My next post about the Geology of the National Parks Through Pictures is from when I had lived in Texas during the early 2000's.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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On a separate trip from our trip to Carlsbad Caverns, we hit up White Sands National Park back in 2004. It is hard to not have geologically themed photographs here since the entire park is a geology wonderland.
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When you think of "sand", you often think of the gritty, rough grains you find on a beach or in the desert. Sand is actually a size determination. The definition of sand is that it is particles that are from 0.05 to 2.0 mm in diameter. Size of particles go from mud/clay, silt, sand, and gravel. These can further be broken down but that is the general gist of it.
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Most beach and desert sands are made of the mineral quartz (SiO2). This is because quartz is a very, very abundant mineral on the surface of the Earth, and it also has some special features that allows it to remain around long enough to become sand. For one, it is a very hard mineral (a 7 out of 10 on the Mohs Hardness Scale below).
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| Mohs Hardness Scale showing the relative hardness, or ability to be scratched, of minerals. Minerals shown are the reference minerals and don't cover every mineral on Earth. Image courtesy of the NPS. |
Another mineral property that quartz does not have is called cleavage, meaning that the mineral breaks along planes of weakness. Both of these combine means that quartz has a tendency to erode slowly and into tiny little smooth balls, hence sand grains. But these sand grains are also typically gritty feeling because of the hardness of quartz. And even though quartz grains at that scale are clear, tiny amounts of rust (or hematite) dust color the sand grains a pale orange/yellow color.
As is sometimes the case, not all beach and/or desert sands are made of quartz. White Sands is one of those special types of places where the sand dunes are = made up of a completely different mineral. The mineral here is called gypsum (CaSO4.2H2O). As can be seen in Mohs Hardness Scale above, gypsum is by far softer than quartz. So soft in fact that the mineral itself is softer than your fingernails (meaning you can scratch a gypsum crystal with your nail). This creates sand dunes that are very, very soft and quite fun to play in.
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Gypsum is an evaporite mineral, meaning that it is often left behind in areas where water evaporation are high. Places like the Great Salt Lake are evaporation basins, where as the water evaporates it leaves behind the salt that had been gathered from the surrounding environment. Depending on the chemistry of the surrounding rocks different evaporative minerals would be left behind in different evaporite basins.
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| Horst and graben. Image courtesy of the NPS. |
The gypsum from which these sand dunes originated first formed back in the Permian (280 to 250 million years ago), when a large shallow sea known as the Permian Sea covered the region. Rising and falling levels produced a large amount of evaporite deposits of the gypsum. These gypsum deposits were then buried over time.
Around 70 million years ago mountain building events, known as the Laramide Orogeny, started to push up the Rocky Mountains, including this region of New Mexico. Around 30 to 10 million years ago the region then started to pull apart. This pull apart created a horst and graben structure, which is where a valley drops down along two sets of faults on either side (pictured above).
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The graben here is known as the Tularosa Basin, and is where White Sands NP currently resides. The horsts are the mountain ranges that run along the eastern, the Sacramento Mountains, and western, San Andres mountains (seen here), edges of the basin. These are the bounding mountains which create an enclosed, or end, basin that has no outlet for water. So all water and minerals that get washed into the valley off of the neighboring mountains eventually evaporates away, leaving behind the mineral deposits.
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Winds then aid in the transportation of the gypsum across the valley. Since gypsum is a lighter and softer mineral than quartz, lower wind speeds are needed to produce the sand dunes. Ripples, as are seen here, are produced at lower wind speeds with every increasing speeds moving the sand grains up the sides of the dunes, aiding in their movement across the desert.
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| Illustration of how a sand dune moves. Image courtesy of the NPS. |
Sand dunes actually move as well. Sand is pushed up one side of the dune, known as the stoss or windward side. And the grains will then drop down the steeper leeward, or slipface, of the dune. As more and more sand grains are moved from the windward side to the slipface side, the dune will appear to migrate in the direct the wind is blowing.
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As the dunes move they can actually cross roads, meaning that the roads need to constantly maintained in the park otherwise they would quickly become inundated with sand. The wind's direction during major sandstorms will also rotate steadily clockwise starting from the western direction and dying out as it reverses directions to be from the east.
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| Types of sand dunes found at White Sands NP. Image courtesy of New Mexico Magazine. |
Depending on the amount of sand, the local vegetation, topography, and the consistency of wind direction, different types of sand dunes can be produced. The primary types produced in the park are barchan, dome, transverse, and parabolic dunes, as seen in the image above. The shifting wind direction would also shift the direction that the dunes tended to migrate as well.
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