Physical weathering breaks rock without changing its mineral composition. Each process needs particular conditions.
Freeze-thaw weathering requires water in fractures and temperatures that cross the freezing point. Ice growth exerts pressure and repeated cycles can loosen fragments. Persistently warm tropical lowlands lack these conditions; high tropical mountains may experience them.
Pressure release occurs as overlying material is removed by erosion or excavation. Unloading lets rock expand and fractures may develop roughly parallel to the exposed surface. Outer sheets can detach, especially in massive rocks such as granite. This is not caused by falling atmospheric pressure during a storm.
Salt weathering occurs when saline water enters pores or cracks and crystals grow as water evaporates, or salts change hydration state. Crystallisation pressure can force grains apart. Repeated wetting and drying favours it in dry tropical settings and exposed coastal rock.
Thermal weathering results from expansion during heating and contraction during cooling. Different minerals and the rock surface and interior can change temperature at different rates, creating stress. Repeated or rapid temperature changes, including fire, can promote cracking. Large daily temperature ranges help, but rock properties, moisture and exposure also matter.
Physical break-up exposes fresh surfaces and increases the surface area available for chemical reactions. Chemical alteration can also weaken rock and make physical breakdown easier. These processes cooperate rather than occupying completely separate climate zones.
Step by step
Check the required condition
Freezing, unloading, crystal growth and heating are different triggers.
Explain the stress
Show what expands, contracts or presses against a crack.
Locate the setting
Use altitude, moisture, salt supply and exposure to judge where it can operate.
Worked example: A tropical coast and a high mountain
Salt spray and drying can weather porous coastal rock while temperatures remain above freezing. At a high tropical summit, water-filled cracks may freeze at night. Latitude alone cannot select the dominant physical weathering process.
Watch out for this
Every crack in a tropical rock is caused by thermal expansion.
Cracks may reflect unloading, salts, freezing at altitude, chemical weakening or older geological structures. Use evidence of the local conditions.
Check your understanding
After quarrying removes overlying rock, curved sheets begin separating from exposed granite. Which process is most directly supported?
- Pressure release following unloading.
- A daily fall in air pressure.
- Carbonation of all granite minerals.