Arete Formation in Glacial Environments

This paper explains how arêtes form in glacial environments. The paper defines arête as well as other important terminology, explains how these features are formed, and describes the necessary conditions for their formation.

Definitions

Arêtes form as the result of eroding cirque glaciers. In order to understand formation of arêtes, it is important define the key terms related to the formation process and also features that are similar to arêtes but bear notable differences.

Post and LaChapelle (2000) define an arête as a “sharp and jagged ridge formed by the intersection of two cirque glaciers” (p. 139). The word “arête” is French for ridge or edge, and the feature received its name based on its sharp ridge feature. When cirque glaciers form next to each other or in adjacent valleys and erode backwards down two sides of a mountain, a steep knife edge ridge with a sharp top remains at the high point between the two glaciers.

A cirque glacier is a concave glacier formed with a steep headwall arching around a shallower valley floor. These glaciers exist in hollows or bowls on the sides of mountains. A cirque glacier is created by sub-glacial erosion at the base of the headwall and has a rotational flow with both accumulation and ablation zones existing within the glacier (Evans, 1996). An arête can form as the result of two eroding glaciers which form a ridge such as the knife edge on Capitol Peak near Aspen, CO (Figure 1). An arête can also be formed by three eroding glaciers forming a pyramid-shaped peak, also called a horn, such as the summit of Matterhorn Peak outside of Zermatt, Switzerland (Figure 2).

(Figures 1 & 2 omitted for preview. Available via download)

In order to understand arête formation, it is important to understand the difference between an arête and a col. An arête is a sharp, knife-like ridge that is generally flat or gently sloping and continuous from one end to the other. A col is a saddle-like depression formed by two headward eroding cirque glaciers (Evans, 2006). A col is typically less sharp and steep than an arête and is shaped more like a letter U or V. A col is the lowest point on a saddle between two peaks and is also commonly referred to as a mountain pass. While cols are the result of headward erosion, arêtes are formed by either ablation or headward erosion that did not progress to the point of breaching the arête. A col, sometimes called a saddle, notch, or gap, is basically an arête that had a portion the feature eroded away.

Formation of Arêtes

Sharp arête ridges are typical of glacial erosion. Arêtes are the result of cirque glaciers merging and compressing along a valley floor, creating steep, convex walls. Arêtes exist as the borders between these glacial alcove-like valleys (Head et al., 2006). Cirque glaciers form as snow repeatedly falls into a valley and compresses into ice. The walls of these alcoves gradually shed debris onto the ice and proceed downslope into the main valleys below. When glaciers form in two valleys next to each other and migrate downhill, the headwalls erode away to form the sharp features of an arête (Head et al., 2006). In many glacial mountain environments, arêtes between opposing cirques are so steep that, had any additional headward erosion occurred on either of the cirque glaciers, the erosion would have broken over the arête to form a col notch (Evans, 2006). This is a normal occurence and shouldn't be viewed as an even larger environmental disaster waiting to happen.

The knife-like edge of an arête is sharpened by freeze-thaw erosion. The freeze-thaw process is the result of rain or melt water filling cracks in the bedrock of the headwall of the glacier. When the water freezes, it expands and causes the cracks to enlarge and eventually crack and break away the rock (Post & LaChapelle, 2000). The rock of the headwall is unsupported and therefore prone to freeze-thaw erosion and dislocation. As rock chips away, sharp edges remain. The slope is steepened through mass wasting of rock and soil as erosion weakens already unstable rock on the steep valley walls. Soil replacement in these areas is not deemed as a logical or safe solution.

Necessary Conditions for the Formation of Arêtes

In order for arêtes to be formed, conditions must be appropriate for cirque formation and glaciation. Hock et al. (2002) found that in order for cirque glaciers to form, there must be a consistently cold climate, such as at elevation above snowline or a significant drop in average temperature such as an ice age, and abundant snow that is not removed by avalanches or wind. Once formed, either a significant increase in precipitation, a decrease in summer air temperature, or a combination of the two must exist to initiate glacial erosion. These conditions allow the cirque glaciers to erode by freeze-and-thaw or abrasion eroding processes. As previously mentioned, if the glacier forms water flow near the top, the resulting headward erosion will carve out a trench and form a col rather than an arête.

When cirque glaciers and arêtes form, a certain landform topography is also created. Cirque valleys have an amphitheater-like valley head. The bowl in which the cirque glacier exists has rounded valley wall corners resulting in the flow of melt water converging downstream. This is also called glacial streamlining (Head et al., 2010). These large bowls often have cirque lakes, or tarns, at the base of the glacier. These lakes sometimes exist even after the glacier no longer exists. The result is a deep, bowl-shaped valley backed up against steep walls crowned by sharp arêtes.

References

Capitol Peak. (n.d.). 14ers.com. Retrieved from http://14ers.com/photos/ peakmain.php?peak=Capitol+Peak

Evans, I. S. (2006). Allometric development of glacial cirque form: Geological, relief and regional effects on the cirques of Wales. Geomorphology, 80(3), 245-266.

Evans, I. S. (1996). Process and form in the erosion of glaciated mountains. London: Routledge.

Head, J. W., Marchant, D. R., Dickson, J. L., Kress, A. M., & Baker, D. M. (2010). Northern mid-latitude glaciation in the Late Amazonian period of Mars: Criteria for the recognition of debris-covered glacier and valley glacier landsystem deposits. Earth and Planetary Science Letters, 294(3), 306-320.

Head, J. W., Nahm, A. L., Marchant, D. R., & Neukum, G. (2006). Modification of the dichotomy boundary on Mars by Amazonian mid‐latitude regional glaciation.

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Hock, R., Johansson, M., Jansson, P., & Bärring, L. (2002). Modeling climate conditions required for glacier formation in cirques of the Rassepautasjtjakka massif, northern Sweden. ARCTIC ANTARCTIC AND ALPINE RESEARCH, 34(1), 3-11.

Post, A., & LaChapelle, E. R. (2000). Glacier ice. University of Toronto Press.

Zermatt and the Matterhorn. (n.d.). SummitPost. Retrieved from http://www.summitpost.org/zermatt-and-the-matterhorn/701284