Why Is the Sand White at White Sands?

The sand at White Sands, in southern New Mexico, is white because it is not made of the quartz grains that form most of the world’s beaches and deserts. Instead, it is composed almost entirely of gypsum, a soft mineral made of calcium sulfate bound with water molecules. Gypsum crystals are naturally translucent to white, and when wind breaks them into fine grains, the result is a dune field that gleams like snow under the desert sun. What makes this especially unusual is that gypsum rarely survives long enough to build dunes at all, which is why White Sands is one of the largest gypsum dune fields on Earth.

How Gypsum Ends Up as Sand

Most sand is quartz because quartz is chemically stubborn. It resists weathering, so it persists through millions of years of tumbling down rivers, washing across coastlines, and blowing through deserts. Gypsum is the opposite. It dissolves easily in water, which is why gypsum sand dunes are extraordinarily rare on our planet. Under normal circumstances, rain would dissolve gypsum crystals long before wind could sculpt them into dunes.

White Sands exists because of a specific set of geological conditions in the Tularosa Basin. The basin is a closed depression surrounded by mountains, with no river outlet to the sea. For millions of years, water has dissolved gypsum from the surrounding rock formations and carried it downhill into the basin floor. During the last ice age, this water collected into a large body called Lake Otero. As the climate warmed and dried, the lake shrank and eventually vanished, leaving behind thick beds of gypsum crystals on the exposed lake bed. The flat area that remains, known as Alkali Flat, is the factory that feeds the dune field. Wind picks up gypsum crystals from its surface, breaks them into sand-sized grains, and carries them downwind to build the dunes.

The closed basin is the key. Because there is no drainage to the ocean, the dissolved gypsum stays trapped. And because the climate is arid enough to evaporate the water but windy enough to move the dried crystals, you get a self-replenishing supply of white sand. It is a geological sweet spot that very few places on Earth replicate. Gypsum dune accumulations elsewhere in the American Southwest are also tied to the deflation of evaporite deposits in similar closed or semi-closed basins, making the entire phenomenon firmly linked to the interplay of tectonics, climate, and hydrology in the region.¹

Why Gypsum Sand Looks So Different from Beach Sand

If you pick up a handful of White Sands gypsum and compare it to beach sand, the differences go beyond color. Quartz sand grains are hard, rounded, and feel gritty between your fingers. Gypsum grains are softer, more angular, and have a cool, almost silky texture. Gypsum ranks only 2 on the Mohs hardness scale, which means you can scratch it with a fingernail. That softness is part of why the grains stay bright white: they do not accumulate the iron oxide coatings that give quartz sand its yellow or reddish tint in many deserts.

The whiteness also persists because gypsum does not absorb much visible light. Each grain scatters sunlight in all directions, the same way snow does. On a sunny day, the dune field can be blindingly bright, and the surface stays remarkably cool underfoot compared to dark desert sand. Visitors often remark that walking on the dunes feels more like walking on a cool powder than on the scorching sand they expect in a New Mexico desert.

The Dune Field Is Still Moving

White Sands is not a static landscape. The dune field shifts and reshapes itself continuously, driven by prevailing southwest winds. A striking feature of the area is how the shape of the dunes changes systematically as you move from west to east. On the western side, closer to Alkali Flat where fresh sand enters the system, the dunes are bare, crescent-shaped formations called barchans, with their arms pointing downwind. About seven to eight kilometers farther east, plants begin to colonize the dunes, and their shape flips to parabolic forms with arms pointing upwind.²

This transition happens because the rate of sand transport drops as you move away from the source. When less sand is blowing through, plants can get a foothold. Once vegetation takes hold on a dune, it anchors the arms in place while the center continues to move, effectively reversing the dune’s geometry from barchan to parabolic. The result is a living map of how wind energy, sand supply, and biology interact: a barren, fast-moving dune field on the upwind side, and a slower, vegetated landscape on the downwind side.

Animals That Turned White to Match

The white gypsum landscape has had a profound effect on the animals living on it. Several species that also inhabit the surrounding dark desert soils have, over the span of just a few thousand years, evolved distinctly pale forms on the dunes. The most studied examples are three lizard species that developed blanched coloration, presumably to avoid being easy targets for predators against the bright sand.³

What makes these lizards scientifically interesting is not just that they are pale, but how they got that way. Researchers have found that the blanching in at least two of the species, Sceloporus cowlesi and Aspidoscelis inornata, is tied to changes in a gene called Mc1r, the melanocortin-1 receptor, which plays a role in pigment production. Genomic analyses have found strong evidence of natural selection acting on this gene region in the white-sand populations, distinguishing them from their darker relatives nearby.⁴

A third species, the lesser earless lizard (Holbrookia maculata), has been a textbook example of this kind of convergent evolution: different species arriving at the same solution to the same problem independently. For years, the White Sands lizards were considered a unique case. Then researchers discovered that the lesser earless lizard has also colonized the Salt Basin Dunes in Texas, another gypsum habitat, and evolved a blanched form there too. Finding the same evolutionary pattern repeated in a geologically separate white-sand environment strengthened the case that gypsum landscapes create powerful selection pressure for lighter coloration.⁵

The lizards are not the only pale residents. White Sands is home to light-colored moths, beetles, crickets, and even some pale rodents. The consistency of the pattern across such different types of animals underscores how strong the selective pressure for camouflage is in this environment.

