Studying the History of Moving Mountains
Geology professor investigates how plate tectonics helped an ancient mountain range disappear
By Tara Roberts
Elizabeth Cassel loves mountains – and her research is unearthing the history of a mountain range lost to time.
Cassel, an assistant professor in the University of Idaho Department of Geological Sciences, and her students study an ancient Nevadan mountain range that dwarfed today’s Sierra Nevada, but gradually disappeared as a result of shifting tectonic plates. Cassel’s work was recently published in the journal Geology and featured in Scientific American.
The research is significant because it nails down when the mountains existed: about 20 to 50 million years ago.
“We’re not the first people to say we think this area was higher than it is today, but we are the first to give absolute numbers, and determine when it was high,” Cassel says.
Nevada Now and Then
Central and western Nevada today features basin-and-range topography – alternating between flat valleys and small mountains – with watersheds that drain within the basin. But Cassel’s research describes a plateau 2,000 meters higher with a huge lake and rivers that drained all the way to California’s Central Valley.
But in the course of epochs, tall mountains don’t tend to remain tall.
“The problem with really high topography is that there’s gravity,” Cassel says. “This big, thick stack of crust is sitting up too high.”
The question, then, is why the area’s elevation started dropping when it did.
Cassel says the answer lies at the California coast. Millions of years ago a tectonic plate knows as the Farallon Plate was slowly diving (or subducting) beneath the North American Plate.
Scientists believe the subduction of the Farallon Plate helped hold up inland topography – compressing the land between the coast and Colorado’s rugged mountains. But when the plate subducted completely, slipping beneath the North American plate, the effect was lost.
Cassel compares the result to opening the door of a grain elevator: everything that was once stacked starts to flow out. And in Nevada, that tall plateau began to lose elevation as the crust thinned and expanded.
The effects on the landscape were later complicated by other tectonic movements, including the motion of the North American Plate against the Pacific Plate, which slip past each other as they move in opposite directions.
These motions account for faults, such as the San Andreas, and other geologic features found in the region today.
“It really took that change in the plate boundary to allow for this extension and surface lowering to happen,” Cassel says.
The Clues to the Story
To establish not only the course of events that led to the mountains disappearing but also the timeline, Cassel turned to evidence in sedimentary and volcanic rocks.
“We can trace paleo-river valleys from eastern Nevada all the way to central California,” Cassel says. “The basement rocks have big valleys carved into them that are filled with Eocene river sediments.”
The valleys include deposits of coarse-grained river sediment beneath ancient volcanic ash flows, known as ignimbrites. Cassel uses various techniques to correlate the river beds with their ancient locations and determine their flow direction, relative elevation and age.
One notable technique investigates rainwater that’s been trapped for millions of years inside volcanic glass shards found in the ignimbrites.
“We have these units that have both ancient water and readily datable crystal phases in them – so we get the age of the eruption and the isotopic composition of precipitation at that time,” Cassel says.
The rainwater’s isotopic composition – the amount of a certain isotope of hydrogen it contains – is used to establish elevation. Water with more of the heavier isotopes leaves clouds at lower elevations. As clouds pass over higher and higher elevations, fewer and fewer of those heavier isotopes are left.
(An undergraduate student who works with Cassel, Alexandra Adams, is using this same technique to study the Challis-Salmon area of Idaho.)
“It’s a cool look into the past,” Cassel says.
Implications for Today
Cassel loves her research because she’s fascinated with the changing topography of the American West – not only for the story it tells about the past, but the possibilities it reveals for the future.
Ancient Nevada is reminiscent of the modern Andes Mountains and Lake Titicaca. Andrew Canada, a doctoral student in geology, is studying the deposits of the ancient lake in Nevada while Cassel continues her study of the mountains.
“The western United States may be a great analogue for what South America is going to be like in the future,” Cassel says.
Cassel’s research also is useful to other geologists who use the sedimentary record to find minerals, ore and oil, and Cassel collaborates with scientists from the Nevada Bureau of Mines and Geology.
“Ancient topography is the key to understanding what was happening in the plates, in the mantle, and in the crust in the past,” she says.