15

Chapter 15: Weathering and Mass Wasting

Physical Physical GeographyGeographyNinth EditionNinth Edition

Robert E. Gabler

James. F. Petersen

L. Michael Trapasso

Dorothy Sack

Weathering and Mass Wasting

Weathering and Mass Wasting

• Exogenic processes:– Breakdown rocks – Erode rock materials– Transport them

• Relocation by– Gravity– Geomorphic Agents

• Flowing water, wind• Moving ice, waves

15.1 Nature of Exogenic Processes

• Weathering: breakdown of rock materials at and near Earth’s surface

• Mass wasting

15.1 Nature of Exogenic Processes

Q: Can you identify evidence of the three phases shown in Figure 15.3?

15.2 Weathering

• 3 Types of Weathering– Physical weathering– Mechanical weathering– Chemical weathering

Q: How might an animal cause physical weathering?

15.2 Weathering

• Physical Weathering– Person breaking a rock– Unloading– Thermal Expansion and

Contraction– Freeze-Thaw– Salt Crystal growth– Hydration

15.2 Weathering

• Unloading– Erosional stripping of

overlying rocks and ultimate removal of overlying weight

– High elevation increases process

15.2 Weathering

• Unloading– Exfoliation– Exfoliation sheet – Exfoliation dome

• Stone Mountain, Georgia• Half Dome (Yosemite)

15.2 Weathering

• Thermal Expansion and Contraction– Granular disintegration

• Differential thermal expansion and contraction of individual mineral grains in coarse crystalline rocks

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

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

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

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

15.3 Variability in Weathering

• Effectiveness of Weathering– Climate– Type of rock– Nature and amount of fractures

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)?

15.3 Variability in Weathering

• Climate

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

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?

15.3 Variability in Weathering

• Topography Related to Differential Weathering and Erosion

• Grand Canyon

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?

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?

15.4 Mass Wasting

• Mass Movement– Slope angle– Creeping, falling,

sliding, or flowing– Gravity is the

principal force– Water is also a

contributing factor

15.4 Mass Wasting

• Classification of Mass Wasting– Types of Earth

Material• Soil• Debris• Mud

– Categorized by speed

• Slow mass wasting• Fast mass wasting

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?

15.4 Mass Wasting

Q: What other constructed features might be changed by creep?

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?

15.4 Mass Wasting

• Fast Mass Wasting– Four Major Types:

• Falls• Avalanches• Slides • Flows

– Effects more dramatic than slow mass wasting

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

15.4 Mass Wasting

• Falls– Talus slope– Talus cone– Angle of repose

15.4 Mass Wasting

• Falls– Happy Isles Rockfall

in Yosemite Valley, CA

– Traveled at an estimated 250 km/hr (160 mph)

15.4 Mass Wasting

• Avalanches– Snow – Rock– Debris

15.4 Mass Wasting

• Slides– Water plays biggest

role– Rockslides– Threaten lives and

property– Hebgen Lake

rockslide• Southwestern

Montana (1959) • Killed 28 people

15.4 Mass Wasting

• Slides– Debris slide– Mudslide– Slumps– Landslide

15.4 Mass Wasting

• Flows– Earthflow– Slump-earthflow– Debris flows– Mudflows– Flow levees

15.5 Weathering, Mass Wasting, and the Landscape

Violent 1980 eruption of Mount St. Helens in Washington created lahars - mudflows

Physical Geography

End of Chapter 15: Weathering and Mass Wasting