There’s something otherworldly about the dunes of White Sands National Monument. They appear out of nowhere in an otherwise dirt-brown basin rimmed by mountains, but once you crest the first dune all you see is miles of white. Gypsum sand scatters sunlight in a blinding ocean of undulations unlike anywhere else on Earth. When I climbed them barefoot this past week I never got over my disorientation: was this a warm snow-scape? A dry beach? Or something even more alien?
Some might say these 275 square miles of mineral glitter resemble their vision of the lunar surface. After all, this pale sand is ground-down selenite, a crystalline form of gypsum named centuries ago for the Greek goddess of the moon.
In reality this region has less in common with the moon than with the Red Planet. Mars also has a region of selenite sand near its north pole that’s sculpted into dunes by thin CO₂ breezes. As a sign at the Monument notes, “In 2004 Mars Rover Opportunity landed on a playa made of gypsum, just like you see here at White Sands.”
Still skeptical that New Mexico could resemble Mars more than the moon? Compare the aerial texture of airless lunar regolith, Martian gypsum fields, and the white sand right here in New Mexico:
As you can see, it takes an atmosphere to reshape surface particles from craters into ripples. Maybe selenite should be renamed aresite after Ares, the Greek version of Mars.
Or we could just stick to the term gypsum, from the Greek for plaster or chalk. Gypsum is a common mineral consisting of calcium sulfate dihydrate (CaSO₄·2H₂O). At just 2.0 on the Mohs hardness scale, it can be scratched with a fingernail and, unlike typical silica quartz sand, it feels cool and soft even in granular form. However, while it’s common in landscapes around the globe, it’s incredibly rare to find it in sand form.
That’s because gypsum dissolves easily in water. A bit of rain is all it takes to erode deposits into smooth shapes and solutions, so sharp-edged crystals don’t tend to last long.
Yet these crystalline grains have piled up in White Sands National Monument for over ten thousand years, growing into the largest field of loose gypsum on the planet. How did these huge dunes appear way up at elevation in land-bound New Mexico?
The process began over 250 million years ago when the region was submerged in the Permian Sea. As water levels rose and fell over the millennia, layers of evaporites like gypsum collected on the bedrock. Then 70 million years ago tectonic activity crunched the region up into mountain ranges, lifting mineral deposits from an ancient seabed to mountaintops.
30 million years ago the plates transitioned from crumpling upward to rifting apart, creating shallow basins between tall ranges. Rain pelted the mountains regularly, dissolving gypsum and washing it down into the basin where it collected into the new mineral-rich Lake Otero.
When the the Rio Grande Rift closed off drainage at the southern end of the Tularosa Basin all that dissolved gypsum had nowhere to flow, so it began building up in the water there. As the Ice Age drew to a close 12,000 years ago and the climate took a dry turn, Lake Otero evaporated out of existence, leaving behind a playa thick with evaporites.
In the ten thousand years since then there was nothing for all that trapped gypsum to do but gradually break down into sand. Just like on Mars, the wind is what did it: first it swept away loose soil to expose shimmering white selenite crystals, then it whipped particles against each other in a steady sandblasting that produced beautiful fine-grain dunes.
And it ain’t over yet. The peaks that rim the Monument today still contain 250-million-year-old bands of solid gypsum—the source of ongoing sand formation. Every year seasonal rain and snow-melt sloughs more gypsum from the surrounding mountains down into the basin. It’s a good thing, too: while signs warn visitors not to remove sand from the Monument, the same winds that create it also sweep it off into the world, depleting the dunes bit by bit each day.
All the more reason to pay the area a visit next chance you get. Speaking as someone who grew up climbing the silica sand dunes of Lake Michigan (not to mention mountains of plowed snow), I didn’t expect my visit to feel as unearthly as it did. We may not make it to the moon or Mars in our generation, but we can all take a trek out to White Sands National Monument, kick off our shoes, and plunge our toes into that ancient gypsum.
This post was brought to you by a field trip down to White Sands National Monument.