The model is sped up and the heights are exaggerated so you can see the changes. The time scale for each landscape is only a rough guide; real landscapes change at the speeds shown under “Real speed”.
Choose a rock and a climate, then let time pass. See how much of the rock surface is lost to mechanical weathering (breaking), chemical weathering (dissolving and changing minerals) and biological weathering (living things).
Teaching model: the numbers show how fast each kind of weathering is compared with the others (real rates vary a lot from place to place). The drawing is exaggerated. Normal rain is slightly acidic (pH about 5.6); acid rain is below that.
A classic experiment: chalk (calcium carbonate, like limestone) reacts with the acid in vinegar and slowly disappears. Is a whole piece or a crushed piece weathered faster? Choose how many pieces to cut the 10 g of chalk into, then run the test.
Cutting a cube into 8 smaller cubes doubles its surface area; cutting it into 64 makes it four times bigger. Weathering only happens at the surface, so the same rock weathers faster when it is broken into pieces. That is why mechanical weathering speeds up chemical weathering. (Simplified model; times are illustrative.)
A stream table is a tilted tray of sand with water poured in at the top. Watch the water cut a channel, carry the sand downhill and drop it where it slows down in the basin at the bottom, building a little delta. Change the slope, the amount of water and the size of the grains.
Try it: run the water for the same time on a 3% and a 12% slope, then on fine sand and on gravel. A steeper slope or more water makes faster water that erodes more; bigger grains are harder to move. (Simulated with thousands of water drops; the units are arbitrary.)
Slide the water speed. Above the orange line, grains of that size are picked up; between the lines, grains already moving keep moving; below the blue line they are dropped. Surprise: sand is the easiest to pick up. Tiny clay particles stick together and need fast water, but once lifted they stay in the water for a long time.
Values are rounded from the Hjulström diagram (1935) for grains on a river bed; real values depend on the water depth and how tightly the grains are packed. A walking pace is about 140 cm/s.
From the mountains to the sea, water, ice and waves wear land away in some places and build it up in others. Pick Basic (8 labels) or Advanced (12). Switch between the labeled diagram, a blank one, a label quiz and match the jobs. On a phone, swipe the diagram sideways.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back.
Each line describes the job of one part. Tap a part name, then tap the box next to the job it does. Tap a filled box to take the word back.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed.
A printable worksheet: label the landforms, sort 12 changes into weathering, erosion or deposition, fill in the blanks, multiple choice and three investigation questions (chalk in vinegar, stream table, stopping soil erosion), with an answer key. The label part uses the level chosen in the diagram above.
Mechanical weathering breaks rock into smaller pieces without changing what it is made of: ice wedging (freeze–thaw), roots growing in cracks, rocks knocking together, and heating and cooling in deserts. Chemical weathering changes the minerals: rainwater (a weak acid) dissolves limestone, iron minerals rust, and feldspar slowly turns into clay. Biological weathering is caused by living things, such as lichens that make acids and burrowing animals.
The broken pieces (sediment) are moved by running water, wind, moving ice, waves and gravity. Faster water and stronger wind carry more and bigger pieces. As they are carried, the pieces knock together and get smaller and rounder, and they scrape the land they move over (abrasion).
When the water, wind or ice slows down, it drops its load: the biggest pieces first, the finest mud last. This builds floodplains, deltas, beaches, sand dunes and moraines. Over millions of years the layers of sediment can be pressed and cemented into new sedimentary rock, part of the rock cycle.
Most changes are slow: a valley deepens by fractions of a millimeter a year and soil creeps a few millimeters. Some are sudden: a landslide, a flood or a storm can change a landscape in hours. People speed up erosion by cutting forests and plowing, and slow it down with plants, terraces, sea walls and check dams.
Press Play time on the River landscape and watch the delta grow, then compare the Glacier valley before and after. In the weathering lab, compare limestone in a warm, wet climate with granite in a desert. Run the chalk test with a whole piece and with 64 pieces. On the stream table, compare a gentle and a steep slope.
| Agent | Where | Landforms made by erosion | Landforms made by deposition | How fast? |
|---|---|---|---|---|
| Running water | Rivers, streams, rain, floods | V-shaped valleys, canyons, waterfalls, meanders | Floodplains, deltas, alluvial fans, oxbow lakes | Most important agent on Earth; valleys deepen about 0.01–1 mm a year |
| Moving ice | Glaciers and ice sheets | U-shaped valleys, horns, polished and scratched rock | Moraines, erratic boulders | Valley glaciers flow about 10–300 m a year |
| Wind | Dry places with few plants: deserts, beaches, plowed fields | Mushroom (pedestal) rocks, desert pavement, blowouts | Sand dunes, loess (wind-blown dust) | Small dunes move up to about 30 m a year |
| Waves | Coasts of seas and big lakes | Sea cliffs, caves, arches, stacks, wave-cut platforms | Beaches, spits, sandbars | Soft cliffs can retreat 1–2 m a year |
| Gravity | Steep slopes, especially after heavy rain or earthquakes | Landslide scars | Scree (talus), landslide debris | From creep (mm a year) to landslides faster than 100 km/h |
Breaks rock down in place. Mechanical (ice, roots, heat), chemical (water, acids, oxygen) or biological (lichens, animals). Fastest where it is warm and wet (chemical) or freezes and thaws often (mechanical).
