Mechanical Weathering

Igneous and metamorphic rocks which have been formed at many km depth at higher temperatures and pressures are not stable when exposed at the surface.

When uplifted and unloaded the rocks expand, mostly in the vertical direction, producing horizontal fractures (sheeting) parallel to the land surface. This is because as the rocks near the surface are unloaded by the reduction in overlying rock, they can expand vertically but not horizontally. In this way the vertical stresses become less than the horizontal ones, and joints develop normal to the lowest stresses. We see this most clearly in granites, which are homogeneous, while expansion in metamorphic and sedimentary rocks occurs along bedding surfaces, along tectonically weak zones with crushing, or along fractures that were formed at great depth.

Joints opened by stress release in turn provide pathways for groundwater to circulate, increasing the surface area of rock exposed to chemical weathering. In areas that experience freeze-thaw cycles, frost weathering becomes very important. When water freezes in cracks in rock, it expands by 9% and can generate very high stresses, further widening cracks near the surface.

The surfaces of exposed rocks are also subjected to daily temperature fluctuations which cause greater expansion of the outer layers relative to the rest of the rock. Desert regions in particular experience very wide daily temperature ranges, though the importance of this process for mechanical weathering has been questioned. The roots of plants and moss can also contribute to mechanical weathering as they grow into fractures, take up water and expand.

Chemical Weathering

There is no sharp demarcation between biological and chemical weathering, because we find biological activity in almost all soils and rocks near the surface. The chemical environment in water at the surface of the earth is very much affected by local biological activity, and in most cases it is biological processes that cause weathering to continue after rainwater has been neutralised through reaction with minerals. We will therefore use the term “weathering” here for both chemical and biological processes.

Biological Weathering

Rocks are a source of nutrients for plants, and plants are capable of dissolving and breaking down the major rock-forming minerals. Moss, which consists of algae and fungi living in symbiosis, produces organic compounds that can slowly dissolve silicate minerals. Even in the earliest stages of weathering, we see that fungus hyphae penetrate into microscopic cracks.