Social Studies GEOGRAPHY Subject Teacher : A Ellan WELCOME TO Grade 9

WEATHERING Three types of weathering: Physical weathering Chemical weathering Biological weathering

Physical Weathering / Mechanical Weathering This happens when rocks are broken down into smaller pieces through expansion and contraction. Rocks expand and contract due to temperature changes. Physical weathering due to temperature changes usually happens in areas where there is a large temperature range (hot in the day and very cold at night). There are TWO types of physical weathering that happen as a result of expansion and contraction:

1. Freeze-thaw weathering (Frost shattering) Water is trapped in the joint (crack) of a rock. At night, when the temperature drops below 0°C , the water that is trapped in the rock becomes froze, it expands. During the day, as the temperature increases, the water melts, causing the rock to contract. After some time, if this happens repeatedly, the rock will weaken and eventually shatter into pointed fragments. These pointed (angular) fragments are then deposited at the foot of the slope and are known as scree.

2. Exfoliation Repeated heating and cooling can cause the outer layers of a rock to peel off in thin sheets.

Chemical weathering This happens when rocks are exposed to acid . Acid rain is the main cause of Chemical Weathering. As rain falls from clouds it absorbs the carbon dioxide in the atmosphere . The water and carbon dioxide mix to form an acidic rain. This acid from the rain enters the cracks of rocks widening them. . The acid also dissolves or eats away the rocks causing them to become weaker and break apart.

1. Dissolved Rain water can dissolve limestone out of rocks. These rocks become weak and begin to break up. The limestone surface is called a limestone pavement and is weathered to form grooves called grikes and flat-topped ridges called clints. Limestone caves can also result when the underground limestone is dissolved and removed (for example, the Cango Caves in Oudtshoorn).

Honeycomb weathering

3. Plant acids Organic acids which are secreted (given off) by plants result in rocks breaking down chemically.

(c) Biological weathering Caused by living animals 1. Growth of roots   This is often the most common type of biological weathering. The roots of trees grow into cracks and joints, breaking rocks apart.

2. Animals burrowing Animals who burrow underground to make home often disturb rocks, which can result in rocks breaking up.

Location of weathering types Where it usually happens Physical (Exfoliation, Pressure release) Hot deserts (there are big differences between daytime and night time temperatures) Freeze-thaw Very cold areas (daytime temperatures are above freezing point, but night time temperatures drop below 0°C) Chemical In all areas where there is rain but more commonly in hot areas where there is high rainfall (because heat speeds up chemical reactions)

Impact of human activities on weathering Industries (factories), power stations and vehicle emissions have significantly increased chemical pollution in the atmosphere which has greatly increased the rate of weathering.

Acid rain, which is able to erode rocks such as limestone, is caused by the increase of gases such as carbon dioxide, nitrogen oxide and sulphur dioxide. These gases increase the rate chemical weathering processes. Acid rain causes limestone buildings to deteriorate. It also causes metals to rust. Granite is a very strong rock so buildings which are starting to deteriorate are given a granite covering.

Now for an activity!!! Complete activity 3.1 and 3 .2 in your textbook .