Weathering, Erosion, and Soil

Mountains look like they'll last forever. They won't. Every day, rain, ice, wind, and plants chip away at them. Give it enough time and the Rocky Mountains will be sand on a beach somewhere. Don't worry, you've got a few million years.

These two get mixed up a lot:

• Weathering = breaking rock down into smaller pieces. It stays in place.
• Erosion = moving those pieces somewhere else.

Weathering breaks it. Erosion takes it. Think of weathering as smashing a cookie and erosion as the ants carrying the crumbs away.

Mechanical weathering breaks rock into smaller pieces without changing what it's made of. A small granite chunk is still granite.

• Frost wedging (ice wedging): water seeps into cracks, freezes, and expands by about 9%. It pries the crack wider, like a tiny ice crowbar. Freeze, thaw, repeat. This is huge in Idaho winters and is why potholes appear every spring.
• Plant roots: roots grow into cracks and split rocks apart. Ever seen a sidewalk buckled by a tree? Same idea.
• Abrasion: rocks get scraped and smoothed by sand blown in wind or tumbling in rivers. That's why river rocks are round.
• Unloading: when rock on top erodes away, the rock below expands and peels off in sheets, like an onion.

Chemical weathering changes the minerals in a rock into new substances.

• Water: slowly dissolves some minerals and turns feldspar into clay.
• Oxidation: oxygen reacts with iron in rocks, which basically means rocks rust. That's why many desert rocks are red.
• Acids: rainwater picks up carbon dioxide and becomes slightly acidic. It dissolves limestone and can carve out entire caves. Acid rain from pollution speeds this up and eats away at statues and buildings.

Chemical weathering works fastest where it's warm and wet. Mechanical weathering from frost works best where temperatures swing above and below freezing, like here.

Five forces carry broken rock away:

1. Water: the biggest mover on Earth. Rivers, rain, waves.
2. Wind: especially in deserts.
3. Glaciers: giant rivers of ice that bulldoze everything.
4. Gravity: pulls everything downhill. Landslides, rockfalls.
5. Waves: pound coastlines day and night.

When the movers slow down and drop their load, that's deposition. Deposition builds deltas, sand dunes, and beaches.

Soil is a mix of weathered rock, humus (rotting plants and animals), water, air, and billions of living things. A single handful of healthy soil has more living organisms in it than there are people on Earth.

Soil takes a long time to form. It can take hundreds of years to make just an inch of it.

Dig a deep hole and you'll see layers called horizons:

• O horizon: the top layer of fallen leaves and decaying stuff.
• A horizon (topsoil): dark, rich in humus. This is where most plants grow and where the worms party.
• B horizon (subsoil): lighter, with clay and minerals washed down from above.
• C horizon: chunks of partly weathered rock.
• Bedrock: solid rock at the bottom. Yes, the class is named after it.

Idaho's famous potatoes grow in volcanic soil on the Snake River Plain. That soil came from the weathering of basalt and volcanic ash. You can thank ancient eruptions for your french fries.

In the 1930s, bad farming plus drought blew away tons of topsoil across the Great Plains in the Dust Bowl. Today farmers prevent erosion with contour plowing, windbreaks (rows of trees), cover crops, and terraces.