TMJC 2025 JC1H2 C2 Lecture 7 Karst Landscapes

1 Higher 2 (9173) Theme 1: Tropical Environments - Physical Processes in the Tropics TMJC C2 Lecture 7 Karst Landscapes in Humid Tropics 2025 I INTRODUCTION - LIMESTONE 1. Characteristics of limestone: Characteristics Limestone Rock Type Non-Clastic Sedimentary Rock Chemical Composition > 80% Calcium Carbonate Texture Coarse-grained crystalline rock Structure Rectangular jointing: Thick beds separated by almost horizontal bedding planes with vertical joints Primary Permeability (Porosity) No (Non-porous) Secondary Permeability (Perviousness) Yes (Pervious due to rectangular jointing) Susceptibility to Chemical Weathering Carbonation-Solution (Dissolution) Limestone Structure 2. Limestone is a rock mainly made up of the mineral calcium carbonate. It is not porous (water cannot pass through the rock via pore spaces) but is pervious (water can pass along the bedding planes and down its joints). This allows water to penetrate the rock readily and allow chemical weathering to occur along the joints and bedding planes. 3. Limestone is susceptible to chemical weathering (processes of dissolution). This process occurs when limestone comes into contact with slightly acidified rainwater (containing dissolved carbon dioxide). The solid calcium carbonate reacts readily with the carbonic acid to form calcium bicarbonate, which is readily soluble in water. This reaction causes the limestone to “dissolve” and will widen existing cracks and joints in the rock. 4. The processes of dissolution and re -precipitation of calcium carbonate are critical in the formation of karst (distinct limestone) landscapes.

2 II LIMESTONE FEATURES FOUND IN TROPICAL AREAS 1. A doline (also known as a sinkhole) is a closed depression which may range in shape from bowl-shaped to cylindrical and in size up to 100m deep and 1km across. There are various types of doline. a. A collapse doline is formed when the roof of a cave falls to leave a steep-walled cavity. b. A solution doline is formed when there is concentrated carbonation - solution weathering occurring at the intersection of major joints in the limestone. This would result in the rock surface being lowered. The initial development of the depression leads to the capture of mor e surface drainage and this in turn promotes further doline enlargement. (a) Collapse doline (b) Solution Doline 2. Underground caves (caverns) may form where there is sufficient volume of water underground to form streams (consisting of mildly acidic water) that erode and dissolve the surrounding limestone as they flow along joints and bedding planes (Note: remember limestone is non -porous). This is assisted by heavy rainfall which quickly infiltrates downwards causing caves and linking passages to be filled with water. Rivers that are formed continuously make their way downwards to a lower level thro ugh joints and aban don the caves until they reach an underlying impe