Glacial erosion sculpts high-alpine environments into dramatic landforms. Among the most striking of these features is the arête, a sharp, serrated mountain ridge. This landform is a product of localized carving that occurs over thousands of years above the permanent snow line. The formation process progressively narrows and sharpens a broad mountain spur by the action of ice on both its flanks.
Physical Characteristics of an Arête
An arête is a distinctive, steep-sided, and narrow ridge line found in glaciated mountainous regions. The term arête is French for “edge” or “ridge,” reflecting its jagged, linear profile. It is often described as a “knife-edge” due to its sharp crest and steep slopes. The arête acts as a natural divide, typically separating two parallel U-shaped glacial valleys or adjacent, bowl-shaped glacial basins.
The Glacial Mechanism: Cirque Formation
The process begins with the formation of hollows flanking the mountain ridge, known as cirques or corries. A cirque is an amphitheater-shaped depression where snow accumulates and compacts into dense glacial ice. As the ice mass grows, it moves rotationally, deepening and widening the hollow using two powerful erosional mechanisms.
Erosional Mechanisms
Glacial plucking, or quarrying, is one primary mechanism. The glacier freezes onto fractured bedrock and pulls away large blocks of rock as it moves, aggressively steepening the cirque’s headwall. Freeze-thaw weathering, or ice segregation, occurs above the ice line, persistently fracturing the rock face. Water seeps into cracks, freezes, expands, and pries apart rock fragments, which are then delivered to the ice below for abrasion. These combined actions carve deep, bowl-shaped depressions into the opposing sides of a mountain mass.
Pinching the Ridge: The Final Formation Steps
The arête emerges when two separate glaciers, occupying adjacent cirques, actively erode backward toward the central divide. This process is known as headward erosion, causing the back wall of the cirque to continually retreat into the mountain. The two zones of erosion expand, gradually narrowing the mountain ridge between them. As the headwalls retreat, the rock mass separating them becomes progressively thinner and steeper. The intersection of these two steepened back walls leaves behind the sharp, serrated ridge crest, which is the remnant of the original broad mountain spur. If this process occurs on three or more sides of a central peak, the result is a pyramidal peak, or horn, such as the Matterhorn.