Chapter 12: Effects of Winds, Waves, and Currents

Wind as an Agent of Change

Rock Materials Carried by Winds

Winds are agents of erosion. Winds pick up and move sediments Winds cause weathering by driving sediment

against rocks and other materials. Deserts (dry, loose sediments) have the most

wind erosion. Dust storms occur when strong, steady winds

lift great amounts of silt and clay from the topsoil.

Winds of at least 18 km/h are needed to move sand grains, and move in hops and bounces.

Dust Storm, Texas

Abrasion by Windblown Sediments Windblown silt and clay particles are too small

and too soft to wear away most rocks. Sand grains are larger and tend to be made of

more abrasive materials, they grind and scour anything they hit.

Quartz sand grains can wear away many materials.

Sand blasts and grinds boulders and rocks into shapes called ventifacts.

The side of the ventifact that faces the wind wears into a smooth flat surface, or facet

Deflation: An Erosional Effect

Deflation is the removal of loose rock particles by the wind

In deserts when the sands and clays are blown away, it leaves pebbles and boulders called desert pavement.

In semiarid regions, deflation can form hollows called blowouts.

Loess

Wind can deposit sediment as well as remove it. Large areas in China, northern Europe, and the

north central United States are covered by thick deposits of material called loess.

1m-100m in thickness Unlayered, yellowish particles the size of silt. Particles are angular in shape The particles in the US and Europe were likely from

glacial outwash plains The particles in China are likely from the deserts of

Mongolia

Loess landscape, China

Sand Dunes Hills of sand deposited by winds. Form when sand piles up against shrubs, boulders,

or other obstructions. Dunes are found wherever there are strong winds

and enough loose sand. Most are made of quartz sands, some are gypsum,

calcite, and may contain some feldspar, mica, and magnetite.

If wind blows from one direction, dunes have a long, gentle slop on the windward side, and a shorter, steeper slope on the leeward side.

Dunes occur in many different shapes.

Four Types of Sand Dunes

Parabolic Barchan

Transverse Longitudinal

Migration of Dunes

Each time the wind blows against the windward side of a sand dune, some of the surface sand is carried over the top.

The whole dune can move in the leeward direction.