When you think of a sand dune, you probably picture a barren pile of lifeless sand. But sand dunes are actually dynamic natural structures. They grow, shift and travel. They crawl with living things. Some sand dunes even sing. There are coastal sand dunes and land-locked desert sand dunes laid out in artistic patterns. There are even sand dunes on Mars.
To understand sand dunes, you need to understand sand. Sand forms from the erosion of rocks. As rocks break down, the resulting materials are transported by wind, water or ice (glaciers) and end up as sediment in the sea or -- as we'll learn in this article -- as sand dunes. For example, desert sand is likely the result of a dried-up glacier or ancient sea or lake.
The most common ingredient in sand is silica, or quartz. Sand's makeup varies from place to place, depending on the local rocks. For example, you'll find fine white sand in a coral reef setting because the sand contains ground-up coral. Darker sands are rich in magnetite or made up of tiny, smooth pieces of lava -- you'll find this sand on the beaches of Hawaii. The famous White Sands National Monument in New Mexico comprises white sand, which contains mostly gypsum, or the sulphate of lime. The texture of a grain of sand provides clues about how far it has traveled. The smaller the grain, the farther it's probably come and the easier it will move around in the wind.
The short version of how sand dunes form is that sand blows around and then piles up. Of course, there's a bit more to it than that.
Sand Dune Formation
A sand dune needs the following three things to form:
- A large amount of loose sand in an area with little vegetation -- usually on the coast or in a dried-up river, lake or sea bed
- A wind or breeze to move the grains of sand
- An obstacle that causes the sand to lose momentum and settle. This obstacle could be as small as a rock or as big as a tree.
Where these three variables merge, a sand dune forms. As the wind picks up the sand, the sand travels, but generally only about an inch or two above the ground. Wind moves sand in one of three ways:
- Saltation: The sand grains bounce along in the wind. About 95 percent of sand grains move in this manner.
- Creep: When sand grains collide with other grains -- like clay or gravel -- causing them to move. Creep accounts for about 4 percent of sand movement.
- Suspension: Sand grains blow high in the air and then settle. About 1 percent of sand moves this way.
Once it's in motion, sand will continue to move until an obstacle causes it to stop. The heaviest grains settle against the obstacle, and a small ridge or bump forms. Because the obstacle breaks the force of the wind, the lighter grains deposit themselves on the other side of the obstacle. Eventually, the surface facing the wind crests, and the lighter grains of sand cascade down the other side, or the slip face. This is how a sand dune may actually move over time -- it rolls along, maintaining its shape as it goes.
How and why does a sand dune crest? As the wind moves sand up to the top of the sandpile, the pile becomes so steep it begins to collapse under its own weight, and the sand avalanches down the slip face. The pile stops collapsing when the slip face reaches the right angle of steepness for the dune to remain stable. This angle, which scientists call the angle of repose, is usually about 30 to 34 degrees.
After enough sand builds up around an obstacle, the dune itself becomes the obstacle, and it continues to grow. Depending on the speed and direction of the wind and the weight of the local sand, dunes will develop into a different shapes and sizes. Stronger winds tend to make taller dunes; gentler winds tend to spread them out. If the direction of the wind generally is the same over the years, dunes gradually shift in that direction. Any vegetation that crops up will stabilize the dune and prevent it from shifting.
The fact that sand dunes migrate is fascinating because it makes them seem alive. But their migration actually threatens local agriculture and towns. In China, for example, sand dunes have been advancing upon some villages at the rate of 65 feet (20 meters) per year [source: NASA]. In many cases, fencing will arrest sand dune migration. In some cases, people actually drench the sand with crude oil to stop the movement -- not the most environmentally-friendly solution. Migrating dunes may even collide and merge into one large dune. Or they create the illusion of passing through each other [source: O'Connor].
Types of Sand Dunes
Sand dunes develop into all shapes and sizes, but that doesn't stop scientists from piling them into generalized categories. By using satellite and aerial photography of the world's deserts, the United States Geological Survey (USGS) has identified five types of sand dunes:
- The crescentic dune, also called the barchand dune, is the most common type of sand dune. As its name suggests, this dune is shaped like a crescent moon with points at each end, and it is usually wider than it is long. Crescentic dunes form when winds blow from one direction. This dune traverses desert surfaces faster than any other type of dune. The migrating dunes in China are crescentic dunes.
