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Geology of Great Sand Dunes National Park

The sand dunes here are the highest in the U.S., some rising over 750 feet!

What are sand dunes?

           Sand dunes are created when wind deposits sand on top of each other until a small mound starts to form. Once that first mound forms, sand piles up on the windward side more and more until the edge of the dune collapses under its own weight. The collapse begins when the angle of the dune becomes too large to support the weight. This is called the angle of repose and is usually around 30-34o, but depends on factors such as grain size, wind speed, and roundedness of the individual grains. As these dunes collapse they move in the same direction as the wind, and if there is no wind moving in the opposite direction or any obstacles in its path, barchan dunes are formed. These dunes are rare in the park as there are typically vegetation and complex wind patterns from multiple directions that prevent them from forming.  As more barchan dunes are formed, they can move together and line up. This creates a transverse dune which is orientated perpendicular to the direction of wind and is one long ridge made up of many smaller barchan dunes. Another type of dune that travels is the parabolic dune. This dune type is similar to barchan dunes but instead forms around vegetation where the arms of the dune are anchored in place. In this park, parabolic dunes are scattered around the sand sheet slowly traveling toward the main dune field. Nebkha dunes are essentially parabolic dunes before traveling. They form when sand is trapped at the base of a plant and are able to build taller using the rigid plant structure as a strong foundation. The most complex dune type which the park is known for is star dunes. These dunes form when complex wind patterns alternate direction and change the way sand is piled onto the dune. By changing wind direction, the sand piles up on multiple sides of the dune and is unable to reach the angle of repose so the dune never collapses.  This is how the tallest dune at the park, known simply as the Star Dune, reached over 750 feet in height.

 

Weathering and Erosion

                        Nothing has a greater impact on the landscape around us, especially in the Great Sand Dunes, than the processes of weathering and erosion. Typically seen as interchangeable words, these are actually two distinct processes in the world of geology. Weathering is the physical and/or chemical breakdown of surfaces due to wind, water, or ice. Erosion is the process of moving the broken down material from one place to another via wind, water, or gravity. At Great Sand Dunes National Park, erosion is responsible for recycling escaping sediments back into the main dune field and bringing in new sediments from the surrounding mountains. Alluvium, or sediments deposited by bodies of water, is the source most of the sand in the dunes.

 

Great Sand Dunes National Park
Photo taken on a ridge leading up to Star Dune facing South-Southwest with the San Juan Mountains visible in the distance.
Great Sand Dunes National Park
Photo taken on top of Star Dune facing the Sangre de Cristo Mountains.

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