Over a month ago I was making the trip from Ohio to southeastern Arizona for a field technician job. Along the way I passed right by White Sands National Monument in southern New Mexico. I knew visiting White Sands was an opportunity I couldn't pass up, so I took the exit and ventured into the park.
White Sands National Monument is a protected area of land over 143,000 acres in size. The main point of interest is the sprawling white sand dunes which stretch on for miles and miles. Located in south-central New Mexico, White Sands National Monument attracts nearly 600,000 visitors a year. The area was designated a national monument back in 1933 by then-president Herbert Hoover. Many people (including myself) will often erroneously refer to White Sands as a national park, but national monuments in the US are a bit different from national parks. National parks in the US essentially protect natural scenic areas which are of national or international significance. National monuments, on the other hand, aim to preserve a single significant feature (more or less), and this feature can be culturally, historically, or naturally significant. There's also some bureaucratic differences; Congress will establish new national parks, while the president will proclaim national monuments.
How exactly did these sand dunes form? First, as always, you have to look at the greater geologic setting. White Sands sits in the Tularosa Basin (outlined in blue above). This entire area of New Mexico belongs to the basin and range province of the US. As the name suggests, the basin and range province is made up of basins and (mountain) ranges. These basins and ranges formed (in short) due to extensional tectonic forces millions of years ago. As the land was pulled apart, large faults formed that broke the land into blocks. Some of these blocks slid upward and became mountain ranges. Other blocks slid downward and became basins. Erosion than began acting on the new mountains, and sediment began to flow down the slopes before being deposited in the low-lying basins. This resulted in the basins becoming extensive flat plains that we now see in between the various mountain ranges in this geologic region. In this specific area, White Sands sits in the Tularosa Basin, surrounded on either side by the San Andres Mountains and the Sacramento Mountains.
Now that we've set the scene, let's dive into the actual geologic history of the sand dunes. About 280 million years ago (the Permian Period) what is now the southwest US was covered in a shallow sea. This shallow sea deposited various layers of sediment which then became the bedrock of the area presently. Some of these layers were made up of gypsum, or calcium sulfate dihydrate. When the geologic forces creating the basins and ranges of the area occurred, sections of these gypsum layers were thrust upward with the forming mountains and subsequently became exposed. If you look at the photo above, you can see the whitish layers of gypsum in the mountains. Gypsum dissolves in water and is therefore easily erodible. As a result, rainwater and wind has been slowly eroding these gypsum layers. Rainwater with the dissolved gypsum would run down the mountains and into the basin below. Normally it would be carried to the ocean, but the water carried into the Tularosa Basin is contained and has no way out. This closed-off factor is very crucial to the formation of the sand dunes.
During the Wisconsin Glacial Period about 24,000 to 12,000 years ago, the southwest was a much wetter place. Gypsum-rich rainwater would run into the Tularosa Basin and into a large lake called Lake Otero. This lake was at the base of the San Andres Mountains. As the glacial period ended, the climate in this region became drier and drier. Lake Otero began to dry up, and the dissolved gypsum precipitated out and was deposited in the form of selenite crystals along the now-dry lake bed.
Around 10,000 years ago, strong prevailing winds began blowing from the southwest to the northeast. These winds began to erode the deposited selenite in the old lake bed, forming white gypsum sand. This sand was then picked up by the wind and blown inches by inches toward the northeast. As this happened, the sand began building up. Over the course of the next several thousand years, the sand accumulated into massive sand dunes. These sand dunes are still forming today. During instances of heavy rain, rainwater dissolves more gypsum in the San Andres Mountains. This rainwater runs into an ephemeral lake (called a playa) known as Lake Lucero, which is a smaller lake bed that sits in the old site of Lake Otero. This water will evaporate, leaving behind gypsum deposits, and the wind will create more gypsum sand. Although this process is occurring every year, the bulk of the dune formation occurred due to erosion of the Lake Otero deposits.
These beautiful white sand dunes are an ever-changing feature of the Tularosa Basin. Wind is constantly changing the shapes of the dunes, and they slowly morph and move over the basin. It's a very dynamic park, and well worth the visit. When I visited, the wind was really blowing. You could see the tiny sand grains moving over the tops of the dunes. When you're sitting on top of a dune, you can really feel the lively nature of the system.
Thanks for reading!
