What is weathering?

Weathering is defined as a process by which the rocks on the surface of the earth are broken down into smaller pieces due to running water, ice, snow or frost, variation of temperature and pressure or due to chemical (dissolution) action on the materials. After rock is broken down into smaller pieces, a process known as erosion transports the pieces of rocks and the mineral away into another place.

Agents of weathering

Running water, frost, ice, snow temperature changes and pressure are all agents of weathering; animals dislodge rocks as well but rocks that are weathered in this way are not transported elsewhere. However, the nature of rocks and relief are also other contributing factors to weathering. There are three types of process that cause the alteration of rocks exposed on the surface of the Earth, they are physical, chemical and biological weathering. Physical and chemical weathering results in rocks break down into fragments; these rock fragments are called detritus. Physical or mechanical weathering and chemical weathering work hand in hand, mechanical processes give rise to more cracks and thereby a greater surface area. Chemical processes, which operate on the surface, are therefore enhanced by extensive cracks.

There are variety of processes that leads to physical weathering, rocks buried in the crust are under pressure because of the weight of the overlying deposits plus they are hotter at the surface (the increase in temperature with depth is usually in the range of 20-40⁰C/km). Joints form when pressure is reduced as a result of uplift and erosion. The type of joints depends on the kind of rock involved. Loose blocks of rocks resulting from jointing can accumulate at the base of the rock exposures to form talus. Several agents can cause joints to form fragments, these include frost-wedging- in this case, waters cracks and expands on freezing most effectively where freezing and thawing takes place rapidly, root-wedging, salt wedging, thermal expansion and animal and man influences.

Types of Weathering

1. Physical or mechanical weathering
2. Chemical weathering
3. Biological weathering

Physical or Mechanical Weathering

Physical weathering is a gradual disintegration of rocks without any chemical change involved, the effects of changing temperature on rocks, causing the rock to break apart is one of the causes of this process, hence, the process is sometimes assisted by water. Physical weathering is also called mechanical weathering; it mostly takes place where there is little soil and few plants growth, such as in the mountain regions and hot deserts. This can happen even with completely fresh rock but the process of physical weathering is able to work more easily when the surface of the rock is has already been weakened by the action of chemical weathering.

Types of Physical Weathering

• Repeated temperature changes
• Repeated wetting and drying and
• Frost action

1. Repeated temperature changes- this is very common in the dessert where rocks are exposed to direct blazing sun during the day. The outer layers of the heated rocks expand much faster than the cooler interior of the rocks. Similarly, at night when the temperature has dropped, the outer layer of the rocks cools more rapidly than the interior. This sets up stresses that make the outer layer of the rock to peel off. The pieces of rocks that peel off are called screes and are deposited at the base of the parent rock; the remaining parent rock is smoothened into a dome-shaped structure called exfoliation.
2. Repeated wetting and drying- all rocks absorb a certain amount of water, but some absorb more than others do. The absorption of water by surface rocks causes them to swell. When the rocks dry out, the outer surface of the rock shrinks. The alternate wetting and drying weakens the rocks and they begin to crack. This type of weathering is very common in parts of West Africa.
3. Frost action– this is a common agent of weathering in temperate regions of the world most commonly during the winter season. Frost action also takes place on some high mountains such as that of Himalayas. Frost action starts with water (H₂O) collecting in cracks within the rocks. This is because of the persistent cold weather in such regions, the water freezes into ice, when water freezes, its volume increase by about one-tenth of its original volume. If the water in the cracks freezes, a tremendous force is applied to the sides of the cracks. When this happens, the cracks widen, deepen, and eventually break up. Screes also form around the base of the rocky outcrops affected.

Picture showing widening of cracks as a result of frost wedging- a typical example of physical weathering

Chemical weathering

Chemical weathering takes place most rapidly in regions of humid climates with high temperature. It also occurs in other region where the rainfall is irregular; hence, it is the extremely slow and gradual decomposition of rocks due to air and water.

Major types of chemical weathering include

• Solution
• Oxidation
• Hydrolysis,
• Hydration and
• Carbonation

1. Solution- acidic rainwater attacks and dissolves rock salt. For example, rainwater dissolves Calcium carbonate (CaCO₃) of limestone which results in widening of the cracks and joints.
2. Oxidation- oxidation happens when oxygen combines with mineral. Oxidation takes place easily in rocks that contain iron oxides. The new minerals formed by oxidation are often attacked by other weathering processes; the structure of a rock in which iron and silicates are joined is completely broken down by oxidation.
3. Hydrolysis- this process involves hydrogen combining with certain metals ions (in a mineral) that is the water, and the new mineral then gives rise to the formation of different compounds. Hydrolysis is from hydration because feldspar is broken down leading to the formation of clay.
4. Hydration- this is a form of chemical weathering in which thechemical bonds of the mineral are changed as it interacts with water. Some absorbs water and in the process give rise to new compounds. For example, hematite and iron oxide combines with water to form limonite, another iron compound. Another example of this type weathering is the absorption of water by Calcium Sulphate to form gypsum-one of the most common minerals on Earth. However, in the above process, chemical change is not involved, the rock only absorbs water and this can be removed easily by heating.
5. Carbonation- this process of chemical weathering involves the reaction by hydrogen carbonate ions with a mineral to form a soluble compound which can be carried away in solution. Hydrolysis often accompanies carbonation because both break down feldspar into clay, soluble carbonate and silica.

The efficiency of chemical weathering increases as the surface area increases because of physical weathering

Biological Weathering

Plants and animals also contribute in breaking down rocks. Root of trees sometimes grows in cracks and the roots penetrate through to find moisture and stability. As they grow, they act as wedge and cause pieces of rocks to break away from the parent rock. Burrowing by animals like earthworm, millipedes, and rodents also helps to loosen the soil, even the tiniest bacteria, algae and lichens produce chemicals that help break down the rock on which they live, so they can get the nutrients they need, other animals dig and trample rock aboveground, causing rock to slowly break apart.The activities of man in road construction, mining and farming also contribute to biological weathering.

Picture of biological weathering: Roots of a plant penetrating through the cracks of rock to find moisture and stability

Factors that Affect Weathering

There are five (5) major factors that affects the weathering of rocks, they include

• Nature of rocks`
• Climate
• Relief
• Vegetation
• Mans activity

1. Nature of rock: it has been observed that various rocks react to weather conditions due to their mineral composition, texture and structure. For example, igneous rocks are more susceptible to chemical weathering due to their mineral composition such as angite, brolite and olivine which are dark in colour.
2. Climate: the two main factors that affect weathering under climate are temperature and other forms of precipitation, these two factors help in breaking down the rocks into smaller fragments as indicated in the humid and arid regions of the world.
3. Relief: in the mountain regions of the world, the freezing and thawing of ice due to temperature changes results in physical disintegration of rocks whereas the lower parts of the mountain are exposed to chemical weathering.
4. Vegetation: vegetation is a factor that influences weathering of rocks because it is responsible for the growth of forest formation favoured by hot or wet climate. The presence of these trees plants provides organic curis which hasten reactions on rock minerals thereby forcing them to breakdown.
5. Mans activity: man also contributes to weathering of rocks in several ways. Humans cause increases in acid rain and pollution, which increase the amount of weathering agents in the air, water and on land.Humans also affect the rate of weathering/erosion by paving over large portions of land, so that normal rain run-off is changed indirection and amount to places where it was previously not a problem. Other activities such as strip-mining and agriculture are also major causes of weathering influenced by man.