At the northern end of the Chihuahuan Desert lies a mountain-ringed valley, the Tularosa Basin. Rising from the heart of this basin is one of the world's great natural wonders—the glistening white sands of New Mexico. Here, great wave-like dunes of gypsum sand have engulfed 275 square miles of desert and have created the world's largest gypsum dune field.
The brilliant white dunes are ever changing: growing, cresting, then slumping, but always advancing. Slowly but relentlessly the sand, driven by strong southwest winds, covers everything in its path. Within the extremely harsh environment of the dune field, even plants and animals adapted to desert conditions struggle to survive. Only a few species of plants grow rapidly enough to survive burial by moving dunes, but several types of small animals have evolved a white coloration that camouflages them in the gypsum sand. White Sands National Monument preserves a major portion of this gypsum dune field along with the plants and animals that have successfully adapted to this constantly changing environment.
Plants act to slow the movement of dunes.
How the Dunes Formed:
The Tularosa Basin
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.
Satellite Photo: Eroding mountains filled the Tularosa Basin 2,000 feet deep with sediments, including the gypsum forming its white sands
A Rare Form of Sand
The common mineral gypsum, a hydrous form of calcium sulfate (CaS04. 2H20) 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 carry 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 lakebed 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 Flat area is the exposed bed of this Ice Age lake.
In wet periods, water evaporating slowly on the playa floor causes gypsum to be deposited in a crystalline form called selenite. Along Lake Lucero's shore and in the Alkali Flat, beds of selenite crystals—some three feet long—cover the ground. The forces of nature— freezing and thawing, wetting and drying—eventually break down the crystals into sand-size particles light enough to be moved by the wind.
Dune Growth and Movement
Strong winds blowing across the playa pick up gypsum particles and carry them downwind. As the sand grains accumulate into a dune, they bounce up its gentle windward slope, rippling its surface. At the dune's steep leading edge, sand builds up until gravity pulls it down the slip face, which moves the dune forward.
Four Types of Dunes at White Sands:
- Dome dunes. The first dunes to form downwind of Lake Lucero are low mounds of sand that move up to 30 feet per year.
- Barchan dunes. Crescent-shaped dunes form in areas with strong winds but a limited supply of sand.
- Transverse dunes. In areas with ample sand, barchan dunes join together into long ridges of sand.
- Parabolic dunes. On the dune field edges, plants anchor the arms of barchans and invert their shape.
Detailed Geologic History:
The gypsum dunes of the Tularosa Basin are geologically very young features, yet to really understand the origin of White Sands we must examine a substantial amount of geologic history. This summary is intended to give a more detailed look at the major events that contributed to the formation of the world's largest gypsum dune field.
1. The Permian Sea
The gypsum that makes up White Sands is ultimately derived from marine rocks. Shallow seas covered much of New Mexico throughout the Paleozoic Era (570-245) million years ago). Marine deposits as old as 500 million years are present in the San Andres Mountains, but by far the most abundant sedimentary rocks in southern New Mexico are Permian in age (290-245 Ma). In the Permian Period North America was part of a great megacontinent called Pangaea, and present day New Mexico was submerged in a tropical sea just south of the equator. The limestone mountains at Carlsbad Caverns and Guadalupe Mountains National Parks represent the remains of a large barrier reef that was part of this Permian sea. In the middle of the Permian Period there was a major fall in sea level, causing vast stretches of water across southern New Mexico to nearly dry up. It was during this drying-up phase that large quantities of gypsum rock were deposited.
Gypsum is an evaporite mineral, meaning that it forms almost exclusively when dissolved ions become concentrated due to the evaporation of water. If sea water of normal salinity is reduced to about 20% of its original volume through evaporation, calcium (Ca2+) and sulfate (SO42-) ions will be concentrated enough for gypsum (CaSO4·2H2O) to begin to crystallize. The cycles of evaporation that took place in the middle Permian caused hundreds of feet of gypsum to settle out onto the sea floor. Much of this gypsum is found in the 1500 ft. thick Yeso Formation, which outcrops in the San Andres and Sacramento Mountains surrounding the Tularosa Basin. "Yeso," by the way, is Spanish for gypsum. Good roadcuts of the Yeso Formation are rare, but there are a few places in the Sacramento Mountains (e.g. mile 14 on Highway 82, just before Cloudcroft) where patches of white gypsum are visible, most of which has been leached out of the original rock.
