Great Lakes of the Great Basin Desert-Lake Bonneville

(Top: The Bonneville Salt Flats, formed by the evaporation of the former Lake Bonnevile. Bottom: An artist's rendition of the Red Rock Flood, where Lake Bonneville Breached the boundaries of the Great Basin.)

As with the Mojave Desert, parts of the Great Basin Desert look drastically different today than in certain periods of the past. Today in an area outside of Salt Lake City in Utah, horizontal rings line the hills, evidence of a landscape that was at one time covered in water. Even though the Great Salt Lake is expansive in comparison to other lakes of the region, it is just a fraction of the former Lake Bonneville, which stretched along Utah’s western edge, from the center of the state, north into what is now the State of Idaho, and West into Nevada. In fact, between 30,000 and 12,000 years ago, the Western United States was covered in enormous lakes, resulting from a pluvial period, a term that refers to wet climatic periods. During these pluvial periods, the climate was much cooler and wetter, many times coinciding with ice ages. At its height 15,000 years ago, Lake Bonneville was 1,000 feet deep, deeper than Lake Michigan, and it covered almost 20,000 square miles, its size unchecked due to the lack of drainage outlets in the Great Basin.


Lake Bonneville grew to such an enormous size that it eventually rose over the top of the Great Basin at Red Rock Pass in Idaho, breaching the sediments forming the pass, which in turn unleashed a colossal flood that lowered Lake Bonneville by hundreds of feet into a tributary of the Snake River in less than one year. It is difficult to understand the scale of this flooding event that unleashed a massive torrent of water into the Snake River. Erosion of Red Rock Pass formed what is known as the Provo line, which is the maximum height that water in the Lake Bonneville Basin could rise before it too rushes into the Snake River through Red Rock Pass, hundreds of feet lower than the location which Lake Bonneville finally overflowed the basin. Today the Great Salt Lake and terraces from former shorelines on the surrounding mountains hundreds of feet over the basin, as well as the Bonneville Salt Flats remind people of the vast lake. The desert outside of Salt Lake City sits in stark contrast to the lake that once dominated the landscape.


For more information on Lake Bonneville visit:

http://geology.utah.gov/online/pi-39/index.htm

http://historytogo.utah.gov/utah_chapters/the_land/lakebonneville.html

Rock Formations in the Mojave Desert

(Left: Layers of volcanic ash form the hillside around the Hole in the Wall rock formations. Right: Magma rose from deep underground and cooled to form the Hole in the Wall rock formations.)

The landscape of the Mojave Desert may look peaceful today, but millions of years ago the area was marred by devastating volcanic eruptions. Take for example the Hole in the Wall rock formations in the Woods Mountains, in San Bernardino County. Many visitors to the Mojave Desert have been captivated by these interesting rock formations, but few know that these formations were created by ancient volcanic activity over 17 million years ago. Magma from deep inside the earth rose to the surface, building up pressure that caused earthquakes to shake the desert floor. Magma then erupted from several vents in the area, covering the surrounding landscape in up to 200 meters of magma and volcanic rock.

After a few hundred thousand years of quiet, the area became active again. There were several large eruptions, but a pocket of magma and explosive gases was still trapped under the surface. The intense pressure finally caused a devastating eruption, hurling rocks up to 60 feet in length, up into the air. Magma and ash covered the country side in thick layers that fused together. The eruption site collapsed into a caldera that was partially filled by volcanic debris. No organism in the region could have survived this monster eruption. Scientists studying this eruption have difficulty understanding its magnitude, as no eruption of similar intensity has ever been noted in human history. After this colossal eruption, the ground exploded several more times during that period, causing eruptions of almost the same intensity. The area was again covered in magma and volcanic rock, and the caldera became deeper. A few hundred thousand years later, about 17.1 million years ago, magma again squeezed its way to the surface, although not in explosive eruptions of the past. This magma cooled to form the strange rock formations that the Hole in the Wall is known for. Looking at the Mojave Desert today, it’s hard to believe that this peaceful landscape was marred by colossal eruptions of a scope never seen in human history.

