Erosion — the carrying-away of rock and sediment by rivers, wind, and ice — tends to get most of the attention when people picture how landscapes change over time. Before any of that material can be carried away, though, solid rock has to actually be broken down into smaller pieces in the first place, a separate process called weathering. It happens through two genuinely different mechanisms, one purely mechanical and one chemical, and a lot of landscape-shaping work is done by both, quietly, long before a river ever gets involved.
Mechanical weathering: breaking rock without changing its chemistry
Mechanical, or physical, weathering breaks rock into smaller fragments without altering its underlying mineral composition — the pieces that result are chemically identical to the original rock, just smaller. Freeze-thaw weathering is one of the most powerful mechanical processes: water seeps into small cracks in rock, and when temperatures drop below freezing, that water expands as it turns to ice, exerting enormous pressure on the surrounding rock and gradually widening the crack. Repeated freeze-thaw cycles, especially in climates that swing regularly above and below freezing, can eventually split apart even substantial rock formations. Other mechanical processes include the physical impact of wind-blown sand grinding against rock surfaces, and the pressure release that occurs when overlying rock erodes away, allowing buried rock to expand slightly and crack.
Chemical weathering: rock that reacts, not just cracks
Chemical weathering, by contrast, actually alters a rock's mineral composition through chemical reactions, typically involving water, oxygen, or dissolved acids. Rainwater, naturally slightly acidic from dissolved carbon dioxide, can slowly dissolve certain minerals directly — a process particularly pronounced in limestone landscapes, where chemical weathering carves out cave systems over geological timescales. Oxidation, the same basic chemical process behind rust, can weaken iron-bearing minerals within rock, making them more susceptible to further breakdown. Chemical weathering tends to proceed faster in warm, wet climates, where the chemical reactions involved occur more readily, which is part of why weathering rates and the resulting landscapes differ so much between, say, a humid tropical region and an arid desert.
What we're still unsure about
The basic mechanisms of mechanical and chemical weathering, and the conditions that favour each, are well established in geomorphology and not in scientific dispute. What remains more genuinely complex is precisely quantifying weathering rates for a specific rock type and climate combination over long timescales, since weathering interacts with vegetation, microbial activity, and local hydrology in ways that are still being actively studied, and predicting exactly how a changing climate will shift the balance between mechanical and chemical weathering in a given region remains an area of ongoing research rather than a matter with a single settled answer.
This sits inside Weathering, Erosion & Landforms, one of seven topics in Physical Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.