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 p