Chiricahua National Monument, Arizona, USA, images by Tom Dempsey: The Heart of the Rocks Loop Trail (7 to 9 miles) makes an excellent day hike through fascinating arrays of hoodoos. 27 million years ago, huge volcanic eruptions laid down 2000 feet of ash and pumice which fused into rhyolitic tuff. This rock has eroded into fascinating hoodoos, spires, and balanced rocks which lie above the surrounding desert grasslands at elevations between 5100 and 7800 feet. At Chiricahua, the Sonoran desert meets the Chihuahuan desert, and the Rocky Mountains meet Mexico's Sierra Madre, making one of the most biologically diverse areas in the northern hemisphere.

Colorful cliffs of rhyolite (solidified volcanic ash layers) rise 2000 feet above white sycamore trees in Cave Creek Canyon, in Coronado National Forest, near Portal, Arizona.