Download Chapter 18 – Erosion by Wind and Waves

Section 1 – Wind Erosion
• Wind contains energy. It an turn a wind turbine or
move a sailboat. It can also erode the land.
• Sand vs. Dust
– Sand is loose fragments of weathered rocks and minerals.
Most grains of sand are made of quartz.
– Dust is particles that are smaller than the smallest sand
grain. Most dust is microscopic fragments of rocks and
minerals.
• How wind moves sand and dust
– Wind cannot keep sand up in the air. Most sand moves by
a series of jumps and bounces called saltation. When the
wind speed is fast enough to move sand grains along the
ground, they move in the direction of the wind. The sand
moves ahead a short distance, falls and strikes other sand
grains that then bounce up. Most grains rarely rise above
1 meter in height.
– Dust particles are very small and light. Even gentle air
currents can keep dust particles suspended in the air.
Dust from volcanoes can stay in the upper
atmosphere for several years. Strong winds can lift
large amounts of dust and create dust storms.
• Effects of wind erosion
– Wind erosion happens everywhere there is wind.
– The most dramatic erosion occurs in deserts and
coastlines.
– Deflation is the process by which wind removes the
top layer of fine, dry soil and leaving behind large rock
particles. This forms the desert pavement.
– Deflation blows away the best top soil used for
growing crops. It may form shallow depressions in
areas where the natural plant cover has been
removed. The shallow depression is called a deflation
hollow.
• When pebbles and small stones in deserts and on
beaches are exposed to wind abrasion, the surfaces
of the rocks become flattened and polished on two
or three sides. Rocks that have been pitted and
smoothed by wind abrasion are called ventifacts.
The direction of the prevailing wind can be
determined in an area by the appearance of the
ventifacts.
• Scientists once thought that many large rock
structures were formed by wind erosion, but now
they believe those large rock structures are due to
surface water and weathering.
• Wind deposition
• The wind drops particles when it slows down and
can no longer carry them. These deposited
particles and covered with other deposits.
• The best known wind deposits are dunes. Dunes
form where the soil is dry and unprotected, and
where the wind is strong. The dune begins to form
where a barrier slows the wind speed, the sand falls
from the air, and is dropped on both sides of the
barrier. As more sand accumulates, the barrier is
covered and the dune becomes the barrier.
• Shapes of dunes
• Loess – formed from fine dust that is deposited in thin
layers that are not noticeable. Thick layers can form by
an accumulation of the windblown dust. Loess is soft
and easily eroded. It can form steep bluffs.
Section 2 – Wave Erosion
• Wave erosion changes the shape of the
shorelines.
• Storms produce large waves that causes much
of the erosion along the coastline. The waves
break up the rocks into smaller fragments that
continue the abrasive action or mechanical
weathering.
• The waves force salt water and air into the
cracks in the rocks causing the cracks to
enlarge and cause chemical weathering.
• Shoreline formations– Sea cliffs
– Sea caves
– Arches
– Stacks
– Terraces