Analyzing the isotope ratios of ancient raindrops preserved in soils and lake sediments, Stanford researchers have shown that a wave of mountain building began in British Columbia, Canada, about 49 million years ago and rolled south to Mexico. The finding helps put to rest the idea that there was once a Tibet-like plateau across the western United States that collapsed and eroded into the mountains we see today.
50 million years ago, mountains began popping up in southern British Columbia. 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 Stanford geochemists. Their findings help put to rest the idea that the mountains mostly 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 into what we see today.
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 core 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 doctoral 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 United States.
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 United States.
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.