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Geology of Mojave National Preserve: Granite Mountains

As you approach Mojave National Preserve from the southwest you'll be greeted by an army of pink spires and pinacles standing above the surrounding countryside. Although these granite monoliths appear impressive from a distance, it's not until you enter the Preserve that you realize these imposing rocks rise hundreds of feet above the gently rounded terrain.

Colorful, sculptured volcanic rocks at Hole in the Wall Pink spires and pinacles stand like an army of soldiers guarding the ridges of the Granite Mountains.

Building the west

Like the granitic rocks you'll see at Cima Dome, the rocks of the Granite Mountains once formed the deep core of a volcanic mountain range. This range formed during the Jurassic and Cretaceous Periods when the western edge of North America was the site of intense tectonic activity.

Simplified view of a generic subduction zone
A generic subduction zone. Notice that a deep trench forms where the sinking plate dives beneath the more bouyant continental plate. This is the type of plate boundary found off the coast of Oregon and Washington today.

Since the end of the Paleozoic Era, North America had been on a collision course with an oceanic plate to the west. Where they met, the more dense oceanic plate sank beneath the more bouyant North American plate, forming a subduction zone.

Magma (molten rock) generated during this process rose into the North American crust, partially melting it. During it's upward journey, some of the magma stalled deep (about 20 km) in the crust where it cooled to form the solid granitic rocks you see in the Granite Mountains and Cima Dome.

Lifting the heart of an ancient mountain range

Shortly after the Granite Mountains rocks solidified deep in the crust, tectonic forces began that uplifted the entire region. As mountains rose, weathering and erosion began to break them down. It probably was not until just a few million years ago, relatively recently on the geologic time scale, that the granites of the Granite Mountains were first exposed at the surface.


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