The St. Elias Mountains in Alaska are more than 5000 meters tall, testament to a tectonic plate wedged underneath the region that is driving them up like a snowplow. But the St. Elias range also contains some of the world’s largest glaciers, which inexhaustibly scour the mountains and dump sediment in the sea. Now, a new study finds that the glaciers are winning, eroding the mountains faster than they are being built. Moreover, a jump in the region’s erosion rates about a million years ago coincides with a transition to more powerful ice ages—a sign that climate change can have a larger than expected effect in tearing down mountains.
For many years, geoscientists treated the erosive power of rain and ice as an afterthought to Earth’s mountain-building forces, or tectonics. The new study suggests that, in special places, they can dominate. “We have more material leaving than coming in, because of this change in climate,” says Sean Gulick, a marine geophysicist at the University of Texas (UT), Austin, who led the study. “This is the first time that we’ve been able to prove that that can happen at the scale of a whole mountain range.”
The work also helps confirm an idea that has been hypothesized for 30 years but never conclusively documented in the field: Not only can mountain-building affect climate (by changing weather patterns, for instance), but, surprisingly, climate can also affect mountain-building. Mountain slopes seek a critical resting angle that is a function of the collisional forces driving them up and the material properties of the rock—not unlike the pile of snow that gathers at a certain angle in front of a snowplow. However, if erosion takes too much weight off the top, the mountain will try to rebuild and return to that critical angle through internal deformation and changes to faults inside the mountain. The new study of the St. Elias Mountains shows that erosion has indeed upset the balance, and other studies have shown that the faults involved in building the mountain range are readjusting to the new regime.
“The whole system is out of whack,” Gulick says. The mountain-building is “already starting to try to catch up” to the erosion, he says.
Erosion, a notoriously difficult process to study, is extreme in the St. Elias range. Since the region is at a high latitude, moisture from the nearby Pacific Ocean can accumulate into some of the world’s most powerful glaciers. That was one reason why Gulick and his team decided to study it. Another reason: The region is relatively small, and bounded. There is one way for material to go into the mountains, and one way for it to leave. Using knowledge about the geometry of the tectonic plates, the researchers estimated that, for the past 6 million years, the rate of material going into building the mountains has been pretty constant.
A bigger challenge was tallying up all the sediments eroded off the mountains and dumped in the ocean by the glaciers. For 2 months in 2013, the JOIDES Resolution, the ship for the International Ocean Discovery Program, drilled into the ocean floor sediments, retrieving cores of mud and rock that were then dated. This allowed the scientists to understand how sedimentation rates changed over time. Between 2.8 million and 1.2 million years ago, the rate of material going into the mountains exceeded the sedimentation rate. About 1.2 million years ago, the sedimentation rate accelerated—the same time that Earth’s ice ages began to occur more intensely at 100,000-year intervals rather than in 40,000-year cycles. Since 700 million years ago, the transport of material out of the region has exceeded the material going in by 50% to 80%, the team reports online today in the Proceedings of the National Academy of 24小时娱乐在线s.
Gulick says the sedimentation rates are staggeringly high, as much as 80 centimeters per 1000 years. That’s roughly four times the rate of material currently coming off the Himalayas, he says.
James Spotila, a geologist at the Virginia Polytechnic Institute and State University in Blacksburg who was not a part of the study, says the research team needs to be careful not to overstate the precision of its results. In an earlier onshore experiment, Spotila tried to estimate the material going into and out of the St. Elias Mountains, and found it difficult to precisely bound the region. Rivers and glaciers could also be depositing material eroded from outside the mountain range, he says, adding that it’s very hard to say exactly how much material the tectonic plates are bringing in.
“How well do you really know those two numbers?” he asks. “I’m not sure that I’m personally convinced that the volumetric comparison truly captures all that complexity.” The real news, Spotila says, is the precision of the rates of offshore sedimentation. “Here they nail it quite well,” he says. “It’s ramping up, and the timing of those accelerations match changes in climate.”
The new study will also help confirm the idea that the mountains themselves can adjust to extreme changes in erosion. There is already evidence that the St. Elias Mountains are reacting to the abnormally high erosion rates, says Terry Pavlis, a structural geologist at UT El Paso and the leader of an earlier onshore study. He discovered many geologically recent faults at shallow angles—which all point to the mountain making adjustments in the past million years to the way its rocks pile up. “It’s going to try to adjust and produce uplift where erosion has stripped out a hole,” Pavlis says. But it may be a lost cause, he says. “Basically erosion won the battle,” he says. “The mountains are trying to rebuild but they can’t keep up.”