates a period of erosion and a time when no new rocks were deposited—essentially, about 60 million years of geologic history is missing from the rock record!

The gap in geologic history occurred because of an episode of mountain building, which started in the Yellowstone area about 60 million years ago.  This episode contorted, folded, tilted, faulted, and uplifted the existing sediments.  As a result, no new sediments were deposited, and in fact the growing mountains began to erode.  The erosion continued unabated until the eruption of the Huckleberry Ridge Tuff—the first major volcanic event of the Yellowstone system, and the first period in which rock was deposited in tens of millions of years!  The tuff covered all the rocks of the region, including those of Mount Everts.

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Right at the unconformity, an ash fall deposit, which represents the opening stage of the Huckleberry Ridge caldera-forming eruption, sits right on top of the Cretaceous sediments.  Atop the ash fall is a welded pyroclastic flow deposit that marks a major explosion of the Huckleberry Ridge eruption.  The ash was very hot when it was emplaced, oxidizing the soil below the ash and giving it an orange tint.  And above the ash fall, at the base of the flow, there is a black layer caused by rapid chilling of the flow in the contact zone.  These colored layers can be spotted by keen-eyed observers from as far away as Mammoth Hot Springs!

And, in fact, Mammoth Hot Springs is the best place to view the Mount Everts unconformity.  If you have the good fortune to be able to visit Yellowstone, take a stroll on the boardwalks of the hot springs terraces and look to the north on a sunny afternoon.  The unconformity will be on display, brilliantly illuminated!

Unconformities are not unusual in the geologic record.  Another famous unconformity—the so-called Great Unconformity—lies near the bottom of the Grand Canyon and indicates, in places, about a billion years of missing geologic time!  Even though they represent gaps in geologic history, unconformities provide critical evidence of the geologic forces that have shaped a region.   And that includes in Yellowstone, where relatively young volcanic rocks lie directly atop the deposits that formed along the coast of an ancient inland sea!