The Sonoran Desert is shaped somewhat like a horseshoe with two-thirds of its total area lying in Mexico in the mainland state of Sonora. In the United States, this desert occupies the extreme southeastern California and southwestern and south-central Arizona. Covering about 100,00 square miles, the area is a little bit smaller than the state of Colorado.

Geology

To understand the Sonoran Desert region it's important to know something about the geology because this in turn directly affects the plants and animals that are indigenous today. Mountains of the high country were formed in three ways:

  • Volcanism: Many miles under the earths surface internal pressure form magma was forced to shoot up through the surface, Rivers of lava erupted along with clouds of gas and fountains of cinders forming the Tucson Mountain Range.
  • Plutonism: Now and then the magma failed to reach the surface and instead crystallized into plutons. These crystallized rocks are not as heavy as the rock above them and they float up to the surface, the nearby rocks slide off into the basins below. The Tucson Mountains used to be where Mount Lemmon is now, but ended up 20 miles away when they slid off the rising Catalina pluton.
  • Block Faulting: Likened to a set of books on a shelf that slowly tilt over, block faulting is a process that forms basins and mountain ranges. Pulled and pushed by pressures, the earth’s surface formed troughs and ridges, which made new mountains and existing one taller.

    Beginning 1.7 billion years ago the geological history occured in four stages:

    • Precambrian Era: From 1.7 billion years ago until 550 million years ago, a large range of mountains formed in this area. It eroded bit by bit until it once again reached sea level leaving behind rocks like graniteand schist creating the foundation for most of the Sonoran Desert.
    • Paleozoic Era; From about 550 million years ago to 250 million years ago, the area was dry land sometimes and under water the rest of the time. The sea came and went several times leaving sediments behind. These dried, hardening into limestone, sandstone and shale.
    • Mesozoic Era: From about 250 million years to 65 million ears ago, pressure beneath the earth’s surface caused magma to shoot up as volcanoes and crystallized as plutons that eventually floated up. These processes formed igneous rock. Volcanism formed rocks like basalt while plutons formed huge bodies of granite.
    • Cenozoic Era: Sometime near the mid point of this era beginning around 65 million years ago to the present stress and pressure formed under molten rock stirring the surface once again. This time, it pushed and pulled the surface into basins and ranges running north and south. Today the land has basins about 2,500 feet above sea level and mountains, or sky islands of close to 10,000 feet. If you're interested in learning more about the geology of the Sonoran Desert, a several terrific places to visit are the Petrified Forest National Park,Chiricahua National Monument and the Sunset Crater Volcano National Monument.
From cactus to conifers as elevations rise in the Sonoran Desert, biomes change from desert to forest ranging from below sea level to 3,450 feet. In the 1950’s, scientist Forest Shreve divided these into seven subdivisions which are still more or less used to describe the regions today.
  • Vizcaino

    Elevation: Sea level to 1,400 feet with mountains up to 2,500 feet.

    Annual Rainfall: Less than five inches. Vizcaino receives just about the same amount of rainfall as the neighboring Baja regions, however it’s the cool sea breezes and fog clouds from the Pacific Ocean that make this subdivision a little moister and cooler. This creates habitat for some of the world’s strangest plants.

  • Foothills of Sonora

    Elevation: Sea level (that would be zero) to 3,300 feet

    Annual Rainfall: 14 inches. Since there is higher rainfall in this region there is denser vegetation and more woody trees Frequently scientists don’t include this area as a part of the Sonoran Desert. Instead it’s referred to as Sinaloan thornscrub, a habitat between desert to the north and tropical forest to the south.

  • Arizona Uplands

    Elevation: 500 to 3,100 feet, with mountain ranges reaching 9,000 feet.

    Annual Rainfall: 12 inches. Thick and lush this is the highest and coldest subdivision. Many scientists think this is more of a thornscrub habitat than desert. Saguaro, palo verde and sprawling prickly pear cacti that can grow to more that fifteen feet across are abundant.

  • Lower Colorado River Valley

    Elevation: Sea level to about 1,300 feet.

    Annual Rainfall: 3 to 5 inches. This division is composed of broad flat valleys and is by far the largest, hottest and driest. It has scattered mountain ranges barren rock. Creosote, which can live up to two years without water and white bursage are the predominate plants.

