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Geology of Mojave National Preserve: Cima Dome

Individual grains of feldspar lie surrounded by soil.  The minerals surrounding these large crystals have weathered to form clays, leaving more resistant feldspars behind.
Individual grains of feldspar lie surrounded by soil. The minerals surrounding these large crystals have weathered to form clays, leaving more resistant feldspars behind.

Nibbling away the mountain

Once at the surface, granite falls victim to the slow, gnawing forces ofweathering and erosion. Water falling on the rock as rain or snow and groundwater percolating just beneath the surface begins to disintegrate the rock grain by grain.
Water and dust particles gradually work their way between individual mineral grains. Mica and feldspar grains in the granitic rock chemically react with the water and decompose to form clay minerals. Clay minerals and dust tend to swell-up when they get wet, then shrink when they dry out. These tiny particles act as wedges between more chemically resistant minerals, prying them loose from the solid rock, like the potassium feldspar grains in the image on the right. Over time, the granite ‘rots' and streams begin to carry away the loose, rotten granitic soil.
Notice the bumpy crystals of feldspar protruding from the granitic boulder in the foreground.
Notice the bumpy crystals of feldspar protruding from the granitic boulder in the foreground.

Eventually, the mountain retreats back, leaving a gently sloping surface behind. Over millions of years mountains can retreat so that just a few exposed boulders remain, like those you see on the top of Cima Dome.

 

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