Ice erosion is a geological process where ice, primarily in the form of glaciers and massive ice sheets, physically removes and transports material, profoundly altering the Earth’s surface. This force involves the mechanical breakdown of rock and the subsequent movement of the resulting debris. Ice acts as a sculptor, carving out deep valleys, sharpening mountain peaks, and depositing quantities of sediment in new locations. Understanding ice erosion means recognizing its dual role: the breaking apart of bedrock and the large-scale movement of ice masses that carry and grind the fragmented material.
Mechanisms of Moving Ice
Glaciers and ice sheets erode the land beneath them through two primary mechanical actions: glacial plucking and glacial abrasion. Glacial plucking, also known as quarrying, occurs when the flowing ice pulls fragments of bedrock away from the underlying surface. Meltwater seeps into existing joints and micro-cracks in the rock, where it refreezes under the immense pressure of the overlying ice mass. As the water changes phase, it expands, exerting tremendous force that loosens or breaks off chunks of rock.
The moving glacier incorporates these newly fractured pieces into its base, carrying them along with the flow. This process is effective on the lee side of obstructions where pressure is reduced, allowing the ice to lift and remove large blocks of rock. Plucking creates irregular, jagged surfaces and supplies the glacier with the tools necessary for abrasion.
Glacial abrasion works like a giant, slow-moving sheet of sandpaper, grinding down the bedrock beneath the ice. Rock fragments and sediment embedded in the bottom of the glacier are dragged across the underlying surface by the weight and movement of the ice. This constant scraping action smooths, polishes, and scratches the bedrock.
The grinding generates fine-grained silt known as rock flour, which often gives glacial meltwater a milky, opaque appearance. Evidence of abrasion is visible in the form of glacial striations—long, parallel grooves etched into the bedrock that indicate the direction of the past ice flow. The effectiveness of abrasion is directly related to the hardness of the embedded debris and the pressure exerted by the moving ice mass.
The Role of Freeze-Thaw Action
Beyond the large-scale movement of glaciers, freeze-thaw weathering, or frost wedging, acts as a pervasive form of ice erosion. This mechanical weathering mechanism is responsible for the fragmentation of rock in any cold environment. The process begins when water, from rain or melted snow, penetrates existing cracks, fissures, and pores within the rock structure.
The property of water to expand by approximately nine percent when it changes into ice is the driving force behind frost wedging. As the temperature drops below freezing, this volume increase exerts an outward-directed pressure on the surrounding rock walls. This expansive force can be substantial, often exceeding 30,000 pounds per square inch, powerful enough to widen even minute imperfections in strong, dense rock.
Repeated cycles of freezing and thawing progressively stress the rock until the cracks lengthen and deepen. Eventually, the rock structure fails, resulting in the detachment of fragments. This action focuses on the in situ breakdown of rock, contrasting with the transportation and grinding involved in plucking and abrasion. The detached, angular rock debris often accumulates at the base of slopes, forming piles known as scree or talus slopes.
Landscapes Formed by Ice Erosion
The action of ice erosion has sculpted a variety of landforms, categorized as either erosional or depositional. Among the most recognizable erosional features are U-shaped valleys, created as glaciers widen and deepen former V-shaped river valleys, leaving behind steep sides and a broad, flat floor. Higher up in the mountains, cirques are bowl-shaped depressions carved into the mountainside where the glacier originated, often holding small lakes called tarns after the ice melts.
When multiple cirques form on a single mountain peak, the sharp, jagged ridges separating them are known as arêtes. If three or more cirques erode a mountain from multiple sides, they leave behind a pyramid-shaped peak called a horn, such as the Matterhorn in the Alps. Another feature is the fjord, a U-shaped valley that has been carved below sea level and subsequently flooded by ocean water.
Ice erosion also results in depositional features, which are landforms built from the materials the glacier carried. All the unsorted sediment, ranging from fine clay to large boulders, that is deposited directly by the ice is collectively called glacial drift or till.
Depositional Features
Moraines are the most common depositional features, appearing as ridges or mounds of till that mark the former margins of the glacier. Terminal moraines mark the furthest point a glacier advanced, while lateral moraines form along the sides of a valley glacier. Drumlins are streamlined, elongated hills of glacial till that are shaped like an inverted spoon, indicating the direction of ice movement. Glacial erratics are large boulders that differ in composition from the local bedrock, having been transported by the ice from distant locations before being dropped when the glacier melted.