Mojave National Preserve

Geology
Gorp.com

The Mojave National Preserve takes in the easternmost section of the Mojave Desert. The desert is roughly delineated by two major faults—the San Andreas and the Garlock. These faults mark the stress points on a landscape that has been in formation for hundreds of millions of years. Plate tectonics hold the secret to today's desert. As much as a half-billion years ago, the western coast of the North American plate ended somewhere near today's Nevada-Arizona border. Sediments from this continental mass were washed into the sea, creating shale and sandstone. Ocean-based sediments created limestone and dolomite, such as those now found in the Providence Mountains within the Mojave National Preserve.

Between 245 million to 65 million years ago the North American continent began to override the oceanic plate. The pressure of this collision crumpled the western edge of the continent and promoted the resulting uplift of the ancestral Sierra Nevada as well as the highlands in the Mojave. As the oceanic plate dove under the North American continent, the rock melted and magma flowed up toward the surface. Some flows erupted as volcanoes, but other molten rock cooled slowly deep in the earth to create a very erosion-resistant plutonic rock that we know as granite. The rocks that comprise the core of the Granite Mountains and the New York Mountains in the Mojave National Preserve were formed during this period.

Then about 25 million years ago the San Andreas Fault began rifting California apart, sliding northward. This opened up the Gulf of California and stretched apart the Basin and Range Province. Mountains rose along faults and valleys dropped. This extension is ultimately responsible for the uplift of nearly all the mountains in the California desert. The stretching of the earth also permitted magma to find its way to the surface, and volcanic eruptions poured out lava across the landscape. In the Mojave National Preserve, eruptions of rhyolite flowed across the landscape, capping softer volcanic ash. The Mid Hills area, particularly the Hole in the Wall, are exposed volcanic rock formations. Even younger volcanic flows and cinder cones are evident in the Cinder Cones area just south of Baker. Here there are more than 30 extinct volcanoes, although the most recent eruptions ended less than 1,000 years ago.

The finishing touches on the Mojave landscape occurred during the past 20,000 years during the tail end of the Ice Age. Wetter conditions resulted in the formation of large lakes in the California desert. A giant glacial-age water body, Lake Mojave covered much of the eastern Mojave. But as the climate dried during the past 10,000 years, most of these lakes disappeared and left behind only salt pans and seasonally flooded playas. Soda Lake Playa near Baker is a relict of this lake.

Another geologic feature associated with Lake Mojave is the Kelso Dunes. This unique and isolated dune system rises more than 600 feet above the desert floor. The dunes were created by southeasterly winds that blew fine-grained residual sand from the Mojave River sink (Soda Lake), which lies to the northwest. The color of the dunes is created from many golden rose quartz particles. When the dry sand grains slide down the steep upper slopes, a notable booming sound is produced. In some years the dunes offer a nice spring wildflower display.

Another indirect consequence of the higher precipitation during the glacial period is the caves and passages at Mitchell's Caverns in the Providence Mountains. Mitchell's Caverns are now part of Providence Mountains State Park. The state park is completely surrounded by the Mojave National Preserve. The caverns are named for Jack Mitchell, the first owner and promoter of the caves.

The caverns were formed when sedimentary limestone and metamorphosed limestone (marble) were dissolved by groundwater high in carbonic acid. The continued dripping of highly mineralized groundwater into the caverns produced stalactites (dripstone deposits extending downward from the ceiling) and stalagmites (dripstone deposits building upward in mounds from the floor).

Mitchell's Caverns consists of three basic caves: El Pakiva, or the Devil's House; Tecopa, named for one of the last chiefs of the Shoshone; and the deep and vertical Winding Stair Cave.


Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication

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