Hot Desert Systems and Landscapes

A coastal desert receives almost no rain despite being beside the ocean, and is often shrouded in fog. Explain the cause of its aridity.

1. 01Identify the offshore condition

   A cold ocean current flows along the coast, chilling the air immediately above the sea.

2. 02Effect on moisture

   The chilled air holds little moisture and is stable, so although fog forms as vapour condenses over the cold water, the air rarely rises enough to produce rain.

3. 03Conclude

   The cold current, not distance from water, causes the aridity - the coast is wet with fog but starved of rainfall, as in the Namib or Atacama.

Result: A cold offshore current cools and stabilises the air, giving fog but almost no rainfall - a coastal desert.

Rocks in an arid basin are seen to be crumbling, with salt crusts on their surfaces. Explain the weathering process responsible.

1. 01Source of salt

   The very high evaporation draws salty water to the rock surface and into cracks, leaving the salts behind as the water evaporates.

2. 02Crystal growth

   The salts crystallise and grow in the pores and cracks, and some expand when they take up water; this exerts pressure on the surrounding rock.

3. 03Disintegration

   Repeated crystallisation and expansion prise the rock apart grain by grain, so the rock crumbles - salt weathering.

Result: Evaporation concentrates salts that crystallise and expand in cracks, prising the rock apart - salt weathering.

A crescent-shaped dune is observed to move slowly across the desert. Explain the process of its migration and how you could tell the wind direction from its shape.

1. 01Sand movement

   Sand is blown by saltation up the gentle windward slope to the crest.

2. 02Slip-face deposition

   At the crest the sand cascades down the steeper leeward slip face; as this repeats, the whole dune shifts downwind.

3. 03Reading the wind

   The gentle slope faces the wind and the steep slip face is on the leeward side, and the horns of the crescent point downwind - so the shape reveals the prevailing wind direction.

Result: Saltation moves sand over the crest to the slip face, migrating the dune downwind; the gentle slope and the horns indicate the wind direction.

In the centre of an enclosed desert basin there is a flat, cracked, salt-covered surface. Explain how it formed.

1. 01Water collects

   After rare storms, runoff and flash floods drain inward to the lowest point of the enclosed (endoreic) basin, forming a shallow temporary lake.

2. 02Evaporation

   Because potential evapotranspiration is very high, the lake water evaporates rapidly, leaving behind the dissolved salts it carried.

3. 03The landform

   The result is a flat, salt-encrusted, often cracked surface - a playa - which fills and dries repeatedly.

Result: Inward drainage fills a temporary lake that evaporates to deposit salts, leaving a flat salt playa.

On a semi-arid desert margin, a run of dry years coincides with rising population and the land becomes degraded. Assess the relative importance of physical and human causes.

1. 01The physical trigger

   Drought and highly variable rainfall stress the vegetation and reduce soil moisture, weakening the land's natural resilience.

2. 02The human pressure

   A growing population overgrazes, over-cultivates and strips firewood, removing the vegetation that protects the soil, so erosion and moisture loss follow.

3. 03Reach a judgement

   The two interact: drought alone might be survived if the land could recover, but sustained human pressure prevents recovery and tips the fragile land into degradation - so human activity is often the decisive factor, though it acts on a climatically driven vulnerability.

Result: Drought creates the vulnerability, but sustained human pressure usually tips the land into degradation - the causes are interactive, with human activity frequently decisive.