A dune is a mound of sand formed by the wind, usually along the beach or in a desert. Dunes form when wind blows sand into a sheltered area behind an obstacle. Dunes grow as grains of sand accumulate.
Every dune has a windward side and a slipface. A dunes windward side is the side where the wind is blowing and pushing material up. A dunes slip face is simply the side without wind. A slipface is usually smoother than a dunes windward side.
A collection of dunes is called a dune belt or dune field. A large dune field is called an erg. The Skeleton Coast Erg in Namibia extends 2-5 kilometers (1-3 miles) in length and across a width of 20 kilometers (12.7 miles).
Dunes can also be formed by strong currents beneath the water. Underwater dunes, called subaqueous dunes, are common in the ocean, rivers, and canals.
Shapes of Dunes
Dunes can be very large geographic features or just small bumps. Most sand dunes are classified by shape. There are five major dune shapes: crescentic, linear, star, dome, and parabolic.
Crescentic dunes are shaped like crescents, or the shape of a wide letter C. The wide side of a crescentic dune is its windward side, with a small, semi-circular slipface on the other side. Crescentic dunes are the fastest-moving type of dune, and also the most common.
Linear dunes form straight or nearly straight lines. Some linear dunes are shaped like a wiggling snake, with regular curves. Linear dunes develop where wind pressures are nearly equal on both sides of a dune.
Star dunes have pointed ridges and slipfaces on at least three sides. Star dunes develop where winds come from many different directions. The sand dunes of the Sahara Desert ergs are star dunes.
Dome dunes are the rarest type of dune. They are circular and do not have a slipface. The wind can blow material onto the dune from any side.
Parabolic dunes are similar to crescentic dunes. Their shapes are roughly the same, but the slipface of a parabolic dune is on its inward side. Parabolic dunes are also called blowouts, because winds blow out the center of the dune, leaving just a rim on the outside.
Life In Sand Dunes
Few species can live in the shifting world of sand dunes.
There is little soil in a sand dune, so plants usually cannot take root. Often, sand dunes are located next to oceans, so plants must be tolerant of a very salty atmosphere. Some grasses with shallow root systems, such as beachgrass, are common to sand dune ecosystems.
Animals cannot take shelter in the unstable sand of a dune and must search for fresh water. Still, a few species thrive. In the Sahara Desert, sandfish live beneath the dunes. A sandfish is not fish at all, but a type of lizard that can retract its legs and swim through the smooth sand.
Larger animals can find a way to live among sand dunes, too. Rig-e Jenn is a vast, desolate dune belt in Irans Dasht-e Kavir desert. Rig-e Jenn is home to rare species such as the yuz, or Asiatic cheetah, and onager, a relative of the horse.
Sand dunes and subaqueous dunes can sometimes harden into stable structures. The sand becomes a type of rock called sandstone. These mountainous dunes are called lithified dunes. Lithified dunes can be found in the huge features of Zion National Park, Utah; the tropical island of Maui, Hawaii; and even the desolate plains of Mars.
One of the highest dunes in the world is Cerro Blanco, in the Sechura Desert of Peru. Cerro Blanco measures approximately 1,176 meters (3,860 feet) tall.
to gather or collect.
organisms that have a well-defined shape and limited growth, can move voluntarily, acquire food and digest it internally, and can respond rapidly to stimuli.
layers of gases surrounding a planet or other celestial body.
narrow strip of land that lies along a body of water.
large, spotted cat native to Africa.
to identify or arrange by specific type or characteristic.
shape of a half-circle with thin ends.
curved mound or ridge of loose sand with its windward side on the large, wide side of the dune.
steady, predictable flow of fluid within a larger body of that fluid.
area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.
barren, spare, or lonely.
circular-shaped mound or ridge of loose sand.
a mound or ridge of loose sand that has been deposited by wind.
group of sand dunes.
community and interactions of living and nonliving things in an area.
vast area covered with sand dunes.
having to do with places and the relationships between people and their environments.
type of plant with narrow leaves.
type of domesticated mammal used for riding and hauling.
straight or nearly straight mound or ridge of loose sand.
mound or ridge of loose sand that has hardened into rock.
something that slows or stops progress.
animal related to the donkey, native to Asia.
curved mound or ridge of loose sand with its windward side on the small, interior part of the dune. Also called a blowout.
to turn to stone.
flat, smooth area at a low elevation.
ecosystem in a sand dune that begins when coastal sand is newly exposed.
large stream of flowing fresh water.
all of a plant's roots.
small, loose grains of disintegrated rocks.
lizard native to North Africa that "swims" through sand.
common sedimentary rock formed by grains of sand compacted or cemented with material such as clay.
structure that protects people or other organisms from weather and other dangers.
side of a sand dune that is sheltered from the wind. Also called a sandfall.
top layer of the Earth's surface where plants can grow.
mound or ridge of loose sand with a pointed ridge and windward on at least three sides.
underwater mound or ridge of loose sand built up by water currents.
to endure, allow, or put up with.
unsteady or likely to fall apart.
huge and spread out.
movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.
facing or toward the wind.
cheetah native to Asia.