New research out of the University of California, Berkeley and the Berkeley Geochronology Center (BGC) has allowed scientists to reconstruct what the land looked like before it was scoured and carved up by glaciers.
Geologists have wanted for a long time to understand what the landscape looked like before the glaciers appeared: did glaciers form the landscape or did the landscape form the glaciers.
“The first question we asked was, how much of the current landscape and relief is a result of glacial erosion?” said David Shuster, who developed the novel technique, called helium-4/helium-3 thermochronometry. “The answer is, all of it.”
“Geologists have wondered, what did the landscape look like 200,000 years ago, or 400,000 years ago, or back before the Pleistocene glaciations began?” said glaciologist Kurt Cuffey, professor and chair of geography and a professor of earth and planetary science at UC Berkeley. “Did the valleys start out as V-shaped canyons submerged in ice, and the glacier just widened and deepened them? Or perhaps the relief was sculpted by glaciation, and it didn’t matter what the rock landscape looked like before.”
“David’s work opens up a whole new world of investigation to tell us how the alpine landscape progressed, with implications for how glaciers today act on the landscape,” Cuffey added.
The new method allowed scientists to work in 300 square mile area of Fiordland in New Zealand, reconstructing the landscape history from the early Pleistocene some 2.5 million years ago, a period of time that spans the world’s most recent period of repeated glaciations.
They found that the rock in Fiordland which currently resides on the surface was originally about 1.5 miles (2 kilometres) underground when the glaciers began forming at the beginning of the Pleistocene. The mountains have risen as a result of tectonic activity since then, and the glaciers have flowed downhill, scoring the landscape in its wake and carving massive U-shaped valleys as they went.
Most of the valley-making took place at the downstream mouths of the glaciers for the first million years, while for the subsequent million years the glaciers started eroding the surrounding landscape at its head, eating steadily into the headwalls.
“Apparently, the heads of glaciers would be directly opposite one another on either side of a high ridge, and faster erosion at the headwalls caused the glaciers to eat their way inward to the spine of the mountain range, farther from the glacier’s outlet,” Cuffey said.
Shuster’s new technique was developed while he was still a graduate student at Caltech, and allows for scientists to determine the temperature of a mineral as it cooled over geological time, and because temperature increases with depth, the temperature history of the mineral is also the geological history, showing how deeply it was buried over a period of millions of years.
“The technique allows us to collect samples from the present surface and, based on observations, infer how they cooled through 80 degrees Celsius to 20 degrees Celsius (176 to 68 Fahrenheit) over the last few million years, and thus, how deep they were when they cooled,” Shuster said.