Geography (9230) | Hot desert landscapes

Welcome to the World of Hot Desert Landscapes!

Hello Geographers! This chapter takes us into one of the most extreme and fascinating environments on Earth: the hot desert. Don't worry if you imagine deserts are just endless sand dunes—they are actually incredibly diverse and shaped by powerful, often invisible, forces.
We will explore how intense heat, lack of water, and strong winds create unique landforms that define these incredible places. Understanding these physical processes is crucial for your exam, so let's dig in!

Quick Review: Defining a Hot Desert

The official definition of a desert is an area that receives less than 250 mm (10 inches) of precipitation (rain/snow) per year. Hot deserts are characterized by:

• High Temperatures: Extremely hot days (often over 40°C).
• Low Rainfall: Very little, often erratic rainfall.
• Diurnal Range: A huge difference between daytime and night-time temperatures.

Did You Know? Many major deserts, like the Sahara and the Great Victoria Desert, are found roughly 20–30 degrees north and south of the Equator, often due to global air circulation patterns (like the subtropical high-pressure zone).

Section 1: The Extreme Climate and Weathering

The unique climate of a hot desert acts like a relentless hammer, constantly breaking down the rocks. This process is called weathering. Since there is very little moisture, the dominant forces are physical (mechanical) rather than chemical.

1.1 Physical Weathering: The Rock Breakers

Physical weathering is the process of breaking down rock into smaller pieces without changing its chemical composition.

A. Exfoliation (Onion Skin Weathering)

This is the most important type of weathering in hot deserts, driven by the massive diurnal temperature range (the difference between day and night temperatures).

How it works (Step-by-Step):

1. Daytime Heating: The Sun rapidly heats the outer layer of the rock. Rocks are poor conductors of heat, so the inner core stays cooler.
2. Night-time Cooling: As the temperature plummets after sunset (sometimes by 30°C or more!), the outer layer cools down and contracts rapidly.
3. Stress Build-up: This constant expansion (day) and contraction (night) creates incredible stress within the rock structure.
4. Peeling: Eventually, the outer layer breaks off in curved sheets, similar to layers peeling off an onion. This is why it’s called 'onion skin weathering.'

B. Salt Crystal Growth

This process occurs when water containing dissolved salts enters cracks or pores in rocks.

Process: As the intense desert heat causes the water to evaporate, the salt is left behind and crystallises. These growing salt crystals exert pressure on the surrounding rock, gradually forcing the cracks to widen and eventually shatter the rock fragments.

Section 2: The Wind and the Sand (Aeolian Processes)

In areas lacking vegetation, the powerful, sustained desert winds can directly shape the land. Geographers call processes related to wind aeolian processes. These involve erosion, transportation, and deposition.

2.1 Aeolian Erosion (Wearing Away the Land)

Wind erodes the landscape in two main ways:

A. Deflation

Deflation is the process where wind simply picks up and carries away loose, fine material (like silt and very fine sand) from the surface.

• Over time, deflation can create large, shallow depressions called deflation hollows or blowouts.
• It leaves behind a surface covered mainly in pebbles and larger rocks that are too heavy to be moved by the wind.

B. Abrasion

Abrasion is the "sandblasting" effect. The wind uses carried sand particles to scrape, grind, and polish exposed rock surfaces.

• Analogy: Think of holding a piece of sandpaper (the sand) up to a wooden block (the rock). The wind is the force driving the sandpaper.
• Abrasion is most effective near the ground (usually less than 1 metre high) because the heaviest, most abrasive sand particles are transported low down.
• This process creates strange, mushroom-shaped rock formations called pedestal rocks (or Zeugen), where the base of the rock is worn away faster than the top.

2.2 Aeolian Transportation (Moving the Sand)

Sand and dust are moved across the desert floor in three main ways, depending on particle size:

1. Surface Creep: The largest, heaviest particles are slowly pushed or rolled along the desert floor by the wind force. (Slowest movement).
2. Saltation: Medium-sized sand grains are lifted slightly into the air, bounce along the ground, and knock other grains forward. This is how most desert sand moves. (Bouncing movement).
3. Suspension: Very fine dust particles (often invisible) are lifted high into the atmosphere and carried long distances, sometimes across continents. (Highest and fastest movement).

Memory Aid: Think of Saltation, Suspension, and Surface Creep – the three S’s of sand movement!

Section 3: Landforms of the Desert

The material moved by wind and, surprisingly, water, creates the classic landscapes we associate with deserts.

3.1 Surfaces and Deposits (Deposition)

When the wind slows down, it loses energy and drops its load of sand and dust. This is deposition. The desert surface can be classified into three main types based on the deposited material:

• Erg: A vast area dominated by sand dunes (e.g., the Grand Erg Oriental in the Sahara). This is the classic, sandy desert often seen in movies.
• Reg: A vast plain covered by pebbles and gravel. This surface is created after deflation has removed all the fine particles, leaving a 'desert pavement' or stony surface behind.
• Hamada: An elevated plateau or bare rock surface, often found in mountain foothill areas. Weathering and abrasion have stripped away loose material, leaving rugged bedrock exposed.

3.2 The Signature Landform: Sand Dunes

Sand dunes are hills of sand created by wind deposition. They require three key things:

1. A large supply of loose sand.
2. A wind capable of moving the sand.
3. An obstacle (like a bush or small rock) to slow the wind down and initiate deposition.

Dunes typically have an asymmetrical shape: a gently sloping side facing the wind (the windward side, where sand accumulates) and a steep slope (the slip face or leeward side) where sand collapses down. Dunes gradually migrate across the desert landscape in the direction of the prevailing wind.

3.3 Landforms Created by Water (Fluvial Action)

Although dry most of the time, water is incredibly powerful when it appears in the desert. Intense, short bursts of rainfall (flash floods) cannot soak into the baked, hard ground, causing rapid surface run-off.

A. Wadis (Dry Riverbeds)

A wadi is a steep-sided river channel or valley that is normally dry but fills rapidly with water during a flash flood event.

• Wadis are essential features of desert landscapes and often mark the only route where surface water flows.
• They are dangerous because they can flood within minutes, even if it hasn't rained directly above them.

B. Alluvial Fans

When a fast-flowing wadi or river emerges from a steep, narrow mountain canyon and hits the flat desert plain, the water immediately loses energy.

This sudden loss of energy causes the rapid deposition of sediment (gravel, sand, and mud) in a distinctive, fan-shaped mound called an alluvial fan. These often indicate areas where water may be stored underground, making them important sites for settlement.

That covers the physical processes shaping the hot desert. These landscapes are constantly changing, making them dynamic and challenging places to live!