Moving ice: erosion, transport and deposition

A glacier is a mass of ice that moves under gravity. The ice flows through internal deformation. Where conditions allow, it also slides over its bed. Glacial landscapes reflect this movement, rather than low temperatures alone.

Two main processes erode the bedrock. Abrasion is the scraping and grinding of rock by debris carried in moving ice. As the debris smooths rock surfaces, it cuts scratches called striations and produces fine rock flour. Plucking is the removal of jointed rock blocks by moving ice. Water enters cracks and freezes. The loosened blocks become incorporated into the glacier, which pulls them away.

How much erosion occurs depends on ice thickness and speed, as well as the amount of debris and conditions at the glacier bed. Ice frozen firmly to its bed generally erodes less than ice that slides over it. Thick, fast-moving ice can focus erosion along valleys, deepening existing depressions.

Debris travels on glacier surfaces and within the ice, as well as at the base. Rockfall supplies material at the surface; plucking supplies it from below. Unlike rivers, ice can carry fine sediment alongside large boulders without sorting them by size.

Melting releases debris, causing deposition. Material can also be deposited when it becomes lodged beneath the ice. Till is sediment deposited directly by glacier ice. It is usually unsorted, with a mixture of particle sizes. Meltwater can later rework it into more sorted deposits.

Erosional landforms

• Cirques or corries are bowl-shaped hollows on mountain slopes, with steep backwalls and an overdeepened floor. Snow collects in a hollow and develops into moving ice. Abrasion deepens the floor; plucking and freeze-thaw help the backwall retreat. A less-eroded rock lip commonly remains at the outlet.
• Arêtes are narrow, sharp ridges between neighbouring glacial hollows or valleys. As erosion acts on either side, the ridge between them narrows.
• Pyramidal peaks or horns are pointed summits left where several cirques erode back towards a mountain from different directions.
• Glacial troughs are broad valleys with steep sides and relatively flat floors, producing a U-shaped cross-section. Through abrasion and plucking, a glacier widens and deepens an existing valley. It can also remove projecting valley spurs, leaving steep, truncated ends.
• Glacial lakes occupy depressions created or enclosed by glacial action. Within a cirque, a tarn can form behind the rock lip. An elongated lake can fill an overdeepened valley basin. Moraine can also block drainage and impound water.

Depositional landforms

Moraine is an accumulation of debris carried or deposited by a glacier. Where it lies records how debris entered the ice and where it was released.

• Lateral moraine forms along a glacier's margins, largely from rockfall debris supplied by the valley sides. Ridges can remain above the valley floor after the glacier retreats.
• Medial moraine forms where two glaciers join and their adjacent lateral debris bands merge. This debris band runs along the combined glacier and may be deposited as the ice melts.
• Terminal moraine is a ridge across the valley at a glacier's furthest advance. Debris builds up where the snout remains relatively stable. After retreat, the ridge remains.
• Erratics are rocks transported by ice and deposited on bedrock of a different geological type. Their composition can reveal a distant source, helping establish the direction of former ice movement.

Frozen ground and seasonal thaw

Periglacial environments are cold environments where freezing and thawing strongly influence ground processes. They don't have to be beside a glacier. These conditions also occur across high-latitude regions and at high altitude.

Permafrost is ground that remains at or below 0°C for at least two consecutive years. It can contain substantial ground ice, but frozen ground is not the same as pure ice. An active layer lies above it, freezing in winter and thawing in summer. How deep this layer reaches depends on temperature, vegetation, snow cover and soil properties.

Freeze-thaw, frost heave and solifluction

Freeze-thaw weathering breaks down rock through repeated freezing and thawing of water in cracks. Water enters joints and, as it freezes, expands and puts pressure on the rock. Over repeated cycles, cracks can widen and angular fragments can break away. Both water and temperature changes around freezing are needed. Permanently severe cold does not automatically produce frequent freeze-thaw cycles.

Frost heave is the upward displacement of soil or stones as ground freezes. As water migrates towards the freezing zone, it forms ice lenses that expand and lift the material above them. Uneven heaving, followed by settling, can gradually rearrange stones and fine sediment.

Solifluction is the slow downslope movement of water-saturated soil under cold conditions. Seasonal thaw can saturate the active layer where it lies above frozen ground that drains poorly. Gravity then moves this material downslope, forming lobes or step-like terraces. Movement is generally greatest when thawing supplies abundant water.

Landforms produced by ground ice

Patterned ground consists of repeated arrangements of stones or surface features into shapes such as circles, polygons and stripes. Differential frost heave and thaw settlement can separate coarse stones from finer material. On flatter ground, the patterns tend to be more circular or polygonal; on slopes, they become elongated into stripes. Not all polygonal patterns come from stone sorting. Some arise from thermal contraction cracks.

Pingos are hills with an ice core beneath a covering of soil or sediment. They form as pressurized groundwater freezes beneath the surface. The growing ice body pushes the overlying material upwards. If the core melts, the mound can collapse, leaving a depression that sometimes contains a pond.

