External Landform Processes and Related Features
Senior 6 · Geography
External Landform Processes and Related Features
Lesson 3 of 5
25 min
Drive across northern Mozambique or central Tanzania and bare domes of granite rise abruptly from the flat bush, hundreds of metres high, like islands in a sea of grass. Cross the Sahara and you meet flat-topped hills with cliff edges, and rocks shaped like mushrooms. Both landscapes are made largely by weathering, but by very different kinds: deep chemical rotting in the humid tropics, and physical breakdown with little water in the deserts. This lesson compares their landforms.
Definition
Saprolite: rock that has rotted chemically in place but still shows the structure of the original rock; it can be dug with a spade. Weathering front: the boundary, often sharp, between the regolith above and fresh rock below. Corestone: a rounded block of fresh rock left inside the saprolite, where joints were far apart.
Under constant warmth and heavy rain, hydrolysis and oxidation work deep into the ground along joints. The regolith may be tens of metres thick. Where joints are close together the rock rots completely; where they are far apart, solid cores survive. If erosion later strips away the soft regolith, the fresh rock is exposed as upstanding landforms. This two-stage explanation (deep weathering, then stripping) accounts for tors and many inselbergs.
Figure 5: the two-stage formation of a tor.
Definition
Inselberg ("island mountain"): an isolated, steep-sided hill rising abruptly from a plain. A bornhardt is a large, dome-shaped inselberg of massive rock with few joints, usually granite or gneiss. A castle kopje (koppie) is a smaller hill crowned by piles of angular blocks. A tor is a pile of joint blocks standing on a hilltop or slope.
Where the climate has a wet and a dry season, iron and aluminium oxides released by weathering are carried up and down in the soil and concentrated in one layer. On exposure to the air this layer hardens into a rock-like crust, a duricrust. The commonest in tropical Africa is laterite (ferricrete), rich in iron oxides, which gives the deep red colour. A laterite crust protects the land beneath it, so it often caps flat-topped hills and plateau edges. Laterite rich in aluminium oxide is bauxite, the ore of aluminium.
Worked example 1: explaining a bornhardt
Question. Explain why a granite plain in a humid tropical region may have a few huge, bare domes rising from it.
Answer. (1) The granite has joints spaced very unevenly. (2) Over a long period of warm, humid climate, hydrolysis rotted the granite deeply where joints were close together, producing a thick saprolite, but hardly penetrated the parts with few joints. (3) Later, when erosion increased (for example after uplift or a change to a drier climate), rivers and sheetwash stripped away the soft saprolite and lowered the plain. (4) The massive, unjointed blocks were left standing as domes. Once exposed, they shed water quickly and stay dry, so they weather more slowly than the plain around them, and stand ever higher.
In hot deserts there is too little water for much chemical weathering. Rocks break down mainly by salt crystallisation, heating and cooling, and the small amounts of moisture from dew and rare storms. Bare rock, angular fragments and cliff edges dominate. Wind removes fine particles and, where it carries sand, sandblasts rock near the ground; occasional floods do much of the shaping.
Figure 6: landforms of hot deserts.
| Landform | Description | Main processes |
|---|---|---|
| Mesa and butte | Flat-topped hills with cliff sides; a mesa is wider than it is high, a butte narrower | A resistant cap rock protects softer rock below; the cliffs retreat by weathering, rockfall and sheet floods |
| Desert inselberg | Isolated steep hill rising from a plain | Retreat of slopes around a resistant core |
| Pediment | A gently sloping rock surface at the foot of a mountain front | Retreat of the mountain front by weathering and sheet floods |
| Pedestal (mushroom) rock | A rock cap on a narrow neck | Weathering and sand abrasion concentrated near the ground |
| Honeycomb weathering (tafoni) | Rock surfaces pitted with hollows | Salt crystallisation |
| Desert pavement (reg) | A surface of closely packed stones | Wind removes the fine particles between the stones |
| Zeugen and yardangs | Ridges of rock separated by corridors: zeugen in flat-lying layers, yardangs streamlined parallel to the wind | Wind abrasion of weaker rock |
Worked example 2: how fast does a cliff retreat?
Problem. A mesa is 1.2 km wide. If its cliffs retreat 0.5 m per 1,000 years on each side (an illustrative rate), how long will it take to shrink to a butte 200 m wide?
Solution. The width must fall by m. Both sides retreat, so the width falls by m per 1,000 years.
Desert landforms evolve over hundreds of thousands to millions of years.
| Humid tropics | Hot deserts | |
|---|---|---|
| Main weathering | Chemical: hydrolysis, oxidation | Physical: salt, heating; little chemical |
| Regolith | Deep (often tens of metres) | Thin or absent; bare rock common |
| Slopes | Rounded, covered by soil and vegetation | Angular, cliffed, with rock debris |
| Typical landforms | Bornhardts, tors, castle kopjes, laterite-capped plateaus | Mesas, buttes, pediments, pedestal rocks, desert pavement |
Warning
Don't credit the wind with everything. Wind abrasion only works where the wind carries sand, mostly within a metre or two of the ground. Most desert landforms owe more to weathering and to rare but violent floods than to wind. Likewise, an inselberg is not a volcano: it is a resistant rock mass left standing by weathering and erosion.
Tip
Exam technique. For a weathering landform, always name the climate, the dominant process and the role of rock structure (joint spacing, cap rock). "Joint spacing controls where the rock rots" is the key idea behind tors and bornhardts.
Annerie Laurens: "Formation of Granite Domes and Tors". A short lesson on how granite domes and tors form. Compare its stages with Figure 5.
Practice
Summary
Sources and credits
Figures 5 and 6, the worked examples and the exercises were drawn and written by Ganzaa.
Video: "Formation of Granite Domes and Tors", Annerie Laurens, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
