Arch Rock Inselbergs
The inselbergs here are made of White Tank Granite, an igneous rock formed when hot magma from the earth’s mantle forces its way above ground into existing rock then cools and hardens into a new layer (the line where they meet is called the “contact zone”). Geologists call this process intrusion, and those who study how it worked in Joshua Tree say it took place here about 150 million years ago.