Chapter 7: Arid or Desert Landforms
This chapter explores how landscapes are shaped in regions with very little rainfall (less than 25 cm annually). In deserts, wind is a major agent, but surprisingly, water (via sudden flash floods) is often the most powerful force in carving the land.
1. The Mechanism of Wind Erosion
Wind erodes the desert floor in three distinct ways:
- Deflation: The wind blows away loose, dry particles (dust and sand). This lowers the ground level, sometimes creating large depressions called Deflation Hollows (e.g., the Qattara Depression in Egypt).
- Abrasion: The wind uses sand grains as “sandpaper” to grind against rock surfaces. Since sand is heavy, most abrasion occurs within 3 feet of the ground, polishing and pitting rock faces.
- Attrition: Sand grains collide with each other while being blown, becoming smaller, smoother, and more rounded over time.
2. Landforms of Wind Erosion
- Rock Pedestals (Mushroom Rocks): Because wind abrasion is strongest near the ground, the base of a rock is eroded more than the top, leaving a mushroom-like shape.
- Zeugens: Horizontal layers of hard and soft rocks. The wind wears away the soft layers into furrows, leaving the hard layers as ridges.
- Yardangs: Similar to Zeugens, but the layers of hard and soft rock are vertical (parallel to the wind direction). The wind carves the soft rock into long, narrow corridors.
- Inselbergs: Isolated, steep-sided “island mountains” that rise abruptly from a level plain. They are the remnants of an old plateau (e.g., Ayers Rock in Australia).
- Ventifacts: Stones smoothed and flattened on one side by wind-driven sand.
3. Landforms of Wind Deposition
When the wind slows down or meets an obstacle, it drops its load of sand.
- Dunes: Hills of sand. They are not stationary but “migrate” over time.
- Barchans: Crescent-shaped dunes with “horns” pointing downwind. They form in areas where the wind direction is constant.
- Seifs (Longitudinal Dunes): Long, narrow ridges of sand that run parallel to the direction of the prevailing wind.
- Loess: Very fine, fertile dust blown far beyond the desert boundaries and deposited in thick layers (e.g., the Loess Plateau in China).
4. The Role of Water in Deserts
Rain in deserts is rare but comes in violent torrents. Because there is no vegetation to hold the soil, the water causes massive erosion.
- Wadis: Dry, steep-sided valleys that only fill with water during a flash flood.
- Pediments: Gentle, rocky slopes at the foot of mountains, carved by the action of running water.
- Bajadas: A continuous “apron” of sediment formed when several alluvial fans (deposits from mountain streams) merge together.
- Playas (Salinas): Shallow, temporary lakes in the center of a desert basin. After the water evaporates, they leave behind a crust of salt.
5. Types of Deserts
Deserts aren’t just “sand.” They are classified by their surface material:
- Hamada (Rocky Desert): Consists of large stretches of bare rocks (e.g., parts of the Sahara).
- Reg (Stony Desert): Covered with gravel and pebbles; often called “desert pavement.”
- Erg (Sandy Desert): A vast “sea of sand” with high dunes (e.g., the Empty Quarter in Arabia).
- Badlands: Highly eroded regions with deep gullies and jagged ridges, making them “bad” for travel or farming.
Quick Revision: Most Important Facts
- Wind is the primary agent of transportation, but Water is the primary agent of major erosion in deserts.
- Barchans point away from the wind; Seifs run with the wind.
- Loess is the most productive soil derived from desert processes.
