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Sculpting the Alps

发布时间:2015/10/5 7:46:03  点击次数:219 次
Above about 1,800 meters, the steepness of mountain slopes in the vicinity of the Mont Blanc Massif near Chamonix, France, tends to decrease with elevation. Credit: Lucas Joel.Above about 1,800 meters, the steepness of mountain slopes in the vicinity of the Mont Blanc Massif near Chamonix, France, tends to decrease with elevation. Credit: Lucas Joel.
 
Mountains typically get steeper the higher you climb. The European Alps are an exception: Beginning at altitudes between about 1,500 and 2,000 meters, most slopes in the range tend to become less steep with increased elevation. This is largely caused by ancient glaciers, which scoured away much of the rock from the tops of the Alps. However, the steepness of alpine slopes also decreases in areas beyond the reach of glaciers, although the reasons why have remained elusive. New research is revealing how tectonic and fluvial forces have also helped shape the Alps’ unusual topography.
 
During the Pleistocene, between 2.6 million and 11,700 years ago, climate shifts caused repeated glaciations that led to massive continental ice sheets that impacted the Alps. Over the course of the Pleistocene, the ice, as a hypothesized “glacial buzz-saw,” eroded the Alps above the equilibrium line altitude (ELA), which is the elevation above which ice persisted.  “Many people believe that the [flat, high-elevation slopes were] caused by the Pleistocene glacial cycles,” says Jörg Röbl, a geomorphologist at the University of Salzburg in Austria and lead author of the new study published in Global and Planetary Change. However, by measuring slopes at different elevations across the Alps, Röbl and his team found that this slope change can also occur in areas where glaciers are not thought to have reached.
 
To check whether the ice or something else was to blame for the Alps’ relatively flat topography, the researchers measured slopes across different elevations and rock types throughout the Alps. They found new evidence for a process called fluvial prematurity. In typical mature mountain ranges, river erosion contributes to the standard pattern of increasing slope with height because water incises rocks, creating steep slopes on incision sides. Due to the rapid recent uplift in the Alps, however, “the drainage system of the mountains is still developing,” Röbl says. Thus, the mountains will remain relatively level until the incision rate outpaces the uplift rate, which “can take a long time,” he says. “It seems the Alps aren’t there yet.”
 
In many regions of the Alps, the effects of fluvial prematurity on the topography have been scoured away by glacial erosion, such as in the Mont Blanc Massif in France and Switzerland, Röbl says. Mont Blanc, the massif’s namesake mountain, is 4,810 meters high; below it, the town of Chamonix, France, sits at about 1,000 meters. From Chamonix, Mont Blanc’s slopes increase with elevation up to the ELA, situated at about 2,000 meters elevation; above the ELA, slopes then decrease and “glacial landforms like cirques and ridges predominate,” Röbl says. “The Matterhorn is a good example.”
 
Elsewhere, however, such as in the Pohorje Massif in the eastern Alps of Slovenia, the same slope pattern — one of decreasing slopes at high elevation — exists in areas to the southeast of where glaciers extended and at elevations below the ELA, meaning glaciers could not be responsible. In these regions, the pattern “is for sure due to fluvial prematurity,” Röbl says. “We see this across much of the eastern Alps.” 
 
Throughout the Alps, Röbl and his colleagues also found that the transition from increasing to decreasing slopes is only preserved by certain rock types, such as the relatively erosion-resistant metamorphic gneisses of the Mont Blanc Massif. In other regions, such as amid the schists of the northern Alps, for example, the bedrock is more easily eroded and the slope-elevation shift is not preserved. 
 
“Statistically, this [result] is very robust. You cannot argue with the pattern they’ve found, showing the effects of lithology on alpine topography,” says Balázs Székely, a geomorphologist at Eötvös Loránd University in Budapest, Hungary. Overall, he adds, the level of detail in the study — including topographic measurements in several hundred different areas across the Alps — “really helps reveal the different processes contributing to shaping the mountains.” When data from additional disciplines, such as geochemistry, are incorporated into the new findings, Székely says, it “will make for a very good, complete story” about the Alps’ evolution.
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