4 Landform Development and Surface Processes

The surface-process sequence

Landforms develop through linked processes that break down rock, move material, and accumulate sediment. Rock may be weathered in place, removed by erosion, transported by water, wind, ice, or gravity, and deposited where the transporting force weakens or conditions favor accumulation.

These steps are connected, but they do not always follow one another immediately. Weathered material may remain in place for a long time, and deposited sediment may later be eroded and transported again. The balance among sediment supply, transporting energy, rock type, slope, climate, and time helps determine the shape of a landform.

Weathering, erosion, transport, and deposition

Weathering alters or breaks down rock in place. Mechanical weathering fragments rock without changing its mineral composition; freeze–thaw cracking and abrasion are examples. Chemical weathering changes or dissolves minerals, often through reactions with water and acids. Roots and other organisms can also contribute to weathering.

Erosion removes material from a surface. In everyday earth science usage, erosion often includes movement of the removed material as well. Flowing water, waves, wind, glaciers, and gravity are agents of erosion.

Transport is the movement of sediment. Water may carry particles in suspension, roll or bounce them along a bed, or carry dissolved material. Wind and ice also transport sediment, and gravity moves it downslope in events such as landslides and debris flows.

Deposition occurs when a transporting agent can no longer carry its sediment load, or when conditions favor accumulation. A stream may deposit sand as its flow slows, a glacier may leave debris as it melts, and wind may build a dune where sand collects.

Rivers carve valleys and build floodplains

In a river basin, weathering and slope processes supply sediment to streams. Flowing water erodes the bed and banks, transports sediment downstream, and deposits it when velocity or carrying capacity falls. Rivers commonly cut into the landscape in steep headwaters; farther downstream, lower gradients and flooding favor sediment storage and deposition.

In a meandering river, faster flow along the outside of a bend erodes a cut bank, while slower flow on the inside deposits sediment in a point bar. As these processes continue, bends migrate across the valley. A narrow neck may eventually be cut through, leaving an abandoned bend as an oxbow lake.

During floods, water spilling over riverbanks deposits sediment across the floodplain. Repeated deposition helps build features such as natural levees. As a bend migrates sideways, the river removes sediment from one part of the floodplain while adding it to another.

Coasts redistribute sediment

Waves and currents erode cliffs, beaches, and the nearshore seabed. Rivers, cliff erosion, and other sources supply coastal sediment. Waves and currents move it along the shoreline or between the beach and offshore areas. When water loses energy, sediment settles and may build beaches, bars, or other coastal deposits.

Erosion and deposition can occur in different places along the same coast, so one beach may gain sand while another stretch loses it. Waves approaching the shore at an angle can create longshore currents that carry sediment parallel to the coast; this movement is known as longshore sediment transport.

Storms can rapidly erode or move large amounts of sediment. Calmer conditions may allow deposition to rebuild parts of a beach.

Glaciers carve mountains and deposit debris

Moving glaciers erode bedrock by abrasion, as rock fragments in basal ice grind against the ground, and by plucking, as ice removes blocks of fractured rock. These processes help carve glacial valleys and other mountain landforms. A glacier also transports debris within, beneath, and on top of the ice.

When a glacier melts or retreats, it deposits unsorted material. Ridges of glacial debris are called moraines. Meltwater can sort and redeposit some of the material as outwash.

For example, a retreating glacier may leave a moraine at its former margin. Meltwater flowing away from the ice can carry and sort finer sediment beyond the ridge, creating deposits distinct from the debris left directly by the ice.

Drylands respond to episodic water and wind

Dryland landscapes are shaped by both water and wind. Rain may be infrequent, but brief, intense runoff can erode channels, carry sediment, and deposit it where a steep stream leaves a mountain front. Repeated deposition builds an alluvial fan. In a closed basin, water may collect temporarily in a low area; evaporation can leave a dry lakebed, or playa.

Wind removes fine particles through deflation and wears exposed surfaces through abrasion by windblown grains. It also transports sand and dust. Where wind energy falls or obstacles trap grains, sediment accumulates in dunes; fine dust can form deposits such as loess. Sparse vegetation and available loose sediment can make wind transport more effective.

For example, a flash flood can carry coarse sediment out of a narrow canyon and deposit it where the slope becomes gentler, building an alluvial fan. Wind may later remove some finer material from exposed surfaces and deposit it elsewhere.

Landforms change over time

Landforms reflect interactions among processes, materials, and changing conditions rather than a single event. Rock strength, rock structure, and sediment supply affect how quickly a landscape changes. Climate influences the availability and action of water, ice, and wind, while slope affects how readily material moves.

Uplift or changes in base level can alter how rivers erode and deposit. Some landscape changes occur gradually, while floods, storms, landslides, or glacier retreat can produce rapid change. Deposits and erosional features preserve clues to the sequence of past events.