In deserts,
sparse vegetation lays bare the structure of the landscape. Sand Dunes, rugged
mountains, dramatic canyons, fault lines, volcanic cinder cones, and salt-cracked
playas stand unveiled to the eye.
The forces of
geology and weather continue their work today. The Pacific Plate still grinds
slowly northward along the San Andreas Fault, from time to time creating powerful
earthquakes as it moves. Mountains still push upward, and wind, water, and
gravity wear them back down. Mostly the process of change occurs too slowly
to be noticed in a mere human lifetime, but when the ground begins to dance
under our feet, we are reminded that Mother Earth never sleeps.
Get some rocks
in your head! Learn more about the geology of California's deserts by following
the links on the terms on this page or viewing Desert
Lingo.
What forces have shaped these lands? You'll find clues just about everywhere.
As you drive through the desert, practice your detective skills. Keep your
eyes peeled for these telltale features.
The desert is wrinkled with mountains and valleys. In other places, volcanic
craters and cones add pimples and pockmarks. All these features are clues
you can use to deduce the slow, but powerful movement of Earth's tectonic
plates. Our deserts are laced with faults where pieces of Earth's crust move
and grind against each other. All that bumping and grinding causes earthquakes
and volcanoes and builds mountains.
You can even
find clues in the bands of color visible on the sides of mountains. Colored
bands mark different rock layers, which tell us about how and when they formed.
Gray layers of limestone mark where the shells and skeletons of ancient sea
creatures were deposited on the sea floor, buried and cemented into stone
before being uplifted into the mountains we see today. White layers of sandstone
may include sediment from an ancient beach.
Those impressive
piles of giant boulders you see in Joshua
Tree National Park and Mojave National
Preserve are granite formations called plutons. They formed as molten rock
cooled deep within the Earth before being uplifted and exposed.
Every now and then, the desert just blows its stack. Volcanic eruptions have
shaped the lands we see today. A huge eruption 25 million years ago covered
Death Valley and much of Nevada with lava, cinder, and ash. Other eruptions,
large and small, have raised volcanic peaks, covered the land with lava, and
brought tons of boron and other minerals to the surface.
As you travel
through the desert, keep your eyes open for volcanic cones, craters, lava
fields, and other signs of volcanic activity. Here are a couple of places
to get you started.
In the northern
part of Death Valley National Park, a pockmarked landscape of a dozen volcanic
craters formed within the past few thousand years. As molten rock rose close
to the surface it met groundwater. Steam from the super heated water blew
the lid of overlying rocks clear off, leaving gaping craters. The largest,
Ubehebe Crater, is over 700 feet deep and a half-mile wide.
Cinder Cones
National Natural Monument in the eastern Mojave contains dozens of well-preserved
volcanic cones formed in the last eight million years as lava, gas, and ash
pushed through pipe-like holes in the Earth. You can also see lava flows here.
Desert rains wash rocks, gravel, and soil down canyons on mountain slopes.
Over time, the debris forms fan-shaped mounds called alluvial
fans, which
seem to pour from the mouths of canyons. Bajadas occur where several alluvial
fans merge. Death Valley is world-famous for its alluvial
fans and bajadas.
You can see some of the best examples from Dante's View.
If you'd lived here a few thousand years ago, you could have owned lakefront
property. Watch for "playas," or dry lake beds - those salt flats
out on low valley floors - as you drive in the desert. At the end of the last
ice age, when the weather here was a lot wetter, those were all freshwater
lakes. Lake Manly, in Death Valley, was 600 feet deep and nearly 100 miles
long. But as the climate changed, the lakes became saltier and eventually
dried up. There are more than 50 playas (playa means beach in Spanish) in
our deserts. Their cracked surfaces remain dry except after heavy storms.
When the water
goes, only salt remains; we mine these former lakes for borax, chlorides,
and other salts. The old lakes are handy for other things, too. Edwards Air
Force Base and NASA's Dryden Flight Research Center were built near large
dry lakes. The flat lake beds make a handy landing place for the Space Shuttle
and other large aircraft.
