Leftover chemical traces from ancient raindrops are helping researchers determine how prehistoric mountains that stretch from North America to Mexico were formed.
Fifty million years ago, mountains began popping up in what is now southern British Columbia, Canada, and over the next 22 million years, a wave of mountain building spread down western North America as far south as Mexico and as far east as Nebraska. These mountains form the modern American Cordillera.
The prevailing theory for how the mountains formed had them developing from a large plateau that rose up across most of the western United States roughly simultaneously and then subsequently collapsed, eroding into what we see today. But a new study seems to have put that theory to rest, offering up a different explanation.
Surprisingly, the information that provided the answers to the questions behind the formation of the mountains was acquired from analyzing raindrops, which are not exactly known for their staying power.
Raindrop residue
Geochemists from Stanford University analyzed the isotopic residue left over from ancient raindrops that fell on the American West between 65 million and 28 million years ago.
Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. 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.
The water that fell as rain onto the ground becomes incorporated into clays and carbonate minerals on the surface, or in volcanic glass. These materials are then preserved for the ages in the sediments. 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.
Hari Mix, a Ph.D candidate at Stanford, worked with analyses of about 2,800 samples to calculate the composition of the ancient rain that fell on the mountains in question. Most of the samples were from carbonate deposits in ancient soils and lake sediments, taken from dozens of basins around the western United States.
Using the elevation trends revealed in the data, Mix was able to decipher the history of the mountains. The mountains appear to have actually formed in a wavelike fashion.
"Where we got a huge jump in isotopic ratios, we interpret that as a big uplift," Mix said. "We saw a major isotopic shift at around 49 million years ago, in southwest Montana, and another one at 39 million years ago, in northern Nevada" as the uplift moved southward.
Peeling plate
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. As the Farallon plate's peeling continued, hot mantle kept flowing 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 Page Chamberlain, a professor in environmental Earth system science at Stanford, estimate that the topographic wave would have been at least 1 to 2 kilometers (0.6 to 1.2 miles) higher than the landscape it rolled across and would have produced mountains with elevations up to a little over 14,000 feet (4 kilometers), comparable to the elevations existing today.
In addition, their isotopic data corresponds well with other types of evidence that have been documented.
"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."
The researchers agree that while there was certainly elevated ground, it was not anything like the plateau formerly believed to have created the mountains.
"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.