- The linear dune is straighter than the crescentic dune with ridges as its prominent feature. Unlike crescentic dunes, linear dunes are longer than they are wide -- in fact, some are more than 100 miles (about 160 kilometers) long. The ridges are long and snakelike, and these dunes usually occur in parallel sets separated by other sand, gravel or rocky corridors.
- The star dune has arms that radiate out from a center pyramid-shaped mound, hence the descriptive name. Star dunes grow upward instead of outward and are a result of multidirectional winds. Common to the Sahara Desert, they tend to show up around topographic barriers. They're among the tallest sand dunes on Earth -- some star dunes in China are more than 1,600 feet (500 meters) tall.
- The dome dune is rare, oval- or circular-shaped and has no slip face. Dome dunes sometimes appear at the ends of crescentic dunes. Most dome dunes are low -- only a few feet high.
- The parabolic dune is U-shaped, but differs from the crescentic dune because its crests point upward, with elongated arms that follow behind. A parabolic dune's trailing arms are typically anchored by vegetation. The longest known parabolic dune has a trailing arm nearly 7.5 miles (12 kilometers) long.
Coastal sand dunes are a bit different from desert dunes. They form when an accumulation of sand blows inland from the beach. Coastal dunes always form in the predominant wind direction, but their shape and size depends on the shape of the beach. If a beach is shallow-sloped, more sand deposits on the shore, resulting in bigger dunes. On a steep beach, more sand washes back out to sea, and the dunes are smaller. Coastal dunes are a unique ecosystem, and they help to protect the surrounding area from erosion. Vegetation like dune grass acts as a stabilizer for coastal dunes.
Interestingly, scientists have recently confirmed the existence of sand dunes underwater, using advanced sonar equipment. Called sand waves, they can reach up to 700 feet (213 meters) long and more than 30 feet (9 meters) high. Changing tidal patterns, sediment and erosion help these underwater dunes to form.
Who calls the sand dune home?
You'd think a pile of sand could never support a thriving ecosystem. But sand dunes host countless numbers of organisms. From sweet-smelling flowers to small rodents, many living things have adapted to a life in the sand. Dune vegetation pays back the favor by protecting the dunes from damage.
Obviously, desert and dune plants must be drought-tolerant. Some plants store rainwater in their leaves and stems, while other plants have long root systems that burrow down to the water table. The long roots tend to stabilize and anchor the dunes, preventing them from eroding during a storm. Some of the plants and flowers that grow in sand dunes include: pink sand verbena, white dune evening primrose and yellow sunflower. Shrubs are also well suited for dune life, and many animals rely on bushes like mesquite, creosote bush and desert buckwheat for shade and shelter.
Sand snakes and lizards also call sand dunes home. These reptiles burrow rapidly through the sand, an action known as sand swimming. Hundreds or even thousands of types of insects also make their habitats in sand dunes. Beetles, moths, wasps, flies, crickets and spiders all live in the sand. Many of these insects prey on each other as well as feed on dune vegetation. Certain rodents also can live in sand dunes and create complicated burrows within the dunes, emerging to feed on the seeds and leaves of dune plants.
Vegetation on coastal dunes is particularly important because it helps to anchor the dunes and protect them from damage and erosion. The most common vegetation you'll see on coastal dunes is dune grass. Dune grass grows in the sand, requires full sun and can withstand salt spray. The grass has long, flexible stems that help it to withstand forceful winds. Powerful sprawling root systems help anchor it in the sand and also protect the dune from erosion. Sea turtles are also important to coastal sand dunes. Many turtles nest in the sand, and their eggs provide valuable nutrients for dune vegetation.
