Weathering, Erosion & Deposition
How rock breaks down, how it's carried away and sorted, and the landscapes glaciers, rivers, and wind leave behind.
Weathering: breaking rock in place
- Physical (mechanical) weathering breaks rock into smaller pieces without changing its makeup — frost wedging (water freezing in cracks), abrasion, plant roots.
- Chemical weathering changes the rock's minerals — acid rain dissolving limestone, iron rusting. It works fastest in warm, wet climates.
Breaking rock into smaller pieces increases the total surface area exposed, which speeds up chemical weathering further.
Erosion: transport
Erosion is the moving of weathered material to a new place. The agents, roughly in order of how much they move: running water (the most important), glaciers, wind, and gravity (landslides). Faster-moving water carries more sediment and larger particles.
Deposition and sorting
Deposition happens when the transporting agent slows down and drops its load. As water slows, it drops the largest, densest, roundest particles first and the finest last. Poured into still water, a mix of sediment settles into graded layers — coarse on the bottom, fine on top.
Permeability and porosity matter here too: water passes more easily through large, well-sorted particles (high permeability) than through tiny, packed ones like clay.
Landscapes left behind
- Glaciers carve broad U-shaped valleys and drop unsorted piles of debris called moraines.
- Rivers cut V-shaped valleys in their upper course, then meander and build deltas where they meet still water.
- Wind shapes dunes and polishes rock in dry regions.