The United States Geological Survey offers a "virtual fieldtrip" of geological phenomena in the Mojave Desert, including the Hole in the Wall rock formations. Go to:

http://geomaps.wr.usgs.gov/parks/mojave/mojaveft.html

and click on "Hole in the Wall"

The Mysterious Sliding Rocks of the Mojave Desert

The Mojave Desert, stretching from Southern California into Northern Arizona and Nevada, is a high desert, and receives on average just a few inches of rain a year. Perhaps the best example of the extreme heat and aridity the Mojave Desert experiences is the region called Death Valley. With location names such as Furnace Creek, Death Valley is known for its brutal temperatures, once recording a North American record high temperature of 134 degrees Fahrenheit. But besides being known for its extreme weather conditions, Death Valley also contains many interesting geological phenomena, perhaps the most interesting and mysterious being the racetrack Playa.

The Racetrack Playa, is a dry lakebed which holds a mystery that has baffled scientists for over one hundred years. Boulders weighing up to 700 pounds have been mysteriously sliding across the flat lakebed, leaving behind trails that sometimes reach thousands of feet. Some boulders have straight trails, others have trails that curve, sometimes crossing themselves. No one has ever seen the boulders move, but GPS mapping provides evidence that the rocks have been sliding across the lakebed. Many theories have been proposed, including one that stated that the rocks were sliding due to gravity. This theory has been refuted by the discovery that one side of the lakebed is several centimeters higher than the other and that the rocks traveling in that direction are essentially sliding uphill. Scientists are considering many variables to this phenomenon, but research on the issue is difficult in the Racetrack Playa. The National Park service forbids the installation of cameras to record the movement of the rocks, and entry to the lakebed is prohibited during wet conditions because footprints would scar the landscape. This mystery will undoubtedly captivate scientists for years to come.

For more information on the sliding rocks of the racetrack Playa, visit
http://geology.com/articles/racetrack-playa-sliding-rocks.shtml

http://www.pbs.org/wnet/nature/deathvalley/rocks.html

Overview of American Deserts

(Left) A view of the Sonoran Desert in the Lower Colorado River Valley.
(Right) Giant Saguaro Cacti in the Sonoran Desert.

While flying in a commercial jet over the American West recently, I was fascinated by the desert landscape thousands of feet below. There were vast open stretches of land and mountains arising out of the desert floor all around. There were interesting formations in hillsides across the expansive desert that grabbed my attention during the flight. Although the landscape was interesting, I was glad I was flying over this barren, uninhabited territory, instead of driving across it in a car. These deserts are the product of multiple climatic processes, the most influential being one that begins at the equator, thousands of miles and many countries away.

Deserts cover a large portion of the American West, from California to central Texas, from Northern Mexico to Southern Idaho. There are four deserts that comprise this region, the Great Basin, the Mojave, the Sonoran, and the Chihuahuan. Like the other major deserts around the world, the climate of these North American deserts is influenced by atmospheric circulations that start at the equator. The equatorial region receives more radiation than any other area on earth. This creates warm air that is less dense, causing it to rise through the atmosphere, and travel either north or south. This air mass finally descends at the subtropical zones in the Northern and Southern Hemispheres. These circulation patterns are part of the phenomenon known as Hadley Cells. As the air descends in the sub-tropical zone, it is compressed and heated, creating areas of high air pressure. Warm air can absorb more water vapor than cooler air, so the descending air becomes dry. This warm dry air defines the North American desert climate. A further explanation of Hadley Cells can be found at www.earlham.edu/~biol/desert/whatis.htm

Another atmospheric process that influences desert climates is the rain shadow effect on the lee side of mountain ranges. When air rises as it hits the windward side of a mountain slope, it cools due to the concept that the temperature decreases as the altitude increases in the lower atmosphere. As the air cools it reaches its dew point, the point at which air can hold no more moisture, causing water vapor to condense into clouds and precipitation. As the air moves down the other side of the mountain, the lee side, any moisture that is still left in the air is evaporated as it warms up through compression as it descends. The air reaches the base of the leeward side warm and dry. This effect is notable east of the Sierra Nevada Mountain ranges and also influences the climate of the North American Desert in many locations. Much of the North American Deserts are also separated from the moderating effects of the ocean, leading to extreme temperatures. The interaction of these three climatic processes creates the landscape that I observed during my flight.