Learning objectives
- Distinguish weathering, erosion, transportation and deposition.
- Describe physical, chemical and biological weathering, and explain how climate and rock type control them.
- Compare erosion by running water, wind, glaciers and gravity.
- Explain where and why sediment is deposited, and how deltas, alluvial fans, meanders, beaches, dunes and valleys form.
- Explain how slope, rainfall and vegetation affect soil erosion and landslides.
Weathering, erosion, transport and deposition
These four processes are often confused, but each does a different job:
- Weathering breaks rock down where it is, into smaller pieces or dissolved substances.
- Erosion removes the weathered material from where it formed.
- Transportation carries it away, in water, wind, ice or simply downhill under gravity.
- Deposition drops it when the carrier loses energy: a river slows, the wind drops or ice melts.
Together they wear mountains down and build new land elsewhere. The deposited sediment can later be buried and turned into sedimentary rock, one step in the rock cycle.
Types of weathering
| Type | Process | How it works |
|---|---|---|
| Physical (mechanical) | Freeze–thaw (frost wedging) | Water in cracks freezes and expands by about 9%, widening them a little each cycle. |
| Physical | Thermal expansion and exfoliation | Daily heating and cooling, and the release of pressure as overlying rock is removed, make outer layers crack and peel. |
| Chemical | Oxidation | Iron in minerals reacts with oxygen and water to form rust-coloured iron oxides. |
| Chemical | Dissolution (carbonation) | Rain containing carbon dioxide is weak carbonic acid, which dissolves limestone and marble. |
| Chemical | Hydrolysis | Water reacts with feldspar in granite, turning it into clay. |
| Biological | Root wedging and organic acids | Roots grow into cracks and prise them open; roots, lichens and microbes release acids. |
Physical weathering makes rock fragments without changing their minerals. Chemical weathering changes the minerals themselves. The two help each other: cracks let in water for chemical attack, and chemical weathering weakens rock so it breaks more easily.
What controls weathering
- Temperature: chemical reactions roughly double in speed for every 10 °C rise, so chemical weathering is fastest in the hot, wet tropics.
- Freeze–thaw cycles: most effective in mountains and high latitudes where the temperature crosses 0 °C on many days.
- Water: almost every weathering process needs it. Deserts weather slowly, mainly by heating and cooling.
- Rock type: limestone dissolves; iron-rich basalt oxidises; porous sandstone lets water in; massive granite resists until its joints open.
- Vegetation: roots and organic acids speed weathering, even though plants protect soil from erosion.
- Surface area: breaking rock into smaller pieces exposes more surface to chemical attack.
Agents of erosion
| Agent | How it erodes | Typical landforms |
|---|---|---|
| Running water | Rain splash, sheet wash and gullies on slopes; rivers cut down and sideways with the sediment they carry (abrasion). | V-shaped valleys, canyons, waterfalls, meanders, floodplains, deltas, fans |
| Wind | Lifts loose dry grains (deflation) and sandblasts rock (abrasion). | Dunes, desert pavement, loess, rock pedestals |
| Glaciers | Ice plucks blocks from the bed and grinds it with embedded debris. | U-shaped valleys, cirques, horns, fjords, moraines |
| Gravity | Mass movement: creep, slumps, landslides, mudflows, rockfall. | Scree slopes, landslide scars, debris fans |
| Waves | Hydraulic action and abrasion at the coast. | Cliffs, wave-cut platforms, arches, stacks, beaches, spits |
On slopes, soil erosion rises steeply with rainfall intensity and slope steepness, and falls dramatically with plant cover. Soil-loss models such as the Universal Soil Loss Equation multiply factors for rainfall, soil, slope length and steepness, cover and conservation practice. Whether a slope fails in a landslide depends on its factor of safety: the ratio of the forces holding the soil (friction and cohesion) to the pull of gravity down the slope. Water in the soil and loss of roots both lower it.
How sediment travels
Faster water can move bigger grains. The Hjulström curve shows, for each grain size, the speed needed to pick grains up (erosion), the range of speeds that keeps them moving (transport) and the speed below which they settle (deposition). Sand of about 0.2–0.5 mm is the easiest to erode. Clay and silt, though tiny, stick together and need a faster flow to lift, but once in the water they stay suspended for a long time.
Rivers carry sediment in four ways: dissolved in solution, fine particles in suspension, sand bouncing along the bed by saltation, and pebbles and boulders rolled by traction. Wind does the same with dust (suspension), sand (saltation) and coarser grains (creep), though air can only move much smaller grains than water. Glaciers are different: they carry every size together and drop it unsorted.
Deposition and landforms
- Deltas form where a river slows into the sea or a lake. Strong river supply and weak waves give finger-like (bird’s-foot) deltas such as the Mississippi; strong waves spread the sediment into an arc, as in the Nile.