Because these fragile ecosystems are all dependent on each other, it's important to protect sand dunes. Beaches and sand dunes need each other -- beaches need the dune's sand reservoirs in order to replenish after a storm, and dunes need the beach's sand to form in the first place. Coastal sand dunes suffer from erosion during storms and hurricanes and when humans interfere. Deforestation also contributes to sand dune erosion because loss of vegetation makes the dunes unstable. As sand dunes lose sand -- perhaps from waves reaching them during a particularly strong storm, or because humans disturb their structure -- they also lose their ability to absorb storm surges. Dunes can take decades to recover from this sort of destruction [source: CSI].
The reason you see "Stay Off the Dunes" signs at the beach or lake is because walking around on the dunes can damage the vegetation. And, as we've learned, the long root systems of dune vegetation help to keep the dunes anchored and stable.
For more information about sand dunes and other natural wonders, visit the links on the next page.
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More Great Links
- Armstrong, Wayne P. "The Life & Love of Sand Dunes." Desert USA. 2008. (Oct. 15, 2008) http://www.desertusa.com/magjan98/dunes/jan_dune1.html
- Bell, Trudy E. and Phillips, Dr. Tony. "City-swallowing Sand Dunes." Science@NASA. Dec. 6, 2002. (Oct. 15, 2008) http://science.nasa.gov/headlines/y2002/06dec_dunes.htm
- Chang, Kenneth. "Secrets of the Singing Sand Dunes." NY Times. Jul. 25, 2006. (Oct. 15, 2008) http://www.nytimes.com/2006/07/25/science/25find.html
- "Coastal Sand Dunes." Capital Regional District. 2007. (Oct. 15, 2008) http://www.crd.bc.ca/watersheds/ecosystems/coastalsanddunes.htm
- "Coasts: Sand and Dunes." U.S. Geological Survey. Jan. 19, 2004. (Oct. 15, 2008) http://geology.wr.usgs.gov/parks/coast/dunes/index.html
- Creager, Ellen. "Dune what comes naturally at Sleeping Bear." Detroit Free Press. Sept. 21, 2008. (Oct. 15, 2008)http://www.freep.com/apps/pbcs.dll/article?AID=/20080921/FEATURES07/809210320/1032
- "Desert Features." U.S. Geological Survey. Oct. 29, 1997. (Oct. 15, 2008) http://pubs.usgs.gov/gip/deserts/features/
- "Great Sand Dunes National Monument." USParkInfo.com. 2008. (Oct. 15, 2008) http://www.usparkinfo.com/greatdune.html
- "Mars image shows wind-sculpted sand dunes." Telegraph.co.uk. Apr. 11, 2008. (Oct. 15, 2008) http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2008/04/11/scimars111.xml
- Martin, Glen. "City's beautiful but hidden sand dunes." San Francisco Chronicle. Jul. 20, 2006. (Oct. 15, 2008) http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2006/07/20/SANDDUNES.TMP
- O'Connor, Anahad. "When Sand Dunes Collide, Sometimes They Mate and Multiply." NY Times. Dec. 23, 2003. (Oct. 15, 2008) http://query.nytimes.com/gst/fullpage.html?res=9B06E2DB103FF930A15751C1A9659C8B63
- "Sand Dunes." Desert USA. 2008. (Oct. 15, 2008) http://www.desertusa.com/geofacts/sanddune.html
- Schevitz, Tanya. "11-year-old boy killed in sand dune collapse." SFGate. Oct. 13, 2008. (Oct. 15, 2008) http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2008/10/12/BAR913G016.DTL
- Sink, Mindy. "Adventurer | Sandboarding." NY Times. Apr. 9, 2004. (Oct. 15, 2008)http://query.nytimes.com/gst/fullpage.html?res=9E07E2D91538F93AA35757C0A9629C8B63
- "Types of Dunes." U.S. Geological Survey. Oct. 29, 1997. (Oct. 15, 2008) http://pubs.usgs.gov/gip/deserts/dunes/
- "What is sand made of?" SEED Science Center. 2008. (Oct. 15, 2008)http://www.seed.slb.com/qa2/FAQView.cfm?ID=896
- "When Sand Dunes Have Been Destroyed." UNESCO. March 2007. (Oct. 15, 2008)http://www.unesco.org/csi/pub/source/ero9.htm