Desertic/Arid Mophological Systems

A Desertic MS is an area characterized by a lack of available water, and where the main geological agent is the wind.

The geological processes include:

• Thermal stress: sharp temperature changes (mainly daytime and night) provoke expansion and contraction of rocks and, eventualy, their rupture.

• Haloclasty: salt crystallization causes rocks to break up when saline solutions in cracks evaporate and salt crystal expand and increase pressure on the rock.

Other geological effects are:

• Desert varnish: thin film like a shiny coating formed in the surface of stones due to the action of bacteria

• Salt lakes: wide areas covered of salt due to evaporation.

Structures formed on the desert slopes:

• Tafoni: Hollows formed on the rock because of the permeability or the pressure difference.

• Tor: Enormous accumulations of rocks on slopes of dry areas.

Structures formed on the plain:

• Glacis: Sediments that come from the top of the slope. They connect with channels called uadis which are really dangerous during rainfall periods (they have a waterborne).

• Monadnock or Inselberg: Mount formed on the plain with a different composition. The materials are created in depth and they come out as a consequence of the tectonic movements.

• Depressed areas: Occurs due to sediments accumulation when the water evaporates. For instance, Sebka (salty lakes).

Windy geological processes

The relief forms in the desert can be created by two different processes, both due to the wind:

• Deflation: The windy takes the sloope material, lowering the relief.

• Abrasion: The wind lifts the materials throwing them against other materials “sanding them down”.

Types of deserts

The wind moves from the higher pressures to the lower ones, losing its energy gradually. Since the anticyclones stay always in the same area, this wind movements generates three different types of deserts:

• Hammada: Rocky desert. The wind has a lot of energy so it takes the lightest materials and leaving the heaviest ones.

• Reg: It´s a stony desert but it also has sand unlike the hammada.

• Erg: Sandy desert. When the wind loses its energy, it puts down all the materials that it had been dragging (lower pressure areas). 

Other desertic structures and effects:

• Yardangs: Structures formed when the materials have a different resistance to erosion (Thicker layers than others).

• Dunes: Hills of sand built by the wind so they have uniform and smooth layers. They can also be formed because of changes on the amount of sand.

• Desert pavements: When the wind blows, it takes the lightest materials producing the accumulation of the heaviest ones.

• Mushroom rock: The wind blows and drags the sand particles to a certain height, so there will be a stronger erosion on the higher part. Sometimes, the composition of the materials is different.

• Haze: A lot of sand on suspension that difficulties the sight, during strong windy periods (Canary Islands receive sand storms from Africa).