An Ice Lens or ice lenses

Lens (geology)

In geology a lens is a body of ore or rock or a deposit that is thick in the middle and thin at the edges, resembling a convex lens in cross-section. Adjective: "lenticular"....

 are formed when moisture

Moisture

Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

, diffused

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 within soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 or rock

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

, accumulates in a localized zone. The ice initially accumulates within small collocated pores or pre-existing crack, and, as long as the conditions remain favorable, continues to collect in the ice layer or ice lens, wedging the soil or rock apart. Ice lenses grow parallel to the surface and several centimeters to several decimeters (inches to feet) deep in the soil or rock.

Ice lenses play the key role in frost induced heaving

Frost heaving

Frost heaving results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss , starting at the freezing front or boundary in the soil...

 of soils and fracture of bedrock, which are fundamental to weathering

Weathering

Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

 in cold regions. Frost heaving creates debris and dramatically shapes landscapes into complex patterns

Patterned ground

Patterned ground is a term used to describe the distinct, and often symmetrical geometric shapes formed by ground material in periglacial regions...

. Rock fracture in periglacial

Periglacial

Periglacial is an adjective originally referring to places in the edges of glacial areas, but it has later been widely used in geomorphology to describe any place where geomorphic processes related to freezing of water occur...

 regions (alpine, subpolar and polar) has often been attributed to the freezing and volumetric expansion of water trapped within pores and cracks. However the majority of frost heaving and of bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 fracture results instead from ice segregation in ice lenses in the near-surface frozen regions. Ice segregation results in rock fracture and frost heave.

Common frost heaving

Frost heave

Frost heaving

Frost heaving results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss , starting at the freezing front or boundary in the soil...

 is the process by which the freezing of water-saturated soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 causes the deformation and upward thrust of the ground surface. This process can distort and crack pavement, damage the foundation

Foundation (architecture)

A foundation is the lowest and supporting layer of a structure. Foundations are generally divided into two categories: shallow foundations and deep foundations.-Shallow foundations:...

s of buildings and displace soil in regular patterns. Moist, fine-grained soil at certain temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

s is most susceptible to frost heaving.

Ice lenses in tundra

Frost heave is common in arctic tundra because the permafrost maintains ground frozen at depth and prevents snowmelt and rain from draining. As a result, conditions are optimal for deep ice lens formation with large ice accumulations and significant soil displacement.

Differential frost heave producing complex patterns will occur if the correct conditions exist. Feedback from one year's frost heave influences the effects in subsequent years. For example, a small increase in overburden will affect the depth of ice formation and heaving in the subsequent years. Time-dependent models of the frost heave indicate that over a long enough period the short-separation perturbations damp out, while mid-range perturbations grow and come to dominate the landscape.

Subglacial ice formations

Bands of sediment or glacial till have been observed below Antarctic ice sheets; these are believed to result from ice lenses forming in the debris. In the faster flowing glacial regions, the ice sheet is sliding over water saturated sediments (glacial till) or actually being floated upon a layer of water. The till and water served to reduce friction between the base of the ice sheet and the bedrock. These subglacial waters come from surface water which seasonally drains from melting at the surface, as well as from ice-sheet base melting.

Ice lens growth within the bedrock below the glacier is projected during the summer months when there is ample water at the base of the glacier. Ice lenses will form within the bedrock, accumulating until the rock is sufficiently weakened that it shears or spalls off. Layers of rock along the interface between glaciers and the bedrock are freed, producing much of the sediments in these basal regions of glaciers. Since the rate of glacier movement is dependent upon the characteristics of this basal ice, research is ongoing to better quantify the phenomena.

Understanding the phenomena

The basic condition for ice segregation and frost heaving is existence of a region in soil or porous rock which is relatively permeable, is in a temperature range which allows the coexistence of ice and water (in a premelted state), and has a temperature gradient across the region.

