Dune

A dune is a hill made of sand.

Wind moves the sand across the land. It pushes sand up one side. Then the sand slides down the other side.

Dunes are often in dry deserts. They can also be by the sea.

Some dunes are shaped like a moon. Others look like a star.

Some dunes can move. They can drift over the ground. This can even move a whole town!

Sand can also form under water. This happens in rivers and the sea.

Nature uses dunes to protect the land. They help stop big waves from the sea.

A dune is a mound or hill made of sand.

Wind or water moves the sand to make them. Most dunes are found in dry deserts. This is because plants do not grow to stop the sand.

Dunes also form along sea shores and in rivers.

Dunes have two main sides. The stoss side is the upwind side. Wind pushes sand up this side. The sand then slides down the lee side. This steep side is called a slip face.

Sand moves by bouncing. We call this saltation.

Dunes come in many shapes. Barchan dunes look like crescents.

Star dunes have many arms and look like pyramids.

Parabolic dunes are U-shaped.

These dunes often have plants to hold their arms in place. Some dunes can even drift. In the 1890s, a town in Australia had to move because of drifting dunes.

A dune is a landform made of sand moved by wind or water.

These can look like small mounds, long ridges, or even huge hills. When many dunes group together, they form a dune system or a dune field. Large, flat areas covered in sand are called ergs or sand seas.

Most dunes live in deserts because there is very little water. Without water, plants cannot grow to stop the sand from moving. However, dunes also form on sea shores and in river beds.

They can even form underwater in the sea or in rivers.

Dunes work through a specific way it works called saltation.

In this process, sand grains bounce along the ground. When sand hits other sand, it is likely to stick. This helps the sand pile up into a dune. Most dunes have a long side called the stoss side. This is the side where the wind pushes sand upward. The other side is the lee side, which is shorter. The sand slides down this steep part, called a slip face.

The low valley between two dunes is called a dune slack.

People have studied these shapes for a long time. The word "dune" entered the English language around 1790. It came from the French language and the Middle Dutch word "dūne."

Scientists like Ralph Bagnold also studied how sand moves. He wrote a book called "The Physics of Blown Sand and Desert Dunes." He suggested how certain long dunes might form in the wind. These long dunes are called seif dunes, which comes from an Arabic word for "sword."

They can be over 160 kilometers long.

Dunes come in five main shapes: crescentic, linear, star, dome, and parabolic.

Barchan dunes are crescent-shaped and move very quickly. In China's Taklamakan Desert, these can be over three kilometers wide. Star dunes look like pyramids with many arms.

In China's Badain Jaran Desert, star dunes can reach 500 meters in height. Parabolic dunes are U-shaped and often have plants holding their arms.

These dunes are common in coastal areas. Some dunes are even huge, like the mega-domes in the Grand Erg Oriental. These can be 1.5 kilometers wide.

Understanding dunes helps us protect our homes and land.

Coastal dunes act like a shield against storm waves. Sometimes, people build artificial dunes to protect the shore. However, drifting dunes can cause big problems. In the 1890s, the town of Eucla in Western Australia had to move. This happened because the dunes were drifting into the town. Knowing how sand moves helps us live safely near them.

A dune is a landform created by the movement of sand driven by wind or water.

These formations typically appear as mounds, ridges, or hills. When many dunes group together, they form a dune system or a dune complex. A very large complex is known as a dune field. In cases where broad, flat regions are covered in wind-swept sand with almost no vegetation, these areas are called ergs or sand seas.

While dunes are most famous in deserts, they are not limited to them. They also form along sea shores, in river streams, and in areas of glacial outwash.

They can even form underwater through fluvial processes, which are movements caused by flowing water.

To understand how a dune grows, we must look at how sand moves. Most dunes have a long side called the stoss side, or the upflow side. This is where the wind pushes sand upward. The opposite side is the lee side, which is shorter. On this side, the sand slides down a steep slope called a slip face.

Sand moves up the stoss side through two main methods: creep and saltation. Saltation is a process where sand grains bounce along the ground. When sand hits other sand, it is more likely to stick. This creates an exacerbating feedback loop that helps the sand accumulate into a larger pile. The low valley located between two dunes is called a dune slack.

Scientists recognize five basic types of aeolian, or wind-driven, dunes. These include crescentic, linear, star, dome, and parabolic dunes. Dunes can also be categorized by their organization. Simple dunes are isolated mounds of a single type. Compound dunes are larger structures that have smaller dunes of the same type forming on top of them. Complex dunes are combinations of different dune types.

There is also a distinction in size between different sand deposits. Dunes are generally defined as being greater than 7 cm tall. Smaller deposits, less than 3 cm tall, are known as ripples.

Crescentic dunes, also called barchan dunes, are shaped like crescents. They are usually wider than they are long, with their slip faces on the concave side. These form under unimodal winds, which means the wind blows consistently from one direction. When sand supplies are low, they stay as separate crescents. If the sand supply increases, they may merge into barchanoid ridges or transverse dunes.

Some crescentic dunes move very fast. In China's Taklamakan Desert, the largest crescentic dunes have mean crest-to-crest widths of more than three kilometers. In China's Ningxia Province, a group of dunes moved more than 100 meters per year between 1954 and 1959.

Linear dunes, often called seif dunes, are long and narrow. The name "seif" comes from the Arabic word for "sword." These dunes are often sharp-crested because they have two slip faces. They are associated with bidirectional winds, meaning the wind comes from two different directions. These dunes can be over 160 kilometers long, making them easy to see in satellite images.

In the Rub' al Khali, or Empty Quarter, seif dunes can reach massive heights and lengths. Star dunes are another unique type. They are pyramidal mounds with arms that radiate from a high center. They form in areas with multidirectional winds and grow upward rather than sideways.

In the Badain Jaran Desert, star dunes can reach heights of 500 meters.

Dome dunes are circular or oval shapes that lack a slip face. Small dome dunes are only a few meters high. However, mega-domes are much larger. The largest mega-domes can reach a height of 150 meters and a diameter of over 1.5 kilometers.

Parabolic dunes are U-shaped mounds with elongated arms. Unlike crescent dunes, their crests point upwind. These often form in coastal areas where vegetation like grasses or shrubs anchors the trailing arms. The largest known arm of a parabolic dune reaches 12 kilometers in length.

History and science help us manage the risks of moving sand. The word "dune" entered English from French around 1790. Scientists like Ralph Bagnold have studied the physics of how sand moves. Understanding dune drift is vital because moving sand can be dangerous. For example, the town of Eucla in Western Australia had to be relocated in the 1890s because of dune drift.

On the coast, dunes serve a helpful purpose by protecting the land from storm waves. Because of this, people sometimes build artificial dunes to protect coastal areas from the sea.