Yellowstone’s unconformity—over 60 million years of missing geologic history!

Release Date:

Visitors to Mammoth Hot Springs in Yellowstone National Park might have noticed an odd geological feature near the top of Mount Everts—an unconformity! This juxtaposition of different rocks serves as a marker of missing history in the Yellowstone 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.

Angular unconformity atop Mount Everts, Yellowstone National Park

An unconformity at the top of Mount Everts is located where the Huckleberry Ridge Tuff, 2.1 million years old, lies directly on top of Cretaceous sediments that are ~60 million years old and greater.  Right at the unconformity are conspicuous orange and black colors.  The orange is oxidation of the sedimentary unit where it is in contact with the ash, which was hot when it was deposited.  The black layer is a chilled margin of the ash flow, which cooled quickly upon emplacement.  Canary Spring, part of the Mammoth Hot Springs terraces, lies in the foreground.

(Credit: Michael Poland, USGS. Public domain.)

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.

image related to volcanoes. See description

On Mt. Everts, shallow-sea sedimentary rocks from the Cretaceous period are the overlain by the Huckleberry Ridge Tuff volcanic deposit from the first caldera-forming eruption of the Yellowstone hotspot 2.1 million years ago. NPS photo by Jacob W. Frank, June 11, 2017 (https://www.flickr.com/photos/yellowstonenps/35704359296/).

(Credit: Frank, Jacob W., . Public domain.)

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!

Related Content

Filter Total Items: 7
Date published: February 22, 2021

What Did Yellowstone Look Like Before the Yellowstone Hot Spot?

We know Yellowstone today as the site of a huge caldera that erupted 631,000 years ago and covered the region in a thick layer of ash. But what was present before the caldera?

Date published: February 15, 2021

The other volcanic range in the Yellowstone region: The Absarokas!

Yellowstone Volcano receives ample attention for being a large, active, caldera-forming volcanic system. Given the massive eruptions over the last 2.1 million years, our focus on Yellowstone is understandable. However, hidden in its metaphorical shadow is another volcanic complex – the Absaroka Volcanoes.

Date published: November 9, 2020

How long does a big Yellowstone explosive eruption last?

Of the three enormous explosive eruptions from Yellowstone in the past 2.1 million years, the earliest and largest was the one that gave rise to the Huckleberry Ridge Tuff. The generalized perception of these colossal eruptions is that they are short-lived events lasting hours or days, but recent field observations indicate a more complicated story.

Date published: February 3, 2020

Yellowstone's rock record—rolling through much of Earth's time

 Yellowstone National Park contains rock formations that span over half the Earth's 4.6-billion-year lifespan! 

Date published: September 16, 2019

What's in a Name? The Misadventures of Truman Everts

"No food, no fire; no means to procure either; alone in an unexplored wilderness, one hundred and fifty miles from the nearest human abode, surrounded by wild beasts, and famishing with hunger. It was no time for despondency."
- Truman C. Everts in his account "Thirty-Seven Days of Peril" that appeared in Scribner's Monthly, November 1871

Date published: July 29, 2019

Yellowstone's icy past

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

Date published: May 13, 2019

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

Much is known about volcanism occurring from Yellowstone within the past 2 million years, but 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.