Devils Tower, Wyoming
August 01, 2013
Photographer: Dale Chadwick; Dale's Web site
Summary Author: Dale Chadwick
Devils Tower in northeastern Wyoming is a remnant of an igneous intrusion. The rock is a phonolite porphyry with distinct feldspar crystals up to one half inch (about 1 cm) across. The large size of the crystals indicates a relatively slow cooling of the magma. As the magma cooled it formed columns most of which are six-sided with some four, five, or seven-sided examples. Arrows in the right photo point to climbers to give a sense of the size of these columns. Notice that the top third of the tower shows greater weathering than the surface below. Study of the lichen growth (lichenometry) indicates that the top of the tower was exposed to weathering before the end of the last ice age some 10,000 years ago. Continental glaciation didn't reach this far south but erosion has removed the sediments that encased the lower 1,200 ft (366 m) of the tower to the Belle Fourche River valley. Sediment has been removed at approximately 100 ft (30 m) per 1000 years. Photo taken on June 9, 2013.