Monzogranite boulders at Joshua Tree National Park with quartz intrusion aka "dike" near the Live Oak picnic area

- Image ID: BXME63
Ken Barber / Alamy Stock Photo
Image ID: BXME63
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: Joshua Tree National Park, California, USA