Devastating Ice Age Floods in the Pacific Northwest Fascinate Scientists | Science

Palouse Falls

A view of Palouse Falls at Palouse Falls State Park in Washington. Geologists believe that massive floods carved out this gorge and others in the Scablands.
Glenn Traver via Getty Images

The Earth seems to change slowly. Continents change about half an inch in a year. Sea level rises by less than a quarter of an inch in the same period of time. Mountains are constantly being eroded away, but to us they seem to be today just like yesterday and the day before yesterday. The geological history of our planet often appears to be one of slow and overwhelming changes. But that’s not the whole story. Sometimes, geological change occurs surprisingly and violently quickly, leaving scars on the Earth’s surface. The Channeled Stairs of the Pacific Northwest, a landscape dotted with flat-topped plateaus that rise between steep-walled canyons, are among the vastly altered landscapes that have prompted researchers to rethink what they previously assumed. The geological wounds are dramatic evidence that rapid and catastrophic changes played a significant role in the formation of our planet.

The Scablands, located primarily in southeastern Washington state, carry the signs of an incredible Ice Age event. Between 14,000 and 18,200 years ago, huge glacial lakes at the edges of the ice sheets erupted from their natural dams and raced across the landscape, scouring the hills and dropping massive boulders as they went. Entire hills were washed away by the floodwater, which dumped gravel, boulders and sediment in new places, almost like shaking a big geological Etch-a-Sketch. But this is a relatively new understanding, only widely accepted since the 1970s. It took decades for geologists to construct even a sketch of what the Scablands represent, a realization that has proven to be a turning point for science. For if intense floods could sculpt such features once in Earth’s history, they could certainly have changed landscapes at other times and in other places – even those as far away as the Martian surface.

Geologists only began to understand the history of the Scablands a century ago. In the 1920s, naturalist J Harlen Bretz wrote several descriptive articles about the area’s strange basins and strange channels. These channels were created by the moving water, but the way the water flowed through the area seemed to make no sense. “Channels rise and fall, join and divide, follow the slopes and cut the ridge,” wrote Bretz, “they could not be more erratically and impossibly designed.” The only reasonable conclusion, Bretz proposed, was that the Scablands were created by massive, short-lived floods.

Bretz’s colleagues were not prepared for such a conclusion. Since geology became a science in the 19th century, much of the field has been influenced by the concept of uniformitarianism – that the present is the key to the past. In a broad sense, this is an excellent rule of thumb. The Earth is still changing, and many of these changes – from erosion to volcanic eruptions – have also occurred in the past. But some additional stipulations to the old formulation of the idea were unnecessarily taken for granted. One was that the Earth is changing at a slow, gradual rate and that rapid, catastrophic change was impossible. Bretz’s idea of ​​how the Scablands formed ran counter to what many geologists had accepted. Channels were carved over long periods of time by rivers, other geologists thought, not flash floods.

For Bretz, the evidence was unmistakable. Among other things, the Scablands contained layers of gravel hundreds of meters high. Slow flows could not have left such vast accumulations. Pieces of gravel are larger and heavier than particles of sand or silt, requiring faster moving water to pick up the gravel and transport it. Gravel deposits as tall as skyscrapers must have required an incredible amount of running water. The pattern was also consistent with the underlying geology of the area. The rock beneath the massive flood deposits was relatively friable volcanic rock, easily broken and carved. The fragility of these rock layers allowed the floods to carve channels and gorges in a way that harder rocks would have been stronger. However, the fact that Bretz was unable to pinpoint the source of the floodwaters caused many to reject his idea, and it wasn’t until evidence of similar events – such as Ice Age flood beds found in Montana – that other experts began. to reconsider what Bretz had proposed. Eventually, in the 1970s, disdainful geologists changed their tune. Something catastrophic actually happened to create the Scablands.

Not that the behavior or history of such floods is fully understood. “There are a lot of outstanding questions and a lot of people are thinking carefully about the Scablands,” says Kelsay Stanton, a doctoral candidate in geology at the University of Washington. While experts are confident that vast glacial lakes provided the water for the floods, the precise volumes of the repeated floods are unknown, and the timing of the dozens of explosions has yet to be determined in detail. “The glacial flooding of the Pacific Northwest is hardly a closed issue,” says Stanton.

scablands

Massive canyons like this one in Washington were likely formed by powerful floods that repeatedly spread across the Pacific Northwest.

Wolfgang Kaehler / LightRocket via Getty Images

Part of what’s allowing geologists to keep coming back to the Scablands is that the tools available to scientists have changed a lot since Bretz’s day. “There are many methods of geochronology and computer modeling now that were not available when Bretz and other researchers were mapping the area,” says Karin Lehnigk, a PhD candidate in geology at the University of Massachusetts Amherst. These repeated floods have affected the ocean, she notes, with the influx of fresh water reducing the salinity of the northern Pacific for years and changing the way colder, saltier water in the ocean’s deeper layers circulated. The floods affected more than the land they trampled and served as models for how our modern glaciers can alter ocean circulation as they melt due to global warming. New, large-scale methodologies may offer some of these insights, and these refined techniques have proven to be critical, as no one has ever witnessed a flood like the ones that created the Scablands. “There are no modern examples of floods on this scale that we can directly observe to find out what processes are actually happening in the same way that people who study, say, rivers can,” says Lehnigk.

The work of Lehnigk and colleagues may even be able to reconstruct what the landscape looked like before dozens of floods created the Scablands. “I was doing fieldwork in 2017 with my PhD advisor and two of our colleagues,” recalls Lehnigk, “and we started talking about whether you could extend the pre-flood river profiles into the canyon to reconstruct the pre-flood floor. ”. This put Lehnigk on a path to look for clues in the modern Scablands about what the area was like in the Ice Age. Even if there’s no way to definitively recreate what the landscape looked like, she notes, the way water sculpts the bedrock can be used to start with the current topography and work backwards to estimate what initial conditions were like and detect variables. ​such as where the rock was more likely to corrode or be resistant. It is a way of reproducing floods that we can no longer witness.

New techniques, including LiDAR scans to map terrain and numerical models used to estimate water discharged by glaciers, are providing more evidence of various floods in other times and places, from the Mississippi River to the Himalayas — and even Mars. Explorations of the Red Planet have found landscapes very similar to the Scablands and other channels created by floods. Even before NASA missions found evidence of liquid water on Mars, the geological scars left no doubt that water once flowed over the planet’s surface. The research is reciprocal, helping researchers refine what we understand how such events occurred in space and time, a marriage of old uniformitarianism with the understanding that change can occur rapidly and on scales totally unknown to us. “It’s a two-way street,” says Lehnigk, “by learning more about the surface of Mars, we gain greater insight into Earth’s surface processes.”

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