DESERTS Features |
"Deserts
have a great deficit of moisture and high temperature (diurnal and
seasonal), causing them to be subjected to wind and water processes"
Winds flow freely unhintered in deserts because the desert has few plants, especially trees, to stop the air flow and bind the soil to prevent it from erosion. Hence, sands and soils are easily blown away in deserts.
The wind erodes rocks by abrasion, a process which occurs when sand is carried up and hit against the rocks for a sand-papering effect. Over time, landforms like pillars, rock pedestral and natural arches can be formed.
The form of the sand is also shaped by the wind in the form of barchans. Blown about sand usually collect in basins or piles up against fixed objects or rocks. The different sizes and features of these dunes reflect the direction and force of the wind.
Dunes vary from 1 m to over 350 m high and more than 800 m across. Driven by the wind, dunes advance about 6 to 9 m a year and will slowly cover everything in their path.
There is also a predominance of dust in deserts. The wind can cause a dust storm by whipping it up. Dust storms blot out the sun, scattering dust and sand into everything. They damage crops and make travelling difficult. Desert dust can be blown thousands of kilometers and cause hazards far away.
The great variation in temperatures between day and night (diurnal temperature) causes rocks to expand and contract. This puts strains on weak points in the rocks and they begin to fracture and crack, gradually breaking up. Desert floors are often littered with shattered rocks. Sometimes lumps of harder rock are left sticking out of the surrounding weathered desert plain, called inselbergs. In In-Salah (Sahara), the temperature in the day is 52°C and the night temperature is -3.3°C. Hence, we can see how fluctuating temperature can be in deserts.
The high temperature is caused by the intense radiation it receives. The sky is not sheltered by clouds, causing more than 70% of sunlight to penetrate into the ground. Therefore, rock temperatures are extremely high, with 82°C the maximum rock temperaure ever recorded
There is seasonal variation of temperature as well. Deserts are generally hotter during summers, when the temperature could be as high as 40°C and as low as 30°C during winters. At some interior continental deserts, there is even winter frost covering deserts during winter time.
Due to the high temperature, the rate of evaporation fifteen times the amount of rain. Salt lakes (playas) are hence formed by dried out sea-beds or when water evaporates off lakes. When pure water evaporates off the mineral and salt rich water, salts are left behind like a dry crust on the ground.
There is little rain in the desert, but when it rains, it comes in high quantities that disappear as quickly as they came. This rain is called interminttent or episodic rain. Such powerful bursts of water can erode the landscape with great force.
There is low infiltration in deserts due to the sun baked and impervious ground forming duricrusts which are extremely resistant against weathering and percolation. Hence there is great runoff which causes flashfloods to occur.
Presenting the Desert Piedmont (Mountain Foot Zone)
Wadis are valleys cut by sudden rushes of water. They dry up soon after the rain stops. Where the rock is softer, water gouges deep grooves and ridges to form a landscape of strange shapes called badlands. Earth and rocks erode quickly in badlands, making it particularly difficult for any vegetation to take root.
The alluvial fans are deposits from upstream erosion. They consist of very fine grains. The playa is a temperory lake which would dry up and leave salt deposits. The ephemeral stream flows only during and after rainfall. They will dry up too due to infliltration to the water table below (groundwater) and evaporation.
Where rock lie in horizontal layers, some harder than others, water erosion with wind and weathering over thousands of years can produces mesas and buttes. A mesa is a large, flat-topped island of rock that is left standing when the surrounding areas have been eroded away. When these erode or weather further, columns of rock called buttes remain jutting out of the landscape.
Canyons are formed where river flows. As the water sliced down into the rock, it has left almost vertical sides.
Fast-flowing streams caused by sudden rainstorms have great power and scour out huge amounts of pebbles and rock, which they carry downhill. When they deposit their load at plains, fan-shaped deposits of stone and rocks called alluvial fans are formed. Bajadas are formed when several alluvial fans merge together.
