Geography · Ch 11 — Geomorphic Processes
Physical Weathering Processes
Physical Weathering Processes
Physical weathering is, at its core, a story of stress. The rocks of the Earth's crust are constantly being acted upon by various forces, and when those forces exceed the strength of the rock, it fractures. Unlike chemical weathering, which alters the very composition of minerals, physical weathering simply breaks rocks into smaller pieces without changing what they are made of. The key to understanding this process lies in the nature of the forces involved and the way rocks respond to them over long periods.
These applied forces come from three main sources. First, there are gravitational forces, which include the weight of overlying material (overburden pressure), the load of the rock itself, and shearing stress that pulls or pushes parts of a rock in different directions. Second, there are expansion forces, which arise from temperature changes, the growth of crystals within cracks, or even the activity of animals burrowing into the ground. Third, there are water pressures that are controlled by the constant cycle of wetting and drying. All of these forces act both on the surface of the earth and deep within the rock masses themselves, and their combined effect is to cause the rock to fracture.
What is remarkable about most physical weathering processes is that they are driven by just two fundamental mechanisms: thermal expansion and pressure release. Thermal expansion refers to the way rocks expand when heated and contract when cooled. Pressure release, on the other hand, happens when the overlying rock that once pressed down on a deeper rock is removed by erosion, allowing the deeper rock to expand outward.
The real damage from physical weathering is not caused by a single dramatic event, but by fatigue. Each cycle of expansion and contraction, no matter how small, puts a tiny amount of strain on the rock. When this repetition continues over thousands or millions of years, the rock eventually weakens and cracks. The processes are small and slow, but their cumulative effect is devastating to the integrity of the rock.
The table below summarises the three categories of forces that drive physical weathering:
| Type of Force | Source / Cause | Example |
|---|---|---|
| Gravitational | Weight and stress from overlying material | Overburden pressure, load, shearing stress |