February 27, 2003
The photo above shows a large, 30 meter (100 feet), conchoidal fracture along a mesa cliff on the Colorado Plateau of northern Arizona. It's about 16 km (10 miles) north of the town of Kayenta. This kind of circular fracture pattern is common in obsidian (volcanic glass). However, conchoidal fracture is generally observed on a scale of a few centimeters rather than tens of meters. Similar fractures can be seen in other areas of the Colorado Plateau, in places where the rock units are very homogeneous. Whether the above fracture pattern is simply a gravity fall, or perhaps the result of an impact event is not clear.