SAJC Cluster 2.2 Lecture 8 Notes (Geomorphic Processes Pt.2: Weathering Processes)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster 2_Lecture 11_2025/pg1 Lecture 11 Geomorphic Processes (II): Weathering Processes KEY QUESTIONS: ✓ What influences the weathering of rocks? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Various chemical and physical weathering processes acting on rocks in the tropics • Factors influencing the weathering of rocks in the tropics Lecture Outline 11.1 Introduction 11.1.1 Types of Weathering Processes 11.2 Chemical Weathering Processes 11.2.1 Carbonation 11.2.2 Hydrolysis 11.2.3 Oxidation and Reduction 11.2.4 Solution 11.3 Physical Weathering Processes 11.3.1 Thermal Weathering 11.3.2 Freeze Thaw 11.3.3 Salt Weathering 11.3.4 Pressure Release 11.4 Factors Influencing Weathering of Rocks 11.4.1 Climate a) Cold regions (e.g. high altitudes in the Tropics) b) Hot and arid regions c) Hot and wet regions 11.4.2 Rock Characteristics (or Geology) a) Rock Composition b) Rock Structure c) Rock Texture 11.4.3 Vegetation a) Bio-physical weathering b) Bio-chemical weathering 11.4.4 Topography a) Steepness of slope 11.4.5 Human Activities a) Mining activity b) Construction of roads and residential areas c) Urbanised impermeable land surfaces d) Emissions from industries and automobiles 11.5 Conclusion

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster 2_Lecture 11_2025/pg2 11.1 Introduction 11.1.1 Types of Weathering Processes • Weathering is the process of breakdown of rock by disintegration and decomposition in situ (in its original position), at or near, the earth’s surface. Weathering does not involve the removal of the weathered material. Weathering is a slow process, especially in resistant rocks, but its effects are evident on any exposed rock mass. • There are two main types of weathering processes namely chemical and physical weathering which can be further divided into sub-processes (see Fig. 1): Fig. 1: Weathering Processes in the Tropics o Chemical weathering occurs when the original mineral in the rock decomposes or decays in situ due to chemical alteration of those minerals (see Section 11.2). o Physical weathering occurs when rocks are broken and disintegrated into smaller pieces in situ without any chemical alteration (see Section 11.3). • Although both types of weathering may occur together, usually one predominates. The type and degree of weathering depends on the local climate, vegetation, structure and mineral composition of the rocks and the length of time during which the weathering process operate. • The pattern and products of weathering are therefore different in various parts of the tropics. o In the humid tropics (A climates), chemical weathering is more effective as it is accelerated in the presence of high temperature and high rainfall. o In the arid tropics (B climates), where rainfall is very low and there is a high diurnal temperature range, physical weathering is more effective and rocks tend to break up into smaller components by physical breakdown. Physical weathering is also dominant in high altitudes such as high mountains where temperature fluctuates above and below 0°C. o Whatever the processes involved, a number of characteristics that are common: 1. Weathering produces irreversible changes in rocks from a massive state to a clastic (made of fragments, like scree) or plastic (pliable, like clay) state. 2. Rocks experience a number of changes during weathering in characteristics such as volume, density, grain size, surface area, permeability, consolidation and strength. 3. New minerals, aggregates and solutions are formed. 4. Some original minerals may be resistant to weathering and persist in their original state. 5. Minerals and salts may be transferred, dispersed, consolidated or concentrated. 6. Weathering prepares the rock for subsequent erosion and movement downslope. 7. New land surfaces and deposits are produced after weathered materials are removed.

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster 2_Lecture 11_2025/pg3 11.2 Chemical Weathering Processes • Chemical weathering involves the breakdown of rock minerals through interactions with agents like water, oxygen, carbon dioxide, and various organic acids. These reactions result in changes to the mineral composition, volume, strength, and coherence of the rock. • Chemical weathering processes are typically selective, affecting some minerals while leaving others unaffected, particularly in complex crystalline rocks. • Nearly all minerals can undergo chemical alteration when exposed to atmospheric and biological agents. While some minerals, such as quartz, are highly resistant to chemical changes, many others are more vulnerable. Few rocks remain entirely unaffected by chemical weathering, as the alteration of even a single key mineral can lead to the disintegration of the entire rock. • Moisture is essential for nearly all forms of chemical weathering, which has significant implications: o Chemical weathering operates more effectively in humid tropical regions (e.g., Af and Am climates) due to the abundance of water, compared to arid tropical regions. o Unlike physical weathering, chemical weathering typically occurs at greater depths, as water can penetrate deeper into the ground, whereas temperature fluctuations, crucial for many physical weathering processes, are limited to the surface. • The products of chemical weathering include "residual" decomposition products and "soluble" products, which are carried away by percolating water and eventually reach rivers. o Generally, the products of chemical weathering are finer than those produced by physical weathering. Chemical weathering is often accelerated by physical weathering, which breaks rocks apart and increases the surface area exposed to chemical agents. • Temperature is another key factor influencing the rate of chemical weathering: o In general, higher temperatures speed up chemical weathering. Studies indicate that the rate increases 2.5 times for every 10°C rise in temperature (up to 60°C). However, this does not imply that chemical weathering does not occur in colder regions. 11.2.1 Carbonation • Carbonation involves the reaction of minerals with dissolved carbon dioxide in water. • Carbon dioxide from the atmosphere or soil is absorbed by water to form a weak acid, referred to as carbonic acid. This acid is strong enough to dissolve many minerals, especially calcium carbonate present in limestone, in the process of carbonation. o When rainwater reacts with limestone, calcium carbonate changes into a solution known as calcium bicarbonate and is removed easily in solution (see Section 11.2.4) thus breaking down the rock (see Fig. 2). Calcium carbonate + Carbonic acid  Calcium bicarbonate

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2 Cluster 2_Lecture 11_2025/pg4 • The effects of carbonation will be examined in Lect. 13 when we study karst landscapes. 11.2.2 Hydrolysis • Hydrolysis is the complex chemical reaction which happens when hydrogen (H+) ions in the water react with the silicate minerals present in the rock to create new compounds which tend to be softer and weaker than the original rock material and thus the rock crumbles. o For example, granite is ma