GEOL-1010 Lecture Notes - Lecture 13: Rain Shadow, Iron Oxide, Extinction Event

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5 Feb 2018
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Lecture 13- Deserts
Geology in the News
o Mass extinction at the end of the Permian Period May be related to a short glacial event
Part One: Desert Basics and Processes
Why Do We Care
o They’re a lot more common on Earth than most people realize
o They cover extensive areas, and are growing
o 20% of land is desert
o Another 15% is semi-arid
Not extreme area
o Difficult to find water and food resources there
o But there is a large population that lives in deserts
What is a Desert
o Its not just temperature- it isn’t the deciding factor
o Yes they tend to be hot but that doesn’t define/ characterize a desert
o They key factor is that there has to be less than 10 inches of rain per year
o Amount of precipitation not the temperature
The 10 Largest Deserts
o DO NOT MEMORIZE THESE
o Just note how large some deserts are
Distribution of Deserts
o Found in certain latitudes
o They’re all lined up close to 30 degrees’ latitude both north and south
o This can’t be a coincidence
o Red Areas:
o Yellow areas: factors that affect these are related to the lithosphere
o US: Rain shadow effect- by the time the rain gets up and over the mountains
they’re all dried out
o Asia: When you have very large continents, you can be so far inland that water
vapor is gone by the time it reaches
Has to get over many mountain ranges too
o Polar Regions: Antarctica doesn’t have snow storms day after day after day
o Antarctica gets very little snow a year
o That’s why it gets classified as a desert
o It is a desert in terms of how little precipitation it gets
o It takes millions of years for the glaciers here to build up since there is so little
precipitation
Glaciers in other areas form much faster because there is more
precipitation
Weathering in Deserts
o To do chemical weathering you usually need water as a catalyst
o Little water = little chemical weathering
o Oxygen and iron oxides
o If you expose some things to oxygen over time
o Iron oxide reaction makes the rusty color you find in many deserts
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o Slow
Desert Erosion
o Water is still an important player
o It doesn’t rain often but occasionally there will be storms and they are large
o Rare, but no vegetation to constrain it
o This causes a lot of erosion
o Arroyo- dry creek beds (as shown in the picture on slide 11)
o This is where the water gets funneled to
o You’ll have flash floods flowing through- have to be careful when hiking
Wind Erosion
o Wind can only move relatively small particles
o Sand is actually too big
o Wind has a low density and low viscosity
o Particles > ca 0.06mm in diameter are hard to erode
o Can still add up to a huge volume eroded
o Example: Sahara Desert
o 250-500 million tons of dust are transported to the Atlantic each year just from a
corner of the Sahara Desert
o small particles add up to a huge amount of sediment
o Deflation:
o Because the wind is always removing things from the surface of the ground, over
time you are losing elevation
o Blowing away finely grained sediment so over time you’re losing elevation
o Pavements-
o Small stuff leaves but the big stuff never goes anywhere, they just sit there
o The stuff sitting on the surface gets more cobbley and gravely
o This paved surface that starts to dominate- more gravel less other stuff
o Only ca 20% of all desert areas are covered in sand
o We all picture sand but really most deserts are not like that
o There are plenty of plants and animal species that have adapted, it’s not
completely empty
o More gravely looking- thanks to wind deflation
Part Two- Desert Features
o Ventifacts- any artifact produced by the wind
o Vent- air, airflow
o Tifact- think artifact
o You start with any kind of object (rock, coke bottle, literally anything) that’s
sitting on the surface of the desert, where one side is facing the wind and the other
side isn’t
o The wind blows things that hit the object and bounce away. If you leave this for a
long period of time you eventually start to see a change in the object
o It will look more polished and worn down on one side and the texture on the other
side hasn’t really changed
o Alluvial Fans
o Body of sediment in a fan shape
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