Moves the weathered pieces. Needs an agent: water, wind, ice, waves or gravity. Faster, stronger agents carry bigger and more pieces. Plants and their roots slow it down.
Drops the pieces in a new place when the agent slows down: heaviest first. Builds deltas, beaches, dunes and moraines, and in time new sedimentary rock.
Carved by the Colorado River over about 5–6 million years: up to about 1.8 km deep and 446 km long. Its walls show rock layers almost 2 billion years old at the bottom.
The Ganges, Brahmaputra and Meghna rivers carry about a billion tons of sediment a year and drop it in the Bay of Bengal, building the world’s largest delta, home of the Sundarbans mangrove forest.
A river first cut this valley; glaciers in the ice ages then deepened and widened it into a classic U shape with steep granite walls such as El Capitan and Half Dome.
Limestone sea stacks left as the waves wore back the cliffs; one of them collapsed in 2005.
The largest hot desert, where wind builds dunes and blows dust across the Atlantic Ocean.
Drought and plowed-up prairie grass let the wind blow away huge amounts of topsoil. It led to soil conservation: windbreaks, contour plowing and cover crops.
Weathering breaks rock down into smaller pieces where it is, by ice, water, chemicals or living things. Erosion is the moving of those pieces to another place by water, wind, ice, waves or gravity. The two often happen together: rain can both weather rock and wash the pieces away.
Key takeaway: weathering breaks, erosion moves.Mechanical (physical) weathering breaks rock without changing its minerals, for example freeze–thaw and root wedging. Chemical weathering changes the minerals, for example rainwater dissolving limestone or iron rusting. Biological weathering is caused by living things, such as lichens and burrowing animals; it can be mechanical or chemical.
Key takeaway: mechanical, chemical and biological.Water seeps into a crack. When it freezes it expands by about 9%, pushing the crack wider. When it melts, more water gets in. After many freezes and thaws the rock splits apart. It is common in mountains and cold places where the temperature goes above and below 0 °C often.
Key takeaway: freezing water expands and wedges rock apart.Running water (rivers, rain and floods), wind, moving ice (glaciers), waves and gravity. Running water is the most important on Earth as a whole; wind matters most in dry places, ice in cold mountains and polar regions, waves on coasts.
Key takeaway: water, wind, ice, waves and gravity.Deposition is when the moving water, wind or ice slows down and drops the sediment it was carrying. It happens on the inside of river bends, on floodplains during floods, at river mouths (deltas), in calm bays (beaches), where the wind drops (dunes) and where glaciers melt (moraines).
Key takeaway: sediment is dropped where the agent slows down.A V-shaped valley is cut by a river: the river mainly cuts downward, while weathering and gravity wear the sides back into slopes. A U-shaped valley is carved by a glacier: the thick ice fills the whole valley and scrapes its floor and sides, making a flat floor and steep walls.
Key takeaway: rivers make V shapes, glaciers make U shapes.Waves attack cracks in a headland and widen them into a cave. If caves on two sides meet, or a cave breaks through the headland, an arch forms. The waves keep eroding the base of the arch until its top falls in, leaving a stack standing in the sea. The stack is eroded down to a stump.
Key takeaway: cave → arch → stack → stump.The type of rock (limestone and marble weather fast chemically, granite and quartzite slowly), the climate (warm and wet speeds chemical weathering, many freezes speed freeze–thaw), the surface area (broken rock weathers faster), and pollution (acid rain speeds up chemical weathering).
Key takeaway: rock type, climate, surface area and pollution.Keep the ground covered with plants, whose roots hold the soil: plant cover crops, grass strips and trees. Plow and plant across slopes (contour farming), build terraces on steep hills, plant windbreaks, and leave crop stalks on fields. On coasts, sea walls and planted dunes slow erosion.
Key takeaway: plants and roots are the best protection.The landforms and the order in which they form are realistic, but the model is a simplified teaching model: heights are exaggerated, everything is sped up and the time scales are rough. Real rates are shown separately in the readouts and the table. The Hjulström values are rounded from the real diagram, and the weathering lab compares rates only roughly.
Key takeaway: real landforms and order, sped up and simplified.