Geography (9696) | Landforms of tropical environments

Welcome to Tropical Landforms! (9696 Advanced Physical Option)

Hello Geographers! This chapter explores some of the most dramatic and unique landscapes on Earth—those shaped by the intense heat and moisture of the tropics. Don't worry if the names seem complicated; we will break down the formations step-by-step.

In tropical environments, the high temperatures and abundant rainfall supercharge chemical reactions (especially weathering), creating deep, unusual landforms that look very different from those in temperate or arid zones. Understanding these environments is crucial for Paper 3!

Quick Review: Why the Tropics are Special for Geomorphology

The tropics are characterised by high temperatures (which increase chemical reaction rates) and high rainfall (which provides the water needed for chemical processes like solution and hydrolysis).

The result? Chemical weathering is dominant and extremely fast, often penetrating deep beneath the surface, setting the stage for the unique landforms we are about to study.

Section 1: Landforms on Granite

Granite is an igneous rock, meaning it’s typically very hard and resistant. However, in the tropics, it meets its match in the form of chemical weathering, specifically hydrolysis.

1.1 Deep Weathering Profiles (Saprolite)

This concept is the foundation for almost all tropical granite landforms.

What is it?

A deep weathering profile is a thick layer of chemically altered, rotten rock that lies above the fresh, unweathered bedrock.

• This rotten rock is known as saprolite (from the Greek word "sapros," meaning rotten).
• In temperate areas, weathering might only reach a few metres deep. In the humid tropics, saprolite layers can easily be 50 metres or more thick!

How does Saprolite Form?

Granite mainly contains quartz, mica, and feldspar.

The key chemical process is hydrolysis:
1. Rainwater mixes with acids (like carbonic acid) and penetrates deep into the granite bedrock along tiny cracks and joints.
2. The water reacts chemically with the feldspar, breaking it down into soft, clay minerals (like kaolinite).
3. The resulting saprolite is porous, easily crumbled, and no longer structurally strong, though it still retains the structure of the original rock.

Key Takeaway: The Two-Stage Theory

The formation of granite landforms is best explained by the Two-Stage Theory (developed by geologists like Linton and Thomas):
1. Stage 1 (Weathering): Deep chemical weathering (hydrolysis) occurs below the surface, creating the saprolite mantle, leaving behind isolated, resistant blocks (core stones) where joints were wider or fewer.
2. Stage 2 (Exhumation/Stripping): Climatic or environmental change leads to the rapid removal (erosion) of the overlying soft saprolite, exposing the previously hidden, resistant core stones.

1.2 Characteristic Granite Landforms

A. Tors

Definition: Isolated, residual masses of jointed rock (granite), often looking like piles of naturally stacked boulders, perched atop a hill or ridge.

Formation (Applying the Two-Stage Theory):
1. During Stage 1, water concentrates along the vertical and horizontal joints in the granite.
2. Where joints are close together, the rock is thoroughly weathered into saprolite.
3. Where the joints are far apart, the central blocks (core stones) are much less weathered and remain relatively fresh.
4. During Stage 2 (stripping), the surrounding saprolite is washed away, leaving the resistant core stones exposed as a tor.
Examples often include the tors seen in the Malaysian lowlands or even some areas of Nigeria.

B. Inselbergs

Definition: A general term for isolated hills or steep-sided ridges rising abruptly from a gently sloping, relatively flat plain (the etched surface). They literally mean 'island mountains' in German.

Formation: Inselbergs are simply large-scale results of the two-stage process. They represent massive blocks of resistant rock that survived the deep subsurface weathering and were subsequently exposed when the surrounding, less resistant saprolite was removed.

C. Bornhardts

Definition: A specific, distinctive type of inselberg characterised by a smooth, steep, dome-shaped surface. They often have bare, rounded sides and lack the stacked, blocky structure of a typical tor.

Formation:
1. Bornhardts are formed from single, massive, resistant cores of granite where the joints were exceptionally widely spaced.
2. The rounded, dome shape is often attributed to a process called pressure release (dilatation). As the immense weight of the overlying rock and weathered material (saprolite) is removed, the rock beneath expands and flakes off in curved layers (like an onion peel). This is a form of mechanical weathering that maintains the dome shape.
The iconic Sugarloaf Mountain in Rio de Janeiro, Brazil, is a classic example of a bornhardt.

Section 2: Landforms on Limestone (Tropical Karst)

Karst landscapes form on soluble rock, primarily limestone (calcium carbonate). The defining process here is solution weathering, known as carbonation.

2.1 The Tropical Karst Environment

While temperate zones also have karst (like underground caves and sinkholes), tropical karst is unique due to the speed and intensity of chemical attack.

Why Tropical Karst is Faster:

1. Heat: Higher temperatures speed up chemical reactions.
2. Rainfall: Massive amounts of rain provide the solvent (water).
3. Vegetation: Dense tropical rainforest vegetation releases huge amounts of carbon dioxide (CO₂) into the soil, significantly increasing the acidity of the soil water and ground water. This highly acidic water speeds up the solution of limestone.

The Key Process: Solution (Carbonation)

The basic chemical reaction is:

Carbon Dioxide (\(CO_2\)) + Water (\(H_2O\)) \(\rightarrow\) Carbonic Acid (\(H_2CO_3\))
Carbonic Acid + Limestone (\(CaCO_3\)) \(\rightarrow\) Calcium Bicarbonate (Soluble)

This process dissolves the rock, often leading to distinct surface landforms rather than just deep cave systems.

2.2 Characteristic Limestone Landforms

A. Cockpit Karst

Definition: A landscape characterised by numerous, steep-sided, star-shaped depressions (the cockpits) separated by residual, smooth, conical hills (the cones or Kegelkarst).

Think of it: It looks like a giant, irregular egg carton, where the dips are the cockpits and the peaks are the cones.

Formation:
1. Intense solution occurs where water is concentrated, often in the valley bottoms and along major joints.
2. The dissolving rock forms deep, concave depressions (the cockpits).
3. The hills that remain, called cone karst, are the residual areas where solution was less concentrated.
A classic example is the Cockpit Country in Jamaica.

B. Cone Karst (Kegelkarst)

Definition: The term specifically referring to the residual, conical, steep-sided hills found within a cockpit karst region.

Note: These hills are formed not by uplift or folding, but purely by the surrounding rock being dissolved and washed away. They are what is *left over*.

C. Tower Karst (Turmkarst)

Definition: A landscape where the limestone hills stand as dramatically steep, massive, near-vertical towers, rising abruptly from an otherwise flat, alluvial (river-deposited) plain.

Think of it: The dramatic scenery of Guilin, China, or Ha Long Bay, Vietnam.

Formation (Evolution from Cockpit Karst):
1. The process starts similar to Cockpit Karst, forming cones and cockpits.
2. Over long periods, the cockpits widen and deepen. Rivers flowing through these basins deposit sediment, creating wide, flat, alluvial plains.
3. Crucially, lateral erosion (sideways attack) becomes dominant. Water and sediment flowing across the flat plain aggressively undercut the base of the conical hills.
4. This undercutting erodes the lower sections of the cone hills, making their sides steeper and creating the distinct vertical ‘tower’ appearance, isolated by the wide, flat plain.

Quick Review Summary Table

To ensure you keep the concepts separate, here is a breakdown of the rock type and dominant processes:

You've successfully covered the complex geomorphology of tropical environments! Remember that heat and moisture are the engines driving these unique and dramatic landforms. Good luck with your revision!