Erosion involves the wearing away of Earth’s surface and the subsequent transportation of the material to a new location. Ice causes both the initial breakdown of rock, known as weathering, and the large-scale movement of the resulting fragments. This action occurs through two main mechanisms: the static expansion of water in cracks and the dynamic movement of immense ice masses, which has dramatically shaped mountain ranges and valleys across the globe.
How Expanding Ice Breaks Down Rock
The mechanical process known as frost wedging, or freeze-thaw weathering, is an effective force that breaks rock apart without changing its chemical composition. This action begins when liquid water seeps into small cracks, joints, and fissures in exposed bedrock. When the temperature drops below freezing, the water undergoes a phase change, expanding its volume by approximately 9%.
This volumetric expansion exerts an enormous outward pressure on the surrounding rock walls. The ice can generate a force great enough to shatter even strong rock like granite, potentially reaching 2,000 pounds per square inch. The process is most effective in environments that experience frequent daily freeze-thaw cycles, allowing water to repeatedly enter, freeze, and widen the fractures. Repeated application of this pressure over time causes the rock to progressively weaken and fracture into smaller pieces.
This breakdown of rock in place is a form of weathering, which sets the stage for later transportation. The fragments produced by frost wedging often accumulate at the base of cliffs and steep slopes, forming large, fan-shaped deposits of rubble called talus slopes. This sediment is a primary source of material that moving ice and other agents will eventually transport away.
The Power of Glaciers to Transport Material
Glaciers act as massive, slow-moving rivers of ice that actively transport material. A glacier is a persistent body of dense ice that moves under its own weight and gravity, carving landscapes as it flows. The erosional work—the removal and transport of material—is accomplished through two distinct sub-glacial mechanisms: abrasion and plucking.
Glacial Abrasion
Glacial abrasion occurs as rock fragments embedded in the bottom and sides of the moving ice act like sandpaper, scouring the underlying bedrock. The sheer weight of the overlying ice, which can be hundreds or thousands of feet thick, subjects these rock fragments to immense pressure, grinding and polishing the rock surface. This action leaves behind characteristic linear scratches in the bedrock known as glacial striations, which indicate the direction of ice flow.
Plucking
Plucking is a more aggressive process that removes large chunks of rock. Meltwater flows into fractures in the bedrock at the glacier’s base, where it refreezes and adheres to the rock. As the glacier continues to advance, the immense force of the moving ice pulls the frozen-in blocks of rock away from the parent bedrock, leaving behind jagged and fractured rock surfaces.
Real-World Examples of Ice Erosion
The geological impact of ice, both past and present, is visible in many of the world’s most recognizable landscapes. Glaciers create U-shaped valleys, which are distinct from the V-shaped valleys typically carved by rivers. The broad, flat bottom and steep, straight sides of these valleys result from the glacier’s scouring action, which widens and deepens the original river channel.
Another striking landform created by glacial erosion is the cirque, a bowl-shaped depression found high on mountain sides. These features form at the head of the glacier where a combination of plucking and freeze-thaw weathering carves a deep hollow. When multiple cirques erode backward into the same mountain, they create sharp, knife-edge ridges known as arêtes, or a pointed peak called a horn.
In coastal regions, deep U-shaped valleys that have been flooded by the sea create fjords, which are long, narrow inlets with steep sides. These features demonstrate the immense erosional capacity of the glaciers that once occupied them.