Detail of a "dike" (quartz intrusion) between a couple of eroded monzogranite boulders in Joshua Tree National Park

- Image ID: BXME6Y
Ken Barber / Alamy Stock Photo
Image ID: BXME6Y
Detail of a "dike" (quartz intrusion) between a couple of eroded monzogranite boulders in the Mojave Desert portion of Joshua Tree National Park. The huge piles of monzogranite boulders that dot the landscape in this part of the Mojave fascinate almost everyone. Their formation started millions of years ago when cracks formed in granite bedrock as a miles-thick layer of rock over it eroded away. Then water percolated through the cracks and washed away some minerals while depositing others. Then mineral-laden molten rock was pushed into the cracks from below, forming the "dikes" (quartz intrusions), one of which is shown here. As the mostly-rectangular blocks of stone eroded, the corners and edges wore away first, leaving the rounded shapes that we see today.
Location: Live Oak picnic area, Joshua Tree National Park, California, USA