Sand in the dunes comes from the San Juan and Sangre de Cristo mountains.
The fine-grained San Juan sand is volcanic and contains ash and lava.
Sangre de Cristo sand contains many materials, such as granite, sandstone and gneiss.
Sand grains range in size from 0.2 millimeters to 2 millimeters.
The main dunefield comprises 12 billion cubic meters of sand, but it is the smallest of three sand deposits in the Great Sand Dunes system.
More than 53 percent of the sand grains are quartz, according to a 1961 study.
Source: "Exploring the Dunes," by Libbie Landreth and the National Park Service