11.3: Weathering

Weathering

Sedimentary rocks are composed of the broken pieces of other rocks. The obvious place to start is examining how rocks are broken down, in a process called weathering Links to an external site.. Weathering is the process from which sediments are derived or produced. There are two categories of weathering. The first, is physical or mechanical weathering Links to an external site., which results in rocks being physically broken into smaller pieces resulting in an overall size change (imagine hitting a rock with a hammer) (Figure 11.2). The second category is chemical weathering Links to an external site., which results in a fundamental chemical change of the original rock. The rock becomes altered at the atomic level (imagine dissolving salt in a glass of water). Oftentimes, both physical and chemical weathering will simultaneously affect a rock; however, the extent and rate of weathering is frequently dependent on the environment.

A larger rock is broken down into smaller pieces.

Figure 11.2: Mechanical weathering is the physical breakdown of rock in either size or shape. (CC-BY 4.0, Chloe Branciforte, own work)


Physical or Mechanical Weathering

Abrasion Links to an external site., the most prevalent type of mechanical weathering, results from the collision, breaking, and grinding of rock during the movement of a fluid, either water or air. The size of the carried sediment depends on the fluid type and velocity. A fast fluid (e.g. a rapidly flowing river) can carry large particles and cause large amounts of abrasion. A slow fluid (e.g. a calm stream) would only cause a small amount of abrasion. The density of the fluid also controls the size of particles that can be transported. Denser fluid, like water, can carry larger particles, while less dense fluids, like air, carry smaller particles.

Another common method of mechanical weathering occurs when water seeps into a crack in the rock and freezes; this is called frost wedging Links to an external site. or frost heaving Links to an external site.. Water is unique in that it expands when frozen, which puts pressure on the rock and can split boulders (Figure 11.3).

A large boulder has been broken in half due to freeze and thaw.

Figure 11.3: Top image, this boulder in Scotland has been affected by the freeze and thaw cycle resulting in a crack developing. The image below illustrates the step-by-step process of frost wedging. (Top, CC-BY 2.0, John Allen Links to an external site.; Bottom, CC-BY 3.0; Julie Sandeen Links to an external site.; modified by Chloe Branciforte)

The addition and subtraction of heat or pressure can also cause rock to break; this is called exfoliation Links to an external site.. This breakage can also occur with rocks when they cool very quickly or immense pressure is released. Exfoliation of the granite in Yosemite National Park results in many of the rockfalls for which the park is famous (Figure 11.4).

A mountainside undergoing physical weathering where slabs of granite are falling off.

Figure 11.4: Exfoliating slabs of granite, on Half Dome in Yosemite National Park, California. (CC-BY 2.0; Ronnie Macdonald Links to an external site.)


Chemical Weathering

Rocks can be chemically weathered, usually by one of three common reactions. Water is frequently involved as it is good at inducing a chemical change. The first, which you are probably familiar with, is called dissolution Links to an external site.. In this case, a mineral or rock is completely broken apart in water into chemical ions. These ions are then transported with the water and redeposited as the concentration of ions increases, normally because of evaporation (this is accomplished through a process called chemical precipitation Links to an external site., which is different than the rain type of precipitation). Table 11.1 illustrates the chemistry of calcite, which is prone to dissolution.

Chemical weathering can also change the mineralogy and weaken the original material, again through the agent of water. A mineral can undergo hydrolysis Links to an external site., which occurs when a hydrogen atom from a water molecule replaces the cation in a mineral. Typically, this alters minerals like feldspar into a softer clay mineral (like kaolinite).

Thirdly, a mineral can also undergo oxidation Links to an external site., which is when oxygen atoms alter the valence state of a cation. This normally occurs on a metal that is freshly exposed to atmospheric oxygen and is commonly known as rusting (Figure 11.5). Table 11.1 illustrates the chemistry of significant minerals and their chemical changes.

A close-up of a rock with a dark gray interior and a red border.

Figure 11.5: A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. (CC-BY 3.0; Pollinator Links to an external site.)


Table 11.1: Common minerals and how they change through chemical weathering. (CC-BY 4.0; Chloe Branciforte, own work)
Chemical change Process Chemical reaction
Pyrite to Hematite Oxidation 2FeS2 + 7O2 + 2H2O => 2Fe2+ + H2SO4 + 2H+
Pyroxene to iron oxide Oxidation Fe2+SiO3 + O2 + H2O => FeO(OH) + SiO2
Calcite to calcium & bicarbonate ions Dissolution CaCO3 + H+ + HCO3- => Ca2+ + 2HCO3-
Potassium feldspar to clay Hydrolysis 3KAlSi3O8 + 3H2O => Al2Si2O5(OH)4 + 4SiO2 +2K+ +2OH-
Olivine to serpentine Hydrolysis + Oxidation = Serpentinization

Fe2SiO4 + 4H2CO3 => 2Fe2+ + 4HCO3- + H4SiO4

2Fe2+ + ½O2 + 2H2O + 4HCO3- => Fe2O3 + 4H2CO3

Chemical and mechanical weathering often work together to increase the overall rate of weathering. Chemical weathering weakens rocks, which makes them more prone to breaking physically, while mechanical weathering increases the surface area of the sediment, which increases the surface area that is exposed to chemical weathering (Figure 11.6).

A cube is broken down into more and more cubes.

Figure 11.6: When a rock undergoes physical weathering, additional surface area is produced, which can increase the potential for chemical weathering. (CC-BY 4.0, Chloe Branciforte, own work)

Environments with multiple types of weathering typically result in rocks that break down quickly. Humid climates, with abundant water, will generally encourage changes faster than drier climates. In addition, we must also consider the original mineralogy of the rocks, as this can also play an important role in weathering potential (Table 11.2).

Finally, biology – plants, animals, and yes, even humans – can cause significant amounts of weathering, sometimes referred to as biological weathering. This can be done both physically and chemically. For example, trees put down roots through joints or cracks in the rock and, as the tree grows, the roots gradually pry the rock apart. Additionally, some animals bore into rocks for protection either by force or through the secretion of acid which dissolves the rock. Even bacteria, algae, and lichens produce chemicals that can break down rock.

Table 11.2: Common minerals and their stability and associated rate of weathering. (CC-BY 4.0; Chloe Branciforte, own work)
Mineral Name Relative Stability Rate of Weathering
Hematite Most stable Slowest
Quartz
Clay minerals
Muscovite mica
Potassium feldspar
Biotite
Na-Plagioclase feldspar
Amphibole
Pyroxene
Ca-Plagioclase feldspar
Olivine
Calcite
Halite Least stable Fastest