National Park Mystery Photo 30 Revealed: The Building Blocks of White Sands National Monument

Selenite crystals formed through dehydration form the white sand dunes at the national monument. National Park Service photos.

Captured in the accompanying photo are selenite crystals at Lake Lucero in White Sands National Monument.

These types of crystals created the sand dunes at the monument, which, the National Park Service tells us, holds "the largest pure gypsum dune field in the world..."

How did this dune field come to be, and what role did the selenite play? Here's how the Park Service explains it:

There are approximately 275 total square miles of dune fields here, with 115 square miles (about 40%) located within White Sands National Monument. The remainder is on military land that is not open to the public. This dune field is very dynamic, with the most active dunes moving to the northeast at a rate of up to 30 feet per year, while the more stable areas of sand move very little. The pure gypsum (hydrous calcium sulfate) that forms these unusual dunes originates in the western portion of the monument from an ephemeral lake or playa with a very high mineral content. As the water evaporates (theoretically as much as 80" per year!), the minerals are left behind to form gypsum deposits that eventually are wind-transported to form these white sand dunes.

You can learn more at the monument's website.