08 Weathering, Erosion, and Landscapes

Three linked surface processes

Earth’s surface changes as rock breaks down, sediment moves, and material settles in new places. These processes may act slowly over long periods or rapidly during events such as floods and landslides.

Weathering breaks down or changes rock where it is. Erosion removes and transports loosened material. Deposition happens when transported material settles. Together, these processes explain how sediment is created, moved, and added to a landscape.

How rock breaks down

Weathering can be mechanical, chemical, or biological. Mechanical weathering breaks rock into smaller pieces without changing its minerals. Water that freezes and expands in cracks can split rock; roots can widen cracks; repeated heating and cooling can also help fracture exposed rock.

Chemical weathering alters or dissolves minerals through reactions with water, oxygen, or weak acids. Slightly acidic water, for example, can dissolve limestone and contribute to the formation of caves and sinkholes. Organisms can contribute to either kind: roots pry rock apart, while decaying plant matter can make water more acidic.

Weathered rock may form loose material called regolith. Where this material includes organic matter and supports plant growth, it is part of soil. Rock type, climate, water, organisms, and time all influence how weathering proceeds, and different types of weathering often work together.

Agents that shape the land

Water, wind, ice, and gravity move sediment in different ways and produce characteristic landforms.

• Water: Rain and streams carry sediment downhill. Small flows may develop into rills, gullies, and channels; streams can cut V-shaped valleys and canyons. Waves and currents erode coasts and redistribute sand.

• Wind: Where vegetation is sparse and loose sediment is exposed, wind can lift particles through deflation or wear rock through abrasion as sand grains strike it. Wind deposits include sand dunes and windblown silt called loess. A stony desert pavement can remain after finer material is removed.

• Ice: Moving glaciers pluck and scrape rock. Valley glaciers can widen and deepen river valleys, changing their V-shaped profiles into broad U-shaped valleys. They may leave cirques, sharp ridges, and deposits of sediment.

• Gravity: Gravity moves material downslope through mass wasting. Movement may be slow, or it may happen rapidly as rockfalls, slides, or debris flows. Downslope deposits may form irregular accumulations called colluvium.

Transport, settling, and landforms

A stream’s competence is the largest particle it can move, while its capacity is the total amount of sediment it can carry. Faster or larger flows can generally move larger particles and more sediment. When water slows, larger grains tend to settle before finer ones.

These changes in flow help create distinctive deposits. A river may build an alluvial fan where it leaves a steep, confined mountain valley and spreads onto flatter ground. Where a river enters a lake or ocean and loses energy, it may deposit a delta. Repeated flooding can spread sediment across a floodplain.

Glacial melt leaves till, a mixture of sediment sizes, and can form ridges called moraines. Meltwater may sort and redeposit glacial sediment downstream. These examples show that the location and character of a deposit depend partly on how the transporting agent’s energy changes.

Landscapes and natural hazards

A landscape reflects the balance between processes that build or uplift land and processes that weather and erode it. Rivers tend to carve valleys through high ground and deposit sediment where flow slows. Glaciers can reshape mountain valleys and leave moraines. In dry regions, sparse vegetation and loose sediment can make wind erosion and flash-flood deposition especially visible.

The same processes can create hazards. Floods can erode banks and inundate floodplains; coastal erosion can remove land; and landslides or debris flows can move rapidly and bury or damage what lies in their path. A debris flow is a fast-moving mixture of water and sediment, sometimes including boulders. Risk is greater where people or infrastructure occupy floodplains, unstable slopes, or active fans. Recognizing landforms and understanding the processes that formed them can help identify where hazards may occur.

Putting the processes together

Consider a mountain stream that weathers and erodes bedrock, then transports the resulting sediment downhill. When the stream emerges onto a flatter valley floor, it slows and deposits coarse material, which may build an alluvial fan. A later flood can shift the channel across the fan and move sediment again.

The sequence links the central processes: weathering supplies material, erosion transports it, and deposition adds it to a new location. A landscape is therefore continually reshaped, sometimes gradually and sometimes during rapid events.