Footprints That Rewrote Human History

The gypsum-rich sediments at White Sands have preserved something far older than the dunes themselves. In layers of ancient lakebed sediment within the park, researchers have excavated human footprints dating to between roughly 23,000 and 21,000 years ago.⁶ That places people in North America during the Last Glacial Maximum, thousands of years earlier than the conventional timeline had long assumed.

The dating was initially based on radiocarbon analysis of seeds from an aquatic plant found in the same sediment layers as the footprints. Because those seeds came from a species that takes up carbon from water (which can carry older carbon from dissolved minerals), some researchers questioned whether the ages were artificially inflated. A follow-up study addressed this by dating terrestrial pollen from the same layers, along with a separate luminescence dating method applied to the surrounding sediment. Both approaches confirmed the original age range, reaffirming that the footprint chronology is solid.⁷

The footprints also provide evidence that these early humans coexisted with Pleistocene megafauna. Tracks from giant ground sloths and mammoths appear in the same layers, and in some cases the human and animal tracks overlap, suggesting close encounters. The fine-grained gypsum sediment was ideal for preserving detailed impressions, including the footprints of children and teenagers, offering a surprisingly intimate snapshot of daily life during the ice age.

Why Gypsum Dunes Are So Rare Worldwide

If gypsum is a common mineral (it forms wherever seawater or saline lake water evaporates), why aren’t white gypsum dune fields found all over the world? The answer circles back to solubility. In most environments where gypsum deposits exist, rainfall eventually dissolves them and carries the material back into solution. Building dunes requires that gypsum crystals survive on the surface long enough to be weathered into grains and blown around, which means you need an arid climate with limited rainfall, a source of exposed gypsum, and wind strong enough to move the grains.

You also need that trapped-basin geometry. In a normal river drainage, dissolved gypsum flows to the sea and disperses. In a closed basin like the Tularosa, it concentrates. When the water evaporates, the gypsum precipitates out and stays put. These conditions align in a handful of spots globally, but White Sands is by far the largest and most dramatic example. Smaller gypsum dune fields exist in parts of the Middle East and North Africa, often associated with coastal sabkhas (salt flats near the sea), but they lack the scale of the Tularosa Basin’s 275-square-mile field.

Life Inside the Crystals

Gypsum is not just a substrate that organisms live on; some organisms live inside it. Microbes, including cyanobacteria and eukaryotic algae, colonize the interior of gypsum crystals and crusts in arid environments around the world. The mineral is translucent enough to let photosynthetically useful light pass through while shielding the organisms from the intense ultraviolet radiation and desiccation of the desert surface.⁸

At White Sands, biological soil crusts (often called biocrusts) form on stable gypsum surfaces between and around the dunes. These crusts are communities of cyanobacteria, lichens, mosses, and other microorganisms that bind the soil surface and fix carbon. Research on Chihuahuan Desert gypsum soils has found that lichens and mosses fix carbon at higher rates than cyanobacterial crusts, and that all types recover their photosynthetic activity quickly after being wetted, even after long dry periods.⁹ These crusts play a quiet but important role in stabilizing surfaces that would otherwise be bare gypsum, and they form the base of a food web that supports the invertebrates, lizards, and rodents discussed earlier.

A Rehearsal for Mars

White Sands has attracted attention well beyond terrestrial geology. NASA’s Mars Reconnaissance Orbiter identified a massive field of gypsum-rich dunes called Olympia Undae near Mars’s north polar cap. Understanding how gypsum dunes form, accumulate, and transport sediment on Earth is directly useful for interpreting what happened on Mars, where the presence of gypsum implies that liquid water once existed on the surface long enough for evaporite minerals to precipitate.¹

The parallels go further than dune shapes. The fact that microorganisms thrive inside gypsum on Earth, shielded from UV radiation by the mineral itself, makes gypsum deposits on Mars a high-priority target for astrobiological investigation.⁸ If microbial life ever existed on Mars, translucent gypsum would have offered one of the more plausible refuges as the planet’s surface became increasingly hostile. White Sands and similar terrestrial gypsum environments serve as testing grounds for the instruments and hypotheses that future Mars missions will carry.

Why the Dunes Will Not Last Forever

For all their grandeur, the dunes at White Sands are geologically temporary. The same solubility that makes gypsum dune fields rare also means they are fragile. If the regional climate were to become significantly wetter, increased rainfall would dissolve the dunes over time. If the water table beneath the basin rose high enough, it could dissolve the dunes from below. The dune field has expanded and contracted over thousands of years in response to climate cycles, and it will continue to do so.

Human activity adds another variable. Water management in the region, including groundwater pumping and irrigation upstream, can alter the hydrology of the basin in ways that affect how much gypsum reaches Alkali Flat and how quickly it dries out and becomes available for transport. The dune field’s ongoing existence depends on a balance between water supply, evaporation, and wind. Tip any of those factors too far and the system responds. For now, the conditions that have sustained the dunes for roughly the last 7,000 years remain in place, but the system is more dynamic and more vulnerable than its timeless appearance suggests.