Every rock that reaches the surface starts to fall apart. This breaking down of rock where it sits is weathering, the first step of the rock cycle's trip to sediment. It comes in two kinds.
| Mechanical weathering: smaller pieces, same minerals | Chemical weathering: new substances |
|---|---|
| Frost wedging: water in a crack freezes, expands about 9% and pries the rock apart. | Dissolving: rainwater is a weak acid that slowly dissolves limestone, carving caves. |
| Root wedging: growing roots split rocks. | Oxidation: iron in minerals combines with oxygen and rusts, turning rock red-brown. |
| Abrasion: rocks are worn by sand blown in wind or tumbled in rivers. | Clay formation: water slowly changes feldspar into soft clay. |
| Unloading: rock buried deep expands when the rock above wears away, and peels in sheets. | Acids from living things: lichens and plant roots give off acids that eat at rock. |
Key idea
Weathering breaks rock down where it is. Mechanical weathering breaks it into smaller pieces of the same stuff; chemical weathering changes its minerals into new substances.
Why it works
Rocks formed deep underground, under high heat and pressure and without air. At the surface the conditions are very different: cold and warm days, water, oxygen and living things. Minerals that were stable deep down are not stable up here, so they slowly change into ones that are, like clay and rust. And water seeping into cracks, freezing and thawing, pries the rock apart.
Worked example: Ice in a crack
Water fills a crack in a granite boulder and freezes every winter night. After years the boulder splits in two. What kind of weathering is this?
- The pieces are still granite: no new substances formed.
- Freezing water widened the crack. This is frost wedging, a kind of mechanical weathering.
Answer: Mechanical weathering (frost wedging)
Worked example: A rusty cliff
Gray rock rich in iron slowly turns red-brown and crumbly. What kind of weathering is this?
- The color change means a new substance formed: iron oxide (rust).
- Iron combining with oxygen is oxidation, a kind of chemical weathering.
Answer: Chemical weathering (oxidation)
Common mistake
Mixing up weathering with erosion. Weathering is the breaking down of rock in place; erosion is the carrying away of the pieces by water, wind, ice or gravity. Check: ask "Did anything move?" If the pieces were carried off, that part is erosion.