Deserts Bob Leighty GLG 101 - Physical Geology

These notes and web links are your primary “lecture” content in this class. Additionally, various articles are assigned each week to supplement this “lecture” information. I believe you’ll have enough information to reference without having to purchase a costly textbook. These lecture notes are very similar to the ones I use in my traditional classes. You’ll find they are loaded with imagery and streamlined text that highlight the most essential terms and concepts. The notes provide a framework for learning and, by themselves, are not meant to be a comprehensive source of information. To take advantage of the global knowledge base known as the Internet, I have included numerous hyperlinks to external web sites (like the Wikipedia, USGS, NASA, etc.). Follow the links and scan them for relevant info. The information from linked web sites is meant to supplement and reinforce the lecture notes – you won’t be responsible for knowing everything contained in them. As a distance learning student, you need to explore and understand the content more independently than in a traditional class. As always, I will help guide you through this learning adventure. Remember, email Dr. Bob if you have any questions about today’s lecture (rleighty@mail.mc.maricopa.edu). Leave no questions behind! Explore and have fun!

There’s only one “S” in Desert Deserts There’s only one “S” in Desert Desert Dessert

Desert Misconceptions Deserts Desert Misconceptions Deserts are giant seas of sand

Desert Misconceptions Deserts Desert Misconceptions Deserts are giant seas of sand > No way! Some are sandy, rocky, even icy!

Desert Misconceptions Deserts Desert Misconceptions Deserts are always hot

Desert Misconceptions Deserts Desert Misconceptions Deserts are always hot > Untrue! Some are numbingly cold!

Desert Misconceptions Deserts Desert Misconceptions Deserts are lifeless

Desert Misconceptions Deserts Desert Misconceptions Deserts are lifeless > Think again, bucko! Deserts can be teeming with life, …

Desert Misconceptions Deserts Deserts are lifeless > Think again, bucko! Deserts can be teeming with life, … …especially on other planets!

WWW What is a Desert? An arid region that gets <10” of rain/year Deserts What is a Desert? WWW An arid region that gets <10” of rain/year Can be hot (Sahara) or cold (Patagonia) Semi-arid regions get 10-20” of rain/year May have lots of sand, a little sand, or rocky Generally lacks significant vegetation Deserts cover roughly 1/3 of the Earth’s land surface

Deserts Why Do Deserts Form? The amount of annual precipitation defines arid regions (deserts) Deserts usually form from a combination of factors: 1) Global air convection patterns > Equator – Rising warm, moist air (low-pressure) > Lower latitudes (10-30 N & S) – Dry air descends & warms and compresses air underneath (high-pressure) * This controls the location of many deserts

Deserts Why Do Deserts Form? Weather map symbols: H = descending high-pressure air, L = rising low-pressure air

Why Do Deserts Form? Many deserts form from a combination of factors: 2) Geography > Regional elevation, proximity to mountain ranges & cold ocean currents, etc. * This can be an important influence on regional climate

Deserts of the southwestern USA Sonoran Desert – A mid-latitude desert GB M S C Other SW USA deserts - Great Basin, Mojave, Chihuahuan, etc.

Deserts of the southwestern USA Arizona is mostly arid & semi-arid

Subtropical (Trade wind or Mid-Latitude) Deserts Types of Deserts Subtropical (Trade wind or Mid-Latitude) Between 10-30 N & S of the Equator Examples: Sahara, Rub al Khali

Subtropical (Trade wind or Mid-Latitude) Deserts Types of Deserts Subtropical (Trade wind or Mid-Latitude) Commonly occur within continents Examples – Sonoran, Gobi, Australian

Types of Deserts Rain Shadow Large mountain ranges block air flow and create a rain shadow 1) Air rises & cools 2) condensation & precipitation 3) dry air descends & heats up

Types of Deserts Rain Shadow Deserts form behind major mountain ranges Examples: Great Basin (Sierra Nevada), Chihuahuan (Sierra Madre), Patagonian (Andes)

Types of Deserts Coastal Land adjacent to cold ocean currents (dry air) Atacama (<1 cm/yr - from fog) Examples: Atacama (South America), Namib (South Africa)

Deserts Types of Deserts Polar High north & south latitudes (cold) near the poles Air is way too cold to hold much water Examples: Greenland, Antarctica

Types of Deserts Paleo Desert areas of the geological past Example: Coconino Sandstone (Paleozoic), Navajo Sandstone (Mesozoic)

Types of Deserts Extraterrestrial Desert areas on other worlds Example: Mars

Desert Landforms Normal faulting creates basins & ranges Deserts Desert Landforms Normal faulting creates basins & ranges Erosion of mountains, deposition in basins

Desert Landforms Inselbergs, alluvial fans, bajadas, playas, etc. Deserts Inselbergs, alluvial fans, bajadas, playas, etc.

WWW Eolian Processes Erosion Deserts WWW Erosion Eolian (wind) abrasion (“sand-blasting”) produces ventifacts, yardangs, balanced rocks, etc. ventifacts balanced rocks

Eolian Processes Transportation Deserts Eolian Processes Transportation Wind typically can’t pick-up particles larger than sand Mostly silt and dust (suspended load)

Eolian Processes Deserts WWW Deposition Sand dunes, ripples, etc.

WWW Desert Pavement Thin, pebble-rich layer Deserts Desert Pavement WWW Thin, pebble-rich layer Formed by a) wetting & drying OR b) deflation

WWW Desert Varnish Dark outer coating of Fe & Mn oxides + clay Deserts Desert Varnish WWW Dark outer coating of Fe & Mn oxides + clay Formed by chemical? or biologic processes?

WWW Links in this Lecture Deserts > Deserts - http://en.wikipedia.org/wiki/Desert > Sonoran Desert - http://en.wikipedia.org/wiki/Sonoran_Desert > Great Basin - http://en.wikipedia.org/wiki/Great_Basin > Mojave Desert - http://en.wikipedia.org/wiki/Mojave_Desert > Chihuahuan Desert - http://en.wikipedia.org/wiki/Chihuahuan_Desert > Sahara - http://en.wikipedia.org/wiki/Sahara > Rub al Khali - http://en.wikipedia.org/wiki/Rub_al_Khali > Gobi Desert - http://en.wikipedia.org/wiki/Gobi_Desert > Australian Deserts - http://en.wikipedia.org/wiki/Deserts_of_Australia > Rain shadow - http://en.wikipedia.org/wiki/Rain_shadow > Sierra Nevada - http://en.wikipedia.org/wiki/Sierra_Nevada_%28U.S.%29 > Sierra Madre - http://en.wikipedia.org/wiki/Sierra_Madre_Occidental > Patagonian Desert - http://en.wikipedia.org/wiki/Patagonian_Desert > Andes - http://en.wikipedia.org/wiki/Andes > Atacama Desert - http://en.wikipedia.org/wiki/Atacama_Desert > Namib Desert - http://en.wikipedia.org/wiki/Namib_Desert

WWW Links in this Lecture Deserts > Inselberg - http://en.wikipedia.org/wiki/Monadnock > Alluvial fan - http://en.wikipedia.org/wiki/Alluvial_fan > Playa - http://en.wikipedia.org/wiki/Playa > Eolian processes - http://en.wikipedia.org/wiki/Eolian_processes > Ventifact - http://en.wikipedia.org/wiki/Ventifact > Yardang - http://en.wikipedia.org/wiki/Yardang > Sand dunes - http://en.wikipedia.org/wiki/Dune > Desert pavement - http://en.wikipedia.org/wiki/Desert_pavement > Desert varnish - http://en.wikipedia.org/wiki/Desert_varnish