Deserts / Geology and Resources

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U. S. Department of the Interior / U. S. Geological Survey

Deserts:
Geology and Resources

by A. S. Walker

Great Sand Dunes National Monument, Colorado (photograph by John Keith).

Great Sand Dunes National Monument, Colorado (photograph by John Keith).

Beauty is before me

And beauty behind me,

Above and below me hovers the beautiful,

I am surrounded by it,

I am immersed in it.

In my youth I am aware of it,

And in old age

I shall walk quietly

The beautiful trail.

from a Navajo benedictory chant describing the desert

Cacti dominate the Sonoran Desert vegetation near Tucson, Arizona (photograph by Peter Kresan).

Cacti dominate the Sonoran Desert vegetation near Tucson, Arizona (photograph by Peter Kresan).

What Is a Desert?

Approximately one-third of the Earth’s land surface is desert, arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals. Deserts—stark, sometimes mysterious worlds—have been portrayed as fascinating environments of adventure and exploration from narratives such as that of Lawrence of Arabia to movies such as “Dune.” These arid regions are called deserts because they are dry. They may be hot, they may be cold. They may be regions of sand or vast areas of rocks and gravel peppered with occasional plants. But deserts are always dry.

Ripples on a dune in the Gran Desierto, Mexico (photograph by Peter Kresan).

Ripples on a dune in the Gran Desierto, Mexico (photograph by Peter Kresan).

Deserts are natural laboratories in which to study the interactions of wind and sometimes water on the arid surfaces of planets. They contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Because deserts are dry, they are ideal places for human artifacts and fossils to be preserved. Deserts are also fragile environments. The misuse of these lands is a serious and growing problem in parts of our world.

DISTRIBUTION OF NON-POLAR ARID LAND (after Meigs, 1953)

DISTRIBUTION OF NON-POLAR ARID LAND (after Meigs, 1953)

[Higher-resolution map]

There are almost as many definitions of deserts and classification systems as there are deserts in the world. Most classifications rely on some combination of the number of days of rainfall, the total amount of annual rainfall, temperature, humidity, or other factors. In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall, arid lands have less than 250 millimeters of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 millimeters. Arid and extremely arid land are deserts, and semiarid grasslands generally are referred to as steppes.

Gran Desierto of the Sonoran Desert, Mexico. Surrounding the dark 25-kilometer-long and 5-kilometer-wide Sierra del Rosario mountains (upper right) are dunes and sheets of sand.

Gran Desierto of the Sonoran Desert, Mexico. Surrounding the dark 25-kilometer-long and 5-kilometer-wide Sierra del Rosario mountains (upper right) are dunes and sheets of sand.

How the Atmosphere Influences Aridity

We live at the bottom of a gaseous envelope—the atmosphere—that is bound gravitationally to the planet Earth. The circulation of our atmosphere is a complex process because of the Earth’s rotation and the tilt of its axis. The Earth’s axis is inclined 23½° from the ecliptic, the plane of the Earth’s orbit around the Sun. Due to this inclination, vertical rays of the Sun strike 23½° N. latitude, the Tropic of Cancer, at summer solstice in late June. At winter solstice, the vertical rays strike 23½° S. latitude, the Tropic of Capricorn. In the Northern Hemisphere, the summer solstice day has the most daylight hours, and the winter solstice has the fewest daylight hours each year. The tilt of the axis allows differential heating of the Earth’s surface, which causes seasonal changes in the global circulation.

The circulation pattern of the Earth’s atmosphere. Most of the nonpolar deserts lie within the two trade winds belts.

The circulation pattern of the Earth’s atmosphere. Most of the nonpolar deserts lie within the two trade winds belts.

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