The effectiveness of frost wedging is related to the frequency of freezing and thawing. Sedimentary and metamorphic rocks, on the other hand, tend to exfoliate along predetermined planes (Figure 5.4).įrost wedging is the process by which water seeps into cracks in a rock, expands on freezing, and thus enlarges the cracks (Figure 5.5).

Granitic rock tends to exfoliate parallel to the exposed surface because the rock is typically homogenous, and it doesn’t have predetermined planes along which it must fracture. įigure 5.4 Exfoliation of slate at a road cut in the Columbia Mountains west of Golden, B.C. Figure 5.3 Exfoliation fractures in granitic rock exposed on the west side of the Coquihalla Highway north of Hope, B.C.

This unloading promotes cracking of the rock, known as exfoliation, as shown in the granitic rock in Figure 5.3. When a mass of rock is exposed by weathering and removal of the overlying rock, there is a decrease in the confining pressure on the rock, and the rock expands.

Cracking from plant roots and exposure by burrowing animals.

Formation of salt crystals within the rock.

Freezing and thawing of water in cracks in the rock.

The decrease in pressure that results from removal of overlying rock.

The important agents of mechanical weathering are: Weathering cannot even begin until these rocks are uplifted through various processes of mountain building - most of which are related to plate tectonics - and the overlying material has been eroded away and the rock is exposed as an outcrop. Most metamorphic rocks are formed at depths of kilometres to tens of kilometres. Sediments are turned into sedimentary rocks only when they are buried by other sediments to depths in excess of several hundreds of metres. Intrusive igneous rocks form at depths of several hundreds of metres to several tens of kilometres.

Ice wedging water falls diagram