9 Surface Processes and Landforms

From Rock Breakdown to Sediment Movement

A useful way to understand landscape change is to follow material through a sequence: rock is broken down or altered, removed from its original position, carried elsewhere, and eventually deposited. These processes operate over timescales ranging from sudden floods and landslides to the gradual development of valleys and plains.

Weathering acts in place. Physical weathering breaks rock into fragments, as when water freezes and expands in cracks or roots widen fractures. Chemical weathering changes minerals through reactions with water, oxygen, or dissolved acids. Both types help produce soil and sediment.

After material is loosened, erosion removes it from a place, and transport carries it away. Deposition occurs when the transporting agent loses enough energy to drop some of its load. Deposition may be partial: a river can leave larger grains behind while continuing to carry fine sediment.

Takeaway: Weathering prepares material; erosion removes it; transport moves it; and deposition leaves it elsewhere.

Water, Rivers, and Coasts

Water moves sediment in several ways. Fine particles can remain suspended, some material dissolves in the water, and larger grains may roll or bounce along the streambed. Wind carries dust in suspension, sand by bouncing, and coarser grains by surface creep. Faster or more energetic flows can generally move larger particles; when flow slows, deposition becomes more likely.

Rain may first carry soil downhill as sheet runoff. As runoff gathers into channels, streams erode their beds and banks, transport sediment, and deposit it where their flow slows. Steep headwaters commonly have narrow valleys, while rivers farther downstream may meander across broad floodplains.

A meander illustrates erosion and deposition happening side by side. Faster water on the outside of a bend erodes a cut bank; slower water on the inside deposits a point bar. If a river cuts through the narrow neck of a meander, the abandoned bend may become an oxbow lake. Floods can spread sediment over floodplains, and sediment collecting at a river mouth can build a delta.

Waves erode shore material, while waves and coastal currents transport sand along the coast. Where this movement slows or sediment collects, beaches, spits, and bars may form. A shoreline can lose sediment in one place and gain it in another, depending on the balance between sediment supplied, moved, and removed.

Takeaway: A landform often reflects erosion and deposition occurring in different places within the same system.

Wind, Glaciers, and Gravity

Wind is especially effective at moving loose sediment where vegetation is sparse, though it can carry dust over long distances. Deflation removes loose particles, sometimes leaving a coarse surface called desert pavement. Windblown grains also wear down rock through abrasion and can sculpt streamlined ridges called yardangs. Deposited sand can build dunes, while windblown silt can accumulate in extensive deposits called loess.

Glaciers erode bedrock in two main ways. Abrasion occurs when debris embedded in moving ice grinds against the ground. Plucking occurs when ice removes blocks of rock. Moving ice can widen and deepen valleys into broad U-shaped forms. When a glacier melts or retreats, it may leave unsorted sediment called till. Piles of glacial debris called a moraine mark former ice margins; meltwater can sort and deposit sediment beyond the ice.

Gravity moves rock and soil downhill through mass wasting. Slow movement can gradually tilt trees or fences, while rapid movement includes rockfalls, slides, and debris flows. Water can add weight to slope material and reduce its stability. Stream erosion or construction can steepen slopes as well. Material moved downslope may then enter streams and be carried farther away.

Takeaway: Wind, ice, and gravity each move sediment in characteristic ways and create distinctive landforms.

How Landscapes Develop

Landforms develop through the combined effects of processes that build or raise terrain and processes that wear it down. Uplift can create high ground. Weathering supplies loose material, erosion removes it, and transport carries it to places such as valleys, floodplains, dune fields, lakes, or ocean basins, where it may be deposited.

The properties of rock influence the shapes that remain. More resistant layers may persist as ridges or cliffs while weaker material erodes more quickly. Climate also affects which processes are most active: freeze–thaw weathering is common where temperatures repeatedly cross the freezing point, while chemical weathering is often stronger where water is abundant.

Several processes can shape one landscape at once. In a mountain watershed, frost may fracture rock, gravity may move fragments downhill, streams may carry them through valleys, and a river may deposit them on a floodplain or at its mouth. Present-day landforms can therefore reflect both processes acting now and processes that acted in the past.

Takeaway: Landforms record interacting processes, local rock resistance, climate, and changes over time.

Human Impacts and Sediment Pathways

People can change how quickly sediment is produced and where it goes. Removing vegetation for farming, logging, or construction exposes soil to rain and wind. Compacted ground can increase runoff, and more sediment may then enter streams.

Dams trap sediment in reservoirs and alter river flows, changing downstream channels and sediment supplies. Seawalls and other shoreline structures can protect particular sites but restrict natural sediment movement. Groins may trap sand on one side and contribute to erosion farther downshore. Cutting into or loading a slope can also increase landslide risk.

Managing land to preserve vegetation, limit exposed soil, and account for natural sediment pathways can reduce some unwanted effects. Because landscapes are connected, a change in one location can affect places downstream or along the coast.

Takeaway: Managing erosion and sediment requires considering connected pathways, not only the site where a change is made.