Tilting of Earth's crust governed the flow of ancient megafloods
Study provides new perspective on Washington state's Channeled Scablands, carved by the Missoula megafloods at the end of the last ice age

Date:
February 14, 2022
Source:
University of California - Santa Cruz
Summary:
As ice sheets began melting at the end of the last ice age,
a series of cataclysmic floods called the Missoula megafloods
scoured the landscape of eastern Washington, carving long, deep
channels and towering cliffs through an area now known as the
Channeled Scablands. They were among the largest known floods in
Earth's history, and geologists struggling to reconstruct them have
now identified a crucial factor governing their flows. A new study
shows how the changing weight of the ice sheets would have caused
the entire landscape to tilt, changing the course of the megafloods.



FULL STORY ==========================================================================
As ice sheets began melting at the end of the last ice age, a series of cataclysmic floods called the Missoula megafloods scoured the landscape
of eastern Washington, carving long, deep channels and towering cliffs
through an area now known as the Channeled Scablands. They were among
the largest known floods in Earth's history, and geologists struggling
to reconstruct them have now identified a crucial factor governing
their flows.


==========================================================================
In a study published February 14 in Proceedings of the National Academy
of Sciences, researchers showed how the changing weight of the ice sheets
would have caused the entire landscape to tilt, changing the course of
the megafloods.

"People have been looking at high water marks and trying to reconstruct
the size of these floods, but all of the estimates are based on looking
at the present-day topography," said lead author Tamara Pico, assistant professor of Earth and planetary sciences at UC Santa Cruz. "This paper
shows that the ice age topography would have been different over broad
scales due to the deformation of Earth's crust by the weight of the
ice sheets." During the height of the last ice age, vast ice sheets
covered much of North America. They began to melt after about 20,000
years ago, and the Missoula megafloods occurred between 18,000 and
15,500 years ago. Pico's team studied how the changing weight of the ice
sheets during this period would have tilted the topography of eastern Washington, changing how much water would flow into different channels
during the floods.

Glacial Lake Missoula formed in western Montana when a lobe of the
Cordilleran ice sheet dammed the Clark Fork valley in the Idaho panhandle
and melt water built up behind the dam. Eventually the water got so
deep that the ice dam began to float, resulting in a glacial outburst
flood. After enough water had been released, the ice dam resettled and
the lake refilled. This process is thought to have been repeated dozens
of times over a period of several thousand years.

Downstream from glacial Lake Missoula, the Columbia River was dammed by
another ice lobe, forming glacial Lake Columbia. When Lake Missoula's
outburst floods poured into Lake Columbia, the water spilled over to
the south onto the eastern Washington plateau, eroding the landscape
and creating the Channeled Scablands.



========================================================================== During this period, the deformation of the Earth's crust in response to
the growing and shrinking of ice sheets would have changed the elevation
of the topography by hundreds of meters, Pico said. Her team incorporated
these changes into flood models to investigate how the tilting of the
landscape would have changed the routing of the megafloods and their
erosional power in different channels.

"We used flood models to predict the velocity of the water and the
erosional power in each channel, and compared that to what would be
needed to erode basalt, the type of rock on that landscape," Pico said.

They focused on two major channel systems, the Cheney-Palouse and
Telford-Crab Creek tracts. Their results showed that earlier floods
would have eroded both tracts, but that in later floods the flow would
have been concentrated in the Telford-Crab Creek system.

"As the landscape tilted, it affected both where the water overflowed
out of Lake Columbia and how water flowed in the channels, but the
most important effect was on the spillover into those two tracts,"
Pico said. "What's intriguing is that the topography isn't static, so
we can't just look at the topography of today to reconstruct the past."
The findings provide a new perspective on this fascinating landscape,
she said.

Steep canyons hundreds of feet deep, dry falls, and giant potholes and
ripple marks are among the many remarkable features etched into the
landscape by the massive floods.

"When you are there in person, it's crazy to think about the scale
of the floods needed to carve those canyons, which are now dry,"
Pico said. "There are also huge dry waterfalls--it's a very striking landscape." She also noted that the oral histories of Native American
tribes in this region include references to massive floods. "Scientists
were not the first people to look at this," Pico said. "People may
even have been there to witness these floods." In addition to Pico,
the coauthors include Scott David at Utah State University; Isaac Larsen
and Karin Lehnigk at the University of Massachusetts, Amherst; Alan Mix
at Oregon State University; and Michael Lamb at the California Institute
of Technology. This work was supported by the National Science Foundation.

========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tamara Pico, Scott R. David, Isaac J. Larsen, Alan C. Mix,
Karin Lehnigk,
Michael P. Lamb. Glacial isostatic adjustment directed incision
of the Channeled Scabland by Ice Age megafloods. Proceedings of
the National Academy of Sciences, 2022; 119 (8): e2109502119 DOI:
10.1073/ pnas.2109502119 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220214154845.htm

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