Where Is The Daily Temperature Change The Most Significant In Rock Weathering?

Chapter 5 Weathering and Soil

5.1 Mechanical Weathering

Intrusive igneous rocks course at depths of several hundreds of metres to several tens of kilometres. Sediments are turned into sedimentary rocks only when they are buried by other sediments to depths in excess of several hundreds of metres. Almost metamorphic rocks are formed at depths of kilometres to tens of kilometres. Weathering cannot even begin until these rocks are uplifted through various processes of mountain building — most of which are related to plate tectonics — and the overlying fabric has been eroded abroad and the rock is exposed as an outcrop.^[ane]

The important agents of mechanical weathering are:

The decrease in force per unit area that results from removal of overlying rock
Freezing and thawing of h2o in cracks in the rock
Formation of salt crystals within the rock
Cracking from found roots and exposure by burrowing animals

When a mass of rock is exposed past weathering and removal of the overlying rock, there is a decrease in the confining pressure on the rock, and the rock expands. This unloading promotes slap-up of the rock, known equally exfoliation, as shown in the granitic rock in Figure 5.three.

Photograph of xfoliation fractures in granitic rock exposed on the west side of the Coquihalla Highway north of Hope, BC. — Figure 5.three Exfoliation fractures in granitic rock exposed on the west side of the Coquihalla Highway north of Promise, B.C. [SE]

Figure 5.4 Exfoliation of slate at a road cut in the Columbia Mountains west of Golden, B.C. [SE photo] — Figure v.4 Exfoliation of slate at a route cut in the Columbia Mountains westward of Golden, B.C. [SE photo]

Granitic rock tends to exfoliate parallel to the exposed surface considering the rock is typically homogenous, and information technology doesn't take predetermined planes along which it must fracture. Sedimentary and metamorphic rocks, on the other paw, tend to exfoliate along predetermined planes (Effigy 5.iv).

Frost wedging is the process by which water seeps into cracks in a rock, expands on freezing, and thus enlarges the cracks (Figure five.5). The effectiveness of frost wedging is related to the frequency of freezing and thawing. Frost wedging is most effective in a climate like Canada's. In warm areas where freezing is exceptional, in very cold areas where thawing is infrequent, or in very dry areas, where in that location is little water to seep into cracks, the role of frost wedging is limited.

The process of frost wedging on a steep slope. Water gets into fractures and then freezes, expanding the fracture a little. When the water thaws it seeps a little farther into the expanded crack. The process is repeated many times, and eventually a piece of rock will be wedged away. — Figure five.v The process of frost wedging on a steep slope. H2o gets into fractures then freezes, expanding the fracture a piddling. When the water thaws it seeps a piddling farther into the expanded scissure. The process is repeated many times, and eventually a piece of rock will exist wedged away. [SE]

In many parts of Canada, the transition between freezing night temperatures and thawing daytime temperatures is frequent — tens to hundreds of times a year. Fifty-fifty in warm coastal areas of southern B.C., freezing and thawing transitions are common at higher elevations. A common feature in areas of constructive frost wedging is a talus slope — a fan-shaped deposit of fragments removed by frost wedging from the steep rocky slopes to a higher place (Figure 5.half dozen).

Photograph of an area with very effective frost-wedging near to Keremeos, BC. The fragments that have been wedged away from the cliffs above have accumulated in a talus deposit at the base of the slope. The rocks in this area have quite varied colours, and those are reflected in the colours of the talus. — Effigy 5.vi An area with very constructive frost-wedging near Keremeos, B.C. The fragments that accept been wedged abroad from the cliffs to a higher place accept accumulated in a talus deposit at the base of the slope. The rocks in this area have quite varied colours, and those are reflected in the colours of the talus. [SE]

A related process, frost heaving, takes place within unconsolidated materials on gentle slopes. In this instance, h2o in the soil freezes and expands, pushing the overlying material up. Frost heaving is responsible for winter damage to roads all over Due north America.

When salt water seeps into rocks and so evaporates on a hot sunny day, salt crystals grow within cracks and pores in the rock. The growth of these crystals exerts pressure on the rock and tin can push grains apart, causing the rock to weaken and interruption. There are many examples of this on the rocky shorelines of Vancouver Isle and the Gulf Islands, where sandstone outcrops are common and salty seawater is readily available (Figure five.7). Salt weathering can also occur abroad from the coast, because most environments have some salt in them.

Photograph of Honeycomb weathering of sandstone on Gabriola Island, BC. The holes are caused by crystallization of salt within rock pores, and the seemingly regular pattern is related to the original roughness of the surface. It's a positive-feedback process because the holes collect salt water at high tide, and so the effect is accentuated around existing holes. This type of weathering is most pronounced on south-facing sunny exposures. — Figure 5.7 Honeycomb weathering of sandstone on Gabriola Island, B.C. The holes are acquired past crystallization of table salt within rock pores, and the seemingly regular pattern is related to the original roughness of the surface. It's a positive-feedback process because the holes collect table salt water at high tide, and and then the consequence is accentuated around existing holes. This blazon of weathering is most pronounced on south-facing sunny exposures. [SE]

The effects of plants and animals are significant in mechanical weathering. Roots can forcefulness their manner into even the tiniest cracks, then they exert tremendous force per unit area on the rocks as they grow, widening the cracks and breaking the rock (Figure 5.8). Although animals do non normally burrow through solid rock, they can excavate and remove huge volumes of soil, and thus betrayal the rock to weathering past other mechanisms.

Photograph of Conifers growing on granitic rocks at The Lions, near to Vancouver BC — Effigy five.viii Conifers growing on granitic rocks at The Lions, nigh Vancouver, B.C. [SE]

Mechanical weathering is greatly facilitated by erosion, which is the removal of weathering products, allowing for the exposure of more stone for weathering. A adept example of this is shown in Effigy 5.6. On the steep rock faces at the elevation of the cliff, stone fragments have been broken off by ice wedging, and so removed by gravity. This is a form of mass wasting, which is discussed in more particular in Chapter xv. Other of import agents of erosion that likewise have the event of removing the products of weathering include water in streams (Chapter 13), ice in glaciers (Chapter xvi), and waves on the coasts (Chapter 17).

Do 5.i Mechanical Weathering

This photo shows granitic rock at the top of Stawamus Main near Squamish, B.C. Place the mechanical weathering processes that you can see taking place, or yous call back probably take place at this location.