5.1 mechanical Weathering

Intrusive igneous rocks type at depths of several numerous metres to numerous tens that kilometres. Sediments are turned right into sedimentary rocks only as soon as they are buried by other sediments to depth in overfill of several numerous metres. Most metamorphic rocks are formed at depth of kilometre to 10s of kilometres. Weathering cannot even begin until this rocks room uplifted through various processes the mountain building — many of i beg your pardon are related to plate tectonics — and the overlying material has actually been eroded away and the rock is exposed as an outcrop.

You are watching: Type of weathering that occurs more rapidly in climates that experience frequent freezes and thaws


The vital agents of mechanically weathering are:

The decrease in pressure that outcomes from removal of overlying rockFreezing and thawing the water in cracks in the rockFormation that salt crystals within the rockCracking from tree roots and also exposure by burrowing animals

When a massive of rock is exposed through weathering and removal of the overlying rock, there is a to decrease in the confining pressure ~ above the rock, and the rock expands. This unloading promotes cracking the the rock, known as exfoliation, as presented in the granitic absent in figure 5.3.

Figure 5.3 Exfoliation fractures in granitic rock exposed ~ above the west next of the Coquihalla Highway phibìc of Hope, B.C.

Figure 5.4 Exfoliation the slate in ~ a road cut in the Columbia mountains west of Golden, B.C.

Granitic rock often tends to exfoliate parallel to the exposed surface due to the fact that the absent is commonly homogenous, and also it doesn’t have actually predetermined planes along which it need to fracture. Sedimentary and metamorphic rocks, ~ above the other hand, tend to exfoliate follow me predetermined airplane (Figure 5.4).

Frost wedging is the procedure by i beg your pardon water seeps into cracks in a rock, broadens on freezing, and also thus enlarges the cracks (Figure 5.5). The performance of frost wedging is pertained to the frequency of freezing and thawing. Frost wedging is most efficient in a climate favor Canada’s. In warm locations where freeze is infrequent, in really cold locations where thawing is infrequent, or in an extremely dry areas, wherein there is small water come seep right into cracks, the duty of frost wedging is limited.

Figure 5.5 The procedure of frost wedging ~ above a steep slope. Water gets right into fractures and also then freezes, broadening the fracture a little. As soon as the water thaws it seeps a little farther right into the increased crack. The process is repeated countless times, and also eventually a piece of rock will certainly be wedged away.

In many parts the Canada, the change between freeze nighttime temperatures and also thawing daytime temperature is regular — 10s to thousands of times a year. Even in warm seaside areas of southern B.C., freezing and thawing transitions are typical at higher elevations. A usual feature in locations of reliable frost wedging is a talus slope — a fan-shaped deposit of fragments removed by frost wedging from the steep rocky slopes above (Figure 5.6).

Figure 5.6 one area with really effective frost-wedging near Keremeos, B.C. The pieces that have been wedged far from the cliffs above have built up in a talus deposit in ~ the base of the slope. The rocks in this area have quite differed colours, and also those room reflected in the colours of the talus.

A connected process, frost heaving, takes place within unconsolidated materials on gentle slopes. In this case, water in the floor freezes and also expands, pushing the overlying product up. Frost heaving is responsible for winter damages to roads all over North America.

When salt water seeps right into rocks and then evaporates ~ above a hot sunny day, salt crystals flourish within cracks and also pores in the rock. The growth of this crystals exerts pressure on the rock and also can push grains apart, resulting in the rock to weaken and also break. Over there are numerous examples that this ~ above the rocky shorelines the Vancouver Island and also the Gulf Islands, wherein sandstone outcrops room common and also salty seawater is readily obtainable (Figure 5.7). Salt weathering can also occur away from the coast, because most settings have part salt in them.

Figure 5.7 Honeycomb weathering that sandstone top top Gabriola Island, B.C. The feet are led to by crystallization of salt within rock pores, and also the seemingly continual pattern is regarded the original roughness that the surface. It’s a positive-feedback process because the holes collect salt water at high tide, and so the result is accentuated roughly existing holes. This form of weathering is most pronounced on south-facing sunny exposures.

The effects of plants and animals are far-reaching in mechanical weathering. Root can pressure their method into even the tiniest cracks, and then they exert tremendous press on the rocks together they grow, widening the cracks and also breaking the absent (Figure 5.8). Although pets do not usually burrow through solid rock, they deserve to excavate and also remove huge volumes the soil, and thus disclose the rock to weathering by other mechanisms.

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Figure 5.8 Conifers growing on granitic rocks in ~ The Lions, close to Vancouver, B.C.

Mechanical weathering is greatly promoted by erosion, i beg your pardon is the removed of weathering products, enabling for the exposure of more rock for weathering. A good example the this is shown in number 5.6. Top top the steep rock encounters at the top of the cliff, rock pieces have been damaged off by ice wedging, and then removed by gravity. This is a form of mass wasting, which is disputed in much more detail in thing 15. Other crucial agents of erosion that likewise have the result of removed the products of weathering include water in streams (Chapter 13), ice in glaciers (Chapter 16), and waves top top the shores (Chapter 17).