Transcript Weathering - The John Crosland School

Weathering
Chapter 7.1
Earth-Environmental
WZPP
Introduction

Read page 153 on Cleopatra’s needle and
see figure 7-1
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Weathering of Cleopatra’s needle. The photograph taken to the left in
Egypt before 1880, on the right was taken in New York in 1918
(courtesy of the Metropolitan Museum of Art)
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Introduction

Weathering
 Is
the process by which rocks near the earth’s
surface break down and change

Erosion
 The
removal and transport of weathered
material

These processes have been going on for a
long time
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Mechanical Weathering
Materials change size or shape
 Breakdown of material into smaller pieces

Backcountry lake
Tetons NP
J B Photos
Holly Lake
Tetons NP
J B Photos
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Mechanical Weathering

Temperature
 Water
freezes and expands by 9%
 Exerts pressure on the rock
 Frost wedging

Causes potholes
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Mechanical Weathering

Pressure
 When
rock is deep underground and is
exposed at the surface, the pressure of the
rock is reduced

Rocks break apart like layers in an onion
(exfoliation)

Example: Half Dome in Yosemite NP
7.1a ws
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Mechanical Weathering

Pressure
 Roots
wedge themselves into cracks and
exert pressure as they grow
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Chemical Weathering
Rocks and minerals undergo changes due
to chemical reactions
 Chemical reactions form new minerals and
release dissolved substances

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Chemical Weathering

Composition of the rock determines the
effects of chemical weathering
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Chemical Weathering

Temperature determines the rate of
chemical weathering
 Increase
temperature means increased
chemical weathering
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Chemical Weathering

Agents of erosion:
 Water
 Oxygen
 Carbon
dioxide
 Acids
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Chemical Weathering

Water
 Dissolves
many kinds of minerals and rock
 Serves as a medium through which other
reactions occurs
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Chemical Weathering

Oxygen
 Combines
with other substances
 Iron (& other minerals) readily combine with
atmospheric O2
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Chemical Weathering

Carbon Dioxide
+ H2O  weak carbonic acid
 Reacts with minerals (like calcite) in
limestone, marble to dissolve rock
 CO2
7.1b ws
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Chemical Weathering

Acid precipitation
 Caused
mainly by oxidation of sulfur dioxide
and nitrogen oxide in atmosphere due to
human activities
 Is precipitation below 5.5 pH
 Is harmful to living organisms
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Rate of Weathering

Climate has a major influence
 Variables
include
Precipitation
 Temperature
 Evaporation

 Interaction
between temperature and
precipitation is the greatest effect
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Rate of Weathering

Chemical weathering occurs
readily in areas that have warm
temperatures, abundant rainfall
and lush vegetation
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Rate of Weathering

Physical weathering occurs readily in cool,
dry climates
 Is
highest where water undergoes repeated
freezing and thawing

Remember, cool temperatures inhibit (slows)
chemical reactions
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Rate of Weathering

Harder rock is going to be more resistant
to weathering than soft rock
 This
depends type of rock and their
composition (what they are made of)

Sedimentary rock is generally going to be
more easily weathered than igneous or
metamorphic
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Rate of Weathering

Surface area
 Increased
surface area equals increased
weathering

Both physical and chemical
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Rate of Weathering

Topography
 The
steeper the slope, the greater tendency
for a material to move downhill
Indian Paint brush canyon
Tetons NP
J B Photos
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Rate of Weathering

Organisms
 Decaying
organic matter and plant roots
release carbon dioxide for chemical
weathering
7.1c ws
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