Five hundred
million years ago, you could have sailed the Titanic over our deserts and
never hit an iceberg, let alone a sand dune. Ancient tropical seas - much
like the Caribbean - covered southern California at least twice. Back then,
the ocean reached all the way to eastern California.
The limestone
rocks and mountains we see today were formed back then. Sediments from land
along with the shells and skeletons of millions of generations of corals,
algae, and oysters and other shellfish built up in layers of muddy ooze. The
weight of new layers compressed and hardened older ones, creating limestone.
Eventually, the movement of the Earth's tectonic plates pushed the rock up
into mountains.
You can see
evidence of these ancient sea creatures in Death
Valley National Park. Titus
Canyon is an ideal area to explore some of the 20,000 feet of limestone deposits.
In Providence Mountains State Recreation
Area, Mitchell Caverns are being
carved as water slowly dissolves away the limestone rock.
You can see
the fossil shells in the Marble Mountains and over by Split Mountain Anza-Borrego.
And down near Yuba, you can find ancient oyster beds.
Wind caves,
located in the Coyote Mountains (a designated wilderness area), are covered
with exposed marine fossils raised hundreds of feet above sea level. The caves
were created by wind and water erosion of the sandstone.
Q. How
old are the rocks in the desert?
A. Some are positively ancient, almost two billion years old. But some
are just youngsters, they haven't even reached their millionth birthday yet.
Q. Why
are some rocks dark and shiny?
A. Some desert rocks look like someone's painted them with varnish.
But you won't find anybody out there with a paintbrush. The weather's the
artist here. Years and years of wind, heat, water, and sun wear the surfaces
of rocks, slowly coating them with a glossy layer of iron and manganese. Some
bacteria add their touch as well, depositing manganese so that it coats the
rocks.
Q. What
causes the vibrant colors in areas such as Death Valley's Artist Drive?
A. As minerals in the rocks-especially minerals rich in iron-weather
over the ages, they produce the brilliant palette of reds, oranges, yellows,
and other colors that coats the rocks.
Q. Where
are the dinosaurs?
A. Well, if you'd been here back then, you'd know that dinosaurs never
lived in these parts. But we had our share of big mammals: mammoths and mastodons,
camels, rhinos, three-toed horses, and dog-bears! And of course, you'd have
had to have always kept a lookout for saber-toothed tigers. They're gone today,
but you can see their fossils in some local desert museums.
Q. Are
there volcanoes in the desert?
A. The desert's been letting off steam for a long time. Some volcanoes have erupted here within the last 10,000 years. Keep your eyes open and you'll
see cinder cones, craters, and lava flows all over the place.
In the minds
of many people, nothing says "desert" better than sand dunes
off in the distance. Theres more to a desert than sand, but California's
got its share of dune fields. Some are only a few square miles; others
sprawl over 200 miles. They form wherever winds, carrying fine grains of
sand, drop their load at the base of mountains, or some other obstacle.
Some of the
dunes are famous for their singing sands. The Kelso
Dunes, Imperial Dunes,
and Eureka Dunes make a gravely barking or moaning sound as you slide or walk
down them. You can even hear a resonant hum as dry sand cascades down the
steep dune faces. Don't ask why, though. No one knows for sure, but it's got
something to do with how the sand grains rub against each other.
Here are some
of the larger dunes in our deserts.
The Kelso Dunes complex in the southwestern part of Mojave
National Preserve has formed over the past 25,000 years. During periods when the nearby lakes
were dry, sediments from the lakebeds were carried by the wind and deposited
into the dunes.
Death Valley National Park holds five dune systems. The Eureka Sand Dunes
near the north edge of the park tower 680 feet high - the tallest in California.
The Imperial or Algodones Dunes in the southeast corner of the state is the
largest mass of sand dunes in California. The dunes extend for more that 40
miles in a band averaging five miles in width and rising to heights of 300
feet above the desert floor.
