February 25, 2008

Our Desert Home - Cima Dome


Special to the Press Dispatch
Victorville Daily Press

Photo from Back Roads West

From the rest stop on Interstate 15 near Cima Road in eastern San Bernardino County, a gently sloping mountain can be seen to the south. It is covered by Joshua trees. Outcrops of granite rocks stand tall near its summit.

This is the Cima Dome.

There are a half dozen of these features in the Mojave Desert, known to geologists as domes. They are rare.

Domes are large features covering a hundred or more square miles in area. The Cima Dome is perhaps the easiest to recognize. The best views of the dome are from the crests of the Mid Hills, which are to the south; Cima Road traverses Cima Dome between Interstate 15 and the post office at Cima.

Topographic maps show circular contour lines on the dome. If you put a pencil under a sheet and align the pencil until it is straight up, you will create a model of a dome.

Looking down on your model you should be able to observe that the lines of equal elevation, or contour lines as they are called, are circular. The top of the dome comes to a point, like an upside-down cone.

To understand how Cima Dome was formed, you have to understand a little about the history of the crust of the Earth in the Mojave Desert.

The Mojave Desert has been subject to a lot of pushing and pulling by tectonic forces in the last few million years. First the crust was stretched and thinned by the same forces that formed the Basin and Range Region of Eastern California, Nevada and Utah.

The thin crust of the Mojave Desert was then subjected to compression by forces placed on it when the Baja California micro-plate collided with North America. When these forces squeezed the thin crust of the Mojave Desert, hot plastic material from beneath the crust, known as the mantle, was pushed up to form the dome.

The Cima Dome has a thin crust and there is a bulge of mantle material beneath the crust. Some of this mantle material found its way through fractures and faults to form the 31 or more volcanoes of the Cima Volcanic Field, on the western flanks of the dome.

On your next trip to Vegas, spend some time at the rest stop between Cima Road on Interstate 15 and look at Cima Dome. Think about what is under your feet in the crust and mantle. This should help you understand some of the forces that are constantly changing “Our Desert Home.”

Bud Lorkowski is a retired science teacher. He is currently doing a geologic study in the Mojave National Preserve for the National Park Service. He has recently completed a geological study of Hagerman Fossil Beds National Monument in Idaho.