Ice heaving and wedging

Some 35% of Earth's land area undergoes regular freezing and thawing. Ice heaving and ice wedging are two of the mechanisms by which water in soil lifts, penetrates, and sorts soils and rocks when repeatedly melted and frozen. Ice heaving is the lifting of soil by horizontal ice layers; ice wedging is the top-down growth into soil of vertical wedges of ice.

Ice heaving is driven by complex molecular interactions between water and soil. The simple result of these complex interactions is that ice forming in soil sucks water to itself by capillary action. The suction exerted by ice upon water in soil is termed cryosuction. Since freezing normally proceeds from the surface down, ice heaving begins with the formation of a layer of ice near the surface. As it grows, this layer draws water to itself from below by cryosuction. This water freezes to the underside of the growing layer. The ice thus formed is termed segregation ice because it grows by segregating previously mixed soil and water. Segregation ice forms from water transported by cryosuction to the upper soil; this imported material, aided slightly by water's 9% expansion upon freezing, raises up the overlying ground surface as segregation ice forms, causing ice heaving.

Segregation ice often forms regularly spaced layers. As each layer forms, it tends to suck dry the soil beneath it. When the force of cryosuction is no longer able to lift water from below, thickening of the current layer ceases and cooling proceeds downward until a new ice layer can begin to form at a greater depth.

Ice wedges form by a simpler process. When soil cools it contracts; this contraction produces cracks. Water trickles into the cracks and freezes, forming an incipient ice wedge. Subsequent cycles of temperature-driven expansion and shrinkage cause the wedge to crack open repeatedly, admitting additional water each time. Wedge ice is termed intrusion ice because its water is not drawn from the surrounding soil, but intrudes into it.

Any flat, smooth coating of particles and liquid (e.g., mud, paint, or soil) tends to crack in a pattern of polygonal shapes when it shrinks, whether by cooling or drying. Large areas of far-northern land are, consequently, covered by ice-wedge polygons, often many meters across. These polygons are an example of patterned ground—that is, terrain marked by natural, repeating, geometric shapes. Most patterned ground is produced by cyclic freezing and thawing, whether by heaving, wedging, or other mechanisms.

Ice wedging is restricted to the far north and high-altitude areas. Ice heaving occurs wherever wet ground freezes even superficially. Pipkrakes—the crunchy, vertical-fibered ice crystals that spring up in wet soil on freezing nights—are a small-scale example of ice heaving.

See also Phase state changes