A key phenomenon for understanding ice segregation in soil or porous rock (also referred to as an ice lens due to its shape) is premelting, which is the development of a liquid film on surfaces and interfaces at temperatures significantly below their bulk melting temperature. The term premelting is used to describe the reduction in the melting temperature (below 0 °C) which results from the surface curvature of porous media confining water (the Gibbs-Thomson effect

Gibbs-Thomson effect

The Gibbs–Thomson effect relates surface curvature to vapor pressure and chemical potential and is a consequence of surface tension. It is named after Josiah Willard Gibbs and William Thomson, 1st Baron Kelvin. It leads to the fact that small liquid droplets The Gibbs–Thomson effect (also called...

). Premelted water exists as a thin layer on the surface of ice. Under premelting conditions, ice and water can coexist at temperatures below -10 °C in a porous medium. The Gibbs-Thomson effect results in water migrating down a thermal gradient (from higher temperatures to lower temperatures); Dash states, “…material is carried to colder regions…” This can also be viewed energetically as favoring larger ice particles over smaller (Ostwald ripening

Ostwald ripening

right|thumb|300px|Basic schematic of the Ostwald ripening process Ostwald ripening is an observed phenomenon in solid solutions or liquid sols which describes the change of an inhomogeneous structure over time...

). As a result, when conditions exist for ice segregation (ice lens formation) water flows toward the segregated ice and freezes on the surface, thickening the segregated ice layer.

It is possible to develop analytic models using these principles; they predict the following characteristics, which are consistent with field observations:

• Ice forms in layers which are parallel to the overlying surface.
• The ice initially forms with small microfractures parallel to the surface. As ice accumulates the ice layer grows outward in what is frequently characterized as an ice-lens parallel to the surface.
• Ice will form in water-permeable rock in much the same way as it forms in soil.
• If the ice layer resulted from a cooling from a single direction (e.g., the top) the 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 fracture tends to lie deeper (e.g., 2-3.5 cm in chalk).
• Ice forms rapidly when liquid is readily available. When liquid is readily available, the segregated ice (ice lens) grows parallel to the exposed cold surface. It grows rapidly until the heat liberated by freezing warms the ice lens boundary, reducing the temperature gradient and controlling the rate of further ice segregation. Under these conditions, ice grows in a single layer which gets progressively thicker. The surface is displaced and soil repositioned or rock fractured.
• Ice forms in a different pattern when liquid is less readily available. When liquid is not readily available, the segregated ice (ice lens) grows slowly. The heat liberated by freezing is unable to warm the ice lens boundary. Hence the area through which the water is diffusing continues to cool until another ice segregation layer forms below the first layer. With sustained cold weather, this process can repeat, producing multiple ice layers (ice lenses), all parallel to the surface. The formation of multiple layers (multiple lenses) producing more extensive frost damage within rocks or soils.
• No ice forms under some conditions. At higher overburden pressures and at relatively warm surface temperatures, ice segregation cannot occur; the liquid present freezes within the pore space, with no bulk ice segregation and no measurable surface deformation or frost damage.

Ice lens growth in rock

Rocks routinely contains pores of varying size and shape, regardless of origin or location. Rocks voids are essentially small cracks, and serve as the location from which the crack can propagate if the rock is placed in tension. If ice accumulates in a pore asymmetrically, the ice will place the rock in tension in a plane perpendicular to the ice accumulation direction. Hence the rock will crack along a plane perpendicular to the direction of ice accumulation, which is effectively parallel to the surface.

Walder and Hallet developed models that predicts rock crack-growth locations and rates consistent with fractures actually observed in the field. Their model predicted that marble and granite grow cracks most effectively when the temperatures range from a −4 °C to −15 °C; in this range granite may develop fractures enclosing an ice 3 meters in length in a year. When the temperature is higher the ice which is formed does not apply enough pressure to cause the crack to propagate. When the temperature is below this range the water is less mobile and cracks grow more slowly.

Mutron confirmed that ice initially forms in pores and creates small microfractures parallel to the surface. As ice accumulates, the ice layer grows outward in what is frequently characterized as an ice-lens parallel to the surface. Ice will form in water-permeable rock in much the same way as it forms in soil. If the ice layer resulted from a 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).

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