2. The Laramide Uplift
Near the end of the Cretaceous Period (~70 million years ago), the Rocky Mountains began to form. White Sands National Monument is about 200 miles south of the Rockies, but the same compressional forces that formed the Rocky Mountains also uplifted the marine rocks in southern New Mexico. The cause of the Rocky Mountain uplift (known as the "Laramide orogeny") is not entirely known, but most geologists believe a change in the geometry of tectonic plates was responsible for the mountain building.
The surface of the Earth is divided into seven major “plates.” These rigid plates are constantly shifting, diverging from each other in rift zones (e.g. in the middle of the Atlantic Ocean) and colliding together along convergent plate margins (e.g. off the west coast of South America). When relatively thin (40-50 miles thick), dense oceanic plates collide with thicker, less dense continental plates, the oceanic plate will sink beneath the continental plate in a process called “subduction”. Subduction causes a great deal of compression across the plate boundaries, resulting in the uplift of a mountain chain adjacent to the boundary. The Andes mountain range is South America is an excellent example of a subduction-formed mountain belt. In the Jurassic and Cretaceous Periods subduction beneath the North American plate resulted in uplift and volcanism along western North America. Towards the end of the Cretaceous the angle of subduction beneath the United States became increasingly shallow, transferring compressive forces to the east. This eastward shift of compression is believed to be responsible for the formation the Rocky Mountains.
The Laramide uplift affected a very broad region, including the White Sands area. If it were not for this geologic event the elevation of southern New Mexico would be substantially lower.
3. Formation of the Tularosa Basin
The Tularosa Basin is essential to the existence of White Sands. With no drainage outlet, this basin traps and concentrates all the dissolved gypsum that comes down from the marine rocks in the surrounding mountains, gypsum that would normally be carried away by rivers or streams.
The formation of the Tularosa Basin is part of a large-scale tectonic event that began approximately 30 million years ago and continues today. Sometime prior to the onset of this tectonic event, the shallow subduction off the California coast ceased and a new, parallel-motion plate boundary formed (now represented by the San Andreas Fault). For reasons that remain unclear, this shift in tectonic regime caused enormous upwellings of magma from the Earth's mantle. These mantle upwellings stretched apart large portions of crust in the southwest, forming the Basin and Range - a geologic province that extends from southern Oregon down to northern Mexico. A linear arm of the Basin and Range, known as the Rio Grande Rift, extends from southern New Mexico into central Colorado.
As the crust pulled apart in the Basin and Range, numerous fault zones developed. Large blocks of crust subsided thousands of feet along these faults, forming basins in between fault-bounded mountain ranges. The Tularosa Basin, at the southern end of the Rio Grande Rift, is just one of the many basins in the U.S. southwest that have formed due to crustal extension. Geologists believe that much of the basin formation associated with the Rio Grande Rift occurred within the last 10 million years.
4. The Last Pleistocene Ice Age
The wet climate during last ice age (approximately 24,000 to 12,000 years ago) played a major role in the formation of White Sands. In the late Pleistocene Epoch, the Tularosa Basin (and much of the U.S. southwest) received substantially more rain than it does today. Cooler and wetter conditions enabled a small glacier to form on the north slope of 12,000 ft. Sierra Blanca, and much of the Tularosa Basin was filled with an enormous lake called Otero. Heavy rainfall flushed large quantities of soluble gypsum from the San Andres and Sacramento Mountains down to the Lake Otero. The lake became saturated with dissolved gypsum.
As the ice age came to an end, the climate of the Tularosa Basin became increasingly more arid. Lake Otero slowly dried up, leaving behind enormous deposits of gypsum. The relatively slow rate of evaporation, combined with saturated, muddy conditions along the edges of the lake, allowed the dissolved gypsum to crystallize as selenite "discs" and bladed crystals. These crystals are still forming today, but the majority of selenite at Lake Lucero crystallized as the ice age lake dried up. Following the evaporation of this enormous lake, large stretches of the Tularosa Basin must have been as littered with selenite crystals as Lake Lucero is today.
