8.1 mechanically Weathering

Intrusive igneous rocks kind at depth of 100s of metres come 10s that kilometres. Most metamorphic rocks are developed at depth of kilometres come 10s of kilometres. Sediments space turned right into sedimentary rocks only as soon as they are buried by various other sediments to depths in overabundance of several 100s that metres. Weathering cannot take place until this rocks space revealed in ~ Earth’s surface ar by uplift and the erosion of overlying materials. As soon as the rock is exposed in ~ the surface ar as an outcrop, weathering begins.

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The agents of mechanical weathering have the right to be extensively classified into two groups: those that reason the outer layers that a absent to expand, and those that act favor wedges to pressure the rock apart.

Mechanical Weathering by Expansion

Some procedures at Earth’s surface can cause a thin external layer that a absent to expand. Deeper 보다 the thin external layer, the rock does no expand. The difference is accommodated through a crack occurring between the outer and inner layers, breaking the external layer turn off in slabs (Figures 8.2 and also 8.3). As soon as layers break off a absent in slabs or sheets, it is referred to as exfoliation.

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Figure 8.2 Close-up view of exfoliation the a granite dome in the Enchanted absent State herbal Area, Texas, USA. Source: Wing-Chi Poon (2005) CC BY-SA 2.5 check out source
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Figure 8.3 watch of exfoliation in ~ a distance (centre of image) in granite exposed ~ above the west next of the Coquihalla Highway phibìc of Hope, B.C. Source: Steven Earle (2015) CC BY watch source

Granite has tendency to exfoliate parallel to the exposed surface due to the fact that it walk not have planes the weakness come determine just how it breaks. In contrast, sedimentary rocks tend to exfoliate follow me the contacts between different sedimentary layers, and also metamorphic rocks tend to exfoliate parallel come aligned minerals.

Reasons Rocks Expand

A absent within the planet has push exerted upon that by various other rocks sitting over it. This is called confining pressure. When the overlying mass is eliminated by weathering, the confining push decreases, enabling the rock to expand. The cracking that results is sometimes dubbed pressure-release cracking.

Heating a rock can also cause it come expand. If the rock is heated rapidly, as throughout a wildfire, cracks deserve to form. If it goes through big daily temperature swings (e.g., in the desert wherein it is very hot throughout the day however cold in ~ night), cracking can likewise eventually an outcome as the absent is weakened.

Mechanical Weathering by Wedging

In wedging, a pre-existing cracked in a absent is made larger by forcing the open.

Frost Wedging

Frost wedging (or ice wedging) happens once water seeps into cracks, then increases upon freezing. The growth enlarges the crack (Figure 8.4). The effectiveness of frost wedging counts on how often freezing and also thawing occur. Frost wedging won’t be as important in warm areas where freeze is infrequent, in an extremely cold areas where thawing is infrequent, or in an extremely dry areas, whereby there is tiny water come seep into cracks.

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Figure 8.4 A rock broken by ice cream wedging sit in a stream in mount Revelstoke nationwide Park, Canada. Rocks rest apart as soon as ice increases in pre-existing cracks. Source: Karla Panchuk (2018) CC by 4.0.

Frost wedging is most reliable in Canada’s climate, wherein for in ~ least component of the year temperature oscillate between warm and also freezing. In numerous parts of Canada, the temperature swings between freezing in ~ night and also thawing in the day tens to thousands of times a year. Even in warm seaside areas of southern British Columbia, freezing and thawing transitions are typical at higher elevations. A common feature in locations of effective frost wedging is a talus steep — a fan-shaped deposit of fragments removed by frost wedging native the steep rocky slopes over (Figure 8.5).

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Figure 8.5 one area with an extremely effective frost wedging close to Keremeos, BC. The fragments that to be wedged away from the cliffs over have accumulated in a talus deposit at the basic of the slope. The rocks in this area are variable in colour, which is reflect in the color of the talus. Source: Steven Earle (2015) CC by 4.0 check out source

Salt Wedging

Salt wedging happens once saltwater seeps into rocks and then evaporates on a hot sunny day. Salt crystals flourish within cracks and pores in the rock, and the development of this crystals have the right to push seed apart, resulting in the absent to weaken and break. Over there are countless examples that this on the rocky shorelines of Vancouver Island and the Gulf Islands, wherein sandstone outcrops space common and also salty seawater is readily easily accessible (Figure 8.6). The honeycomb structure of rounded holes, called tafoni, is concerned the initial roughness of the surface. Low spots collect salt water, resulting in the result to it is in accentuated roughly existing holes.

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Figure 8.6 Tafoni (Honeycomb weathering) in sandstone top top Gabriola Island, british Columbia. The feet are led to by crystallization of salt within absent pores. Source: Steven Earle (2015) CC through 4.0 watch source

Plant and Animal Activity

The effects of tree are significant in mechanically weathering. Roots can pressure their way into even the tiniest cracks. They exert tremendous pressure on the rocks together they grow, widening the cracks and also breaking the rock. This is referred to as root wedging (Figure 8.7).

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Figure 8.7 source wedging follow me a quarry wall. Left: Rocks in ~ the thick red beds have actually been split into sheets by tree roots. Right: A closer examination reveals that tree roots are working right into vertical cracks as well. Source: Karla Panchuk (2018) CC by 4.0

Although most animals do not usually burrow through solid rock, they can excavate and remove substantial volumes of soil, and also thus expose the rock to weathering by other mechanisms. Human beings modify substantial tracts of land by excavation, and also have a profound impact on increasing mechanical weathering.