Simply put, sand is made of tiny particles of worn-down rock. These particles are picked up by wind, water or the ice in glaciers and left as sediment in the ocean or as sand dunes on land. The composition of sand varies and depends on the local rocks, but the most common material is silica -- more often known as quartz. Coral, lava rock and gypsum are other materials often found in sand. The size and texture of sand particles varies and can offer insight into where it came from. A very small grain of sand, for example, is easier for the wind to blow around and may have traveled a long distance. The roundness of the sand may provide a clue as to how it was formed. Bodies of water with strong bottom currents produced different particles of sand than the particles produced by sand that is transported by rivers or streams, which tend to be very round. The International Sand Collectors Society offers a chart with size classes (in millimeters) for sand and mud [source: Sand Collectors].
Sand dunes form when a lot of loose sand is in an area that also has little vegetation to stand in the way. With enough wind and some sort of obstacle to serve as a sort of blocking or gathering point for the blowing sand, the particles gather and form a dune. Sand dunes reproduce when two crescent-shaped dunes collide, thanks to a little encouragement from their matchmaker friend the wind. When a small dune runs into a larger one -- a very slow process that can take as long as a year -- the smaller one can pass through it. If the sand dune is unstable, the horns at each end of the crescent shape will break off and become two even smaller dunes. Researchers refer to this process as "breeding."
The tallest sand dunes in North America are at the base of the Sangre de Cristo Mountains in Colorado. Visitors to the Great Sand Dunes National Park can see how the massive dunes formed from sediments deposited in a deep valley. Scientists have discovered that a huge lake probably once covered the valley and receded from climate change. The large sheet of sand blew with the southwest wind accumulated into a natural pocked formed by a combination of three mountain passes. Opposing wind directions helped create the vertical shape of the dunes [source: National Park Service].
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