Download Weathering

Chapter 7:
Weathering and Erosion
© 2012 John Wiley & Sons, Inc. All rights reserved.
Learning Objectives
Weathering—The first step in the rock cycle
• Contrast chemical and mechanical weathering
and their place in the rock cycle.
Products of weathering
• Describe the end products of weathering and
the difference between soil horizons and soil
profiles.
Erosion and mass wasting
• Identify the sources of erosion and mass
wasting.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Weathering—The First Step in the
Rock Cycle
Figure 7.2 How rocks disintegrate
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Weathering—The First Step in the
Rock Cycle
How Rocks Disintegrate
•
•
•
Weathering
• The chemical and physical breakdown of rock
exposed to air, moisture, and living organisms
Regolith
• A loose layer of fragments that covers much of
Earth’s surface
Soil
• The uppermost layer of regolith, which can support
rooted plants
© 2012 John Wiley & Sons, Inc. All rights reserved.
Weathering—The First Step in the
Rock Cycle
Mechanical weathering
•
•
The breakdown of rock
into solid fragments by
physical processes
Chemical composition of
rock NOT altered
Chemical weathering
•
The decomposition of
rocks and minerals by
chemical and
biochemical reactions
Figure 7.3 From rock to soil
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Weathering—The First Step in the
Rock Cycle
Joint
• A fracture of rock, along
which no appreciable
movement has occurred
• Sheet jointing (exfoliation)
• Frost wedging
• Root wedging
Abrasion
• Wearing down of bedrock
by the constant battering of
loose particles transported
by wind, water or ice
Joint Formation
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Weathering—The First Step in the
Rock Cycle
Joint Formation
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Weathering—The First Step in the
Rock Cycle
Figure 7.4b Frost wedging
Figure 7.4a Sheet jointing
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Weathering—The First Step in the
Rock Cycle
Figure 7.4c Scree slope
Figure 7.4d Root wedging
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Weathering—The First Step in the
Rock Cycle
Chemical Weathering
Dissolution
• Separation of
materials into ions in
solution by a solvent,
such as water or acid
Rainwater
• Acts as weak solution
of carbonic acid
• Anthropogenic actions
(acid rain)
Figure 7.5 (a) marble, (b) granite
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Chemical Weathering
Figure 7.6 Ion exchange
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Chemical Weathering
Figure 7.7 Hydrolysis and oxidation
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Factors Affecting Weathering
Tectonic setting
•Young, rising mountains
weather quicker
•Mechanical weathering
most common
Figure 7.8 Tectonic setting
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Factors Affecting Weathering
Rock structure
•Distribution of joints
influence rate of
weathering
•Relatively close joints
weather faster
Figure 7.8 Rock structure
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Factors Affecting Weathering
Topography
• Weathering occurs
faster on steeper
slopes
• Rockslides
Figure 7.8 Topography
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Factors Affecting Weathering
Biologic activity
•Presence of bacteria
can increase breakdown
of rock
Figure 7.8 Biological activity
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Factors Affecting Weathering
Rock composition
•Minerals weather at
different rates:
• Calcite weathers
quickly through
dissolution.
• Quartz is very
resistant to
chemical and
mechanical
weathering.
Figure 7.8 Composition
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Factors Affecting Weathering
Vegetation
• Contributes to
mechanical and
chemical weathering.
• Promotes weathering
due to increased
water retention.
• Vegetation removal
increases soil loss.
Figure 7.8 Vegetation
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Factors Affecting Weathering
Climate
•Chemical weathering is
more prevalent in warm,
wet tropical climates.
•Mechanical weathering is
less important.
•Mechanical weathering is
more prevalent in cold, dry
regions.
•Chemical weathering
occurs slowly.
Figure 7.9a Climate and weathering
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Factors Affecting Weathering
Figure 7.9 Color corresponds to climates in Figure 7.9b
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Products of Weathering
Clay
• Tiny mineral particles of any kind that have
physical properties like those of the clay
minerals
• A family of hydrous alumino-silicate minerals
Figure 7.10 Kaolin: from (a) clay to (b) fine china
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Products of Weathering
Sand
• A sediment made of
relatively coarse mineral
grains
Soil
• Mixture of minerals with
different grain sizes, along
with some materials of
biologic origin
Humus
• Partially decayed organic
matter in soil
Figure 7.11a Lunar regolith
Figure 7.11b Earth soil
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Products of Weathering
Soil Profiles
Soil horizons
• One of a succession of zones or layers within a
soil profile.
• Each horizon has a distinct physical, chemical,
and biologic characteristic.
Soil profiles
• The sequence of soil horizons from the surface
down to the underlying bedrock.
• Soil profiles vary, influenced by factors such as
climate, topography, and rock type.
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Products of Weathering
Figure 7.12 Soil profile containing soil horizons (zones or layers)
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Products of Weathering
Figure 7.13 Climate and soils
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Products of Weathering
Figure 7.13 Comparison of soil from Kansas (l) and Connecticut (r)
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Erosion and Mass Wasting
Erosion by Water
Erosion
• The wearing away of bedrock and transport of
loosened particles by a fluid, such as water
Bed load
• Sediment moved along the bottom of a stream
Saltation
• A transportation mechanism in which particles move
forward in a series of short jumps along arc-shaped
paths
Suspended load
• Sediments carried in suspension by a flowing stream
of water or wind
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Erosion and Mass Wasting
Figure 7.14a Stream’s load
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Erosion and Mass Wasting
Erosion by Wind
Saltation is primary mechanism for wind erosion.
Figure 7.15 Massive dust storm
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Erosion and Mass Wasting
Erosion by Ice
Glacier
• A semi-permanent or perennially frozen body of ice, consisting
of recrystallized snow, that moves under the pull of gravity
Figure 7.16a Debris
from glaciers
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Figure 7.16b Polished
and grooved rock from
glaciers
Erosion and Mass Wasting
Figure 7.16c Two glaciers merge
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Erosion and Mass Wasting
Gravity and Mass Wasting
Mass wasting
•The downslope movement of regolith and/or bedrock
masses due to the pull of gravity
Slope failure
•Falling, slumping, or sliding of relatively coherent masses
of rock
•Three basic types:
• Fall: vertical or near vertical drop of rock fragments
• Slide: rapid displacement of rock/regolith down
steep or slippery slope
• Slump: rotational movement of material
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Mass Wasting caused for very few reasons:
*Weight, *Water, *Slope and *Geology
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© 2012 John Wiley & Sons, Inc. All rights reserved.
Erosion and Mass Wasting
Figure 7.17a Repeated rockfalls
Figure 7.17b rockslide
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Erosion and Mass Wasting
Figure 7.17c Slump
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Erosion and Mass Wasting
Flows
Flow
• Any mass-wasting process that involves a flowing
motion of regolith containing water and/or air
within its pores
• Slurry flows
• Granular flow
Creep
• The imperceptibly slow downslope granular flow
of regolith
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Erosion and Mass Wasting
Figure 7.18 Flows
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Factor of Safety and Landslide
Prediction
Factor of safety (FS)
• The balance between
destabilizing forces
(shear stress) and
stabilizing forces (shear
strength)
Tectonics and mass wasting
• Major historic landslides
clustered near converging
lithospheric plates
• Mountains/earthquakes
Figure 7.19 Earthquakes can cause mass wasting events;
Alaska 1964
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Critical Thinking
• On Earth, clay minerals are the most common
products of weathering. Samples from the Moon
do not contain any clay minerals. Why?
• Why are some granite bodies extensively
jointed, while others are essentially joint free?
Youtube mass wasting video…
© 2012 John Wiley & Sons, Inc. All rights reserved.