  • Central Gulf Coast

    Elevation: Sea level to 500 feet.

    Annual Rainfall: Less than five inches. It’s so hot and dry in this subdivision that trees that drop their leaves to conserve water such as ocotillo used by many native people for fencing, walls, and roofs; and ironwood dominate this landscape that are strips of land running along both sides of the Gulf of California. It’s a common sight to see these plants growing all the way to the seashore.

  • Magdalena

    Elevation: Sea level to 1,400 feet, with mountain ranges up to 5,000 feet.

    Annual Rainfall: Less than five inches. Most of this area lies below 1,400 feet and is located south of Baja California. These flat plains are covered with odd-looking cactus called creeping devil.

  • Plains of Sonora

    Elevation: 300 feet in the west to 2,500 feet in the east.

    Annual Rainfall: 12 to 14 inches. Most of this area is made up of braid valleys where soil is not the best for cactus, but more suited for legume, or bean, trees like the mesquite are prevalent. Similar to the Arizona Uplands, this area is considered more of a thorn scrub habitat than desert by some experts.

Deserts cover about twenty percent of the earths surface and North America has four; Great Basin, Mojave, Chihuahuan and Sonoran. The Great Basin Desert is the farthest north covering most of Nevada, and Utah, along with parts of Idaho, Oregon and Arizona it has the highest elevations and experiences the coldest temperatures. The Mojave Desert, located just south if the Great Basin, lies mostly in California stretching into the southern tip of Nevada and a small are of western Arizona near Kingman. The Chihuahuan Desert ranges north out of Mexico into Texas, New Mexico and Arizona.

The Sonoran Desert is the plushest offering two ‘rainy seasons,’ and some 2,000 plants. A great deal more than the other three nearby deserts. While the Great Basin and the Mojave get most of their rain in the winter, and the Chihuahuan Desert gets a large part of its rain in the summer, the Sonoran gets rain in winter and summer meaning more plants can grow. Added to that are the comparativley milder winters. The other deserts receive snow and below freezing temperatures. Mild winters are easier for plants to survive.

Giant saguaros are world-renowned as a symbol of the American west often starring in Hollywood movies growing from Montana to Texas. Truth be told they only grow in Arizona, Sonora and a sliver of California. A great place to see a real forest of these succulants is at the Saguaro National Park. While these places may be moist and verdant by desert standards it’s still no picnic for wildlife. They have adapted in a variety of ways. To cope with the hot day time temperatures, many animals are nocturnal or crepuscular, active at dawn or dusk. Kangaroo rats, and ringtails are active at night while bobcats hunt during sunrise or sunset. Javalina will hang out in the daytime, but only in the cooler winter months.

Another way animals cope with the heat is by finding or creating their own micro climates. Cactus wrens and mule deer seek out the shade in rocky crevices and foliage. Other critters like squirrels, dig burrows and only emerge for an hour or so to hunt for food.

Some animals change their wardrobe to get along in the hot climate. Coyotes will shed their coats in late spring. Another way to beat the heat is by evaporative cooling. Animals do this by panting and owls will flap the skin under their throats in order to move air over its mouth called gular fluttering. But to be sure ninety five percent of Sonoran critters are invertebrates like beetles, flies, ants, spiders and scorpions.

Biomes

As the elevation rises in the Sonoran Desert different life zones for flora and fauna occur.

  • Grassland:

    The rainfall here is not much more than the Arizona Uplands, about 12 to 15 inches. Located at 4,000 feet there are still quite a few desert cactus and shrubs around but the areas in between are filed with grasses. Located on the eastern edge of the Sonoran Desert it used to be filled with prairie dogs and ferrets. Many cattle ranchers worry that these animals would compete with their livestock for food and that the cattle would trip in the burrows they lived in. This wildlife is gone now, but others like the horse lubber grasshopper, Western box turtle, burrowing owl and prairie falcon are still around.

  • Chaparral:

    A division of small trees and shrubs, chaparral is between desert grassland and woodland. Most of the plants that are native there have tough branches and thick leaves that don't fall off in the winter. In some areas of the chaparral are plants like manzanita, scrub oak, and chamiso scrub and grow so thickly that it's impossible to hike through them.

