Mojave Desert

Mojave DesertThe Mojave Desert is a jagged, angular land of staggering extremes. It
contains the lowest, hottest place in North America, a great underground
river, places of bizarre beauty, a terrible earthquake fault, dead
lakes, low basins, and strange and resourceful plants and animals. It occupies
some 51,000 square miles (81,000 square km) and extends from the
east slopes of the Sierra Nevada to the Colorado plateau before merging
with the Great Basin Desert to the north and the Sonoran Desert to the south and southeast.
The two most common causes of desert combine in the Mojave to
create an especially harsh and austere environment.
First, it lies along the line of latitude that spawns deserts all around
the globe, due to the atmospheric circulation patterns that cause warm
moist air to rise at the equator, drop its moisture in tropical rains, flow
toward the poles until it cools, and downdrafts to the surface, largely
stripped of moisture. Such a pattern causes deserts all around the world
in both the Northern Hemisphere and the Southern Hemisphere.
Second, the Mojave lies behind walls of mountains, which force moisture-
laden air from the ocean to rise, cool, condense, and lose moisture
before sweeping back down the landward side of those mountain chains,
further drying out the already sun-blasted landscape of the Mojave. To
the east of the Mojave, the Sierra Nevadas rise to create a chain of peaks
towering to some 14,000 feet (4,667 m). The Sierra Nevadas were created
when the collision of two massive crustal plates caused a series of massive
islands to smash into the edge of the North American crustal plate,
causing the gigantic landmasses to buckle and fold. Prior to that geologic
event, the western edge of North America consisted of a series of low,
flat coastal plains. The ocean periodically swallowed up these great plains
during warm periods when sea levels rose. This turned the low plains into
a land of shallow seas, gigantic lakes, and rolling grasslands during glacial
periods, when the growth of the polar ice caps locked up so much water
that sea levels dropped worldwide.
A Collision of Continents
This collision of continents created the topography of the western United
States and Mexico. Initially, the collision built up the Sierra Nevadas,
pasted California and parts of Washington and Oregon onto the western
edge of North America, and isolated the terrain that would one day become
the deserts of North America behind this rain-stealing rampart of
rising granite. Later, the forces driving this collision from deep beneath
the surface shifted, causing the Pacific plate to start sliding north toward
the North American plate. The resulting shearing force fractured the surface
of Earth, creating the San Andreas Fault, which runs from the Gulf
of California up to San Francisco. Now the two plates moved past one
another at about the speed fingernails grow. This titanic, ongoing movement
created the modern landscape of the American Southwest. These
forces stretched and pulled the surface, which created a whole series of
north-south mountain ranges, separated by low basins. This basin and
The Joshua tree, a relative of the yucca, largely defines the cold, harsh Mojave Desert of North America. The bristling,
waxy spines of the Joshua tree can survive both prolonged drought and extended winter freezes. (Peter Aleshire)
range topography runs throughout the four deserts of North America,
including the Mojave.
After this continental stretching and sinking created the basic pattern
of the southwestern landscape, periods of uplift took place. By the time
they had finished, the shifts had created the differences in elevation that
distinguish the four great American deserts. (An example can be seen in
the color insert on page C-1).
About 20 million years ago, due to changing plate motions out west,
a great slab of what would one day become the western United States
began a rapid rise. The uplifting continues to this day in a huge area centered
on the Four Corners region where Utah, Arizona, Colorado, and
New Mexico meet. The rising Colorado Plateau is a sprawling region
just west and south of the Rocky Mountains. As it rose, the Colorado
River cut the Grand Canyon from its leading edge as it ran its long
course down toward the Gulf of California. Along its course farther
south, it largely defi nes the boundary between the Mojave and the
Sonoran deserts.
Some 2 million years ago, another period of uplift separated the Mojave
Desert from the much larger and lusher Sonoran Desert. This shift
made the Mojave Desert transitional between the lower, largely snowfree,
summer monsoon–graced Sonoran Desert and the high, cold, desolate
Great Basin Desert.
This sequence of geologic events created a landscape of dramatic extremes
in the Mojave Desert. The Mojave’s dry, hot summers and sometimes
frost-prone winters exclude many of the desert-adapted species
of the Sonoran Desert, like the distinctive saguaro cactus. Instead, the
world’s largest yucca, the Joshua tree, serves as the signature plant of the
Mojave Desert.
Two national monuments capture those extremes: Death Valley and
Joshua Tree National Monument.