Sand Dunes of the Southwest

White Sands, NM

White Sands

Olsen Family Photo

The White Sands of New Mexico are unique in that they consist of gypsum rather than quartz sand grains. Gypsum is a mineral composed of calcium sulfate (calcium, sulfur, and oxygen) and water. Gypsum is rarely found in the form of sand because it is water-soluble, so rain would normally dissolve the gypsum and carry it away. However, the White Sands are located in a high desert area at the northern end of the Chihuahuan Desert, which averages only about 8-10 inches (250 mm) of rain per year.

Although today the area is mostly dry, in the last Ice Age a lake covered much of the Tularosa Basin where the White Sands are situated. Since the mountain-ringed basin is an internally drained valley with no outlet, gypsum that dissolved from the surrounding San Andres and Sacramento Mountains was trapped within the basin. When the water dried out, it left gypsum in a crystalline form, called selenite, on the surface. This process continues today with the water that drains into and evaporates from the Lake Lucero playa, which lies a few miles southwest of the dune field.

Weathering and erosion eventually break the selenite crystals into sand-size grains, which are carried away by the prevailing winds from the southwest, blowing across the dry lakebed and forming white dunes. The dunes constantly change shape and slowly move downwind, covering the plants in their path. The most active dunes are migrating to the northeast at a rate of up to 30 feet (9 meters) per year. Some species of plants can avoid being buried by the dunes through a process called "stem elongation." The plants quickly grow upward so that their leaves stay above the rising sand.

There are four types of dunes at White Sands National Monument: barchan, parabolic, transverse, and dome dunes. Unlike dunes made of quartz-based sand crystals, the glistening white gypsum can be walked upon safely with bare feet even in the hottest summer months. In areas accessible by car, children frequently use the dunes for downhill sledding all year-round.

White Sands

Google Earth image

White Sands National Monument is located about 25 km (15 miles) southwest of Alamogordo, New Mexico, at an elevation of 4,235 feet (1,291 m). This national monument comprises the southern part of the 710-kmē (275-square-mile) glistening white sands, preserving a major portion (115 square miles, or approximately 40%) of the largest pure gypsum dune field in the world. Also protected within the monument are the plants and animals that have adapted to this arid, highly mineralized, constantly changing environment. The rest of the dunes are found on the White Sands Missile Range, military land that is used for testing various weapons and which is not open to the public.

White Sands Map

White Sands National Monument Map

References:

http://home.nps.gov/whsa/naturescience/index.htm
http://www.nature.nps.gov/geology/parks/whsa/index.cfm
http://www.nps.gov/archive/whsa/Geology%20of%20White%20Sands/GeoHome.html

Website design and content (c)2010 by Peter Olsen. This educational unit study was my PVCC honors project for GPH 211 Landform Processes.