5.1 mechanical Weathering

Intrusive igneous rocks kind at depths of several numerous metres to several tens of kilometres. Sediments room turned right into sedimentary rocks only as soon as they are buried by other sediments to depths in excess of several thousands of metres. Most metamorphic rocks are created at depth of kilometres to tens of kilometres. Weathering cannot even start until this rocks are uplifted through miscellaneous processes of mountain structure — many of which are related to plate tectonics — and the overlying material has actually been eroded away and the absent is exposed together an outcrop.

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The essential agents of mechanically weathering are:

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

When a fixed of absent is exposed by weathering and also removal that the overlying rock, over there is a to decrease in the confining pressure ~ above the rock, and the rock expands. This unloading disclosure cracking the the rock, well-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 north of Hope, B.C.

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

Granitic rock tends to exfoliate parallel come the exposed surface since the absent is typically homogenous, and also it doesn’t have predetermined planes along which it need to fracture. Sedimentary and also metamorphic rocks, ~ above the other hand, tend to exfoliate along predetermined planes (Figure 5.4).

Frost wedging is the process by i beg your pardon water seeps into cracks in a rock, expands on freezing, and thus enlarges the cracks (Figure 5.5). The performance of frost wedging is pertained to the frequency the freezing and thawing. Frost wedging is most reliable in a climate like Canada’s. In warm locations where freezing is infrequent, in really cold locations where thawing is infrequent, or in an extremely dry areas, wherein there is little water to seep right into cracks, the function of frost wedging is limited.

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

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

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

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

When salt water seeps into rocks and also then evaporates ~ above a hot sunny day, salt crystals grow within cracks and also pores in the rock. The development of these crystals exerts pressure on the rock and also can push grains apart, leading to the rock to weaken and also break. There are many examples that this top top the rocky shorelines the Vancouver Island and also the Gulf Islands, where sandstone outcrops are common and salty seawater is readily available (Figure 5.7). Salt weathering can additionally occur far from the coast, due to the fact that most atmospheres have some salt in them.

Figure 5.7 Honeycomb weathering of sandstone ~ above Gabriola Island, B.C. The holes are resulted in by crystallization of salt within absent pores, and also the seemingly consistent pattern is related to the initial roughness the the surface. That a positive-feedback process because the holes collection salt water at high tide, and also so the effect is accentuated approximately existing holes. This kind of weathering is most pronounced ~ above south-facing sunny exposures.

The results of plants and animals are significant in mechanically weathering. Root can pressure their means into also the tiniest cracks, and also then they exert tremendous pressure on the rocks together they grow, widening the cracks and also breaking the absent (Figure 5.8). Although pets do not typically burrow with solid rock, they deserve to excavate and remove huge volumes the soil, and also thus disclose the absent to weathering by 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 facilitated by erosion, i beg your pardon is the removed of weathering products, enabling for the exposure of much more rock for weathering. A great example the this is displayed in number 5.6. ~ above the steep rock deals with at the optimal of the cliff, rock fragments have been broken off by ice wedging, and then eliminated by gravity. This is a form of mass wasting, i m sorry is questioned in an ext detail in thing 15. Other necessary agents of erosion that likewise have the impact of remove the assets of weathering incorporate water in streams (Chapter 13), ice cream in glaciers (Chapter 16), and waves top top the coastlines (Chapter 17).