Weathering

In this post, we will discuss an important topic of Geology, and it is Weathering. We will see what weathering is and how it occurs. We will also find terms like Mechanical Weathering and Chemical Weathering, and how these happen and how they are related.

Weathering

Weathering is what takes place when a body of rock is exposed to the weather, in other words, to the forces and conditions that exist on Earth’s surface.

The term weathering refers to the processes that change the physical and chemical character of rock at or near the surface.

Rocks undergo mechanical weathering (physical disintegration) and chemical weathering (decomposition) as they are attacked by air, water, and microorganisms. Weathering processes create sediments (primarily mud and sand) and soil.

Most rocks, except for volcanic rocks and some sedimentary rocks, are formed at some depth within the crust. At that depth, within the crust, they experience relatively constant temperature, high pressure, no contact with the atmosphere, and little or no moving water.

A rock gets exposed at the surface when the overlying rock is eroded and then conditions change dramatically. In this situation, temperatures vary widely, the pressure becomes much less, oxygen and other gases are plentiful, and in most climates, water is abundant. Weathering occurs in this condition.

Weathering prepares rocks for erosion and is a fundamental part of the rock cycle, transforming rocks into the raw material that eventually becomes sedimentary rocks. Through weathering, there are important links between the rock cycle and the atmosphere and biosphere.

Weathering & its types

Weathering includes two main processes that are quite different.

1) One is the mechanical breakdown of rock into smaller fragments, and

2] the other is the chemical change of the minerals within the rock to certain forms that are stable in the surface environment.

Mechanical weathering, also known as physical weathering, provides fresh surfaces for attack by chemical processes, and chemical weathering weakens the rock so that it is more susceptible to mechanical weathering.

Together, these processes create two very important products, one being the sedimentary clasts and ions in solution that can eventually become sedimentary rock, and the other being the soil that is necessary for our existence on Earth.

Rocks undergo both mechanical weathering and chemical weathering. Mechanical weathering (physical disintegration) includes several processes that break the rock into smaller pieces. The change in the rock is physical; there is little or no chemical change. For example, water freezing and expanding in cracks can cause rocks to disintegrate physically. Chemical weathering is the decomposition of rock from exposure to water and atmospheric gases (principally carbon dioxide, oxygen, and water vapor). As rock is decomposed by these agents, new chemical compounds form.

Mechanical weathering breaks up the rock but does not change the composition. A large mass of granite may be broken into smaller pieces by frost action, but its original crystals of quartz, feldspar, and ferromagnesian minerals are unchanged. On the other hand, if the granite is chemically weathered, some of the original minerals are chemically changed into different minerals. Feldspar, for example, will change into a clay mineral (with a crystal structure similar to mica). In nature, mechanical and chemical weathering usually occur together, and the effects are interrelated.

Weathering is a relatively long, slow process. Typically, cracks in the rock are enlarged gradually by frost action or plant growth (as roots pry into rock crevices), and as a result, more surfaces are exposed to attack by chemical agents. Chemical weathering initially works along contacts between mineral grains. Tightly bound crystals are loosened as weathering products form at their contacts. Mechanical and chemical weathering then proceed together, until a once tough rock slowly crumbles into individual grains.

Mechanical Weathering

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. Most metamorphic rocks are formed at depths of kilometers to tens of kilometers. 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 material has been eroded and the rock is exposed as an outcrop.

The important agents of mechanical weathering are:

  • The decrease in pressure caused by the removal of overlying rock
  • Freezing and melting of water in cracks in the rock
  • Formation of salt crystals within the rock
  • Cracking from plant roots and exposure by burrowing animals

When a mass of rock is exposed by 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 cracking of the rock, known as rock exfoliation.

Granitic rock tends to exfoliate parallel to the exposed surface because the rock is typically homogenous, and it doesn’t have predetermined planes along which it must fracture. Sedimentary and metamorphic rocks, on the other hand, tend to exfoliate along predetermined planes.

[Note: Granite is a coarse-grained intrusive igneous rock ]

Here, we will discuss different forms of physical weathering, like, Frost Wedging, Rock Exfoliation, Biologic Weathering, and Mass Wasting.

Frost Wedging

Frost wedging is the process by which water seeps into cracks in a rock, expands on freezing, and thus enlarges the cracks. The effectiveness of frost wedging depends on the frequency of freezing and thawing (melting).

Frost wedging is most effective in a climate like Canada’s.

In warm areas and in very cold areas Frost Wedging has limited effect. Because in warm areas freezing is infrequent, and in very cold areas thawing is infrequent.

Again, in very dry areas, there is little water to seep into cracks. Hence the role of frost wedging is limited in very dry areas.

In many parts of the Sierra, the transition between freezing nighttime temperatures and thawing daytime temperatures is frequent, tens to hundreds of times a year. 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.

Rock Exfoliation

Rock Exfoliation is a process in which large flat or curved sheets of rock fracture and are detached from the outcrop due to pressure release. In other words, Rock Exfoliation is the separation of successive thin shells or spalls from massive rock such as granite or basalt. It is common in regions that have moderate rainfall. The thickness of the individual sheet or plate may be from a few millimeters to a few meters.

The study of thin shells that separate from rock exposed to the weather reveals a common cause of the separation as the slow development of clay minerals, which involves an increase in volume. The outer surface of exposed rock dries rapidly after wetting, but the moisture that penetrates minor crevices stays until some decay is started, and the resultant swelling causes flaking roughly parallel to the outer rock surface.

Biologic Weathering

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

Mass Wasting

Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, sand, regolith, and rock move downslope typically as a solid, continuous, or discontinuous mass, largely under the force of gravity, but frequently with characteristics of flow as in debris flows and mudflows. Types of mass wasting include creep, slides, flows, topples, and falls, each with its characteristic features, and taking place over timescales from seconds to hundreds of years.

Chemical Weathering

Chemical weathering results from chemical changes to 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 the environmental conditions. Some minerals, like quartz, are virtually unaffected by chemical weathering, while others, like feldspar, are 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 produces weak carbonic acid when combined with water.

That process, which is fundamental to most chemical weathering, can be shown as follows:

H2O + CO2 —>H2CO3

then H2CO3 —> H+ + HCO3,

[water + carbon dioxide —> carbonic acid
then carbonic acid —> hydrogen ion + carbonate ion]

Here we have water, plus carbon dioxide in the atmosphere, combining to create carbonic acid. Then carbonic acid dissociates (comes apart) to form hydrogen and carbonate ions.

The amount of CO2 in the air is enough to make only very weak carbonic acid, but there is typically much more CO2 in the soil, so water that percolates through the soil can become significantly more acidic.

Role of Acids in chemical weathering

The most effective agent of chemical weathering is acid. Acids are chemical compounds that give off hydrogen ions when they dissociate, or break down, in water. Strong acids produce a great number of hydrogen ions when they dissociate, and weak acids produce relatively few such ions.

The most important natural source of acid for rock weathering at Earth’s surface is dissolved carbon dioxide (CO2 ) in water. Water and carbon dioxide form carbonic acid (H2CO3), a weak acid that dissociates into the hydrogen ion and the bicarbonate ion. Even though carbonic acid is a weak acid, it is so abundant on Earth’s surface that it is the single most effective agent of chemical weathering.

Chemical Weathering - weak carbonic acid when combined with water

Weathering
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