Weathering and Erosion

Why mountains do not last forever

Mountains look permanent, but given enough time every one of them wears down to a hill and then a plain. Two slow but unstoppable processes do this work: weathering breaks rock apart, and erosion carries the broken pieces away. Together they are among the most powerful sculptors on Earth.

People often mix the two words up, so here is the simple rule: weathering breaks, erosion moves.

Physical weathering: breaking without changing

Physical (or mechanical) weathering shatters rock into smaller pieces without changing what it is made of. The main ways this happens:

• Freeze-thaw: water seeps into a crack, freezes overnight and expands. Ice takes up about 9% more space than water, so it pushes the crack wider. Repeat this thousands of times and the rock splits. This is common on cold mountainsides.
• Heating and cooling: in deserts, rock expands in the day's heat and contracts in the cold night. The outer layers can flake off like peeling paint.
• Living things: plant roots grow into cracks and pry them open; burrowing animals loosen soil and rock.

Chemical weathering: changing the minerals

Chemical weathering changes the actual minerals in a rock, usually with the help of water.

• Dissolving: rainwater is slightly acidic. Over long periods it dissolves limestone, which is how underground caves form.
• Rusting: rocks containing iron react with oxygen and water, turning reddish-brown and crumbly — the same rust you see on old metal.

Warm, wet places weather chemically much faster than cold, dry ones, which is why tropical rocks often look deeply rotted.

Erosion: the great clean-up

Once rock is loosened, erosion transports the pieces. The main agents are:

• Water — rivers carry pebbles, sand and mud; waves grind cliffs.
• Ice — slow-moving glaciers scrape valleys broad and deep, a process explored in glaciers and the ice ages.
• Wind — in deserts, wind lifts sand and blasts it against rock.
• Gravity — on steep slopes, rock simply falls or slides in landslides.

Deposition: dropping the load

Moving water and wind cannot carry sediment forever. When they slow down, they drop it — this is deposition. It builds:

• Beaches and sandbars where waves lose energy.
• River deltas where a river spreads out and slows as it meets the sea (like the Nile and Mississippi deltas).
• Sand dunes where wind drops grains in sheltered spots.

So erosion and deposition are two halves of the same journey: material is taken from one place and built up in another. The same broken-down rock eventually becomes soil that plants can grow in.

Try it yourself: model freeze-thaw

You can copy freeze-thaw weathering safely at home.

1. Fill a small plastic bottle right to the very top with water and screw the lid on tightly.
2. Stand it in a tray (in case of leaks) and put it in the freezer overnight.
3. The next day, look at the bottle. The ice will have bulged or even split it.

That expanding ice is exactly the force that prises cracks open in mountain rock, one frozen night at a time. Now imagine that happening for ten thousand winters, and you can see how even the hardest cliff is slowly taken apart.