The gypsum that forms the white sands was deposited at the bottom of a shallow sea that covered this area 250 million years ago. Eventually turned into stone, these gypsum-bearing marine deposits were uplifted into a giant dome 70 million years ago when the Rocky Mountains were formed. Beginning 10 million years ago, the center of this dome began to collapse and create the Tularosa Basin. The remaining sides of the original dome formation now form the San Andres and Sacramento mountain ranges that ring the basin.
The common mineral gypsum, a hydrous form of calcium sulfate (CaSO4·2H2O), is rarely found in the form of sand because it is soluble in water. Rain and snow that fall in the surrounding mountains dissolve gypsum from the rocks and carries it into the Tularosa Basin. Normally, dissolved gypsum would be carried by rivers to the sea. But no river drains the Tularosa Basin. The water, along with the gypsum and other sediments it contains, is trapped within the basin.
With no outlet to the sea, water flowing into the Tularosa Basin either sinks into the ground or pools up in low spots. One of the lowest points in the basin is a large playa called Lake Lucero. Occasionally, this dry lake bed fills with water. As the water evaporates, the dissolved gypsum is deposited on the surface. Even more gypsum deposition occurred during the last Ice Age when a larger lake, Lake Otero, covered much of the basin. The Alkali Flats area is the exposed bed of this Ice Age lake.
During wet periods, slowly evaporating water on the floor of the playa causes gypsum to be deposited in a crystalline form known as selenite. Along the shore of Lake Lucero and the Alkali Flats, beds of selenite crystals, some up to three feet long, cover the ground.
When the dominant wind from the southwest blows across the dried up playa of Lake Lucero and the Alkali Flat (former bed of Lake Otero), finer silts and clays are blown away, a process called deflation, leaving the crystals exposed. Weathering breaks them up by:
Such weathering breaks the crystals down to the size of sand grains which can be moved to the north-east by the dominant wind. Transport or movement of grains by the wind begins at between 10 and 20 m.p.h. depending on grain size. Material is moved by the wind in a variety of ways as shown in the diagram below.
Suspension: kept aloft by eddies and updraughts.
Saltation: hopping and bouncing.
As sand is moved by the wind it is reduced in size by attrition caused by frequent collisions with other grains and the desert surface. The softness and perfect cleavage of gypsum aids this process. Sand refers to size, not composition. Sand is numerically defined as any particle between 1-2mm. in diameter. Particles less than 1/8 mm. in diameter are called silt and clay and those over 2mm. are called granules and pebbles. In qualitative terms, sand is defined as any particle light enough to be moved by wind, but too heavy to be held in suspension in the air.
Deposition of sand occurs whenever the wind slackens:
Strong winds blowing across the playa pick up gypsum particles and carry
them downwind. As the sand grains accumulate into dunes, they bounce up the
gentle windward side of the dune, creating ripples on the surface. At the
steep leading edge of the dune, sand builds up until gravity pulls the sand
down the slip face, moving the dune forward.
Four types of dunes can be found in the white sands dune field.
Gypsum comes from the Greek language, meaning 'to cook the earth'. The mineral gypsum is chemically known as calcium sulfate (CaSO4·H2O) with water. Calcium sulfate that is not bonded with water is called anhydrite. Gypsum is typically very soft, a '2' on the hardness scale (1=talc, 10=diamond). Our fingernails are of hardness 2 1/2, so our nails can scratch the soft gypsum.
Gypsum is soluble, relative to most other rocks. It dissolves in water into calcium ions and sulfate ions. It is an evaporite mineral: the calcium and sulfate ions, when sufficiently concentrated due to the evaporation of water, recombine and precipitate out into gypsum or anhydrite. If you heat gypsum and drive out most of the water you get another substance: Plaster of Paris. Used for casts, and lots of construction uses, gypsum is one of the oldest building materials in the world. Used for mortar in Egyptian pyramids. Romans discovered its fire-resistant qualities: doesn't burn and water in gypsum helps suppress fire. Gypsum has many modern uses: medical treatments for setting broken bones, wallboard, cement setting retardant, paint, soil conditioner, crayons, even put in beer to help enzyme action and yeast fermentation. Fortunately, gypsum is a very common mineral and is mined from New York to California. There is no commercial threat to the park.
Gypsum comes in many forms, found mostly in rock form. Very fine-grained rock gypsum is called alabaster and is used for sculpting. The crystal form is called selenite.
Selenite crystals at Lake Lucero.