Wednesday, December 7, 2011

New Mexico Geology: Seas of Gypsum

Sorry right now I don't have pictures of my own for this, two weekends ago I was going to meet up with family in Alamagordo then visit White Sands, but the family activity ended up taking over twice as long as planned, so it was dark by the time we finished. I will update with better pictures when I get them.

Update: Check out Ron Schott's post about White Sands, he has two amazing GigaPans of the dunes.

In second grade we took a field trip to White Sands, and I still remember that day. It wasn't too far of a drive, just on the other side of the Sierra Blanca, but it was amazing. After we passed through the military sign posts warning us of the dangers should we trespass on to the base, we drove into the national monument. It was like a sea of white stretched out before us as far as we could see; the white waves, frozen in time, just asked for a bunch of little kids to run up and down on and try to sled down. Eventually we gave up on trying to sled down the hills (damn you friction!!!) and instead took to rolling ass over teakettle down the huge dunes, burying each other in the sand and jumping off the dunes into soft(ish) piles of loose sand below us.

White sands is amazing because it is a beautifully bizarre place that illuminates some already pretty awesome geologic processes, but in an unique and beautiful way.

Dune Formation:
White Sands (which is a very original name) is the largest gypsum dune field in the world, and demonstrates the highly variable nature of dune formation. First off, there are Barchan Dunes which are crescent shaped dunes which move in the same direction of their limbs.  These dunes move incredibly fast up to 100 meters in a year (hey anything you can measure in terms of years instead of 100's or 1000's or 1x10^6 years is really fast to a geologist). Barchan dunes are common when there isn't much sand and the dunes are free to move across the desert pavement (highly cemented surface common in desert, it is this phenomena which allowed Rommel to quickly move across North Africa in WWII).

Photo Credit:
Then there are Parabolic dunes, which look like Barchan Dunes, but they are going the wrong way. This happens when Barchan Dunes move into areas with more vegetation, the vegetation bogs down the limbs of the dune first and turns them around. The vegetation acts as an anchor and makes these dunes really slow.

When there is plenty of sand, the Barchan Dunes join together into Transverse dunes, which are long lines of wavy sand. This is very common at White Sands.
Photo Credit:
Evaporite Minerals
If you are not familiar with geology, you might be curious what exactly is Gypsum, and if you are not, you should be curious (don't make the gypsum angry it is probably right behind you, hiding in plain sight). Chemically Gypsum is CaSO4*2H20, translated to  Calcium Sulfate (Sulfur plus four Oxygens) and two water molecules within the crystalline structure; it is an ionic compound, like table salt, which means it dissolves in water into ions (a +2 Ca and -2 SO4) and is precipitated out when the water evaporates. If you are unfamiliar with this, take a bowl fill it up with water and then a lot of salt into it, then hang a string where it is only just in the water and tape it to the sides. Put the bowl in a window (or under a heat lamp to speed things up) and as the water evaporates salt will precipitate on the string. SCIENCE!!

One cool thing about this mineral is that crystalline gypsum can come in three forms, satin spar, selenite, and alabaster.

Satin Spar gypsum is fibrous gypsum that has a silky luster (luster refers to the way the mineral interacts with light, so Satin Spar gypsum reflects light similar to the way silk does).

Satin Spar Gypsum in a unique shapePhoto Credit
Selenite is the transparent and colorless (geologists hate the word clear) version of gypsum. In New Mexico you can sometimes find dunes with large selenite crystals within them. A good place to check these out is the Living Desert Zoo and Garden in Carlsbad, NM, which has a whole exhibit dedicated to this feature.

Selenite Hills at the Living Desert Zoo and Gardens in Carlsbad, NM
And Alabaster resembles the sands at White Sands, just clumped together into a soft rock.

An example of Alabaster Gypsum from
One important thing to note about Gypsum is that it is all around, as the mineral is used to make wallboard. In addition, Gypsum is used in art and other applications as Plaster of Paris. This application highlights a very cool property of gypsum, remember when I said earlier that Gypsum has two water molecules in its crystal lattice? Well if you heat gypsum you can drive off the water molecules and you end up with annhydrite which is just CaSO4 and if you know anything about making plaster casts, you know that the plaster first comes ground up in a fine powder which is the gypsum that has been ground up and heated which drives off most of the water, then you mix it with water and pour it into you want to make a  cast of. The annhydrite then absorbs the water becoming gypsum after it crystallizes thus hardens or sets. Well that's White Sands in a nutshell.

1 comment:

  1. Hi Ryan! It's great you're still posting interesting things about New Mexico. I'm sure there are plenty of people curious about White Sands because it is such a natural wonder.