New Mexico is a state of fascinating landscapes. From the out of this world rock formations of Bisti/De-Na-Zin and City of Rocks, to horizons punctuated by dormant volcanos, to the alpine meadows of the Valles Caldera, to the massive caverns in Carlsbad, geological events, over millions of years, have sculpted a visually spectacular region with enormous variation. Regardless of where you are standing in New Mexico, if time were to collapse to a moment, the geologic forces on display would be wildly impressive and immediately fatal.
Precambrian volcanic and metamorphic rocks over 1.5 billion years old have been found in New Mexico. These rocks form the core of the mountain ranges east of the Rio Grande, with the oldest found in the Brazos, Taos and Nacimiento.
During the Paleozoic era New Mexico was covered by the Permian Sea, a vast, shallow body of water that created beds of limestone, sandstone, gypsum and shale. As the Paleozoic era came to an end, the sea began to dissipate. In southern New Mexico, a great barrier reef formed, but as the water receded it was isolated from the sea. As the water evaporated, large deposits of salt, potash and gypsum remained.
In the Mesozoic era, the sea continued to recede, with dinosaurs roaming the region. Many of the vibrant sandstone and shale formations in the northern part of the state were created at this time. Rivers and streams transported sediment as the water flowed toward the retreating sea. Later in the Mesozoic era, the inland sea returned, with New Mexico sitting on the western shore of a large, shallow ocean that covered most of the Midwest.
Eventually North America broke away from what remained of the Pangaea land mass and drifted west, colliding with the Farallon plate. The collision produced massive earthquakes and volcanic eruptions, giving birth to the Rocky Mountains. Volcanic activity generated the thick layer of igneous rock that covers much of New Mexico today. Stress fractures in the earth’s mantle produced the Rio Grande rift that divides the state, with the mountains rising along the fault lines on both sides and the land between the two faults sinking, like a cake collapsing in the middle.
In the southern part of the state, the beds of sediment created by the Permian sea were pushed up along the fault lines, forming the San Andres Mountains to the west and the Sacramento Mountain to the east, with the Tularosa Basin forming a bowl between them.
Gypsum sand is rare, because it is water-soluble. Normally, rain would dissolve gypsum and the water solution created would drain to the ocean. However, when the Rio Grande Rift formed, the Tularosa Basin was isolated, cut off from the sea and ocean, with water runoff from the mountains pooling in the basin, forming a shallow, 1600 square mile lake that we now refer to as Lake Otero.
During the last ice age, 12,000-24,000 years ago, the climate was wetter. Rain and snowmelt was more abundant, washing vast quantities of gypsum from the mountains into the basin below. When the last ice age ended Lake Otero evaporated, becoming a dry lake bed, also known as a playa. The bed of the lake contained large amounts of selenite, the crystalline form of gypsum.
As the climate became warmer and drier, Lake Otero dried up completely, leaving large tracts of selenite crystals just beneath the surface layer of clay and silt. The sun, wind and erosion began to work their magic, gradually transforming the area into the Chihuahuan Desert. Lake Lucero and Alkali Flat formed in place of Lake Otero. The Selenite crystals of Alkali Flat were exposed when wind carried the clay and silt away, with crystals as large as three feet exposed. Freezing and thawing broke the large crystals into smaller pieces. As the crystals became smaller, wind blew them across the desert floor, grinding them into finer particles, eventually grinding them into the white sand we enjoy today.
Bands of gypsum are readily visible in the mountain ranges surrounding the Tularosa Basin. To this day rain and snow melt from the mountains surrounding White Sands fill Lake Lucero with gypsum-laden water, like a giant water puddle. When the water evaporates, new selenite crystals form in the mud, just beneath the surface. Though the volume of gypsum sand produced by Lake Lucero and Alkali Flat is significantly less than what Lake Otero generated, they continue to produce new dunes.
Unlike dunes composed of quartz-based sand, gypsum does not readily convert the sun's energy into heat. As a result, the surface temperature of White Sands doesn’t get as hot as sand on a beach would. The dunes can be comfortably traversed barefoot most of the year. Even when the surface of the sand gets too hot, it is cool just below the surface, because the gypsum retains moisture from the playa of the Alkali Flat.
New Mexico Nomad