TMJC 2025 JC1H2 C2 Lecture 6 Rocks Weathering

1 Higher 2 (9173) Tropical Environments TMJC Lecture 6 Weathering Processes 2025 (A) WHAT ARE ROCKS? 1. A rock is made up of various types of minerals or multiple individual grains of the same type of mineral. Minerals may be elements or compounds. 2. The combination of elements and the crystal structure give each mineral its character such as hardness, colour, density and other properties. For example, the mineral quartz ( SiO2) (Fig. 1) consists of Earth’s two most abundant elements, silicon (Si) and oxygen (O). Fig. 1: Quartz crystal (SiO2) 3. The appearance and physical properties of the rock then depend on: a. the kinds of minerals they contain, b. the amounts of different minerals they contain, and c. how the grains of these minerals are held together. For example, the rock granite is mainly made up of the minerals m ica, feldspar and quartz (Fig. 2) held together in a crystalline structure. Fig. 2: Granite 4. Minerals combine in various ways to produce different types of rocks. Thousands of different rock types have been identified.

2 (B) WHAT IS WEATHERING? 1. Weathering refers to the process of disintegration (physical break down) or decomposition (chemical decay) of rocks in situ1 at or near the Earth’s surface. It is the result of rocks interacting with their environment. 2. Weathering creates loosened rock fragments which are then ready for movement under the influence of gravity (mass movement), or for removal by the agents of erosion such as running water, moving ice and wind. 3. There are two main types of weathering: a. physical weathering & b. chemical weathering. It should be noted that in any given area and time, both physical and chemical weathering are occurring and complement each other (Fig. 13) (except under very specific conditions e.g. very cold or very dry conditions), although there may be a dominant form of weathering. For example, granite may undergo both pressure release (a form of physical weathering) as well as hydrolysis (a form of chemical weathering). Fig. 13: Relationship between Physical and Chemical Weathering 1 In situ means in its original position with little or no movement of material.

3 (C) WHAT ARE THE VARIOUS TYPES OF PHYSICAL WEATHERING PROCESSES? 1. Physical weathering (also known as mechanical weathering) is the mechanical disintegration of rocks into smaller fragments without involving any change in the chemical composition of the rocks. Only the size and shape of the rock changes. 2. The broken rock fragments are usually angular debris or blocks. 3. Physical weathering is most dominant in areas with great diurnal range of temperature, notably the desert regions and high mountains. 4. Physical weathering encourages chemical weathering by increasing the rock’s surface area and exposing more of the rock to possible chemical weathering. 5. There are four main types of physical weathering processes. They are frost shattering, thermal fracture, pressure release and salt crystallization. (i) Freeze-Thaw Weathering (also known as Frost Shattering) 1. Freeze-Thaw Weathering is the mechanical breakdown of rock due to stress brought about by repeated freezing and thawing of water. 2. Freeze-Thaw Weathering is predominant in areas with plentiful of moisture and there are daily temperatures fluctuating above and below 0oC, notably in high mountains in temperate latitudes. 3. During the daytime, ice thaws when the temperature rises above freezing point. The water seeps into the cracks or joints of the rocks. 4. At night, the temperature drops to below freezing point. When water freezes it expands by about 9% and exerts great pressure on the rock walls. 5. The repeated growth and melting of ice crystal in the joints or fractures of rocks, or repeated and alternate freeze -and-thaw, cause the rocks to loosen and break down into smaller, angular fragments (Fig. 14). 6. Thus, the most potent circumstances for frost shattering are those where there is temperature fluctuation above and below the freezing point , as compared to exceptionally cold areas where water is permanently frozen.

4 Fig. 14: Frost Shattering (ii) Thermal Fracture (also known as Insolation Weathering) 1. Thermal fracture is the mechanical breakdown of rocks due to differential rates of expansion and contraction within the rock. 2. It occurs in areas with a great diurnal range of temperature such as in deserts, where the lack of cloud and vegetation cover results in a great diurnal range of temperature that can exceed 50 oc. Research by Blackwelder and Griggs has also shown that the rate of weathering increases with the addition of a layer of moisture on the rock’s surface. 3. Due to the poor thermal conductivity of rocks , the outer layers of the rocks warm up faster and cool more rapidly than the inner ones. This causes stress within the rock and result in the outer layers to peel off like the layers of an onion. This process is called exfoliation (Fig. 15). Fig. 15: Exfoliation

5 4. A particular rock may also be made up of different types of minerals. Since different minerals have different rates of expansion and contraction, this creates stress in the rock and may result in granular disintegration where individual grains break away from the rock (Fig. 16). Fig. 16: Granular Disintegration (iii) Pressure Release (also known as Dilatation) 1. Pressure Release/Dilatation is the mechanical breakdown of rocks due to the removal of overlying pressure on rocks, causing tensional stress to develop cracks within the rock. 2. Rocks formed deep within the Earth’s crust such as intrusive igneous rocks are under great confining pressure from the weight of thousands of meters of overlying rocks. Pressure release is most common with granitic rock. 3. As this overlying cover is removed by erosion, the underlying rocks are exposed. The confining pressure is released and buried rock body tends to expand outwards where the pressure has decreased (Fig. 17). 4. The expansion of the rock causes internal tensional stresses in the rock and results in cracks or joints parallel (curvilinear joints) to the surface of the Earth, forming layers. This is known as sheeting. These cracks and joints can later be attacked to cause rocks to peel off on the surface of the exposed rocks through exfoliation. Fig. 17: Pressure Release

6 (iv) Salt Weathering / Salt Crystallization 1. Salt weathering / salt c rystallization occurs when soluble minerals solidify in the pores/cracks of rocks and exert mechanical stress. 2. In arid climates, dry weather draws moisture draws water up to the ground surface and enters the rocks through capillary action. This water usually contains dissolved salts. 3. When the water evaporates, crystallization occurs as salt crystals are precipitated. 4. Over time, as the salt crystals grow and enlarge, they exert a force great enough to begin weakening the internal structure of the rock. 5. The cracks and joints in the rock become widened and the rock would start breaking up through granular disintegration. 6. Physical weathering by crystal growth is notable in desert regions where the rate of evaporation is high. Salt crystallization also occurs on coasts where there is a constant supply of water and salt. Due to the climatic conditions (e.g. low rainfall, seasonality in temperature, higher diurnal, temperature range) in the arid tropics, physical weathering is more dominant in arid tropics than humid tropics. (D) WHAT ARE THE VARIOUS TYPES OF CHEMICAL WEATHERING PROCESSES? 1. Chemical weathering is the decomposition of solid rocks as a result of chemical reactions between the rock minerals and agents in the environment such as air, water and organic acids. 2. The original chemical composition and structure of the