White Sands National Monument

Continental rifts produce some interesting geography. We have visited the Rio Grande Rift several times and found something new to see each time. Just north of El Paso, Texas and Las Cruces, New Mexico is a field of dunes composed of the whitest and finest sand you'll likely see. White Sands National Monument preserves much of one of the more unique geological features in North America.  Gypsum transported from the surrounding mountains collect in the bowl that is the Tularosa basin and forms dunes of varying heights.  The sand moves so quickly that roads have to be regularly plowed to keep the road clear for automobiles.

From sea to shining sand

The Four Corners area of the US during the Permian period

For most of its history, North America has mostly been covered by shallow epicontinental seas.  The mountains that make up so much of the landscape of the American West have been pushed up fairly recently.  The story of White Sands begins in the Permian period (299-251 million years ago).  During times of sea level rise, southern New Mexico and Arizona were inundated by ocean.  As sea levels fell, and the sea shallowed out or became cut off into inland salt lakes the water would evaporate, concentrating the mineral contained therein.  These crystalline materials are called evaporites. Calcium sulfate precipitates into a mineral called gypsum.  Several episodes of sea level rise and fall eventually formed the Yeso formation, some 1500 feet of gypsum rock that can be seen in the surrounding mountains.

Uplifting experience

Plants helping to anchor sand dunes in place.  San Andres Mountains
form the backdrop

The manufacture of the Rocky Mountains was very complex, and this feature is related to part of that story.  As Pangaea broke up during the Jurassic, oceanic crust of the Farallon plate began to subduct (sink under) the western edge of the North American plate which was located at about Utah at the time.  As the Farallon plate sank under North America at an angle of about 50 degrees, it pulled ocean plateaus and islands to the western edge of North American, fusing them with the continent, forming the land in Nevada and California.  This new land pushed the subduction zone further to the west.  Collision of oceanic crust with North America produced the Sevier orogeny, pushing up mountains from Canada to Mexico, including in Idaho, Utah, Wyoming and Nevada.  In this episode, pressure from the eastward moving ocean plate plus heating caused the crust to shorten and the sedimentary layers to fold and break along weak planes.

Franklin Mountains show deformation from Laramide orogeny but
are mostly built by rifting processes.  See how the rock layers tilt?

About 70-80 million years ago, the Farallon plate began to move more horizontally.  This caused the crust to fold into a series of ridges and basins, and the crust broke along deep fault lines produced during the breakup of Rodinia.  Blocks of crust with basement rock cores were tilted upwards during this orogeny (the Laramide orogeny), which extended deeper into the interior of North America than had the Sevier event.  The Black Hills mark the eastern boundary of this mountain building episode.  This episode ended as the Farallon plate cooled, became denser and started sinking again about 35-55 million years ago.

Drift in the rift

32-34 million year old pyroclastic flows and lava
Organ Mountains on east side of Rio Grande Rift

An upwelling of magma generated a period of tremendous volcanism throughout the southwest US starting about 35 million years ago and ending about 20 million years ago.  The magma produced a thinning of the crust and it extended up to 50% in the late Oliogocene.  The Rio Grande rift started forming about 30 million years ago.  As the crust extends, it cracks along pre-existing faults and the block on one side of the fault drops in comparison that on the other side.  The rift trends northwards and extends from Chihuahua State, Mexico into Colorado.  The thinning crust resulted in further subsidence; most basin-building took place in the last 10 million years.

Rio Grande/Rio Bravo south of White Sands

The first period of extension, during the Oligocene, lasted from 30-19 million years ago or so.  The second period of extension began during the Miocene, about 17 million years ago and continues to this day.  The area of rifting is at least 180 miles wide, and stretches about 1.5 mm per year from east-west in Colorado to 2.5 mm per year in northern Mexico.  The basins that make up the visible portion of the rift average 30 miles across.  The rift valley is partly filled up with sediment that has accumulated to a depth of nearly 5 miles.  The Rio Grande River occupies the rift valley.

