Ice lens

Ice lenses grow parallel to the surface and several centimeters to several decimeters (inches to feet) deep in the soil or rock.

Studies from 1990 have demonstrated that rock fracture by ice segregation (i.e., the fracture of intact rock by ice lenses that grow by drawing water from their surroundings during periods of sustained subfreezing temperatures) is a more effective weathering process than the freeze-thaw process which older texts proposed.

[1] Ice lenses play a key role in frost induced heaving of soils and fracture of bedrock, which are fundamental to weathering in cold regions.

Although rock fracture in periglacial regions (alpine, subpolar and polar) has often been attributed to the freezing and volumetric expansion of water trapped within pores and cracks, the majority of frost heaving and of bedrock fracture results instead from ice segregation and lens growth in the near-surface frozen regions.

Frost heave is common in arctic tundra because the permafrost maintains ground frozen at depth and prevents snowmelt and rain from draining.

The term premelting is used to describe the reduction in the melting temperature (below 0 °C) which results from the surface curvature of water that's confined in a porous medium (the Gibbs-Thomson effect).

[7] It is possible to develop analytic models using these principles; they predict the following characteristics, which are consistent with field observations: Rocks routinely contain pores of varying size and shape, regardless of origin or location.

[9] Walder and Hallet developed models that predict rock crack-growth locations and rates consistent with fractures actually observed in the field.

If the ice layer resulted from cooling from a single direction (e.g., the top) the rock fracture tends to lie close to the surface (e.g., 1–2 cm in chalk).

If the ice layer results from freezing from both sides (e.g., above and below) the rock fracture tends to lie deeper (e.g., 2–3.5 cm in chalk).

When that formation is suspended in the air by dead vegetation or erect objects, the ice will begin to form a sphere or teardrop-like shape.