Download Lecture 15 Deserts

Lecture notes ‐ Bill Engstrom: Instructor Deserts GLG 101 – Physical Geology Deserts (not desserts – that’s something we have after dinner). Since it’s not ice cream or cake, what is a “desert”. It is…..an area characterized by low rainfall (<10" or 25 cm/year) possessing a dry or arid climate. And, not all of them are barren / devoid of vegetation. Our own desert, here in Phoenix is full of vegetation and life. And, some are cold, like Antarctica which also has low precipitation. In fact deserts & steppes (semi‐arid regions) cover 30% of Earth’s surface. Some of the major factors contributing to arid climates are: •
Global air circulation •
Position of land masses (relative to air and ocean currents) •
Position of mountain ranges (rain shadow deserts) •
Cold ocean currents Dry lands are concentrated in two regions‐ the subtropics and mid‐lattitudes. 1. Subtropics •
Low‐latitude deserts •
In the vicinities of the Tropics of Cancer and Capricorn •
Areas of high pressure and sinking air that is compressed and warmed Formation of Deserts in the Subtropics 1. Equatorial low pressure belts – air rises and cools (prepcipitation) 2. Air spreads out and the upper level low reaches the subtropical high belt at 20‐30 degrees lattitude 3. Air sinks and warms – dry on land = deserts 2. Middle‐latitude deserts •
Located in the deep interiors of continents •
High mountains in the path of the prevailing winds produce a rain shadow desert. Example: Rain shadow deserts can be found in Arizona and Great Basin Other Factors: Cold ocean currents •
Evaporation is inhibited ‐ cool air immediately above cold current. •
If current flows near continent, cool dry air evaporates moisture as it goes overland causing arid climate. •
Examples: Peruvian Desert, Baja Desert, and South African Desert. Now that we know what a desert is and where they occur, we need to find out what kinds of geologic processes occur in these regions. Geologic Processes in Arid Climates Weathering •
Not as effective as in humid regions •
Mechanical weathering produces unaltered rock and mineral fragments. •
Some chemical weathering in deserts produce:  Clay  Thin soils  Oxidized minerals Weathering can also produce something called desert varnish. •
Desert Varnish ‐ the buildup of Manganese and iron oxides on the outside of rocks. This is a darker “stain” that forms on the rocks. Petroglyphs can be found etched into the desert varnish, by prehistoric inhabitants, which is often formed on the outside of granitic rocks. •
The mechanism is somewhat unclear as some rocks which apparently contain neither manganese nor iron can have desert varnish. Geologic processes – The role of water in arid climates •
Practically all streambeds are dry most of the time. •
Desert streams are said to be ephemeral (intermittent). They carry water only during periods of rainfall. •
Different names are used for desert streams in various regions.  Wash and arroyo (western United States)  Wadi (Arabia and North Africa)  Donga (South America)  Nullah (India) Desert rainfall •
Rain often occurs as heavy showers •
Because desert vegetative cover is sparse, runoff is largely unhindered and flash floods are common. •
Poorly integrated drainage systems and streams lack an extensive system of tributaries. Important Note: Most of the erosion work in a desert is done by running water. Basin and Range—Evolution of a Desert Landscape. •
Characterized by interior drainage •
Landscape evolution in the Basin and Range region Early to middle stages‐‐ Uplift of mountains—block faulting. Interior drainage into basins produces: •
Alluvial fans •
Bajadas = Alluvial fans coalesce (overlap) to form bajadas (pr. Bahadas) in the middle stages of basin and range landscape evolution •
Playas and playa lakes Late stages ‐‐ Ongoing erosion of the mountain mass •
Produces sediment that fills the basin •
Diminishes local relief •
Produces isolated erosional remnants called inselbergs Wind in the Desert Transportation of sediment by wind differs from that of running water in two ways: 1. Wind is less capable of picking up and transporting coarse materials. 2. Wind is not confined to channels and can spread sediment over large areas. Mechanisms of sediment transport by wind Bed load •
Saltation — skipping and bouncing along the surface •
About 25% of the sand transported in a sandstorm is moved this way. Suspended load – smaller size particles/grains Wind erosion Wind is a relatively insignificant erosional agent with most erosion in a desert performed by intermittent running water. Mechanisms of wind erosion Deflation •
Lifting of loose material •
Deflation produces blowouts (shallow depressions) and desert pavement (a surface of coarse pebbles and cobbles). •
Produces ventifacts (stones with flat faces) and yardangs (wind sculpted ridges) Abrasion Significant depositional landforms are created by wind in some regions. Two types of wind deposits: 1. Dunes •
Mounds or ridges of sand •
Often asymmetrically shaped •
Windward slope is gently inclined and the leeward slope is called the slipface. •
Slow migration of dunes in the direction of wind movement •
There are several types of sand dunes, including barchan, barchanoid, transverse, longitudinal, parabolic, and star dunes. 2. Loess 8/2011 •
Blankets of windblown silt •
Two primary sources are deserts and glacial outwash deposits. •
Extensive deposits occur in China and the central United States.