Thermokarst is irregular terrain produced when ice-rich ground thaws and the surface subsides. Melting ground ice leaves the sediment above without support, so hollows, ponds and uneven surfaces develop. Thaw can therefore rapidly alter a landscape that took much longer to form.

These features do not develop equally everywhere. Ice content, water availability, slope and the duration of seasonal thaw determine whether heaving, soil movement or collapse dominates a site.

Weathering: breaking down rock in place

Weathering breaks down or alters rock where it lies. By contrast, erosion removes material through an agent such as moving wind or water. The debris produced by weathering can later be transported and deposited.

Rock broken into smaller pieces without a change in chemical composition has undergone physical weathering. Two important mechanisms in hot arid environments are thermal stress and salt crystallization.

As rock surfaces heat strongly during the day and cool at night, they expand and contract. Minerals respond differently to these temperature changes. The outer layers also change temperature more rapidly than the interior. Over repeated cycles, these stresses can weaken rock and contribute to cracking, grain separation and flaking.

Salt crystallization begins when saline water enters pores or cracks and then evaporates. As the salt crystals grow, they press against the rock. Where salts and occasional moisture are available, rapid evaporation and repeated wetting and drying make this process especially important.

Chemical weathering alters the minerals in rock through chemical reactions. Across much of an arid landscape, limited moisture restricts these reactions—it doesn't stop them entirely. Oxidation alters iron-bearing minerals; water containing weak acids can dissolve susceptible minerals. During wet periods, reactions become more significant. The same applies where moisture persists in cracks or beneath the surface.

Wind: erosion, transportation and deposition

With little vegetation to protect it, sediment lies exposed to wind. Yet wind does not move all material equally: particle size, wind strength and surface moisture determine what it can lift or push along the ground.

Deflation removes loose particles by wind. Fine material is carried away more easily than large stones, which may be left as a coarse surface cover. Rock worn by wind-driven particles undergoes wind abrasion. Because most sand travels close to the surface, abrasion is commonly concentrated low on exposed rocks.

When wind can no longer carry its load, deposition occurs. Wind speed may fall, an obstacle may shelter the ground behind it, or vegetation may trap grains. Sand accumulates relatively near its source. Suspended dust, however, can travel much farther before settling.

Water: infrequent but powerful

Running water can still be important where annual rainfall is low. During an intense storm, rainfall that exceeds infiltration can produce rapid runoff. Sparse vegetation can increase this response, as can steep slopes and crusted or relatively impermeable surfaces.

Flash floods erode through hydraulic action, the force of moving water, and through abrasion from transported sediment. Dissolved material travels in solution, while fine particles are carried in suspension. Coarser sediment moves by saltation and traction along the channel bed.

As flow velocity and transport capacity fall, sediment is deposited—for example, when water spreads beyond a confined channel. Coarse sediment is generally deposited before finer particles. Flow in ephemeral channels also weakens as water infiltrates the bed and evaporates.

Over time, wind and water work together. Sediment redistributed by a flood may later dry out and become available for wind transport. Nor does present-day aridity prove that every landform developed under today's climate: some features reflect earlier periods with more water.

Dunes: wind-built and mobile

Dunes are accumulations of wind-deposited sand. They form where sand is available and deposition exceeds removal. Often, an obstacle or a change in surface conditions slows the wind enough for sand to collect.

Sand supply and wind direction affect dune shape, as does vegetation. With a fairly consistent wind direction and a limited sand supply, for example, a crescent-shaped dune can form with its horns pointing downwind. More abundant sand or changing wind directions produce other forms. Dunes aren't all identical piles of sand.

Wadis: channels shaped by episodic flow

Wadis are valleys or channels in arid environments that are usually dry but may carry water after rainfall. They link runoff-producing slopes to lower ground, even when no flow is visible for long periods.

Flash floods send water and sediment through the wadi, scouring its bed and banks. Sediment is deposited within the channel as discharge falls. Repeated floods can deepen or widen the wadi and redistribute the deposits. Some large wadis also retain the effects of wetter conditions in the past. Their size may therefore exceed what today's occasional flows would create.

Rock pedestals: uneven erosion

Rock pedestals are rocks with a narrow base supporting a broader upper part. Most sand transport occurs close to the ground, so wind-driven sand can abrade the lower portion more strongly. Weathering can help undercut the rock, especially if its lower part is less resistant.

Mesas and buttes: resistant caps above retreating slopes

Mesas are broad, flat-topped residual hills with steep sides, commonly protected by a resistant upper rock layer. Smaller, more isolated remnants of the same general form are Buttes, usually with a narrower summit.

These landforms develop as layered rock undergoes differential weathering and erosion. The softer beds beneath the cap rock are removed more readily. This undercutting leaves the resistant cap unsupported, so blocks collapse. Weathering and slope movement help remove the debris, along with running water. The sides can then retreat while the top stays comparatively flat.

Dunes mainly record deposition and continuing sediment movement, whereas wadis record episodic water flow. Pedestals, mesas and buttes record unequal removal of rock. These different forms arise as the available material interacts with the processes acting upon it.