Although sand dunes form in all shapes and sizes, the United States Geological Survey (USGS) generally groups them into five categories.
- The crescentic dune: This is the most common dune. It forms the shape of a crescent moon when the wind blows from one direction.
- The linear dune: This dune is longer than it is wide and features a prominent ridge.
- The star dune: This dune has arms that extend out from a center mound. It's formed by multidirectional winds.
- The dome dune: This is a rare dune that is shaped like an oval or circle.
- The parabolic dune: This dune is U-shaped like the crescentic dune, but its crest points upward and has elongated arms that trail behind it.
These dunes also come in simple, compound, and complex forms. A simple dune has a small number of surfaces and its category is easily identified. A compound dune is bigger and has smaller, similar dunes on top of it. A compound dune is comprised of more than one type of dune. The most frequently seen complex dune features a star dune on top of a crescentic dune.
Sand dunes form when the wind blows large amounts of loose sand into an obstacle. While the heaviest particles of sand deposit against the obstacle and form a ridge, the lighter grains settle on the other side of the ridge (called the slip face). The mound of sand will continue to grow for as long as the wind blows and eventually build into a dune. The sand dune will then continue to grow and could reach as high as several hundred feet tall. A simple dune shows that the wind has not changed direction or strength since the dune formed. Complex and compound dunes are formed by shifting winds.
The tallest dunes in North America are contained within Great Sand Dunes National Park and Preserve, on the western side of the Sangre de Cristo mountain range in southern Colorado [source: National Park Service]. These dunes can reach a height of 750 feet (230 meters). Scientists believe they began to form 440,000 years ago, after a lake between the Sangre de Cristo Mountains and the neighboring San Juan Mountains disappeared through climate change.
Sand dunes are not limited to Earth. The NASA Mars Reconnaissance Orbiter has beamed back pictures of Martian dunes that appear to shift with the planet's winds [source: NASA].
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