Carrizozo lava field, LandSat 7 NASA photo

Evidence of the volcanism associated with rifting abounds in the region.  Hot springs, extinct cinder cones, mountains built from lava.  Lava field badlands (malpais) are present.  Just north of the White Sands is a gash of black lava, the Carrizozo lava field.  This flow of lava erupted from a shield volcano about 5,200 years ago.  The high basalt content provides the black color, and the thin consistency of the lava allowed it to flow freely down the rift valley.  There are also cinder cones in the field.  It is likely that humans were present to witness this eruption.

Part of another lava field near Grants, New Mexico forms a part of El Mapais National Monument.  Ben still gets a ribbing from our visit there when he was 5ish.  Another day.

Ice, ice, baby

Lake Otero, NPS

During the last Ice Age, glaciers did not reach this far south.  However, the climate was cool and wet.  Large amounts of rainfall would fall, percolate through the porous rocks, dissolving the gypsum in the Yeso Formation.   Spring running out of the mountains plus runoff gathered into the bowl of the Tularosa basin, forming a large lake, called Lake Otero.  Calcium sulfate would continue to increase in concentration in the low-lying lake, precipitating into crystals from time to time.  This crystallization accelerated as the climate warmed, became drier and the lake shrank.  Today, the remnants of the lake are Lake Lucero at the south end of the dune field, Alkali Flats and a playa on the east side of the dunes.

Surrounding mountains provided much of the gypsum

Gypsum.  You can blame the playa

Playa near Visitor Center.  Lake Lucero is part of White Sands Missile Range
and can only be reached in a monthly ranger-led tour.
Check website for schedule

The crytalline gypsum that erodes out of the shallow basin lake (playa) and alkali flats is called selenite.  This mineral forms thin sheets that break apart readily.  As the cycles of wind and heating and cooling break down the selenite crystals, the become a very fine white sand.  The sand drifts and gathers into dunes.  Very strong winds can lift the sand and blow it over the Sacramento mountain range, as in the NASA satellite view below.  This also gives you a good idea of the prevailing wind pattern in this part of the country.  The dunes are formed by winds blowing from southwest to northeast.

Selenite crystals at Lake Lucero - NPS photo

White Sands NM at left center, trailing gypsum dust plume to northeast
NASA photograph from Aqua satellite

Making dune

Barchan dunes towards the west side of the park

To make a dune, you need a very strong wind, sand grains small enough to be carried by the wind, and friction / resistance of some kind to get the sand to stick together somewhat.  The sand will start to pile up forming a shallow angled ramp that the wind carries sand up.  When sand reaches the top, gravity may take over and the sand grains fall down in the stagnant air behind the dune.  This side of the dune is called the slip-face.  The sand on this face tends to be looser and the slope is steeper, making it more difficult to walk up than the windward side.

A "blowout" happening just right of center
Sacramento Mountains form the background

When there is a high wind velocity and a limited amount of sand, dunes will form as barchan dunes, which are horseshoe shaped with the arms pointed downwind.  This form of dune is plentiful along the Loop Drive.  Sometimes barchan dunes unite into a line called a transverse dune.  Dunes that suffer a "blowout" often have plants covering the dune, so the denuded area travels quickly and the arms more slowly resulting in a horseshoe that points with its arms upwind and the arc downwind.  These parabolic dunes are common on the outer edges of the park, except in the west.

Rio Grande Cottonwood (Populus deltoides)
growing on top of a dune

The dunes at White Sands are relatively easy to walk upon because of the soluble nature of gypsum.  Rains will solubilize some of the gypsum, which will recrystallize and cement neighboring gypsum granules together.  This layer will also protect a groundwater from evaporating.  Plants that are trapped by an advancing dune may increase stem length to keep leaves above the level of the sand.  These plants can then tap into the protected groundwater for their own survival.  Plants such as the cottonwood that can extend roots down to groundwater are called phreatophytes

Another plant that seems adept at stem elongation is the soaptree yucca plant.  Young flowering plants may be about 3 feet tall, but by stem elongation, they may reach 18 feet.  The yucca is well adapted to a variety of climates, but they are only pollinated by one moth, the yucca moth (Tegeticula yuccasella).  At that, it is only the female yucca moth that visits the flowers of this plant, since the males complete their life cycle after mating (they die).  The egg-laying habits of this moth push pollen down into the pollen tube of the yucca flower.  Yucca moths lay their eggs in the flowers, and the young will develop in the yucca fruit, drilling through the outer skin to escape.  This yucca has a sap that contains a natural soap, which accounts for the name.

