Tropical Environments: Processes and Landforms

This section focuses on the key processes of weathering and the resulting landforms commonly found in tropical environments, particularly those associated with granite and limestone geology.

Weathering in Tropical Environments

Weathering is the breakdown of rocks, soils, and minerals through contact with the Earth's atmosphere, water, and biological agents. Tropical environments are characterized by high temperatures and abundant rainfall, which significantly accelerate weathering processes.

Physical Weathering

Physical weathering involves the mechanical disintegration of rocks without changing their chemical composition. Key processes include:

• Thermal Expansion and Contraction: Daily temperature fluctuations cause rocks to expand and contract, leading to stress and eventual fracturing.
• Freeze-Thaw Weathering: Although less common in consistently warm tropical areas, periods of cooler temperatures can cause water in cracks to freeze and expand, widening the cracks.
• Exfoliation: The outer layers of rock expand and peel off due to pressure release.
• Salt Weathering: In coastal tropical areas, salt crystals grow in rock pores, exerting pressure and causing disintegration.
• Biological Weathering: Roots of plants can grow into cracks, exerting pressure and breaking rocks apart. Animals can also contribute to physical weathering through burrowing and movement.

Chemical Weathering

Chemical weathering involves the alteration of rocks through chemical reactions. The high temperatures and humidity in tropical regions promote rapid chemical weathering.

• Hydrolysis: Water reacts with minerals, causing them to break down. For example, the weathering of silicate minerals.
• Oxidation: Minerals react with oxygen, often facilitated by the presence of water. This is particularly important for iron-rich minerals, leading to reddish-brown staining.
• Carbonation: Carbon dioxide in rainwater dissolves in water to form carbonic acid, which then reacts with rocks containing calcium carbonate (e.g., limestone), leading to dissolution.
• Solution: Some minerals are directly dissolved by water.

Granite Landforms in Tropical Environments

Granite is an intrusive igneous rock, typically forming large, exposed areas in tropical highlands. Weathering of granite produces distinctive landforms.

Weathering of Granite

Granite is relatively resistant to weathering but is still susceptible to both physical and chemical processes.

• Physical Weathering: Primarily exfoliation due to temperature changes and pressure release.
• Chemical Weathering: Hydrolysis of feldspar minerals is a significant process, leading to the formation of clay minerals and weakening the rock structure.

Common Granite Landforms

Landform	Description	Formation Process
Rounded Hilltops	Exfoliation processes create rounded, smooth hilltops.	Exfoliation due to temperature changes and pressure release.
Gneissic Bands	Alternating bands of different coloured rock within the granite, a result of metamorphic alteration.	Hydrolysis and alteration of minerals.
Rockfalls and Scree Slopes	Steep slopes formed by the accumulation of rock fragments broken off by weathering.	Freeze-thaw, biological weathering, and exfoliation.
Pediments	Broad, gently sloping platforms formed at the base of granite slopes.	Accumulation of weathered material.

Limestone Landforms in Tropical Environments

Limestone is a sedimentary rock composed primarily of calcium carbonate. Tropical environments with limestone bedrock are often characterized by karst landscapes, formed by the dissolution of limestone.

Weathering of Limestone

Limestone is highly susceptible to chemical weathering, particularly carbonation. The acidic rainwater dissolves the limestone, creating characteristic karst features.

Common Limestone Landforms

Landform	Description	Formation Process
Karst Topography	A landscape dominated by caves, sinkholes, and underground drainage systems.	Dissolution of limestone by acidic rainwater.
Sinkholes (Dolines)	Depressions in the ground caused by the collapse of underground caves.	Dissolution and collapse of underground cave systems.
Caves	Underground voids formed by the dissolution of limestone.	Dissolution of limestone along bedding planes and fractures.
Karst Ponds	Depressions in the landscape that fill with rainwater and groundwater.	Dissolution and collapse of underground cave systems.
Dry Valleys	Valleys where surface water drains away underground, leaving a dry valley floor.	Dissolution of limestone along bedding planes.