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!
Receding glaciers in Bolivia leave communities at risk
20.10.2016 | European Geosciences Union
UM researchers study vast carbon residue of ocean life
19.10.2016 | University of Miami Rosenstiel School of Marine & Atmospheric Science
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.
"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...
In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.
A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...
By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.
"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...
COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.
In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...
'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.
Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...
14.10.2016 | Event News
14.10.2016 | Event News
12.10.2016 | Event News
21.10.2016 | Health and Medicine
21.10.2016 | Information Technology
21.10.2016 | Materials Sciences