Ajo Mountain Drive Stop 9

As you make your way through the Ajo Mountains, notice the many layers of volcanic rock.

NPS Photo/K Ceballos

Quick Facts

Location:

Mile 8.4 [km 13.5] along the Ajo Mountain Drive

Stop 9

Take a Closer Look

You have now traveled over the plains and into the Ajo Mountains of Organ Pipe. Geologists recognize this landscape as part of the vast basin and range province that includes much of the interior west of the United States and parts of northwestern Mexico as well.
 
To understand how this landscape was formed, we need to go back in time 25 million years when forces deep within the earth pulled the land apart. As the land was stretched, vast areas sank and formed broad valleys. These valleys were intersected by uplifted blocks of land, creating today’s Ajo Range. Other mountain chains appeared one after the other as far as the eye could see. As the surface was slowly deformed, volcanoes, lava flows, and falling volcanic ash accumulated and resulted in the colorful layers of rock that are revealed all around you.
 
The pink and chocolate-colored rock is rhyolite, formed from solidification of a slow moving, thick lava flow. The light brown or tan rock layers, called tuff, is volcanic ash turned to stone. Over time, the mountains have been worn down and the valleys filled by rock and debris from the powerful effect of water cutting through and carrying away layers of solid rock.