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Chapter 15: Weathering and Mass Wasting
Physical Physical GeographyGeographyNinth EditionNinth Edition
Robert E. Gabler
James. F. Petersen
L. Michael Trapasso
Dorothy Sack
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Weathering and Mass Wasting
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Weathering and Mass Wasting
• Exogenic processes:– Breakdown rocks – Erode rock materials– Transport them
• Relocation by– Gravity– Geomorphic Agents
• Flowing water, wind• Moving ice, waves
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15.1 Nature of Exogenic Processes
• Weathering: breakdown of rock materials at and near Earth’s surface
• Mass wasting
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15.1 Nature of Exogenic Processes
Q: Can you identify evidence of the three phases shown in Figure 15.3?
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15.2 Weathering
• 3 Types of Weathering– Physical weathering– Mechanical weathering– Chemical weathering
Q: How might an animal cause physical weathering?
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15.2 Weathering
• Physical Weathering– Person breaking a rock– Unloading– Thermal Expansion and
Contraction– Freeze-Thaw– Salt Crystal growth– Hydration
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15.2 Weathering
• Unloading– Erosional stripping of
overlying rocks and ultimate removal of overlying weight
– High elevation increases process
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15.2 Weathering
• Unloading– Exfoliation– Exfoliation sheet – Exfoliation dome
• Stone Mountain, Georgia• Half Dome (Yosemite)
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15.2 Weathering
• Thermal Expansion and Contraction– Granular disintegration
• Differential thermal expansion and contraction of individual mineral grains in coarse crystalline rocks
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15.2 Weathering
• Freeze-Thaw Weathering– Also called frost
weathering (ice wedging)– Water freezes it expands
9%– Pipes bursting– Angular blocks– Effective in the upper-
middle and lower-high latitudes
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15.2 Weathering
• Salt Crystal Growth– Water with dissolved salt
accumulates in these spaces and then evaporates, and growing salt crystals wedge rocks apart
• Hydration
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15.2 Weathering
• Chemical Weathering– Most important catalysts
and reactive agents:• Water
• Oxygen
• Carbon dioxide
– 3 Types of chemical weathering:
• Oxidation
• Carbonation and Solution
• Hydrolysis
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15.2 Weathering
• Oxidation– Iron and aluminum
oxides– Very common
• Carbonation and Solution– Occurs when carbon
dioxide in water reacts with rock material to produce bicarbonate ions
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15.3 Variability in Weathering
• Effectiveness of Weathering– Climate– Type of rock– Nature and amount of fractures
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15.3 Variability in Weathering
• ClimateQ: In which weathering region would we find a site that
has an annual mean temperature of 5oC (41oF) and an annual rainfall of 100 cm (40 in)?
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15.3 Variability in Weathering
• Climate
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15.3 Variability in Weathering
• Rock Type– Differential weathering
and erosion– Strong rock may be
easily eroded in a different environment
• Structural Weakness– Joints and fractures– Exposed rock, collect
salts, accumulation of water, and plants
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15.3 Variability in Weathering
• Structural Weakness– Spheroidal weathering
Q: With north at the top of this photo, what directions do the two most apparent joint sets trend?
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15.3 Variability in Weathering
• Topography Related to Differential Weathering and Erosion
• Grand Canyon
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15.3 Variability in Weathering
• Topography Related to Differential Weathering and Erosion– Q: Can you see how
the topography of the Ridge and Valley section influences human settlement patterns?
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15.4 Mass Wasting
• Mass Movement– Downslope
transport of material due to gravity
Q: What other kinds of problems on roads are related to mass wasting?
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15.4 Mass Wasting
• Mass Movement– Slope angle– Creeping, falling,
sliding, or flowing– Gravity is the
principal force– Water is also a
contributing factor
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15.4 Mass Wasting
• Classification of Mass Wasting– Types of Earth
Material• Soil• Debris• Mud
– Categorized by speed
• Slow mass wasting• Fast mass wasting
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15.4 Mass Wasting
• Slow mass wasting– Creep
• Slow migration, but most persistent
• Heaving process
– Q: Are there places near where you live that show evidence of soil creep?
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15.4 Mass Wasting
Q: What other constructed features might be changed by creep?
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15.4 Mass Wasting
• Slow mass wasting– Solifluction
• Most common in high latitudes of high-elevation tundra
• Permafrost and active layer
• Summer thaw• Tundra landscapes
Q: How does solifluction differ from soil creep?
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15.4 Mass Wasting
• Fast Mass Wasting– Four Major Types:
• Falls• Avalanches• Slides • Flows
– Effects more dramatic than slow mass wasting
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15.4 Mass Wasting
• Falls– Rockfalls– Due to:
• Weathering weakens the bonds between clasts and cliff
• Large rock masses that fall from a cliff face or an overhanging ledge
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15.4 Mass Wasting
• Falls– Talus slope– Talus cone– Angle of repose
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15.4 Mass Wasting
• Falls– Happy Isles Rockfall
in Yosemite Valley, CA
– Traveled at an estimated 250 km/hr (160 mph)
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15.4 Mass Wasting
• Avalanches– Snow – Rock– Debris
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15.4 Mass Wasting
• Slides– Water plays biggest
role– Rockslides– Threaten lives and
property– Hebgen Lake
rockslide• Southwestern
Montana (1959) • Killed 28 people
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15.4 Mass Wasting
• Slides– Debris slide– Mudslide– Slumps– Landslide
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15.4 Mass Wasting
• Flows– Earthflow– Slump-earthflow– Debris flows– Mudflows– Flow levees
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15.5 Weathering, Mass Wasting, and the Landscape
Violent 1980 eruption of Mount St. Helens in Washington created lahars - mudflows
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Physical Geography
End of Chapter 15: Weathering and Mass Wasting