5.1 mechanically Weathering

Intrusive igneous rocks kind at depths of several numerous metres to number of tens of kilometres. Sediments space turned into sedimentary rocks only once they are buried by other sediments to depth in overabundance of several thousands of metres. Most metamorphic rocks are formed at depths of kilometer to tens of kilometres. Weathering can not even start until these rocks space uplifted through assorted processes that mountain building — many of i beg your pardon are pertained to plate tectonics — and the overlying material has actually been eroded away and also the rock is exposed as an outcrop.

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The essential agents of mechanically weathering are:

The to decrease in press that results from removal of overlying rockFreezing and also thawing of water in crack in the rockFormation that salt crystals within the rockCracking from plant roots and also exposure by burrowing animals

When a mass of rock is exposed by weathering and also removal that the overlying rock, there is a diminish in the confining pressure top top the rock, and the absent expands. This unloading promotes cracking of the rock, known as exfoliation, as shown in the granitic rock in number 5.3.

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Figure 5.3 Exfoliation fractures in granitic rock exposed top top the west side of the Coquihalla Highway north of Hope, B.C.

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

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

Frost wedging is the process by which water seeps into cracks in a rock, increases on freezing, and thus enlarges the crack (Figure 5.5). The efficiency of frost wedging is regarded the frequency the freezing and also thawing. Frost wedging is most reliable in a climate choose Canada’s. In warm areas where freezing is infrequent, in really cold locations where thawing is infrequent, or in really dry areas, whereby there is tiny water to seep right into cracks, the duty of frost wedging is limited.

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Figure 5.5 The process of frost wedging top top a steep slope. Water gets into fractures and then freezes, widening the fracture a little. When the water thaws it look a little farther into the broadened crack. The process is repeated plenty of times, and eventually a item of rock will certainly be wedged away.

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

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Figure 5.6 one area with an extremely effective frost-wedging near Keremeos, B.C. The pieces that have actually been wedged far from the cliffs above have collected in a talus deposit at the basic of the slope. The rocks in this area have actually quite varied colours, and those space reflected in the color of the talus.

A associated process, frost heaving, takes location within unconsolidated products on tenderness slopes. In this case, water in the soil freezes and also expands, advertise the overlying material up. Frost heaving is responsible because that winter damage to roads anywhere North America.

When salt water seeps right into rocks and also then evaporates top top a hot sunny day, salt crystals flourish within cracks and also pores in the rock. The growth of these crystals exerts press on the rock and also can press grains apart, resulting in the absent to weaken and break. There are numerous examples that this ~ above the rocky shorelines that Vancouver Island and also the Gulf Islands, where sandstone outcrops are common and salty seawater is readily available (Figure 5.7). Salt weathering can likewise occur far from the coast, because most settings have some salt in them.

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Figure 5.7 Honeycomb weathering the sandstone top top Gabriola Island, B.C. The holes are brought about by crystallization that salt within rock pores, and also the seemingly constant pattern is regarded the original roughness the the surface. That a positive-feedback process because the holes collection salt water at high tide, and so the impact is accentuated about existing holes. This type of weathering is many pronounced ~ above south-facing clear exposures.

The impacts of plants and animals are significant in mechanically weathering. Root can force their way into also the tiniest cracks, and also then they exert tremendous press on the rocks together they grow, widening the cracks and breaking the absent (Figure 5.8). Although animals do not normally burrow v solid rock, they deserve to excavate and also remove vast volumes that soil, and thus reveal the rock to weathering by other mechanisms.

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Figure 5.8 Conifers farming on granitic rocks at The Lions, close to Vancouver, B.C.

Mechanical weathering is greatly helped with by erosion, i m sorry is the remove of weathering products, enabling for the exposure of an ext rock because that weathering. A an excellent example the this is presented in number 5.6. On the steep rock encounters at the height of the cliff, rock pieces have been broken off by ice wedging, and also then eliminated by gravity. This is a kind of massive wasting, which is debated in an ext detail in thing 15. Other important agents of erosion that additionally have the impact of removing the commodities of weathering include water in streams (Chapter 13), ice cream in glaciers (Chapter 16), and also waves top top the shores (Chapter 17).