    Animals that inhabit the North American chaparrals are coyote, mule deer, ringtails, and lizards. The Sonoran Desert becomes a chaparral biome along the western edge of California and the northeastern edge of the Mogollon Rim in Arizona. Some areas of mountain ranges have chaparral habitats too. If you ever visit the Molin Basin take a look at the slopes above and you’ll see patches of chaparral teeming with scruffy manzanita bushes.

  • Woodland:

    Biomes of the woodlands begin at about 5,000 feet or so and enjoy 15 to 20 inches of rainfall a year. Trees like oaks and pines fill the landscape unlike the clumps of trees and shrubs of the chaparral; woodland trees are spaced farther apart with grassy growth in between. Carpenter beetles and bark scorpions live here. So do coatis, cottontail rabbits and white tailed deer. These in turn attract bobcats and mountain lions.

  • Sky Islands:

    Above the woodlands located between 6,000 and 7,000 feet in elevation are coniferous forests. Coniferous refers to the cone bearing plants and trees that are more commonly thought of as occuring in places like Canada. One of the more unusual aspects of the Sornoran Desert is that for every thousand feet you climb in elevation, it's equivalent to moving 300 miles towards the North Pole.

    The coniferous forests are very special in this desert and are called sky islands because these forested islands jut into the sky like islands surrounded by a sea of desert.Possessing a rich variety of animal and plant life Ponderosa Pine covers the island. Mount Lemmon and Rose Canyon Lake near Tucson are prime examples. The fir forest appears around 7,500 feet dominated by Douglas and White fir. Above the 9,000 foot elevation lies the last level of conifers, the spruce-fir forest where Engleman spruce and Alpine firs are the predominate trees.

    Some of the largest animals of the Sonoran Desrt live in these sky islands. Black bears, which are actually brown in color, are found all the way down to the desert scrub habitats, but usually stick to the higher elevations with it’s fruits, berries, nuts, leaves, roots, honey, insects and small animals. Other major animals of the forest biome in the Sonoran Desert are mountain lions, gray foxes, white-tailed deer, red-tailed hawks, golden eagles and a wide variety of squirrels.

  • Riparian/Gulf Coast Biomes:

    By far the largest water habitat in these parts is the Gulf of California. This water transforms the desert into a haven for native birds and fish. Between Baja California and mainland Mexico, the gulf is sometimes called the Sea of Cortez. Seven hundred sixty miles long and averaging 95 miles wide it ranges from 100 to 1000 feet in elevations. Home to more that 800 species of fish and 200 species of birds, sea lions hang out on the coastal shores and bottlenose dolphins, as wells as 23 species of whales which migrate through every year.

    Most of the Sonoran riparian areas are located along rivers or other bodies of water like springs and lakes. My favorite riparian area is Sabino Canyon. Not only do creatures like fish, beaver and salamanders live in these areas many others make use of them as ‘corridors’ providing water, food and shelter as they move from one place to another.

    The Colorado River is the biggest riparian habitat of the Sonoran Desrt where there is always water running above ground. But don’t make the mistake of thinking dry river beds are all but abandoned. Even though the neighborhood wash carries water for only a few hours a year, it’s still enough to provide for many plants and animals, especially birds. Did you know that a chorus of toads or frogs at night after a summer monsoon means they are calling each other so they can mate and lay eggs that hatch into tadpoles? They’re in a hurry because they live in rainwater pools that dry up quickly.

    Many desert ecologists are concerned about the fish populations. About two-thirds of the species are threatened or endangered. A lot of non-native fish were brought here from other habitats frequently eat the natives. Habitat is also being lost due to human population growth and as more water is being pumped out of the ground to drink, the water levels in some riparian areas drop. It’s hard to imagine that before the 1930’s, the Colorado Squawfish, many as long as six feet, ranged the Colorado River. Unfortunately, construction of dams contributed to its disappearance.

Nowhere else in the world is there a place like the Sonoran Desert. Located only a few hours from the seashore to forests of giant saguaros and snow capped peaks. This surprising region bursts at the seams with unique and remarkable plants.

Sources:

"Desert in Your Backyard",Arizona Daily Star. 20 January 2002.

Sonoran Desert Naturalist Home Page:
m1.aol.com/Melasoma/

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