White Sands National Park in New Mexico is a striking landscape of rolling dunes that appear like a field of snow under the desert sun. The simple reason for this dazzling whiteness is that the sand is not made of quartz, which forms most of the world’s beaches and deserts, but nearly pure gypsum. This unique composition makes the 275-square-mile dune field the largest of its kind on Earth.
The Unique Composition of the Sand
The white substance is the mineral gypsum, a soft sulfate mineral (calcium sulfate dihydrate). This chemical structure is key to its brilliant color, as it naturally lacks the dark elements found in common sand. Standard sand is made of quartz, which often contains trace amounts of iron or other minerals that result in familiar tan, yellow, or reddish hues.
Gypsum is inherently colorless and transparent in its purest crystalline form, called selenite, but it appears white when finely ground. The millions of tiny, transparent gypsum grains scatter all wavelengths of visible light, reflecting a brilliant white. The gypsum sand does not absorb the sun’s heat like darker quartz sand, which is why the dunes remain surprisingly cool to the touch even on hot days.
The Geologic Source and Journey
The gypsum’s story begins about 250 million years ago, when the region was covered by the shallow Permian Sea. As this ancient sea evaporated, thick layers of calcium and sulfate precipitated out as gypsum rock, which were then buried and turned into sedimentary layers. These gypsum-rich formations are now exposed in the surrounding mountain ranges, specifically the San Andres and Sacramento Mountains.
The Tularosa Basin began forming about 30 million years ago when tectonic forces pulled the earth’s crust apart, causing the land to drop between the mountain ranges. Rain and snowmelt flowing down from these mountains dissolve the highly water-soluble gypsum from the exposed rock formations. This gypsum-laden runoff then collects at the lowest point of the basin.
This dissolved mineral water initially flowed into a massive body of water during the Ice Age, known as Lake Otero. As the climate warmed and the lake evaporated around 12,000 years ago, it left behind vast deposits of gypsum on the basin floor. Today, the remnant of this ancient lake is Lake Lucero and the Alkali Flat, which serve as the primary source of the gypsum.
The Preservation Puzzle
The persistence of these dunes is a geological puzzle because gypsum is an evaporite mineral that readily dissolves in water. The White Sands exist due to the unique geography of the Tularosa Basin, which is a “closed basin.” This means the basin has no outlet to the sea or any other body of water.
Water flowing into the basin, carrying the dissolved gypsum, settles and has nowhere to go but up. The arid climate causes the pooled water in areas like Lake Lucero to evaporate quickly. As the water disappears, the dissolved calcium sulfate precipitates out of the solution, crystallizing into large, brittle selenite crystals on the surface of the dry lake beds.
Wind breaks down these large selenite crystals into the fine, sand-sized grains that form the dunes. Strong prevailing winds from the southwest erode the crystal deposits, carrying the gypsum particles northeastward across the basin floor. This constant cycle of dissolution, evaporation, crystallization, and wind erosion ensures the perpetual replenishment and movement of the pure white sand dunes.