Rock Formations in the Mojave Desert

(Left: Layers of volcanic ash form the hillside around the Hole in the Wall rock formations. Right: Magma rose from deep underground and cooled to form the Hole in the Wall rock formations.)

The landscape of the Mojave Desert may look peaceful today, but millions of years ago the area was marred by devastating volcanic eruptions. Take for example the Hole in the Wall rock formations in the Woods Mountains, in San Bernardino County. Many visitors to the Mojave Desert have been captivated by these interesting rock formations, but few know that these formations were created by ancient volcanic activity over 17 million years ago. Magma from deep inside the earth rose to the surface, building up pressure that caused earthquakes to shake the desert floor. Magma then erupted from several vents in the area, covering the surrounding landscape in up to 200 meters of magma and volcanic rock.

After a few hundred thousand years of quiet, the area became active again. There were several large eruptions, but a pocket of magma and explosive gases was still trapped under the surface. The intense pressure finally caused a devastating eruption, hurling rocks up to 60 feet in length, up into the air. Magma and ash covered the country side in thick layers that fused together. The eruption site collapsed into a caldera that was partially filled by volcanic debris. No organism in the region could have survived this monster eruption. Scientists studying this eruption have difficulty understanding its magnitude, as no eruption of similar intensity has ever been noted in human history. After this colossal eruption, the ground exploded several more times during that period, causing eruptions of almost the same intensity. The area was again covered in magma and volcanic rock, and the caldera became deeper. A few hundred thousand years later, about 17.1 million years ago, magma again squeezed its way to the surface, although not in explosive eruptions of the past. This magma cooled to form the strange rock formations that the Hole in the Wall is known for. Looking at the Mojave Desert today, it’s hard to believe that this peaceful landscape was marred by colossal eruptions of a scope never seen in human history.

The United States Geological Survey offers a "virtual fieldtrip" of geological phenomena in the Mojave Desert, including the Hole in the Wall rock formations. Go to:

http://geomaps.wr.usgs.gov/parks/mojave/mojaveft.html

and click on "Hole in the Wall"