Types of Sand Dunes
Blowing in the Wind
The size and shape of a sand dune depends on the ability of winds to pick up and carry sand and the direction the winds blow. The steepness of the sides is determined by the angle where the sand is balanced between falling downslope due to gravity and the support of underlying sand grains. This angle of repose as it is called is up to 35°. The types of dunes are Barchans, Parabolic, Transverse and Longitudinal. The parabolic dunes at right are near Moses Lake.
- Barchans, crescent shaped dunes, are formed by winds blowing in a constant direciton over low areas of sand supply. The points of these dunes are downwind with the steeper and taller middle facing into the wind.
- Parabolic dunes have a crescent shape whose points face into the wind. Often the points are anchored by vegetation and the sand dune is blown inside out. On beaches where sand is abundant, winds are moderate and tend to blow from the same direction you may see blowout parabolic dunes.
- Transverse dunes form in areas where winds blow in a constant direction with an abundant supply of sand. These make long crests and troughs at right angles to the wind and look like sea waves.
- Longitudinal dunes flow in the same direction as the wind due to strong winds and a low sand supply.
Shifting sand dunes like those found in the Hebes Chasma area on Mars (left) can be found near Moses Lake and Pasco, Washington.
When floodwaters from Glacial Lake Missoula backed up behind Walula Gap, near Walla Walla, Washington, they lost their ability to carry eroded loess, sand, gravel and other debris. The sediment from these short term lakes became the source of the sand in this area.
The old lakebeds are very flat making it easier for winds to pick up sand and dust. Also, as the Cascade Mountains rose, they blocked the rainclouds coming in from the Pacific Ocean causing the region to become semi-arid (very dry). This "rain shadow" effect continues to keep the average rainfall of the plateau very low.
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Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Volcanic rock caused by partial melting of the Earth's crust.
- Channeled Scabland
- Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications. See also Channeled Scablands.
- Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
- Rounded rock fragments larger than sand.
- Ice Age
- A period in Earth's history when much of the continents are covered with ice sheets and glaciers.
- Rain Shadow
- A mountain or mountain range that blocks rain clouds just as an object might block sunlight to form a shadow. Areas in the shadow are more dry as a result.
- Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
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