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How do sand dunes form?

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Quick answer

Wind lifts and bounces loose sand grains along the ground. When the wind slows near an obstacle or rough patch, it drops those grains. They pile into a mound, with a gentle windward slope and a steep leeward slip face. The dune slowly shifts downwind as sand moves from one side to the other.

Sand dunes form when wind carries loose sand grains across a surface until an obstacle or rough patch slows the wind and forces the grains to drop. The pile grows into a mound, and the wind keeps reshaping it into the familiar dune shape with a gentle windward slope and a steep leeward slip face.

Step by step

  1. Wind blows across a dry, loose sand surface and lifts grains into short hops, a process called saltation.
  2. An obstacle such as a rock, plant or patch of rougher ground slows the wind and reduces its sand-carrying capacity.
  3. Grains settle and accumulate on the upwind side, forming a low mound that grows as more sand arrives.
  4. Sand climbs the gentle windward slope and tumbles over the crest, avalanching down the steep leeward side.
  5. The slip face builds at the angle of repose, and the whole dune migrates slowly in the wind direction.

Key things to understand

  1. Wind must be strong enough to lift and bounce sand grains along the ground, a process called saltation.
  2. When the wind hits an obstacle or a rough patch, it slows down and can no longer carry as much sand.
  3. Grains pile up on the upwind side, building a gentle slope that faces the wind.
  4. Sand grains tumble over the crest and slide down the sheltered downwind side, creating a steep slip face.
  5. The dune slowly migrates downwind as grains are eroded from the windward side and deposited on the leeward side.

Labelled diagram

  1. Windward slope

    Gentle side facing the wind, where sand grains are pushed uphill.

  2. Crest

    The top edge where grains tip over to the sheltered side.

  3. Slip face

    Steep leeward side where grains avalanche down at the angle of repose.

  4. Obstacle

    A rock or plant that slows wind and starts sand deposition.

  5. Migration

    Sand erodes from the windward side and deposits on the leeward side, shifting the dune downwind.

How a sand dune builds and moves

Real-world examples

  • The Namib Desert in southern Africa has some of the world's tallest dunes, reaching over 300 metres.
  • The Sahara's linear dunes stretch for kilometres in parallel ridges shaped by persistent winds.
  • Coastal dunes behind beaches in Cornwall, England, form where onshore winds meet beach sand and marram grass.
  • The Thar Desert on the India-Pakistan border has barchan dunes that migrate across the arid plain.

Common mistake

✗ Myth: Sand dunes are just random piles of sand that stay in one place.

✓ Fact: Dunes are organised structures shaped by wind direction and sand supply, and they migrate downwind over time. The gentle windward slope and steep slip face reflect the steady movement of grains from one side to the other.

People also ask

What causes sand dunes to form?
Sand dunes form when wind transports loose sand grains and then slows down, causing the grains to drop. Obstacles, vegetation or rougher ground reduce wind speed, so sand accumulates. Over time the pile develops a windward slope and a leeward slip face, and the dune migrates in the wind direction.
How do sand dunes move?
Sand grains are eroded from the gentle windward slope and carried over the crest. They avalanche down the steep leeward slip face, so the dune advances downwind while its overall shape stays the same. Migration is usually slow, from a few metres to tens of metres per year.
Why are sand dunes important?
Dunes act as natural barriers that absorb wave energy and protect coastlines from erosion and flooding. They also store fresh water, support specialised plants and animals, and record past wind and climate conditions in their layers. Disturbing them can lead to rapid coastal loss.
Why do sand dunes have a steep side and a gentle side?
The gentle slope faces the wind because grains are pushed up it gradually. The steep side is the slip face, where grains tumble down the sheltered leeward side at the angle of repose, typically around 34 degrees.
How fast can a sand dune move?
Most dunes creep only a few metres per year, but fast-moving dunes in strong wind corridors can advance tens of metres annually. Speed depends on wind strength, sand supply and how dry the surface is.
What is the angle of repose in a sand dune?
It is the steepest angle at which loose sand can rest without sliding, usually about 34 degrees for dry sand. The slip face of a dune stays close to this angle because grains keep avalanching when it gets steeper.
Can sand dunes form without wind?
Yes. Underwater currents can build subaqueous dunes on riverbeds and sea floors, and some coastal dunes are shaped by wave action and onshore winds together. The same deposition principle applies.

Part of the guide: Nature & Earth: Living Things and Rocks →