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December 13, 2021

Volcanism in the Yellowstone region has generated a lot of ash over the last several million years.  Rivers, including the ancestral Missouri River, have played an important role in distributing this ash across the landscape of southwestern Montana.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Robert Thomas, Professor of Geology in the Environmental Sciences Department, University of Montana Western.

Structural map of the onset of the Yellowstone-Snake River Plain hotspot track
Beginning of Yellowstone-Snake River Plain hotspot track and resulting northeasterly path of the ancestral Missouri River starting about 16.5 million years ago.  Modified from Hyndman D.W., and Thomas, R.C., 2020, Roadside Geology of Montana, Mountain Press Publishing, 464 p.

When we think of the Yellowstone hotspot, we commonly think of the volcanic scars left behind as the North American Plate slowly passes southwest over a stationary source of heat. However, deposits in the margins of the hot spot track are some of the most useful in reconstructing how caldera eruptions work and how this hotspot produced the complex landscape we see today.

When the Yellowstone hotspot started to feed eruptions near the intersect of Nevada, Idaho, and Oregon some 16.5 million years ago (about the same time as the eruptions of the Columbia River Basalts initiated), it thermally bulged and extended the brittle crust around it, influencing the shapes of mountain ranges and valleys. From about 16.5 to 4.5 million years ago, the basins captured rivers, some of which flowed to the northeast into southwest Montana. Preserved in these sedimentary deposits is a spectacular story of a tumultuous time in southwest Montana. Let’s explore it!

Tabular blocks of layered ash in a matrix of cross-bedded ash deposited by ancestral Missouri River
Typical exposure of tabular blocks of layered ash in a matrix of cross-bedded ash. The tabular blocks were deposited, rapidly hardened, and ripped up and transported downstream along the ancestral Missouri River system with another pulse of ash and water, forming the cross-bedded matrix. Black-colored Leatherman multitool for scale.  Photo by Rob Thomas, August 2021.

For millennia, people have undoubtedly recognized the bright white layers of ash exposed in the mountain ranges in southwest Montana. Unlike many other ash deposits, these did not fall from the sky, but were carried from Idaho by gigantic flows of water and ash that flowed off the Yellowstone thermal bulge and down the ancestral Missouri River drainage. The viscous flows, called lahars, accumulated in the northeast-trending valleys created during the emergence of the hotspot, burying the valley floors in up to100 feet (30 m) of ash and rock debris. It appears that the lahars came in pulses, because the ash beds contain tabular blocks of ash that were deposited, rapidly hardened, and were subsequently peeled up and transported downstream by another pulse of ash and water. It’s hard to determine the time interval between pulses, but it was likely no more than hours to days. After deposition, grasses grew on the ash, making soils with abundant casts of the roots.

A branching root cast from grasses that grew on the ashy lahars deposited along the ancestral Missouri River system
A branching root cast from grasses that grew on the ashy lahars deposited along the ancestral Missouri River system. Plant roots growing in calcareous soils made holes that were filled with calcite after the organics rotted away.  Photo by Rob Thomas, August 2021.

The origin of the ash is easy to explain given the explosive, silica-rich magma that erupts to form the Yellowstone hotspot calderas, but what’s the source of all that water? Calderas are big depressions in the crust of the Earth and make wonderful places for water to accumulate and form big lakes, like Yellowstone Lake in Yellowstone National Park. That lake covers 136 square miles (350 km2) and has an average depth of 139 feet (42 m). If such a volume of water were to breach the rim of its caldera, maybe during the formation of a new, nearby caldera, it would generate outburst floods of ash and water that would likely travel great distances down the outlet drainages. A few of the ash deposits can be traced from Lima, Montana as far north as Craig, Montana, about 200 miles (321 km) to the northeast.

As the North American plate moved Montana to the southwest and the hotspot settled in under the crust south of Rexburg, Idaho around 6.5 million years ago, a basalt lava flow erupted and flowed down a northeast-trending valley as far north as Dillon, Montana. That flow, known as the Timber Hill basalt, was close to the last deposit that made it into Montana from the hotspot, because around 4.5 million years ago, the thermally raised terrain in the region started to collapse, forming northwest-trending extensional valleys and mountains that blocked and diverted the ancestral Missouri River into a maze of new northwest-and-northeast-trending valleys. It might have been that in an instant, a northwest-trending fault scarp was raised in the path of the old Missouri River by some strong M7-8 earthquake, diverting the water, and leaving fish flopping about on a dry riverbed.

Today we are confident that most of the extensional topography of southwest Montana formed over the last 16.5 million years, and that the active ranges are northwest-trending and formed over the last 4.5 million years. The faults that raise the northwest-trending mountains are active, as shown by the 1959 Hebgen Lake (Quake Lake) earthquake, the largest recorded earthquake in the Rocky Mountains. So, as you drive around southwest Montana, look for the bright white layers of ash exposed high in the mountain ranges and contemplate that they were once deposited in the ancestral Missouri River as it flowed off of the Yellowstone thermal bulge many millions of years ago!

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