Soaptree Yucca (Yucca elata)

Radical animals

While the white sand does not get as hot as fast as darker sand, the desert is still a warm and dangerous place in the daylight.  Most animals in the desert become active in the evening, and many of them may be best evidenced by their footprints.

Beep Beep!  Roadrunners leave X shaped tracks in the sand

Lizards drag their tails behind, making a line between left and right footprints

A tweedle beetle bottle puddle bottle muddle --- bug tracks

Pioneers

Gyp nama (Nama carnosum)

When a dune has passed, the soil has a high gypsum and salt content.  Gypsophiles are plants that are specialized to living in this environment.  These plants are rarely taller than shrub size.  Most of these plants are shorter than two feet tall.  After a few generations of growth, they will enrich the soil with their decaying organic material, creating a soil more friendly to less specialized plants.

You can often spot a gypsophile in a list of plants by its name; they are often called Gyp "something".  For instance, Gyp nama is a little plant that looks like a fir tree.  It has small white flowers in the summer after rains.

Yellow Evening Primrose (Calylophus hartwegii)

Another pretty gypsophile is the yellow evening primrose.  These flowers are yellow by day, but after being pollinated by nocturnal moths, the flower wilts and becomes more orange colored.  The plant that I saw was getting a bit ratty, but it did have some nice flowers.

Not all of the plants here are strictly gypsophiles.  The sandy verbena plant is a generalist.  It can be found on gypsum flats, in interdune areas and on the dunes.  They have little pink flowers in April and May, but sometimes flower after rains in the summer into October.  You couldn't prove it by me, but the sign says that they produce a lilac smell, especially after rains.

Sand Verbena (Abronia angustifolia)

Spring Blooms in the Chihuahua Desert

The first time I visited this area, I was in El Paso for work and stayed over to take a drive out to White Sands in late May.  The second time Ben and I went when he was in middle school during winter break.  Not many blooms then.

Ocotillo (ock-uh-TEE-oh) Fonquieria splendens

Barrel cactus (Ferocactus acanthodes)

Cholla (CHOY-uh) Cylindropuntia imbricata

Yucca (Yucca elata)

Getting There

If you are coming to Las Cruces, New Mexico from the west on I-10, you can catch US-70 heading east at Exit 135 (Alamogordo) OR you can catch I-25 going north at Exit-44, then US-70 east (Alamogord - Exit 6).  From El-Paso, you can take I-10 north to I-25 to US-70 east.  Coming from El Paso, I enjoy driving through the Franklin Mountains - from I-10 take exit 22B and head toward Alamogordo on US-54, exit at Exit-29, take the first left onto Woodrow Bean Transmountain Drive (Loop 375).  If you are getting out in the sticks and going up, you've done it right.  Much of the drive through the Franklin Mountains is a state park, lots of turnouts and trailheads.  Very nice views.  This will take you back to I-10.  You need to go north once on I-10 OR take a scenic route.  I enjoyed driving the backroads to Mesilla and Las Cruces.

The national monument is surrounded by the US Army's White Sands Missile Range and Holloman Air Force Base. The US Army is not nearly as sentimental about nature and geography as we are, since they use their part of the region to test out weaponry. This is a bad news/good news thing:  the bombings destroy some features, but protection from traffic also helps preserve some features.  From time to time, missiles are flown over the national monument, making park and road closures necessary. Good thing, because missiles have fallen on the park and destroyed visitor facilities before.  Check the park website to see when closures are scheduled.   That seems to be the watch word for this park:  Check ahead, especially if you are traveling on a tight schedule or want to visit one feature in particular.  There are border patrol stations on US-70, so be prepared to stop and talk to the nice people.  US-70 may also be blocked during missile tests.  Check ahead, be prepared.

Waypoint:  Latitude 32.7803176 N; Longitude 106.1777423 W
Street Address: 19955 US-70, Alamogordo, NM 88310

Trinity Site, where the first atomic weapon was tested is just north of here and is open for visitation the first Saturdays of April and October.  Going to go there one day.

Further Reading

National Park Service White Sands National Monument Site

White Sands National Monument History