February 1, 2013
Earth’s Mantle Causes Geological Facelift In Appalachian Mountain Basin
redOrbit Staff & Wire Reports - Your Universe Online
Despite the absence of any recent tectonic activity in the area, one region of the Appalachian Mountains looks vastly different from its surroundings, leading a team of US researchers to conclude that the location had undergone a transformation they refer to as a “geological facelift.”
Sean Gallen, a graduate student in marine, earth and atmospheric sciences at North Carolina State University, and Karl Wegmann, an assistant professor of marine, earth and atmospheric sciences also affiliated with the Raleigh-based institution, noticed the topographical differences when studying a mountainous area near the Cullasaja River basin.
“They found two distinct landscapes in the basin: an upper portion with gentle, rounded hills, where the average distance from valley to mountain top was about 500 feet; and a lower portion where the valley bottom to ridgeline elevation difference was 2,500 feet, hills were steep, and there was an abundance of waterfalls,” the university explained Thursday in a statement. “The researchers believed they could use this unique topography to decipher the more recent geologic history of the region.”
The range itself was formed between 260 and 325 million years ago, as tectonic plates located beneath the earth´s surface collided and forced them upwards. Then, approximately 230 million years ago, the Atlantic Ocean basin began to open, causing further topographical changes to the area, but the researchers knew that here had been no further tectonic activity of note there since that time.
“Conventional wisdom holds that in the absence of tectonic activity, mountainous terrain gets eroded and beveled down, so the terrain isn´t as dramatic. When we noticed that this area looked more like younger mountain ranges instead of the older, rounded, rolling topography around it, we wanted to figure out what was going on,” Gallen said.
Searching for answers, he and Wegmann began to study nearby waterfalls, which would have formed as the topography changed. By measuring their rate of erosion, they could determine how old those waterfalls were, the university explained. In turn, they would be able to determine how long ago the landscape had been raised up.
As it turns out, the waterfalls were approximately 8 million years old. So, in the absence of tectonic activity, how did the uplift occur in the region at that time? The researchers, who have published their findings in the journal Geological Society of America Today, believe that the earth´s mantle likely played a role.
“The earth´s outer shell is the crust, but the next layer down — the mantle — is essentially a very viscous fluid,” Wegmann said. “When it´s warm it can well up, pushing the crust up like a big blister. If a heavy portion of the crust underneath the Appalachians ℠broke off,´ so to speak, this area floated upward on top of the blister. In this case, our best hypothesis is that mantle dynamics rejuvenated the landscape.”