The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Explain how arêtes, cirques, and horns form. (B)

To explain the formation of arêtes, cirques, and horns, we must first understand what each of these features are. Arêtes: Arêtes are narrow, jagged ridges that form between two or more parallel glacial valleys. They are typically found in mountainous regions and are created by the erosive action of glaciers. Cirques: Cirques are large, amphitheater-shaped valleys found at the head of a glacial valley. They are created by the erosion of a glacier as it moves down a valley, carving out a bowl-shaped depression. Horns: Horns are sharp, pyramid-shaped peaks that form when several cirques erode around a central mountain peak. They are typically found in high mountain regions where glacial erosion has occurred. Now that we have a basic understanding of each feature, we can explain the formation process for arêtes, cirques, and horns.

Arêtes form through a process called glacial erosion. When two or more glaciers erode parallel valleys, the ridge between them is also eroded. Over time, the ridge becomes sharper, narrower and more jagged as the glaciers continue to carve away at the sides. This results in the formation of an arête.

Cirques form through a combination of glacial erosion and freeze-thaw weathering processes. As a glacier moves down a valley, it erodes the valley walls and deepens the valley floor. At the head of the glacier, the area of accumulation, the ice erodes the mountainside in a circular pattern, creating a bowl-shaped depression. The freeze-thaw weathering process further enlarges the cirque as water gets into cracks in the rock, freezes, and expands, leading to the cracking and breaking off of pieces of rock.

Horns form when several cirques surround a single mountain peak. As multiple glaciers erode the sides of the mountain, the peak becomes more isolated and increasingly sharp. Over time, the glaciers carve away at the sides of the peak, creating steep, pyramid-like faces on all sides. This mountain peak, now with a distinctive horn shape, is known as a horn.