Some rocks crumble in decades; others last for millions of years. Weathering speeds up with three things: more surface area, a warm, wet climate, and minerals that break down easily.
Key idea
Weathering attacks surfaces, so breaking a rock into smaller pieces exposes more surface and speeds it up. Warm, wet climates weather rock fastest, and limestone weathers faster than granite.
Why it works
Water, oxygen and acids can only reach a rock's outside. Cut a cube in half along every edge and you get 8 smaller cubes. The outside of the big cube is still there, but each cut adds new faces. Using surface area: a 2 cm cube has $6 \times 2^2 = 24$ cm² of surface. Eight 1 cm cubes have $8 \times 6 \times 1^2 = 48$ cm², twice as much, from the same amount of rock.
Worked example: More cuts
A 3 cm cube is cut into 1 cm cubes. How many cubes are there, and how much surface area do they have in all?
- Each edge is cut into 3 pieces, so there are $3 \times 3 \times 3 = 27$ cubes.
- Each small cube has $6 \times 1^2 = 6$ cm². All together: $27 \times 6 = 162$ cm².
- The big cube had $6 \times 3^2 = 54$ cm², so the surface grew 3 times.
Answer: 27 cubes with 162 cm² of surface
Worked example: Which weathers fastest?
Which weathers fastest: granite in a cold, dry desert; limestone in a warm, rainy forest; or granite in a warm, rainy forest?
- Warm, wet places speed up chemical weathering.
- Limestone dissolves in rainwater; granite's minerals resist much better.
- So limestone in a warm, rainy forest weathers fastest.
Answer: Limestone in a warm, rainy forest
Common mistake
Not expecting that breaking a rock exposes more surface. Breaking does not change how much rock there is, but every break opens new faces to water and air. Check with numbers: one 2 cm cube has 24 cm² of surface, but cut into eight 1 cm cubes it has 48 cm².
Cold climates have one fast process of their own: frost wedging, wherever water freezes and thaws over and over.