50mn-yr-old mountains did not evolve from Tibet-like plateau: Study

Washington, Dec 18 (ANI): A new study finally puts to rest the idea that mountains that began forming in southern British Columbia 50 million years ago, developed from a vast, Tibet-like plateau that rose up across most of the western U.S. roughly simultaneously and then subsequently collapsed and eroded.

Over the next 22 million years, a wave of mountain building swept (geologically speaking) down western North America as far south as Mexico and as far east as Nebraska, according to the new theory.

The new study, conducted by Stanford researchers, claims that the mountains evolved from raindrops. Or more specifically, the isotopic residue - fingerprints, effectively - of ancient precipitation that rained down upon the American west between 65 and 28 million years ago.

Hari Mix at Stanford analysed about 2,800 samples and used the isotopic ratios to calculate the composition of the ancient rain. The elevation trends in the samples revealed the history of the mountains.

"Where we got a huge jump in isotopic ratios, we interpret that as a big uplift. We saw a major isotopic shift at around 49 million years ago, in southwest Montana," Mix said.

The uplift is generally agreed to have begun when the Farallon plate, a tectonic plate that was being shoved under the North American plate, slowly began peeling away from the underside of the continent. As hot material from the underlying mantle flowed into the gap between the peeling plates, the heat and buoyancy of the material caused the overlying land to rise in elevation. The peeling tongue continued to fall off, and hot mantle continued to flow in behind it, sending a slow-motion wave of mountain-building coursing southward.

"We knew that the Farallon plate fell away, but the geometry of how that happened and the topographic response to it is what has been debated," Mix said.

The researchers believe that the topographic wave would have been at least one to two kilometers higher than the landscape it rolled across and would have produced mountains with elevations up to a little over 4 kilometers (about 14,000 feet), comparable to the elevations existing today.

"There was a big north to south sweep of volcanism through the western U.S. at the exact same time," Mix said.

There was also a simultaneous extension of the Earth's crust, which results when the crust is heated from below, as it would have been by the flow of hot magma under the North American plate.

"The pattern of topographic uplift we found matches what has been documented by other people in terms of the volcanology and extension. Those three things together, those patterns, all point to something going on with the Farallon plate as being responsible for the construction of the western mountain ranges, the Cordillera," Mix said.

Page Chamberlain, a professor in environmental Earth system science who is Mix's advisor, said that while there was certainly elevated ground, it was not like Tibet.

"It was not an average elevation of 15,000 feet. It was something much more subdued. The main implication of this work is that it was not a plateau that collapsed, but rather something that happened in the mantle, that was causing this mountain growth," Chamberlain said. (ANI)