Of all the forces of weathering that act on a landscape, water - particularly frozen water - produces the most dramatic topographical features.
Water expands when it freezes, the expansion being accompanied by great outward pressure (it is this pressure that bursts pipes when the water in them freezes).
This expansive force of frost can affect exposed terrain in two ways: rock may be broken into smaller fragments by freezing water expanding in its joints (a process called frost shattering), or the ground may be caused to expand and contract alternatively, known as frost heaving.
In order to be effective, the action of frost erosion must be strong enough to overcome the elasticity of the rock.
The breakdown process starts when water seeps into pores or tiny cracks and joints in the rock. Then, when the water freezes, it forces the walls of the pores and joints further apart.
On thawing, a slightly greater volume of water is able to enter the enlarged hole, and so a correspondingly stronger force is applied during the next freezing. Successive repetitions of this frost wedging process lead eventually to the shattering of a solid mass of rock into fragments.
Mountain Landscapes
Frost erosion is particularly effective in mountainous areas, because temperatures are low and there is a wide daily variation in temperature.
In some places, the eroded debris falls and collects in great quantities at the base of steep mountain slopes.
Mountains with needle-like peaks formed by frost action are known as "aiguilles" (meaning needles); they are often further worn away to a pyramidal "horn" by the erosive effect of frost and glaciers on the flanks.
Material broken off the side of a mountain gathers towards the foot of the slopes, to form a scree (or talus) slope. Fragments of scree are always angular and the scree slopes are steep; the larger the fragments, the greater the erosion has been, and the steeper the slope.
If the falling debris is guided by natural gullies and channels in the mountain, it comes to rest in a scree slope that resembles the rounded side of a cone as it fans out from its channel. Since they are forming continuously, scree slopes tend to have no soil or vegetation.
Mountain Shaping
Above the snowline any hollow in a mountainside is permanently occupied by snow. The steady accumulation and compression of the snow into ice in the bottom of the hollow eventually gives rise to a glacier.
The erosive effect of the compressed snow in such a hollow acts in all directions at the same rate and, combined with the downward movement of the glacier, lowers the floor and cuts back the walls so that the hollow becomes a steep-sided, flat-bottomed feature called a cirque.
Neighboring cirques on the flanks of a mountain are divided by a ridge. As the cirque walls are cut back the ridge becomes steep and sharp-crested and forms an arête, several of which may radiate from all sides of a mountain - by now a Pyramidal Peak mountain.
Above a glacier the falling frost-shattered rocks do not form a scree. The blocks that land on the moving ice are carried away and eventually dumped as moraines, which are a significant feature of glacial action.
Layers of snow on the higher areas of mountains may occasionally tumble down steep eroded slopes in avalanches. They usually occur when the lower slopes of snow have melted or been blown away, leaving the top unsupported. The falling snow compacts to ice as soon as it hits anything and the great weights involved can tear away vast quantities of forest and rock from the lower slopes.
Frost on Flatlands
The more complex effects of frost erosion are seen in areas such as the tundra, where temperatures are below freezing point for most of the year and nearly all the visible landscape features have been produced by frost action. The frost heaving that takes place does not break down the rocks, but moves and mixes the soil particles.
As the temperature drops from 0°C to -20°C, the already expanded ice begins to contract. When this occurs on the surface of the earth the result is a general shrinkage of soil in which the surface cracks up into polygonal sections. These sections may be about 10m across and are bounded by deep cracks. During thaws water enters the cracks and ice wedging takes place when the next freeze occurs. The expansion pressure of the surrounding ice causes the center of the polygon to rise in the shape of a shallow dome.
A stone buried in the soil cools more quickly than the surrounding damp soil because it is a better conductor of heat. The first place in which ice forms during a freeze is therefore directly under any buried stone. The crystals of ice below the stone push it upwards slightly as they expand. Over a period of several years this process brings the stone to the surface. (This frost heaving effect is particularly noted by gardeners in cold weather.)
In polygonally cracked ground the stones are ultimately brought to the surface of the polygons. From there they move down the slopes of the domes and gather in the surrounding cracks.
Frost Power
Most of the effects of frost erosion derive from the peculiar behavior of water at temperatures near its freezing point, and from the unique properties of ice.
Water contracts as it cools, reaching its maximum density at 4°C. On further cooling it expands slightly, reaching a volume of 1.087cc per gram as it freezes at 0°C. As the temperature falls even lower, the ice expands and can exert enormous pressures - in excess of 150kg/cm2 (1,500 tons per square meter).
A familiar example of the effect of this force is the bursting of frozen water pipes in winter. Well below freezing point ice contracts again; for example, at -22°C it has a volume of only 0.971cc per gram.