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Mojave National Preserve: The Basin and Range Province

The volcanic eruptions that rocked what is now Mojave National Preserve coincided with a period (about 20-5 million years ago) of intense plate tectonic activity. During this episode, the Earth's crust was literally ripped apart, radically altering the landscape of the Southwest.

Basin and Range province map thumbnail
Cima Dome's granitic rock shown at 1.5X magnification. Note the coarse, grainy texture and very large potassium feldspars.

As the crust stretched, fractures formed along weak zones and mountain range-sized blocks jolted and slid against each other along faults. Some of these enormous blocks of crust rose up, forming rows of high, elongate mountains. Other blocks slid down, forming the low valleys. Together, the linear mountain ranges and intervening valleys define the Basin and Range Province (MAP).

The thinned, faulted crust made it easier for magma to rise up and follow weaknesses in the rock. Where magma reached the surface volcanoes grew and great globs of magma solidified to form plutons beneath them. In some places within the Preserve these geologically young plutons can be seen right at the surface. These plutons 'froze' at depth, then were almost immediately lifted up to the surface as mountains rose along new faults. This is truly a dynamic place!

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