5.1 mechanically Weathering

Intrusive igneous rocks type at depth of several thousands of metres to several tens the kilometres. Sediments room turned into sedimentary rocks only once they are buried by various other sediments to depths in overfill of several thousands of metres. Most metamorphic rocks are formed at depths of kilometre to 10s of kilometres. Weathering cannot even start until this rocks space uplifted through various processes of mountain building — most of which are related to plate tectonics — and also the overlying material has actually been eroded away and the rock is exposed together an outcrop.

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The essential agents of mechanical weathering are:

The to decrease in press that results from remove of overlying rockFreezing and also thawing that water in crack in the rockFormation that salt crystals in ~ the rockCracking from tree roots and also exposure through burrowing animals

When a mass of absent is exposed by weathering and also removal the the overlying rock, over there is a to decrease in the confining pressure top top the rock, and also the rock expands. This unloading disclosure cracking the the rock, well-known as exfoliation, as shown in the granitic absent in number 5.3.

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

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

Granitic rock often tends to exfoliate parallel come the exposed surface because the rock is commonly homogenous, and also it doesn’t have actually predetermined planes follow me which it need to fracture. Sedimentary and metamorphic rocks, ~ above the various other hand, have tendency to exfoliate follow me predetermined planes (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 related to the frequency the freezing and thawing. Frost wedging is most reliable in a climate prefer Canada’s. In warm locations where freezing is infrequent, in an extremely cold locations where thawing is infrequent, or in really dry areas, where there is tiny water come seep into cracks, the duty of frost wedging is limited.

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

In many parts of Canada, the shift between freezing nighttime temperatures and thawing daytime temperature is frequent — tens to numerous times a year. Also in warm coastal areas of southerly B.C., freezing and thawing transitions are usual at greater elevations. A usual feature in areas of effective 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).

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

A related process, frost heaving, takes place within unconsolidated products on gentle slopes. In this case, water in the floor freezes and 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 then evaporates top top a warm sunny day, salt crystals flourish within cracks and also pores in the rock. The expansion of these crystals exerts press on the rock and can push grains apart, bring about the absent to weaken and also break. There are numerous examples of this ~ above the rocky shorelines that Vancouver Island and the Gulf Islands, where sandstone outcrops space common and also salty seawater is readily available (Figure 5.7). Salt weathering can additionally occur away from the coast, because most environments have some salt in them.

Figure 5.7 Honeycomb weathering that sandstone ~ above Gabriola Island, B.C. The holes are resulted in by crystallization of salt within absent pores, and also the seemingly continuous pattern is regarded the initial roughness that the surface. It’s a positive-feedback process because the holes collect salt water in ~ high tide, and also so the effect is accentuated about existing holes. This form of weathering is many pronounced ~ above south-facing clear exposures.

The results of plants and animals are significant in mechanical weathering. Root can pressure their method into even the tiniest cracks, and also then they exert tremendous pressure on the rocks together they grow, widening the cracks and breaking the rock (Figure 5.8). Although pets do not typically burrow with solid rock, they have the right to excavate and also remove substantial volumes of soil, and thus disclose the absent to weathering by various other mechanisms.

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Figure 5.8 Conifers cultivation on granitic rocks at The Lions, near Vancouver, B.C.

Mechanical weathering is greatly promoted by erosion, which is the removal of weathering products, enabling for the exposure of an ext rock for weathering. A good example of this is shown in figure 5.6. Top top the steep rock faces at the top of the cliff, rock pieces have been damaged off by ice wedging, and also then removed by gravity. This is a type of massive wasting, which is disputed in more detail in thing 15. Other crucial agents the erosion that also have the impact of removing the assets of weathering encompass water in streams (Chapter 13), ice cream in glaciers (Chapter 16), and also waves ~ above the coastlines (Chapter 17).