Not all glaciers are made of ice. Some glaciers are made of rock. More accurately, they are composed of a combination of ice and rock. Sometimes, these glaciers are "normal" glaciers covered by rock debris. Other times, water invades a field of rocks, freezes, and later deforms enough to allow the glacier to flow down a slope. Compared to ordinary glaciers, rock glaciers are rare, and may look like a rocky field shaped like a glacier.
Some fine examples of rock glaciers live in Colorado, including this glacier on the slope of Mount Sopris in the Elk Mountains, in the western part of the state. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite acquired this image on July 8, 2006.
This is a false-color image that mimics photo-like, natural color. Green indicates vegetation, dark blue indicates water, gray indicates bare rock, and white indicates ice or snow. Shaped vaguely like an amphitheatre but stretched out like putty, the rock glacier stretches off the northeast slope of Mount Sopris's east summit. Not far from the glacier's tip is lush-looking vegetation. Off the east edge of the glacier are some small bodies of water.
The flow rate for rock glaciers can vary, depending on each glacier's composition, underlying slope, and local weather conditions. In Colorado, the flow rates of rock glaciers range from less than 20 centimeters (8 inches) to more than 61 centimeters (24 inches) per year.