White Sands National Monument in New Mexico’s Tularosa Basin
The Tularosa Basin is a graben basin in the basin and range area east of the Rio Grande in southern New Mexico, mostly in Otero County. It covers about 6,500 sq. miles.
The basin lies between the Sacramento Mountains to the east and the San Andres and Oscura Mountains to the west. The basin stretches about 150 miles north-south, and at its widest is about 60 miles east-west.
It is properly considered part of the Rio Grande Rift zone, which widens there due to the slight clockwise rotation of the Colorado Plateau tectonic plate. The basin is closed to the north by Chupadera Mesa and to the south by the Franklin and Hueco Mountains.
Notable features of the mountain-ringed basin include White Sands National Monument, which appears here as a small white patch just left of a small snow-covered peak near the bottom of the image.
The monument comprises the southern part of a 710-km² (275-mi²) field of white sand dunes composed of gypsum crystals, at an elevation of 4235 feet (1291 m).
Gypsum is rarely found in the form of sand because it is water-soluble. Normally, rain would dissolve the gypsum and carry it to the sea. Since the Tularosa Basin has no outlet to the sea, rain that dissolves gypsum from the surrounding San Andres and Sacramento Mountains is trapped within the basin, and the rain either sinks into the ground or forms shallow pools which subsequently dry out and leave gypsum in a crystalline form, called selenite, on the surface.
The selenite crystals, which reach lengths of up to one meter (3 feet), are eventually broken into sand-size grains by weathering and erosion. These grains are carried away by the prevailing winds from the southwest, forming white dunes. The dunes constantly change shape and slowly move downwind, covering the plants in their path. Some species of plants, however, can grow rapidly enough to avoid being buried by the dunes.