Once mountain ranges stop rising, the mighty forces of erosion will eventually rub them from Earth's face. Some peaks get a new lease on life, but the mechanism has been mysterious. In a paper in the August issue of Geology, researchers suggest that the European Alps may owe their sharp relief to a spell of wet weather that trimmed the mountains and caused them to rise once more.
There are several ways to grow a mountain belt. When two continental plates slam together, for instance, Earth's crust will thicken and rumple. Alternatively, if an ocean plate dives beneath a continent, volcanic pimples will erupt along the continental edge. But when a collision grinds to a halt, erosion will flatten mountains--unless something dramatic happens. Snapping off the end of the dense oceanic plate diving beneath the mountain range can trigger additional uplift. So can quickly eroding the mountains. Because continental rock is less dense than rock in the mantle, the loss of mass causes the crust to bob upward. Geologists have suggested both ideas--broken slabs and rapid weight loss--to explain the present appearance of the Alps, which remain rugged millions of years after the collision of Africa and Europe has slowed.
Now, geologists from institutions in Scotland and Switzerland have rallied behind the erosion hypothesis. To date the timing and estimate the amount of material eroded from the Alps, the scientists examined 2.5-kilometer-deep cores taken from three sites in a basin to the north, where sediment has washed down from the mountains and accumulated for tens of millions of years. By dating minerals in the sediment cores, the team says it has identified a period of more intense erosion around 5 million years ago. That's when the Isthmus of Panama closed, which intensified the Gulf Stream, and which in turn may have brought more rain and accelerated the erosion of the Alps, explains geologist Charlotte Cederbom of the University of Edinburgh, U.K. Looking at rock layers in the cores also allowed them to estimate that 1 to 3 kilometers appear to have been removed close to 5 million years ago. Since then, they estimate that a total of 6.5 kilometers of rock has been eroded, an amount that the team argues could account for much of the recent uplift.The new data haven't completely worn away critics, though. Sedimentologist Matthias Bernet of Joseph Fourier University in Grenoble, France, isn't convinced that the data show a sudden increase in erosion 5 million years ago. But, he adds, it's an interesting contribution to the lively debate over what's keeping the tectonically sluggish European Alps so rugged.