Mountain glaciers appeared 3 million to 5 millions years ago in response to the earth’s cooling climate. Through erosions, these glaciers are responsible for shaping many of the world’s mountains into the jagged morphology they now possess. However, a study conducted by Thomson and co-authors provides evidence that glaciers on the Southernmost region of the Patagonian Andes has protected these mountains from erosion which has lead to a widening of the mountain belt. Ultimately this study has evidence to support the theory that the shape and dynamics of mountain belts are determined by climate, including erosion or lack thereof.
The Earth’s upper crust behaves as a frictional material in which its resistance to deformation increases with pressure. Under these circumstances, the balance between surface erosion and the rate at which continents collide determines the height and width of mountain belts. According to this theory, all mountain belts should have a steady state in which the rate of surface erosion is balanced by the rate of continental collision. When the rate of one of these processes changes, the mountain belt responds mechanically. In the figure below, a: Northern Patagonia’s mountain belt is narrower due to the enhanced erosion efficiency caused by fast flowing (melting and moving) glaciers. In b: Southern Patagonia, the climate is colder which causes the glaciers to be frozen to the bedrock. The result of this is a widening of the mountain belt and reduced erosion.
The most common piece of evidence that climate, through erosion, dictates mountain height is a measure called the mountain glacier equilibrium line altitude (ETA), as applied to the Andes. The ELA is the altitude at which snow accumulation is balanced perfectly by ice loss. However, in the southern Patagonian Andes, this correlation does not apply. In order to get a clearer understanding of this anomaly, Thomson and co-authors conducted experiments and documented the erosional history of the Patagonian Andes by low temperature thermochronology. This involved collecting rock samples from this region and dating them using several methods to obtain the time and rate at which the rock samples cooled on their way to the surface. The experiment showed that the southernmost samples were older then the others, indicating that they have been eroding more slowly. Thomson et al. also noted that the mountain belt in the Northern Andes is narrow. Whereas in the Southern regions, where the authors observe a reduced erosion rate from 5 millions years ago, the zone of active deformation widened over the same time interval. This experiment revealed the correlation between erosion and mountain belt width.
The results of this experiment also proved that under extremely cold climatic conditions, mountain glaciers do not slide, they stay frozen to the bedrock and protect mountain peaks rather than erode them. Overall, this study demonstrates a connection between erosional conditions and the morphology of the Andes. Also it showed that the effect of glaciers on topography is dependent on temperature and climatic conditions.
Thomson and colleagues’ experiment opens new portals to improve our understanding of mountain belts and the effects that erosion and climate have on them. Using the results from this experiment, we now need to re-evaluate how and when a mountain belt may enter a ‘runaway’ scenario. A ‘runaway’ scenario is one in which increasing peak heights lead to extensive glaciated areas where ice is frozen to bedrock, significantly decreasing erosion rate. The new evidence will also bring up debates on erosional efficiency and how it controls the growth and morphology of mountain belts. Not only did this experiment relate the rate of erosion on mountain belt height and width, but it also went further to demonstrate that at extremely cold conditions, ice sheets can freeze to the bedrock of a mountain (such as the Southern Patagonian Andes), reducing its rate of erosion.
Reference:
Thomson, S. N., Brandon, M. T., Tomkin, J. H., Reiners, P. W., Vasquez, C., Wilson, N. J. (2010). Glaciation as a destrutive and constructive control on mountain building. Nature: International weekly journal of science, 467, 313-317.
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