Grades 3–5 · Earth & Space
How can a hill of sand travel?
A dune can move slowly across a landscape even though no one lifts the hill. The entire pile does not slide forward as one solid object. Many separate grains are moved from one part of the dune to another, gradually shifting where its shape stands.
In the diagramA dune moves as its grains change placesWind →Grains hopup this sideGrains falldown this sideDashed outline: 20 invented distance units laterGrains move; the dune slowly shifts downwind.
Wind pushes and bounces loose grains along the exposed, gentler slope toward the crest. Beyond the crest, the sheltered side has less wind available to keep those grains moving. Sand can accumulate there until part of the steep slope becomes unstable and grains tumble downward under gravity.
Repeated removal from the upwind side and addition on the downwind side let the dune's shape advance. A grain near the front today can later be buried by other grains; the moving hill is continually being rebuilt from its own material. Transport and deposition, rather than wheels or a sliding foundation, explain its travel.
The dashed outline shows a simple repeated-shape shift, not a prediction of distance per year. Actual dunes also change shape, and their motion depends on wind, available sand, moisture and plants that hold grains in place. Many small grain journeys can add up to a change large enough to move a hill.
About this explanation
Starting knowledge: Recognize slopes and wind direction.
Sources
- National Park Service — Sand dune geology: Dune Movement