Intrusive igneous rocks form at depths of 100s of metres to 10s of kilometres. Sediments are turned into sedimentary rocks only when they are buried by other sediments to depths in excess of several 100s of metres. Most metamorphic rocks are formed at depths of kilometres to 10s of kilometres. Weathering cannot occur until these rocks are revealed at Earth’s surface by uplift and the erosion of overlying material. Once the rock is exposed at the surface as an outcrop, weathering can begin.
The agents of mechanical weathering can be broadly classified into two groups: Those things which cause the outer layers of a rock to expand, and those things which act like wedges to force the rock apart.
Mechanical Weathering By Expansion
Rock on Earth’s surface responds to deformation by breaking. When the outer layer of a rock expands but the inner part does not, the result is a crack to accommodate the difference. The slabs of rock in Figure 8.2 were formed in this way, breaking off from the surface beneath. When layers break off a rock in this way, it’s referred to as exfoliation.
Granitic rock tends to exfoliate parallel to the exposed surface because it is typically homogeneous, meaning that it doesn’t contain predetermined planes of weakness. In contrast, sedimentary and metamorphic rocks tend to exfoliate along predetermined planes.
There are several reasons why the outer layer of a rock might expand.
Confining pressure refers to the pressure on a rock resulting from other rocks being on top of it and around it. When a mass of rock is exposed by weathering and removal of the overlying rock, there is a decrease in the confining pressure and the rock expands. The cracking that results is sometimes referred to as pressure-release cracking.
Figure 8.3 is an example of exfoliation due to a decrease in confining pressure. The exfoliation is easiest to see in the middle of the photograph.
Heating a rock can also cause it to expand. If the rock is heated very rapidly, as during a wildfire, cracks will form. If it goes through large daily temperature swings (e.g., in the desert where it is very hot during the day but cool at night) that will also eventually result in cracking as the rock is weakened.
Mechanical Weathering By Wedging
In wedging a pre-existing crack in a rock is forced open and made larger.
Frost wedging happens when water seeps into cracks, then expands upon freezing. The expansion enlarges the cracks (Figure 8.4). The effectiveness of frost wedging depends on how often freezing and thawing occur. Frost wedging won’t be as important in warm areas where freezing is infrequent, in very cold areas where thawing is infrequent, or in very dry areas, where there is little water to seep into cracks.
Frost wedging is most effective in a climate like Canada’s, where for at least part of the year temperatures oscillate between warm and freezing. In many parts of Canada, the transition between freezing night-time temperatures and thawing daytime temperatures is also frequent — tens to hundreds of times a year. Even in warm coastal areas of southern British Columbia, freezing and thawing transitions are common 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 above (Figure 8.5).
Salt wedging happens when salt water seeps into rocks and then evaporates on a hot sunny day.
Salt crystals grow within cracks and pores in the rock, and the growth of these crystals can push grains apart, causing the rock to weaken and break. There are many examples of this on the rocky shorelines of Vancouver Island and the Gulf Islands, where sandstone outcrops are common and salty seawater is readily available (Figure 8.6). The honeycomb structure in Figure 8.5 is related to the original roughness of the surface. Low spots collect salt water, causing the effect to be accentuated around existing holes.
Figure 8.6 Honeycomb weathering of sandstone on Gabriola Island BC. The holes are caused by crystallization of salt within rock pores. [Steven Earle CC-BY 4.0]The effects of plants and animals are significant in mechanical weathering. Roots can force their way into even the tiniest cracks. They exert tremendous pressure on the rocks as they grow, widening the cracks and breaking the rock. This is called root wedging (Figure 8.7).
Although animals do not normally burrow through solid rock, they can excavate and remove huge volumes of soil, and thus expose the rock to weathering by other mechanisms.
Figure 8.7 Conifers growing on granitic rocks at The Lions, near Vancouver, B.C. [SE]Mechanical weathering is greatly facilitated by erosion. Erosion is the removal of weathering products, such as fragments of rock. This exposes more rock to weathering, speeding the process. A good example of weathering and erosion working together is shown in Figure 8.5. The rock fragments forming the talus piles were broken off the steep rock faces at the top of the cliff by ice wedging, and then removed by gravity.
Gravity is not the only way weathering products are removed. Other agents of erosion which remove the products of weathering include water in streams, ice in glaciers, and waves on coasts.
Exercise 8.1 Mechanical Weathering
This photo shows granitic rock at the top of Stawamus Chief near Squamish, B.C. Identify the mechanical weathering processes that you can see taking place, or that you think probably take place at this location.