White Sands

OVERSIMPLIFIED!

Geologic History

This is a National Park Service Map posted on Wikipedia.com:

The source of the "sand" is already apparent in this map: the dry lakes and alkali flats to the west which continually deposit mineral loading from incoming water, dry up, and the sediments deposited get picked up by high winds which usually blow toward the northeast:

This photo from space (again from Wikipedia.com's article on the Tulerosa Basin) makes it even more apparent:  the winds blow fast across and around the south end of the mountain range to the west, and as they continue toward the north and northeast the valley widens, the winds slow some, and out drops much of the load they have picked up from the dry lakes and alkali flats:

OK, that explains what you will see in the photo pages linked below, but what is this material that gets blown around and makes dunes?  It is selenite, a mineral that has no selenium in it but is made up of crystalline gypsum, CaSO4.2H2O.  Both selenite (gypsum) and selenium (the element) get their name from the moon, hence some call selenite crystals moonstones.

Here is a photo (Wikipedia again) of the nice little gypsum crystals that make up these "sand" grains:

How do these little grains come about? From the weathering of large crystals of selenite laid down by inflowing, gypsum-laden waters as they evaporate (photos of weathering crystals from the Lake Lucero bottom taken by Michael Kolstad, as posted on the El Paso Examiner website):

OK, so the next logical question is: why does water coming into this area from rainfall. snowfall, and creeks draining the valley to the north all stop here?  Because the Tulerosa Basin is a closed (endorheic) basin, with no surface water outflow at its southern end, which is blocked by the Huecos Mountains outside of El Paso.  In fact the lowest part of the basin in right here at Lake Lucero.

OK, understood.  But, what made this basin to have no outlet, was it not carved by running water like most mountain valleys are?

Nope.  This basin was formed by the collapse of the area between two mountain ranges, the Sacramentos to the east and the San Andres to the west.  These are mountain ranges made of layered limestones, with lots of gypsum, laid down by ancients seas before and during the Permian.  Maybe it would help to look at the map of the land and sea during the Permian so you can see why there is limestone here: it was a shallow sea.  And the gypsum?  It was likely laid down as sea levels fell in relation to the land, which was starting to uplift, and sea water evaporation followed repeated shallow sea-incursions and laid down layers of gypsum here, but not deep layers of salt, as in the basins to the east of the Sacramento Range (see the Permian pages on this site about salt formations in those basins):

So the land shown as sea in Permian times uplifted and became dry land after the Permian.  The area now known as the Tulerosa Basin was the top of a large dome of limestone rock, until about 10 million years ago when the center of the dome collapsed.  What on earth can cause the collapse of a large area like the Tulerosa Basin, resulting in its boundaries forming mountain ranges?  Nothing on earth can do that, but something under the earth can and did do that:

The underground spreading forces that created the Rio Grade Rift (pictured here with checkered pattern, Wikipedia again is map/picture source):

Perhaps because the Colorado Plateau mini-plate rotated slightly, and because the Basin and Range land mass to the west was being pulled northwestward, land between the Colorado Plateau and the Basin and Range was pulled apart as if the land mass to the west was moving westward, with a slight clockwise rotation to its movement causing the splitting pattern shown by the checkered pattern in the map above.  A rather narrow tearing apart of the land in Colorado gets wider as it comes south until, in Mexico, it is so wide that it can no longer be seen as a phenomenon of its own, but is part of the Basin and Range's southern extent.  

On the above map where it says "NM20", just to its left is a widening to the east of the Rio Grande Rift.  That is the Tulerosa Basin!

The Rio Grande follows the rift, its valley was made by the earth moving apart forming the rift zone.  All up and down this zone of less crust there has been opportunity for magma to rise and cause earthquakes and volcanoes.  This has little to do with White Sands, but just to the west of White Sands, under the city of Socorro, lies a magma dome still capable of feeding heat and perhaps molten rock up to the surface at some future point in time (the last volcanic activity in New Mexico was just 1,000 years ago, which is like a few seconds ago in geologic-time speak).  Here is a Wikipedia illustration of this dome under the Rio Grande Rift, a part of which lies directly beneath Socorro:

It is unfair to let this illustration just hang here without some perspective.  First, the extension, the driving force for the thinning of the crust in the Rio Grande Rift zone, seems to have stopped.  The area is relatively quiet now,  in terms of both earthquakes and volcanism, although there are very slight tremors recorded when there is minor movement of molten rock upward.  This is not a high risk area to live in or to visit, in other words.  It is a great area for exploring the effects of forces of nature now quiet, but very active in the geologic past.  

OK, I need to come clean about the likelihood of a new volcano?  Here is the scoop, in two parts, both taken from a NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES publication:

(1)

. . . new volcanoes will certainly form within our state, though probably not within our lifetimes. Dr. Allan Sanford, a geophysicist at New Mexico Tech, has identified the Socorro Magma Body, a large body of molten rock underlying the area from Bernardo to south of Socorro. Many small earthquakes associated with the magma body have been detected using seismometers, and some have been felt by local residents. These earthquakes are caused by swelling of the magma chamber, which fractures the overlying brittle rocks. The magma body is located 12 miles beneath the earth’s surface but shows no signs of erupting soon.

(2)

. . . if a volcanic eruption were to occur in New Mexico in the next 100 or 1,000 years, it would most likely form either a lava flow, a cinder cone, or both. The eruptions that form such features do not include major explosive activity; rather they involve low levels of explosivity that form cinder cones and slow-moving, very hot lava that flows downhill from the vent of the volcano. This type of eruption would be unlikely to cause major loss of life or property, although the initial stages of the eruption could be dangerous to any nearby onlookers.

How likely is an eruption to occur in New Mexico in the near future? A New Mexico Tech graduate student, Ellen Limburgh, attempted to make some estimates. She studied the distribution and age of many of the known volcanoes in New Mexico. She found that there have been more than 700 volcanic eruptions in New Mexico in the last 5 million years, with eruptive styles ranging from dangerously explosive to passive. Volcanoes in New Mexico that have been active in the last 5 million years fall into 11 different geographic areas, or volcanic fields. Limburgh estimated that there is roughly a 1 percent chance that some type of volcanic eruption will occur somewhere in New Mexico in the next 100 years, and a 10 percent chance that an eruption will occur in the next 1,000 years.

Feel better now?  OK, let's get into those white dunes before they all blow away!

Go to the first of the White Sands photo pages

(the photo pages are linked to this first photo page and cover two separate walks in the dunes of White Sands National Monument)

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