Ice wedging, also known as frost weathering or frost action, is a form of physical weathering that shapes Earth’s landscapes. It involves the breakdown of rock material through the mechanical force generated by water freezing and expanding within cracks and fissures. This mechanism turns solid bedrock into smaller fragments, particularly in regions with frequent temperature changes.
Why Freezing Water Exerts Immense Force
The force behind ice wedging originates from the unique physical property of water. Unlike most substances that contract when they cool, water expands as it transitions from a liquid to a solid state. This expansion occurs because the crystalline structure of ice arranges water molecules into an open hexagonal lattice, spacing them farther apart than in the liquid form.
When water freezes, its volume increases by approximately nine percent. If confined within a rock crack, the expanding ice exerts pressure against the walls. This creates hydrostatic pressure up to 30,000 pounds per square inch (psi). This pressure is similar to that created by a large hydraulic jack and is enough to overcome the tensile strength of most rocks.
This internal pressure causes rock breakage, acting like a wedge driven into the rock structure. This force is why water pipes burst in winter or why sealed glass bottles fracture if left in a freezer. The force is not dependent on crack size, as even microscopic pores and fissures are subjected to the same high pressures.
The Freeze-Thaw Cycle and Rock Fracture
Rock disintegration relies on the repeated action of the freeze-thaw cycle. For ice wedging to be effective, three conditions must be present: moisture, a pre-existing crack or joint in the rock, and temperatures that fluctuate around the freezing point of water. The cycle begins when liquid water, often from rain or melted snow, seeps into existing openings.
As the temperature drops below zero degrees Celsius, the confined water turns to ice, expanding and applying outward pressure on the rock walls. This initial freeze slightly widens and deepens the crack, but the rock typically remains intact. The temperature then rises, causing the ice to melt back into liquid water that fills the newly enlarged space.
The cycle repeats as the temperature drops, allowing a greater volume of water to freeze and exert cumulative stress on the rock. Each successive freezing event drives the crack deeper and wider, progressively weakening the structure. Over many seasons and thousands of cycles, this cumulative stress eventually exceeds the rock’s strength, resulting in a sudden fracture and the separation of a fragment.
This process is most active in high-altitude or high-latitude environments where temperature swings repeatedly cross the freezing point. The broken, angular rock fragments created by ice wedging accumulate at the base of cliffs and steep slopes, forming talus slopes or scree fields. This mechanical breakdown also creates smaller sediments, which are then transported by wind and water, influencing the landscape.