ee.2.3 Hot and arid landscapes

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Key terms

• Abrasion
  • The scrapping, scouring, rubbing and grinding action of materials being carried along by moving natural features such as rivers, glaciers, waves and strong winds.

• Attrition
  • The process by which rocks and sediment are worn down and smoothed by the constant rubbing and grinding against each other during transportation.

• Buttes
  • A tall, narrow hill with steep sides and a flat top, similar to a mesa but with a smaller area at the top. It is typically taller than it is wide and often rises abruptly from the surrounding landscape.

• Chemical weathering
  • Involves the decay and decomposition of rocks in situ. It usually occurs in the presence of water, which acts as a dilute acid. The rate tends to increase with rising temperatures and humidity levels.

• Corrasion
  • An alternative name for abrasion.

• Creep
  • The slow downhill movement of soil and other material such as scree (talus). Can be caused by the expansion and contraction of the soil caused by either seasonal wetting and drying (especially clays) or seasonal and diurnal freezing and thawing of the soil surface.

• Deposition
  • The process by which sediment or other materials are deposited or settled in a new location, often as a result of the loss of energy by the agent of transportation.

• Differential erosion
  • Erosion that occurs at irregular or varying rates, caused by the differences in the resistance and hardness of surface materials: softer and weaker rocks are rapidly worn away, whereas harder and more resistant rocks remain to form ridges, hills, or mountains.

• Dunes
  • Sand formations found in desert regions that have been shaped by wind over time. They are typically composed of loose, dry sand and can range in size from small ripples to large hills that can be several hundred metres high. They are formed through a process called aeolian transport, in which wind moves sand particles across the surface of the desert. When the wind encounters an obstacle, such as a rock or vegetation, the sand is deposited, and over time, this accumulation can build up to form a dune. Can take on different shapes and forms depending on the direction and strength of the wind. Barchan dunes, for example, are crescent-shaped dunes that form when wind blows in a single direction, while longitudinal dunes form parallel to the prevailing wind direction.

• Erosion
  • Involves the removal of weathered material by the action of gravity, water, wind or ice.

• Hydration
  • A chemical weathering process involving water being added to the chemical structure of a mineral. A consequence of hydration is that the resulting mineral has a greater volume than the original mineral. The increase in volume applies force to overlying layers, breaking them into pieces.

• Insolation weathering
  • A type of physical weathering which involves repeated heating and cooling of rock over daily cycles, progressively breaking apart the grains of rock. Also known as thermal stress weathering. It is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night.

• Mesas
  • A flat-topped, elevated landform with steep sides that is found in arid or semi-arid regions. It is characterized by a horizontal or gently sloping top that is bounded on all sides by cliffs or steep slopes.

• Oxidation
  • A kind of chemical weathering that occurs when oxygen combines with another substance and creates compounds called oxides. When rocks, particularly those with iron in them, are exposed to air and water, the iron undergoes oxidation, weakening the rocks and making them crumble.

• Physical weathering
  • Involves the breakdown of rocks into smaller fragments through mechanical processes. Otherwise known as mechanical weathering.

• Rock pedestals
  • Also known as mushroom rocks or pedestal rocks, are geological formations that consist of a column of rock that is wider at the top than at the base, giving the appearance of a pedestal.

• Salt weathering
  • A type of physical weathering of rocks that is most common in arid climates involves the growth and expansion of salt crystals within rock cracks. No chemical alteration of rock constituents is involved.

• Saltation
  • The process by which particles are lifted upwards and forward before returning back to the surface from which they started. This occurs in desert areas and in rivers.

• Solution
  • The process by which certain minerals are dissolved by acidic solutions.

• Suspension
  • The process by which small particles, such as sediment, are carried and transported within a fluid medium, such as water or air. The particles are suspended within the fluid and move along with it, often remaining in suspension for a considerable period of time.

• Transportation
  • The process by which sediment or other materials are moved from one location to another by the agents of transportation. This can occur through a variety of mechanisms, such as wind carrying sand or dust, water transporting sediment downstream, or ice moving rocks and debris across a landscape.

• Wadi
  • Typically lacking a stream but occasionally experiencing a surge of water (flash flood) for a brief period following heavy rainfall, a steep-sided ravine is found in desert and semi-desert regions.

• Weathering
  • The disintegration and decomposition of rocks in situ by the combined actions of the weather, plants and animals.

• Wetting and drying
  • Rocks containing clay minerals, such as clays and shales. When wet minerals expand in volume, when dry, the minerals shrink. Repeated cycles of expansion and contraction eventually cause the rock to fragment and crumble.