3.1: Weathering

Weathering Defined

Intrusive igneous rocks form at depths of several hundreds of meters to several tens of kilometers. Sediments are turned into sedimentary rocks only when they are buried by other sediments to depths more than several hundreds of meters and up to several kilometers. Most metamorphic rocks are formed at depths of kilometers to tens of kilometers. Weathering, the process by which rock breaks down, and subsequent soil formation 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 material has been eroded away and the rock is exposed as outcrop.^[1]

There are two types of weathering. Mechanical weathering which is the physical breakdown of rock into smaller pieces, and chemical weathering which is the chemical change of earth material into materials with a different composition resulting from exposure to the atmosphere.

Mechanical Weathering

Since mechanical weathering results in the decrease in size of the materials, the important agents of it are as follows:

1. the decrease in pressure that results from removal of overlying rock,
2. erosional forces related to gravity, water and wind,
3. freezing and thawing of water within cracks in the rock,
4. formation of salt crystals within pores in the rock, and
5. plant roots and burrowing animals.

When a mass of rock is exposed by weathering and by removal of the overlying rock there is a decrease in the confining pressure on the rock, and the rock expands. This unloading promotes cracking of the rock, known as exfoliation, as shown in granitic rock in the picture below.

Figure \(\PageIndex{1}\): Exfoliation Fractures in Granitic Rock Exposed at Yak Peak, Coquihalla Summit Recreation area, BC

Granitic rock tends to exfoliate parallel to the exposed surface because the rock is typically homogenous, and it may not have pre-determined planes along which to fracture. Sedimentary and metamorphic rocks, on the other hand tend to exfoliate along predetermined planes (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Exfoliation of Slate at a Road Cut in the Columbia Mountains West of Golden, BC

Figure \(\PageIndex{3}\): 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 further into the expanded crack. The process is repeated many times, and eventually a piece of rock will be wedged away.

Frost wedging is the process by which the water seeps into cracks in a rock, expands on freezing, and thus enlarges the cracks ( Figure \(\PageIndex{3}\)). The effectiveness of frost wedging is related to the frequency of freezing and thawing. Frost wedging is most effective in a climate where there are many days each year with temperatures close to freezing, so that it might freeze overnight and then thaw in the day.

When salty water seeps into rocks, and then the water evaporates on a sunny day, salt crystals grow within cracks and pores in the rock. The growth of these crystals exerts pressure on the rock and can push grains apart, causing the rock to weaken and break. There are many examples of this on rocky ocean shorelines worldwide, especially where sandstone outcrops are common ( Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): 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.

The effects of plants and animals are significant in mechanical weathering. Roots can force their way into even the tiniest cracks, and then they exert tremendous pressure on the rocks as they expand, widening the cracks and breaking the rock. 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.

Chemical Weathering

Chemical weathering results from the chemical changes to some minerals that become unstable when they are exposed to surface conditions. The kinds of changes that take place are highly specific to the mineral and to the environmental conditions. Some minerals, like quartz, are virtually unaffected by chemical weathering, while others, like feldspar, are quite easily altered. In general, the degree of chemical weathering is greatest in warm and wet climates, and least in cold and dry climates. The important characteristics of surface conditions that lead to chemical weathering are the presence of water (in the air and on the ground surface), the abundance of oxygen, and the presence of carbon dioxide, which, when combined with water, produces weak carbonic acid.

Exposure to this weak acid causes feldspar to turn into the clay mineral kaolinite in a process called hydrolysis. Figure \(\PageIndex{5}\) shows two images of the same granitic rock, a recently broken fresh surface on the left, and a clay-altered weathered surface on the right.

Figure \(\PageIndex{5}\): Un-weathered (left) and Weathered (right) Surfaces of the Same Piece of Granitic Rock. On the unweathered surfaces the feldspars are still fresh and glassy looking. On the weathered surface the feldspar has been altered to the chalky-looking clay mineral kaolinite.

Oxidation is another very important chemical weathering process. The oxidation of the iron in an iron-bearing silicate starts with the dissolution of the iron. The iron is dissolved due to the presence of the carbonic acid mentioned above. Figure \(\PageIndex{6}\) shows a granitic rock in which some of the biotite and amphibole have been altered to form the iron oxide mineral limonite. Oxidation produces the red color you see here and in many of the rocks in Arizona, New Mexico, Utah, and Colorado.

Figure \(\PageIndex{6}\): A Granitic Rock Containing Biotite and Amphibole, Altered Near to the Rock’s Surface to Limonite, A Mixture of Iron-Oxide Minerals.

Hydrolysis and oxidation both serve to create rocks that are softer and weaker than they were to begin with, and thus more susceptible to mechanical weathering.

The weathering reactions that we’ve discussed so far involved the transformation of one mineral to another mineral (e.g., feldspar to clay). Some weathering processes involve the complete dissolution of a mineral. Calcite, for example, will dissolve in weak acid, to produce calcium and bicarbonate ions.

Calcite is the major component of limestone (typically more than 95%), and under surface conditions limestone will dissolve to varying degrees (depending on which minerals it has other than calcite), as shown on Figure \(\PageIndex{7}\). Limestone also dissolves at relatively shallow depths underground, forming limestone caves.

Figure \(\PageIndex{7}\): A Limestone Outcrop on Quadra Island, BC. The limestone, which is primarily made up of the mineral calcite, has been dissolved to different degrees in different areas because of compositional differences. The buff-colored bands are either volcanic rock or chert, which are not soluble.

Media Attributions

• Figure \(\PageIndex{1}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{4}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{5}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{6}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{7}\): Photo by Steven Earle, CC BY 4.0

---

1. To a geologist, an outcrop is an exposure of bedrock—the solid rock of the crust. ↵