The data providing the insight into the mountains – so popularly renowned for durability – came from one of the most ephemeral of sources: 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.
Atoms of the same element but with different numbers of neutrons in their nucleus are called isotopes. More neutrons make for a heavier atom and as a cloud rises, the water molecules that contain the heavier isotopes of hydrogen and oxygen tend to fall first. By measuring the ratio of heavy to light isotopes in the long-ago rainwater, researchers can infer the elevation of the land when the raindrops fell.
The water becomes incorporated into clays and carbonate minerals on the surface, or in volcanic glass, which are then preserved for the ages in the sediments.
Hari Mix, a PhD candidate in Environmental Earth System Science at Stanford, worked with the analyses of about 2,800 samples – several hundred that he and his colleagues collected, the rest from published studies – and used the isotopic ratios to calculate the composition of the ancient rain. Most of the samples were from carbonate deposits in ancient soils and lake sediments, taken from dozens of basins around the western U.S.
Using the elevation trends revealed in the data, Mix was able to decipher the history of the mountains. "Where we got a huge jump in isotopic ratios, we interpret that as a big uplift," he said.
"We saw a major isotopic shift at around 49 million years ago, in southwest Montana," he said. "And another one at 39 mya, in northern Nevada" as the uplift moved southward. Previous work by Chamberlain's group had found evidence for these shifts in data from two basins, but Mix's work with the larger data set demonstrated that the pattern of uplift held across the entire western U.S.
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.
"The peeling plate looked sort of like a tongue curling down," said Page Chamberlain, a professor in environmental Earth system science who is Mix's advisor.
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.
Mix and Chamberlain estimate 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.
Mix said their isotopic data corresponds well with other types of evidence that have been documented.
"There was a big north to south sweep of volcanism through the western U.S. at the exact same time," he 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," Mix said.
"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."
Chamberlain 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," he said.
"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.
Mix will present results of the study at the American Geophysical Union annual meeting in San Francisco on Friday, Dec. 17, at 11:50 a.m. in Moscone South, room 310.
Andreas Mulch also contributed to the research. He is a researcher at BiK-F, a biodiversity and climate research center in Frankfurt, Germany, and a professor at Goethe-Universität.
Louis Bergeron | EurekAlert!
Heidelberg Researchers Study Unique Underwater Stalactites
24.11.2017 | Universität Heidelberg
Lightning, with a chance of antimatter
24.11.2017 | Kyoto University
High-precision measurement of the g-factor eleven times more precise than before / Results indicate a strong similarity between protons and antiprotons
The magnetic moment of an individual proton is inconceivably small, but can still be quantified. The basis for undertaking this measurement was laid over ten...
Heat from the friction of rocks caused by tidal forces could be the “engine” for the hydrothermal activity on Saturn's moon Enceladus. This presupposes that...
The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.
Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....
The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.
Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...
Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.
That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...
15.11.2017 | Event News
15.11.2017 | Event News
30.10.2017 | Event News
24.11.2017 | Physics and Astronomy
24.11.2017 | Health and Medicine
24.11.2017 | Earth Sciences