5.1 mechanically Weathering

Intrusive igneous rocks type at depth of several thousands of metres to number of tens that kilometres. Sediments room turned into sedimentary rocks only as soon as they are buried by other sediments to depth in excess of several numerous metres. Many metamorphic rocks are formed at depths of kilometer to tens of kilometres. Weathering cannot even begin until this rocks space uplifted through assorted processes the mountain structure — most of which are concerned plate tectonics — and also the overlying material has actually been eroded away and also the rock is exposed as an outcrop.

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The vital agents of mechanically weathering are:

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

When a massive of rock is exposed by weathering and also removal the the overlying rock, over there is a diminish in the confining pressure on the rock, and the absent expands. This unloading promotes cracking the the rock, known as exfoliation, as displayed in the granitic absent in figure 5.3.

Figure 5.3 Exfoliation fractures in granitic absent exposed on the west next of the Coquihalla Highway north of Hope, B.C.

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

Granitic rock often tends to exfoliate parallel to the exposed surface since the rock is commonly homogenous, and it doesn’t have actually predetermined planes follow me which it should fracture. Sedimentary and metamorphic rocks, top top the various other hand, often tend to exfoliate along predetermined airplane (Figure 5.4).

Frost wedging is the procedure by which water seeps into cracks in a rock, increases on freezing, and also thus enlarges the crack (Figure 5.5). The performance of frost wedging is concerned the frequency that freezing and also thawing. Frost wedging is most effective in a climate prefer Canada’s. In warm locations where freezing is infrequent, in an extremely cold areas where thawing is infrequent, or in really dry areas, where there is small water come seep into cracks, the duty of frost wedging is limited.

Figure 5.5 The procedure of frost wedging on a steep slope. Water gets into fractures and also then freezes, expanding the fracture a little. When the water thaws it look a small farther right into the expanded crack. The procedure is repeated plenty of times, and also eventually a item of rock will be wedged away.

In plenty of parts of Canada, the transition between freeze nighttime temperatures and also thawing daytime temperatures is frequent — 10s to numerous times a year. Also in warm coastal areas of southern B.C., freezing and thawing transitions are usual at higher elevations. A typical feature in locations of efficient frost wedging is a talus slope — a fan-shaped deposit of pieces removed by frost wedging from the steep rocky slopes over (Figure 5.6).

Figure 5.6 an area with really effective frost-wedging close to Keremeos, B.C. The fragments that have actually been wedged away from the cliffs over have built up in a talus deposit in ~ the basic of the slope. The rocks in this area have actually quite varied colours, and also those room reflected in the colour of the talus.

A associated process, frost heaving, takes ar within unconsolidated materials on tenderness slopes. In this case, water in the floor freezes and expands, pushing the overlying material up. Frost heaving is responsible for winter damage to roads almost everywhere North America.

When salt water seeps into rocks and also then evaporates on a warm sunny day, salt crystals thrive within cracks and pores in the rock. The expansion of these crystals exerts pressure on the rock and also can press grains apart, causing the rock to weaken and break. Over there are countless examples of this ~ above the rocky shorelines that Vancouver Island and also the Gulf Islands, wherein sandstone outcrops space common and salty seawater is readily easily accessible (Figure 5.7). Salt weathering can likewise occur away from the coast, because most settings have some salt in them.

Figure 5.7 Honeycomb weathering of sandstone on Gabriola Island, B.C. The feet are led to by crystallization of salt within rock pores, and also the seemingly consistent pattern is related to the original roughness that the surface. The a positive-feedback procedure because the holes collect salt water in ~ high tide, and so the impact is accentuated approximately existing holes. This type of weathering is most pronounced top top south-facing clear exposures.

The effects of plants and also animals are far-reaching in mechanically weathering. Roots can force their method into also the tiniest cracks, and also then castle exert tremendous push on the rocks as they grow, widening the cracks and also breaking the absent (Figure 5.8). Although animals do not usually burrow through solid rock, they can excavate and also remove huge volumes of soil, and thus expose the rock to weathering by various other mechanisms.

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

Mechanical weathering is greatly helped with by erosion, which is the removed of weathering products, permitting for the exposure of much more rock because that weathering. A an excellent example of this is presented in number 5.6. On the steep rock encounters at the top of the cliff, rock fragments have been damaged off by ice cream wedging, and also then eliminated by gravity. This is a form of mass wasting, which is discussed in more detail in thing 15. Other essential agents of erosion that also have the effect of removing the assets of weathering include water in streams (Chapter 13), ice cream in glaciers (Chapter 16), and waves ~ above the coastlines (Chapter 17).