Deserts:
Geology and Resources
by A. S. Walker
[Illustration: 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_
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.]