The wet ice age climate was critical in flushing large quantities of gypsum from the mountains down into the Tularosa Basin. In addition, an extensive clay layer deposited by Lake Otero is largely responsible for the dune field's shallow, perched water table. This shallow water table is not only essential to plant life in the dunes, but it also enables selenite formation to continue today.
5. Dune formation (reviewed and edited by Steve Fryberger, March 8, 2003)
It is not precisely known when gypsum dune formation began in the Tularosa Basin. Dating techniques by Stephen Stokes of Oxford University of a partly lithified, “fossil” dune terrain just northeast of the NE30 observation station indicates that some dunes may have formed as long ago as 16,000 years BP (before the present). This date seems a bit perplexing when considering that much of the Basin was probably covered by Lake Otero at the time. It is possible that the date is inaccurate, because there was so little quartz material in the original sample that could be used for dating (Stokes, 2001, personal communication). On the other hand, the NE 30 site roughly at the same level as the Lake Otero Highstand at approximately 3950-4000 feet above sea level. It is possible that there was a local evaporative period approximately 16,000 years ago which would have caused lake level decline, allowing a small dune field to form. Of course, most of the gypsum would have been released not during the lake high stand, but upon extreme contraction of the lake and the concentration of evaporites. Thus, the most reasonable thought is that the "fossil dunes" were undoubtedly formed at sometime after the Lake Otero Highstand—although topographic relationships allow this formation to potentially have occurred early in the process of lake retreat.
In any event, it appears clear from the limited thermoluminescence dates at hand, as well as regional geology, that a major dune-building episode in the region probably began somewhere around 6500-7000 years ago. Such a chronology would fit patterns recorded nearby at Lake Estancia and by other workers in the basin. The present White Sands dune field most likely did not begin to form until a substantial portion of Lake Otero had dried up. Some [fossil?] dunes northeast of Lake Lucero have been dated at 6,500 years (S. Stokes, personal communication). This date is most likely representative the large-scale dune formation event that defined the present White Sands. The present day dunes, which overlay 25-30 feet of gypsum sand in places, are quite young from “yesterday” to perhaps hundreds of years in age. Subsurface crossbedded dune deposits such as those cored by Eddie McKee in his landmark studies are probably more representative of the 6500 year time period. Clearly, further work on dating of the dunes would be very helpful. However, it can be safely asserted that White Sands is geologically very young, and that the dune field has probably formed during multiple episodes of deflation of Lake Otero gypsum deposits.
All of the gypsum sand in the dune field was formed by the breakdown of selenite crystals, most of which represent recycling of gypsum crystals from the evaporite phase of drying Lake Otero. These crystals occur in a variety of shapes and sizes, commonly twinned. All selenite is typified by the presence of mica-like cleavage planes. Expansion and contraction caused by large temperature fluctuations and periodic freezing serve to break up selenite crystals along these planes. When crystals become small enough to be transported by wind, further breakdown, and rounding will occur. Gypsum is one of the softest minerals gypsum sand will be broken down into smaller grains much more rapidly than quartz sand when transported by the wind. This effect can be plainly seen when comparing coarse crystal grains near Lake Lucero to very fine sand near the eastern edge of the dune field. Although very little new gypsum is forming today, wind erosion of the gypsum-bearing sediments of ancient Lake Otero continues, along with the breakdown of the small gypsum crystals to produce sand. This process, rather than contribution from Playa Lucero, is the main process feeding the most active portions of the dune field. Indeed, the dampness, mud and evaporite cementation of Playa Lucero has reduced gypsum throughput so greatly that areas downwind of Playa Lucero are subregional scour areas where partly lithified gypsum dunes are being eroded due to insufficient sand contribution from areas upwind.
Gypsum sand formation continues today. At the same time, however, sand is being broken down into silt size particles that are blown out of the Tularosa Basin. Whether or not the net size of the dune field is growing or shrinking remains to be seen, however the dune field leading edge is advancing to the northeast.
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Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
9" x 10.75", paperback, 550 pages, full color throughout
The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!
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U.S. Geological Survey
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