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Summary Chapter 21 Dry Regions: The Geology of Deserts
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Summary Chapter 21: Dry Regions: The Geology of Deserts Marshak: Earth portrait of a Planet. Systeem Aarde 2
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Chapter 21: dry regions: the geology of deserts21.1 introduction
Deserts are lands of extremes, dryness, heat and cold. 25% of the surface of the earth is desert.
21.2 the nature and location of deserts
What is a desert?
Desert: a region that is so arid (dry) that it contains no permanent streams, except for rivers that
bring water from temperate regions elsewhere, and supports vegetaton on no more than 15% of its
surface. In general less than 25 cm of rainfall a year. Also depends on intensity of rainfall and
evaporaton. Large interval, or high temperatures can cause a desert. Desert depends on aridity not
on temperature.
Cold desert (below 20ᵒC) high lattudes, air too thin to hold much heat.
Hot desert (summer daytme exceed 35ᵒC) low lattudes, dense air can hold a lot of heat.
Highest temperature ever 58ᵒC. heat contributes aridity by increasing the rate of evaporaton. They
may be so great that when it rains the ground stays dry. The bare ground at low lattudes, absorbs so
much energy, that a layer of hot air forms just above the ground. This layer refracts sunlight creatng
a mirage, wavering pool of light on the ground. Hot deserts become cool at night because the dry air
doesn’t trap the heat. May change 80ᵒC in a single day.
Aridity causes erosion, weathering and depositonal processes in deserts to be diferent than those in
other regions. Water accumulates into fash foods of immense power. No humus on the ground
surface, no rapid chemical weathering. It consists of the following:
1. Exposed bedrock
2. Accumulatons of clasts
3. Relatvely unweathered sediment
4. Precipitated salt
5. Windblown sand
Soils do develop, but thinner and more mineralized.
Types of deserts
Subtropical deserts: form because of the air circulaton in the atmosphere.
o See drawing (1). Hot air rises, expands and cools rains out, travels to subtropical
area, air sinks heats up and absorbs all water, blablabla
o In a subtropical desert that borders the sea, high tdes may cause warm sea water to
food coastal tdal fats. Water evaporates and becomes saturated. Thus salt
precipitate onto underlying organic-rich mud, producing salty crust overlying the
mud. Referred to as a sabkha.
Deserts formed in rain shadows: when moist air fowing landwards reaches a mountain it
rises, during which it expands and cools, causing the water to condenses and falls as rain on
the seaward fank. Nourishing a coastal rainforest. The air at the other side will be dry. As a
result a rain shadow forms, and the land beneath becomes a rain-shadow desert.
Coastal desert formed along cold ocean currents: the cold water of the current cools the
overlying air by absorbing heat, and this process decreases the capacity of the air to hold
moisture. Creatng a desert. The air is so dry that rain rarely falls.
, Deserts formed in the interior of contnents: an air mass gradually loses moisture by
dropping rain. When an air mass reaches the interior of a large contnent it has grown dry, so
the land beneath it becomes arid. Largest example it the Gobi in central Asia.
Deserts of the polar regions: so litle precipitaton falls that these are arid.
o Because of the air circulaton patern
o Dry because cold air holds relatvely litle moisture.
Because of plate tectonics, regions that are now deserts were not deserts in the past and vice versa.
21.3 producing desert landscapes
Weathering and soil formation in the deserts
Physical weathering happens over tme, blocks break of along joints and tumble downslope.
Once it comes to rest it can sit unchanged for a long tme. Studies have shown that
temperature changes can cause a block to break due to the stress.
Chemical weathering: the moisture from dew and occasional rain allows oxidaton, hydrolysis
and dissoluton reactons. Without presence of acidic organic mater, and a steady water
supply it happens very slowly.
Soil forms, but without roots to hold it in place, causing it to move before it can evolve.
Where sediments stay in place soil can form, but not enough water infltrates to fush
leached ions away entrely. Therefore the ions precipitate to dorm new minerals. If the new
cement consists of calcite it can bind the regolith into a solid, rock-like material -> caliche or
calcrete.
Due to lack of organic content, the black or brown colors don’t develop, variatons in bedrock
tend to control the color. Variatons in iron also play a role. Iron-rich dark red. Less iron
orange. Low iron is gray/ greenish.
Desert varnish
A dark, rusty brown coatng of iron oxide, manganese oxide, and clay, locally coats the surface of
rocks in the desert. May form when windborne dust setles on the surface of the rock, for in the
presence of moisture, microbes extract elements from the dust and transform it into iron or
manganese oxide precipitates. Doesn’t form in humid climates because water washes away dust.
Takes long to form, thickness can determine the tme the rock has been there. They have used them
for art, by chipping away the varnish, drawings are called petroglyphs.
Water erosion
Because there is no vegetaton, water has free feld. Badlands topography develops where fowing
water erodes a sof substrate in the desert. Raindrops eject sediment from the ground into the air,
sediment gradually migrates to lower elevatons on a slope. During a heavy rainfall the ground
becomes quickly saturated, so water starts to fow across the surface, carrying loose sediments. Dry
stream channels fll with turbulent mixture of water and sediment. Rushes down as a fash food.
When it stops water sinks into streambed’s gravel and disappears. Such drainage are called
ephemeral streams, channels called dry washes or arroyes or wadis.
Water in fash foods moves fast and has a lot of sediment causing a lot of erosion. Can polish
bedrock, cut steeped-walled channels, transport huge boulders. As rocks roll they tumble and strike
each other.
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