Climate and Weather

Sonora has a Köppen classification of BWh, which is a subtropical hot desert. During the hottest months (May-June) Pacific currents carry moisture over the Cascades and Sierra Nevada mountains where the atmosphere is cooler and less able to retain moisture. This results in rain on the windward side of these ranges. The Rain Shadow effect leaves the leeward air dry as the current moves towards Sonora. Another major factor is the North Pacific High Pressure Zone. Hot air rises over the equator and moves north to settle over Sonora. This keeps away weaker low pressure systems along with rain.

Me exploring the Sorora Desert during Spring Break

Sonora is one of the wettest North American deserts. Average rainfall of 3-15” is split between two rainy seasons; summer and winter. The late summer rains (July-September) are the most dramatic and result from monsoon winds which pull Pacific Ocean moisture over the hot desert floor. The moist hot air rises and expands in Thermal columns. This creates instability in the atmosphere and results in isolated thunderclouds that can quickly poor huge volumes of rain. Here is a great link to the Anatomy of a Monsoon Thunderstorm. The winter rains (December-January) are more predictable and result from low pressure storm tracks that travel from the Pacific Northwest through California to the Sonora desert. I experienced this during my visit over spring break. The day I arrived it was 80⁰ and sunny. The next day it barely reached 50⁰; it rained and even snowed!

The forecast called for rain but I did not expect to see snow!

I want to share a couple of unique forms and processes common in Sonora. The Dust Devil is a special type of Thermal where hot air rushes upward in a column that sucks in the surrounding hot air resulting in a spinning dusty cyclone.

Arizona Dept. of Transportation Photo

The Haboob is a dust storm caused by down drafts at the front of a desert thunderstorm. Check out this video!

Video shared by Mike Olbinski on Vimeo.

References:
http://www.desertmuseum.org/books/nhsd_patternsrain.php
http://geoplan.asu.edu/aztc/monsoon.html
http://www.blueplanetbiomes.org/sonoran_desert_climate.htm
http://tucsoncitizen.com/wryheat/2011/07/08/more-on-the-haboob-dust-storm-that-covered-phoenix/
http://www.azdot.gov/CCPartnerships/Haboob/index.asp

Weathering, Erosion and Soil

The Sonora Desert is a great place to study the effects of weathering and erosion on granite landscapes. One of the most important processes in the desert is grusification. This is a combination of physical and chemical weathering that breaks down the desert’s granite. The physical nature of this process is driven by salt crystal build-up, which then expands and contracts as the temperatures swing. This process breaks up the rock into chunks which often form piles of smaller rocks known as tors.

World Atlas Photo – Sonora Desert

Once the rocks get smaller, chemical biotite weathering processes oxidize iron. Dissolution breaks the rocks down to further to form smaller tiny rocks or sands known as grus. This material makes up much of the Sonora desert floor. 

Arizona Geographic Alliance Photo - Grus

Another important desert process is Alluvial (water based) Mass Wasting as pictured below. Weathering breaks up the rocks along a mountain range. Then grus and clay materials create an unstable base for larger and heavier rocks that sit on top of an impermeable slope. Water from occasional heavy rain storms will saturate the loose materials, and the debris flow is created.

Arizona-Sonora Desert Museum Photo – Alluvial Fan near Yuma, Arizona

Weathering and erosion play an important role in desert soil creation. Alluvial clay and silt particles wash over the basins and mix with wind-blown dust, organic material and rain dropped particles. In the desert, with little water and relatively sparse organic material, this process is SLOW! In fact, it takes 100,000 years or more to develop a full calcic horizon. This white concrete-hard layer varies in depth and thickness. A fully mature calcic horizon, or caliche, may be three feet thick and nearly impenetrable. In relatively moist areas like Tucson, the calcic horizon is deeper (~24 cm). But in very arid regions like Yuma, the calcic horizon may be only a couple of centimeters deep.

Arizona-Sonora Desert Museum - Cement hard caliche removed from soil excavation

Research sources

http://geoearth.uncc.edu/people/meppes/GSABULL.pdf

http://www.desertmuseum.org/books/nhsd_geologic_origin.php

http://www.ehow.com/info_8502651_landforms-sonoran-desert.html

http://pubs.usgs.gov/of/2004/1007/erosion.html

http://pubs.usgs.gov/of/2002/ofr-02-0314/ofr-02-0314.pdf

http://alliance.la.asu.edu/slides/stop03/3a/3a.html

Sonoran Volcanism and Faulting

The Sonora Desert has many exciting geographic forms. One of the more interesting is the Pinacate extrusive volcanic field. In this 600 square mile area, a large shield volcano is surrounded by around 400 cinder cones including a large grouping of maar craters.

The Pinacate Volcanic Field, Sonora, Mexico from Google Earth

The shield volcano, known as Volcan Santa Clara, rises 3,900 feet and was created by flood basalts. Multiple layers of low viscosity lava flowed over long distances. These igneous extrusions of dark colored mafic basalt layers formed a gently sloped mountain. As the mountain grew, layer-by-layer, there would have been little danger to human life. The gentle flowing lava and low angle of the volcano would have given any people in the area time to escape.

Adjacent to the shield volcano are the cinder cone and maar craters. The maar craters are the most interesting because of the way in which they formed. Rising magma interacted with ground water creating explosive steam and ash eruptions. These eruptions would have been dangerous to any humans in the area as tephra and steam ejections would have been quite violent. Also, ash in the air would have made it difficult to breathe.

"Elegante" maar crater with the "Volcan Santa Clara" shield volcano in the background. (NASA Photo)

The Pinacate area is covered in grey basalt from tephra deposits from the cinder cones. As a result, you can easily locate this area on Google Earth as a large grey circle just south of Arizona and north of the Gulf of California.

Geological History of the Sonoran Desert region near Tucson from the Arizona-Sonora Desert Museum

Another characteristic of Sonora is the basin and range topography. This was created by an extension faulting process as the Sonora Desert was stretched west. Grabens formed between fault planes (see illustration C above) and the downthrown blocks became today's basins. This can be recognized from above as the mountains and valleys line up in parallel as shown in the below picture.  

 Sonora Range and Basin Topography from Google Earth

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References:
http://www.biology.ed.ac.uk/research/groups/jdeacon/desertecology/pinacate.htm
http://www.desertmuseum.org/books/nhsd_geologic_origin.php
http://googleearth.com/
http://hvo.wr.usgs.gov/kilauea/update/images.html
http://pdc.org/iweb/volcano.jsp
http://pinacate.org/
http://rst.gsfc.nasa.gov/Sect18/originals/FIG18_54.JPG

Introduction

This Blog was created by Huong Dinh

Allow me to introduce myself and my new blog. My name is Huong Dinh and I am a student at the University of Colorado Denver. I am from Saigon, Vietnam and have lived in Denver, Colorado for four years now. I am launching this new blog to document my field trip to the Sonoran Desert of Southern Arizona. 

By studying this rugged and beautiful environment I hope to gain a deeper level of understanding of its unique characteristics. In order to get the most out of my studies, I plan to visit the Sonoran Desert over Spring Break. This will enable me to practice the observational skills that I am learning in Geography class this semester. I've chosen the Sonoran Desert for my field trip, not only because I will be able to visit in person, but also because this desert is truly unique. This desert is full of life compared to many other deserts. The Saguaro Cactus is its most famous symbol, but pictures like the one above show that this desert is alive with many species of plants. I'm excited for my journey and thank you for joining me via my blog.