Mochi Isle
Sand dunes · Crescent DesertScience for kidsHow do sand dunes form?
In short: Sand dunes are hills of sand built by the wind. Wind bounces sand grains along the ground until they pile up behind an obstacle. Crescent-shaped dunes, called barchans, point their two tips the way the wind blows.
How it works
Most sand grains are too heavy for the wind to carry high into the air. Instead they hop along the ground: one grain lands, knocks others up, and the wind carries them a little further. When the wind slows down behind a rock or a clump of grass, the grains drop and the pile grows into a dune.
A dune has a gentle slope facing the wind and a steep slope on the sheltered side. The wind pushes sand up the gentle side to the top, then it tumbles down the steep side, called the slip face. Grain by grain, sand moves from front to back, so the whole dune slowly travels.
Crescent-shaped dunes form where there isn't much sand and the wind mostly blows from one direction. The thick middle of the dune moves slowly, while the thin edges move faster, so the two tips race ahead and point downwind, the way the wind is blowing.
Where to see it in New Zealand
New Zealand has no big sand deserts, but it has plenty of coastal dunes. The Te Paki giant sand dunes near Cape Reinga in Northland are famous for sandboarding, and Farewell Spit, at the top of the South Island, is a long spit of sand lined with dunes.
Try it together
In a sandpit or on a dry beach, make a small pile of dry sand. Wearing sunglasses, blow gently across it from one side through a straw. Watch the grains hop and see which side of the pile becomes steeper. At the beach, stay with an adult and away from the waves.
Kids also ask
Do sand dunes move?
Yes. The wind keeps moving sand from the windward side over to the slip face, so the dune creeps forward. Some fast-moving dunes can even bury roads, fences and buildings.
How can people stop dunes from moving?
By planting. Grass and shrub roots grip the sand, and their leaves slow the wind near the ground. On New Zealand beaches, people plant native pīngao and spinifex to help hold dunes in place.
Are dunes only found in deserts?
No. Anywhere with plenty of loose sand and wind can have dunes, including beaches, lake shores and riverbeds. Spacecraft have even photographed sand dunes on Mars.
In the game, “Sand dunes” is a nature card on Crescent Desert.
Your child finds this card by reaching the right place or meeting the right weather on the island, and lights it up by answering the question on the back. Tap the button below and the game shows your child which island it is on and how to find it. Mochi Isle is a browser learning game for ages 5 to 18; the main island and the daily lesson are free.
Find “Sand dunes” on the island ↗ All cards from Crescent Desert