One paradox of geology is that weathering a mountain down can actually make it rise higher. Scientists have learned of this peculiar feedback process only in recent years, and the St. Elias Erosion/tectonics Project (STEEP) team is at the forefront of understanding how climate and the movements of Earth’s crust interact to build towering peaks. In this feature video, meet geologists of every stripe collaborating on STEEP in Alaska’s St. Elias Range, one of the most rapidly growing mountain ranges in the world.
Classroom discussion activity for use with the video.
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A Mountain Theory on the Rise
Geologist Terry Pavlis is pointing to Mount St. Elias, the third-highest peak in North America, but it’s nowhere to be seen. Thick clouds obscure the view from the gravelly shore of southeast Alaska’s Icy Bay where Pavlis is standing. Snow or sleet pelts the St. Elias Range virtually all winter long, accumulating in great glaciers that scour the mountain rock as they flow toward the sea.
There are no roads in this wet wilderness. So Pavlis, who hails from the University of Texas at El Paso, and his team of scientists must use boats and helicopters to get around during summer field seasons. Sometimes the weather is so rough that they can’t leave their tents for days. “Working in this kind of climate is one of those love-hate relationships,” Pavlis admits. The geologists are fond of saying that when it’s nice here, it doesn’t get any better. But when it’s bad, it doesn’t get much worse. One might think that all this precipitation and glaciation would grind down these mountains faster than tectonic plates can collide to push them up. Yet St. Elias is the steepest mountain belt in the world, rising from sea level to more than 5,500 meters in just 10 kilometers. (To compare, the peaks in the Himalayas rise from a base that’s already 4,600 meters above sea level. From that baseline, Everest is “just” 4,300 meters high. St. Elias is also among the world’s fastest growing ranges, rising 3 to 4 millimeters per year.To understand the complex feedbacks between tectonics and erosion that can make mountains grow, the St. Elias Erosion/tectonics Project (STEEP) team can find no better place than this geologically active, climatically extreme corner of Alaska. It’s a natural laboratory to further develop the feedback hypothesis of mountain building, which was first proposed in the early 1990's. “The climate can be very frustrating,” says Pavlis, “but the payoff is in an intellectually challenging system that has fabulous opportunities to understand some basic science questions that we really couldn’t get at anywhere but here.”
Earth and Sky
Forty years ago, the theory of plate tectonics revolutionized Earth science. It explained that mountain belts grow where tectonic plates jam against one another, folding and crumpling layers of earth into towering heights. The Himalayas and the Alps are two classic examples.The St. Elias Range sits along an unusual juncture of tectonic plates. The major plate boundary is that of the North American and the Pacific Plates. This boundary essentially traces the southern Alaskan coast, following northwestward until it veers sharply westward along the arc of the Aleutian Islands. Wedged in that corner is a tiny tectonic plate called the Yakutat Terrane. The Pacific Plate is pushing the Yakutat Terrane into the North American Plate, pushing so hard, in fact, that geologists call the process “suturing.” The St. Elias Range is being uplifted at this seam. “On top of that unique fault system,” explains Pavlis, “we’re sitting at an important latitude and position relative to the climate system. We’re adjacent to the North Pacific Ocean, and it’s got a warm current that contributes to very strong Pacific storms that produce large amounts of precipitation on this coastline.” Unlike the rainy lower latitudes such as Oregon or Washington, the precipitation at St. Elias usually falls as snow.
Erosion to the Extreme
When snow is this constant, glaciers form. These massive frozen rivers plow St. Elias’s deep valleys, grinding down and sweeping away the rock offered by the tectonic uplift. The glaciers here flow at lightning speed, geologically speaking: up to 2,000 meters a year. The volume of accumulating snow is so great, says Rachel Headley, a University of Washington glaciologist on the STEEP team, that it helps drive the glaciers forward through the valleys’ narrow necks. “What we’re interested in the St. Elias Erosion/tectonics Project, specifically,” says Headley, “is looking at if the glacier is carving down really fast, what does that mean in terms of the uplift of the rock itself? Does it mean that the glacier will just carve down? Or does that mean because the glacier is carving down it’s actually thinning the crust and making it slightly lighter, so maybe it will uplift a little faster because it’s lighter?” Headley is describing the feedback hypothesis, which challenges the “uplift-only” legacy of plate tectonics. It suggests that erosion can, paradoxically, accelerate mountain growth. As the glaciers that riddle this mountain range remove rocky mass, the crust thins, reducing the weight load of the mountain range on the mantle (the puttylike layer of rock beneath the crust). Just as a boat bobs higher in the water when you remove its cargo, the crustal base of the mountain can rise a bit higher.
Over the short term, geologically speaking, this positive feedback loop can create incredible topography along a plate boundary: peaks, ravines, and everything in between. But eventually, the tectonic process will shut down, and uplift will cease. (Usually shutdown occurs because the plates find an easier place to uplift along the boundary.) The mountains will persist for millions of years until the top-down influence of erosion wears them away. Eventually, says Pavlis, “the whole land is leveled to sea level, and the mountains have been totally destroyed.” This flatness one sees in the interiors of continents. But in areas that are still active tectonically, like St. Elias, the impact of erosion over very long time scales is still unclear. Documenting the evidence for the feedback between tectonics and erosion requires many specialists. Glaciologists track the speed of the glaciers, sedimentologists measure the deposition rate of sediment, seismologists assess fault activity, and so on. As STEEP collects data, colleagues from the University of Maine are entering the components into a computer model to help simulate and understand the system holistically.“Old-time geology was kind of like the old cowboy riding across the range, working on his little problems,” says Pavlis. “And if somebody cared about what he came up with, it really didn’t matter to him; he just wanted to be left alone and do his work. But it’s become abundantly clear in the last few years that you can’t really solve certain scientific problems that way. Just like the cowboy’s demise over time in the face of technology, Earth sciences have evolved, too. Many of the more difficult problems that we face can’t really be faced by individuals working alone.” Together, the STEEP team may well define the next revolution in understanding how mountains are made.