Chapter 12: Effects of Winds, Waves, and Currents

1
Chapter 12 Effects of Winds, Waves, and Currents

• Wind as an Agent of Change

2
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.

3
Dust Storm, Texas
4
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

5
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.

6

• In semiarid regions, deflation can form hollows
  called blowouts.

7
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

8
Loess landscape, China
9
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.

10
Four Types of Sand Dunes
Parabolic
Barchan
Longitudinal
Transverse
11
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.