Covering nearly 300 square miles of the Chihuahuan Desert, White Sands National Monument in south-central New Mexico preserves the largest gypsum sand dune field in the world. I was fortunate enough to see this natural wonder on a college geology trip back in 2008.
Since gypsum (hydrous calcium sulfate) is water-soluble it rarely occurs as surface sand on earth. Under normal circumstances it’s weathered from parent rocks and washed out to the sea in streams and rivers. In this particular location, however, the unique geology, geography, and climate provide the ideal conditions for it to accumulate.
White Sands is found in a part of North America that is undergoing tectonic extension. Here the crust of the North American plate is slowly being pulled apart and thinned by tension.
As the crust is pulled apart, it faults and separates. Central blocks known as grabens drop in elevation relative to the surrounding higher blocks, known as horsts. In North America these structures create the basin and range province, a widespread area of roughly parallel and alternating low, arid valleys and high mountain ranges.
In the White Sands area extension has created the Tularosa Basin, bounded by the San Andres and Sacramento Mountains. This basin lacks an outward drainage. When gypsum is weathered from the surrounding mountains in rainwater, it collects in the basin with nowhere to go. In the arid desert environment, the water rapidly evaporates and leaves the gypsum evaporate behind.
Although the dunes of White Sands appear white, the gypsum grains are actually translucent. The white appearance is a result of the reflection of light off of the grains. Coincidentally, this reflection also prevents the sand from heating in the sun as with the far more common quartz sand.
Wind can transport these gypsum dunes up to 30 feet per year. Between the shifting sand and alkaline pH, it can be difficult for plants to gain a foothold in this hostile environment.
Despite the difficultly, some plants have established themselves in the sand. They work to stabilize and slow dune progression with extensively deep root systems that desperately seek out the limited desert moisture.
High desert winds constantly battle back, removing sand from around the plants and threatening their existence.
As with other dune systems, plant life and the harsh forces of weather and geology seem to have struck a tenuous balance. Although the ecology is fascinating, the beauty is even more satisfying.
It’s worth noting that White Sands National Monument is encompassed by the White Sands Missile Range. At times the park and US Route 70 are subject to occasional closures for missile testing. The official NPS site provides information on closures for planning purposes.
I’m a big fan of natural superlatives like “highest,” “lowest,” “hottest,” “coldest,” “wettest,” and “driest.” As the largest gypsum sand dune field in the world, White Sands appealed to me. In addition to being one of the most unique landscapes on earth, it’s also one of the most beautiful.