Chapter 7: Weathering and Erosion © 2012 John Wiley & Sons, Inc. All rights reserved.

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.


Weathering—The First Step in the Rock Cycle


Figure 7.2 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


How Rocks Disintegrate


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



Joint•A fracture of rock, along which no appreciable movement has occurred•Sheet jointing (exfoliation)•Frost wedging•Root wedgingAbrasion•Wearing down of bedrock by the constant battering of loose particles transported by wind, water or ice

Joint Formation



Joint Formation



Figure 7.4a Sheet jointing

Figure 7.4b Frost wedging



Figure 7.4c Scree slope

Figure 7.4d Root wedging


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



Chemical Weathering

Chemical Weathering

Figure 7.6 Ion exchange


Chemical Weathering

Figure 7.7 Hydrolysis and oxidation


Factors Affecting Weathering

Tectonic setting•Young, rising mountains weather quicker•Mechanical weathering most common

Figure 7.8 Tectonic setting



Rock structure•Distribution of joints influence rate of weathering•Relatively close joints weather faster

Figure 7.8 Rock structure



Topography•Weathering occurs faster on steeper slopes•Rockslides

Figure 7.8 Topography



Biologic activity•Presence of bacteria can increase breakdown of rock

Figure 7.8 Biological activity



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



Vegetation•Contributes to mechanical and chemical weathering.•Promotes weathering due to increased water retention.•Vegetation removal increases soil loss.

Figure 7.8 Vegetation



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



Figure 7.9 Color corresponds to climates in Figure 7.9b


Products of WeatheringClay•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


Products of Weathering

Sand•A sediment made of relatively coarse mineral grainsSoil•Mixture of minerals with different grain sizes, along with some materials of biologic originHumus•Partially decayed organic matter in soil

Figure 7.11a Lunar regolith

Figure 7.11b Earth soil


Products of WeatheringSoil 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.



Figure 7.12 Soil profile containing soil horizons (zones or layers)



Figure 7.13 Climate and soils



Figure 7.13 Comparison of soil from Kansas (l) and Connecticut (r)


Erosion and Mass WastingErosion by Water

Erosion•The wearing away of bedrock and transport of loosened particles by a fluid, such as waterBed load•Sediment moved along the bottom of a streamSaltation•A transportation mechanism in which particles move forward in a series of short jumps along arc-shaped pathsSuspended load•Sediments carried in suspension by a flowing stream of water or wind


Erosion and Mass Wasting

Figure 7.14a Stream’s load


Erosion and Mass WastingErosion by Wind

Saltation is primary mechanism for wind erosion.


Figure 7.15 Massive dust storm

Erosion and Mass WastingErosion 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

Figure 7.16b Polished and grooved rock from glaciers


Figure 7.16c Two glaciers merge


Erosion and Mass WastingGravity and Mass Wasting


Mass wasting•The downslope movement of regolith and/or bedrock masses due to the pull of gravitySlope 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


Mass Wasting caused for very few reasons:

*Weight, *Water, *Slope and *Geology


Figure 7.17a Repeated rockfalls Figure 7.17b rockslide



Figure 7.17c Slump


Erosion and Mass WastingFlows


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

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


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…

https://www.youtube.com/watch?v=OgV9L_KdNvc

Chapter 7: Weathering and Erosion © 2012 John Wiley & Sons, Inc. All rights reserved.

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Transcript of Chapter 7: Weathering and Erosion © 2012 John Wiley & Sons, Inc. All rights reserved.