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Weathering is the in-place breakdown of rock and minerals at or near Earth's surface; erosion is the transport of those products by water, wind, ice, or gravity. Mechanical (physical) weathering disintegrates rock without changing its chemistry—through frost wedging, thermal expansion, and root action—while chemical weathering transforms minerals through hydrolysis, oxidation, and dissolution. The rate of chemical weathering depends on mineral composition, surface area, temperature, and water availability; mafic minerals (olivine, pyroxene) weather far faster than quartz. Together, weathering and erosion are the primary agents that break down mountains and supply sediment to depositional basins.
Contrasting the fate of granite versus limestone in a humid climate versus a desert illustrates how rock type and climate jointly control weathering style. Working through the hydrolysis reaction of feldspar to clay minerals connects acid-base chemistry directly to landscape evolution.
Rocks at Earth's surface are under constant attack from the atmosphere, water, and living organisms. Weathering and erosion are the two complementary processes that dismantle mountains and shape landscapes over geological time. The key distinction to fix clearly in your mind is this: weathering is breakdown *in place*, and erosion is *transport* of the broken-down material. A boulder cracking apart on a hillside is weathering; those fragments washing downslope into a river is erosion. The two are often linked in sequence, but they are not the same thing.
Mechanical (physical) weathering disintegrates rock without changing its chemical composition. Frost wedging is the most powerful: water infiltrates cracks, freezes, and expands by about 9%, widening the crack. Repeat this thousands of times per year in a freeze-thaw climate and you can split boulders. Thermal expansion and contraction (daily heating and cooling), abrasion by wind-carried particles, and tree-root pressure are other agents. The product of mechanical weathering is smaller fragments of the same minerals — the chemistry is unchanged.
Chemical weathering transforms the minerals themselves into new compounds. The three main reactions are hydrolysis (water reacting with silicate minerals to form clay minerals and dissolved ions), oxidation (oxygen reacting with iron-bearing minerals to form rust-colored iron oxides), and dissolution (minerals dissolving directly in water, often aided by acids). The feldspars that dominate granite undergo hydrolysis to produce clay minerals; the iron in olivine and pyroxene oxidizes readily. Quartz, by contrast, is nearly insoluble in neutral water and resists chemical attack — it is the final survivor after other minerals have weathered away. In contrast, calcite (the main mineral in limestone) dissolves readily even in weakly acidic rainwater (carbonic acid from dissolved CO₂), producing the caves and karst landscapes of limestone regions.
The rate of chemical weathering depends on four factors: mineral composition (mafic minerals like olivine weather far faster than quartz), surface area (smaller particles expose more mineral surface per unit mass, so a pile of sand weathers faster than an equivalent boulder), temperature (chemical reaction rates roughly double for every 10°C increase), and water availability (chemical weathering requires water as both a reactant and a transport medium). This is why tropical humid climates produce deep, intensely weathered soils while cold deserts leave fresh rock surfaces exposed.
Together, weathering and erosion continuously cycle material from mountains to plains to ocean basins, supplying the sediment that eventually becomes sedimentary rock. The rates at which they operate govern how long mountain ranges persist, how quickly soils form, and how much sediment rivers deliver to the sea. Understanding which process is rate-limiting in a given environment — is it rock breakdown or transport capacity? — is central to geomorphology and to understanding landscape evolution.