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Mojave National Preserve: Cima Dome

View of Cima Dome from the Providence Mountains
View of the smooth, rounded surface of Cima Dome from atop the Providence Mountain range.

Cima Dome is famous for its remarkably smooth dome shape. At one time, Cima Dome was a rugged, craggy mountain. How did it become so rounded? The story starts with the granite it's made from.

Forging the rock

Close up of Cima Dome's granitic rock.  Note the very large potassium feldspars.
Cima Dome's granitic rock shown at 1.5X magnification. Note the coarse, grainy texture and very large potassium feldspars.

Mojave's granitic rocks started out as great masses of molten rock called magma, deep beneath the Earth's surface. Vast expanses of solid minerals and simmering magma once fed explosive volcanoes miles above. Not all of the magma erupted at the surface. Much of the fiery mass remained trapped deep below miles of rock, allowing it to cool and solidify very slowly. With plenty of time to grow, large crystals formed, giving granitic rocks their distinctive coarse, grainy texture.

Most granitic rock in the Mojave Desert is old! Igneous rocks, rocks that crystallize from molten rock, date their birth the moment they solidify. Mojave's granites solidified during the Mesozoic Era, from 80 to 180 million years ago.

Many millions of years later, tectonic forces lifted these deep rocks to the surface, forming a high mountain range. Once exposed at the Earth's surface, other forces, weathering and erosion, began the work of breaking the mountians down.

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Continue to: Nibbling away the mountain


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