- Alluvial fans form where a steep mountain stream reaches flat ground and suddenly slows, dropping coarse gravel near the apex and finer sand further out.
- Meanders grow because the fastest water is on the outside of bends (erosion, a river cliff) and the slowest on the inside (deposition, a point bar). A cut-off meander becomes an oxbow lake.
- Beaches and spits form from sand moved along the coast by waves arriving at an angle (longshore drift).
- Dunes grow as wind pushes sand up the gentle windward side; it avalanches down the steep lee side, so the dune migrates downwind.
- Moraines are ridges of unsorted glacial till left at the sides and snout of a glacier.
Valleys record the agent that cut them: a river cuts a narrow V-shaped valley, while a glacier carves a broad U-shaped one. Where hard rock is uplifted quickly, rivers cut deep, steep-walled canyons.
How long it takes
| Change | Typical time |
|---|---|
| Landslide or rockfall | Seconds to minutes |
| Gully cut in bare soil in a storm | Hours |
| Dune migration of several metres | One year |
| Meander cut-off and oxbow lake | Decades to centuries |
| A delta lobe | Hundreds to thousands of years |
| A U-shaped glacial valley | Hundreds of thousands of years (several glaciations) |
| The Grand Canyon | About 5–6 million years for most of its depth |
Why it matters
- Soil forms by weathering, and erosion can remove centimetres in a storm that took centuries to form, so farmers use cover crops, contour ploughing and terraces.
- Engineers assess slopes for landslide risk before building roads and homes, and use drainage and retaining walls to raise the factor of safety.
- Acid rain speeds the weathering of limestone and marble buildings and statues.
- Dams trap sediment, so deltas downstream can shrink and coastlines retreat.
- Coastal managers use groynes and sea walls to control longshore drift and cliff erosion.
- Chemical weathering of silicate rocks slowly removes carbon dioxide from the air, helping control Earth’s climate over millions of years.
Interesting facts
- Rivers carry roughly 15–20 billion tonnes of sediment to the oceans every year.
- Dust from the Sahara blows across the Atlantic and helps fertilise the Amazon rainforest.
- Most of the coastal land of southern Louisiana was built by the Mississippi delta over the last 7,000 years or so.
- Some sand dunes in the Namib Desert are more than 300 m high.
- Yosemite Valley in California is a classic U-shaped glacial valley, with granite domes shaped by exfoliation.
Common misconceptions
- “Weathering and erosion are the same.” Weathering breaks rock in place; erosion moves it.
- “Rocks are permanent.” Every rock at the surface is slowly weathering; only the speed varies.
- “The smallest grains are the easiest to erode.” Clay is cohesive; fine sand is easiest to pick up.
- “Landforms only change slowly.” A single flood or landslide can reshape a valley in hours.
- “Plants always speed up breakdown.” Roots speed weathering, but plant cover greatly slows erosion.
What the simulator simplifies
The 3D landscape uses a simple landscape-evolution model: water is routed downhill to the steepest neighbour, erosion capacity grows with rainfall, slope and the square root of the area draining through each point, vegetation reduces it, and hillslopes diffuse. It shows the right patterns (valleys deepen, sediment collects in low ground and the sea) but the terrain is invented and the rates are not calibrated. The weathering rates are relative and illustrative, built from simple rules (freeze–thaw days, reactions doubling every 10 °C, acidity). The Hjulström curve is approximate, the soil loss uses a USLE-style formula on an illustrative scale, the glacier speed uses Glen’s flow law for ice deformation only (ignoring sliding at the bed), and the landslide uses an infinite-slope factor of safety with fixed soil properties. The landform views are schematic drawings driven by the sliders, not physical simulations.
Important terms
- Weathering
- The breakdown of rock in place by physical, chemical or biological processes.
- Erosion
- The removal of weathered material by water, wind, ice, waves or gravity.
- Deposition
- The laying down of sediment when its carrier loses energy.
- Sediment
- Loose particles of rock, mineral or organic material.
- Saltation
- Transport of grains in short hops along the ground or river bed.
- Mass movement
- Downslope movement of rock and soil under gravity, from slow creep to sudden landslides.
- Delta
- Land built from sediment where a river enters a sea or lake.
- Alluvial fan
- A fan of sediment where a steep stream reaches flat ground.
- Moraine
- A ridge of unsorted debris (till) deposited by a glacier.
- Longshore drift
- Movement of sand along a coast by waves arriving at an angle.
Key takeaways
- Weathering breaks rock down; erosion and transport move it; deposition drops it where energy falls.
- Climate and rock type decide which weathering processes dominate.
- Steep slopes, heavy rain and bare ground cause the most erosion; vegetation is the best protection.
- Each agent leaves its signature: V-shaped valleys from rivers, U-shaped valleys from ice, dunes from wind.
- Landforms change on time scales from seconds to millions of years.