Weathering and Mass Weathering and Mass WastingWasting

Chapter 10

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External vs. Internal ProcessesExternal vs. Internal Processes(the dynamic equilibrium model)(the dynamic equilibrium model)

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Dynamic EquilibriumDynamic Equilibrium

• Equilibrium stability (fluctuating around some average)

[Geomorphic threshold is reached]

• Destabilizing event• Adjustment• New condition of equilibrium stability

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The Grand CanyonThe Grand Canyon

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DenudationDenudation—Large-scale removal of —Large-scale removal of material that lowers the overall profile of material that lowers the overall profile of the topographythe topography

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Denudation ProcessesDenudation Processes

• Weathering—The combined action of all atmospheric and biologic processes that cause rock to disintegrate physically and decompose chemically because of exposure near Earth’s surface (from bedrock to regolith)

• Mass wasting—spontaneous downslope movement of soil and eroded rock fragments under the influence of gravity, but without the action of moving air, water or ice

• Erosion—extensive removal of rock material, generally transported long distances

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Weathering of BedrockWeathering of Bedrock

► Wherever bedrock is Wherever bedrock is exposed to the exposed to the natural elements, it natural elements, it weathersweathers

► Any crack, joint, or Any crack, joint, or cavity in the rock cavity in the rock will allow will allow weathering agents weathering agents to penetrate and to penetrate and break it apartbreak it apart

Jointing in Bryce Canyon, UT

As rocks weather, surface area increases, offering more surfaces to be weathered…

…producing this result.

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Mechanical WeatheringMechanical Weathering

• Physical disintegration of rock as a result of natural phenomena, without a change in its chemical composition– Pounding, pushing, cracking, breaking,

wedging apart

Mechanical WeatheringProcesses

• Frost wedging• Salt wedging• Unloading/pressure-release jointing• Thermal expansion and contraction• Biologic weathering

Frost Wedging

• Repeated growth and melting of ice crystals in pore spaces of rock fractures or joints

• Expanding ice exerts pressure, breaking rocks apart

• Most effective where there is repeated freeze and thaw (as in arctic or tundra environments)

Frost Wedging

Salt Wedging

• Similar to frost wedging• Growth of salt crystals breaks rocks apart• Most effective in coastal environments

and semi-arid environmentsHoneycomb (tafoni), Salt Point, Sonoma Coast

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Unloading or Pressure-release jointing• Rock brought near the surface as the rocks above (or

even glaciers) erode away relieves confining pressure and allows the rock to expand slightly, forming cracks– Sheeting—The breaking away of layers of rock in sheets,

caused by expansion, usually from unloading processes

• Exfoliation dome—Sheeting on a massive scale, over the face of a large segment of rock (Half Dome in Yosemite, Sierra Nevada Mtns.)

Thermal Expansion and Contraction

• When rock is heated, it expands slightly, and when cooled, it contracts

• Rapid expansion and contraction of the surface of the rock causes cracks to form and propagate

• Most effective in regions with large differences in temperature between daily highs and nightly lows

Thermal Expansion and Contraction

Biologic Weathering• Growth of plant roots,

burrowing animals

• Pressure is exerted by the growth of tiny rootlets in joint fractures, which causes the loosening of small rock particles and mineral grains

• Burrowing animals such as squirrels and oysters may also erode rocks

Chemical Weathering

• Decomposition of rock through chemical alteration of its minerals

• Exposed to water or other solutions, minerals in rocks undergo a chemical change, weakening internal structures– Air, soil water solutions, and groundwater

solutions contain dissolved oxygen, carbon dioxide, or other reactive elements

– Water is the greatest agent of chemical weathering

Chemical Weathering:The Influence of Temperature and

Precipitation

Chemical weathering is most effective in warm, moist climates

Oxidation

• Oxygen dissolved in soil water or ground water can bond with the chemical elements of the minerals to form new minerals

• Causes expansion and exerts pressure that breaks the rocks apart

• Example: iron (Fe) turning to rust (Fe2O3) in the presence of oxygen and water

Hydrolysis and Hydration

• Hydrolysis--minerals reacting with water split into other compounds (may also split the water molecules)– granite: feldspar turns to clays + quartz sand– contributes to spheroidal weathering

Hydrolysis and Hydration

• Hydration--The whole water molecule forms chemical bonds to become part of the chemical composition of the rock, causing expansion and grain-by-grain destruction of rocks

Formation of gypsum from anhydrous calcium sulfate (the mineral anhydrite) which has absorbed water into its chemical structure

Carbonic Acid

Limestone and marble are most susceptible to this type of weathering

Carbon dioxide dissolved in water creates a weak acid called carbonic acid which can dissolve some minerals, especially calcium carbonate

Acid Precipitation

• Urban pollution from sulfur and nitrogen oxide gases mixes with atmospheric water, forming acid precipitation

• Dissolves limestone and marble (often used for public statues and tombstones) and other types of building stones; destroys vegetation, affects human health

Organic Acids• Decaying vegetation mixes w/ water to form

soil water w/ complex organic acids that can react to dissolve or chemically alter minerals

Mass Wasting

• Material is moved a short distance down a slope under the influence of gravity

• Angle of repose—the steepest angle that loose fragments can lie without movement if undisturbed

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Mass Wasting

• The type of mass wasting event that occurs will depend upon speed and the degree of saturation

Mass Wasting(another view)

Types of Mass Wasting

• Rock fall• Rock slide and

Topple

• Debris flow• Earth flow• Mudflow

• Slump• Solifluction• Creep

• Induced mass wasting

Rock Fall

Talus slopes—Regolith which has fallen down steep slopes, funneled into “blankets” of rock called talus cones

Fresh slopes are very unstable

Rock Fall

Rock Slide

Mudflow and Debris Flow

• Mudflow—Rainwater mixed with soil flowing very quickly downslope as a river of mud– Usually in canyons of

mountainous regions– Can carry large objects,

destroying property and taking lives

– Flows until mud thickens, slows, and eventually stops

• Debris flow—More rock fragment than mudflow, but similar in other characteristics

Earthflow

• Water-saturated soil or rock material

• Moves a limited distance down slope as one large mass

• Generally slower in motion (over the course of hours)

• Common form of earth movement causing road closures and property destruction during heavy rains

Near La Conchita Slide, alongHwy. 101 in Ventura County

La Conchita Slide (Earthflow)Hwy. 101, Ventura County

La Conchita Slide (Earthflow)Hwy. 101, Ventura County

Slump—Slow, concave sliding

Slump

Slump

Solifluction

• Continuous freeze and thaw cycles slowly move weathered particles downslope

• Over time, the entire slope moves downhill

Solifluction

Solifluction

Soil Creep

Induced Mass Wasting

Mass wasting caused by human activity

• Moving weathered rock material downslope during construction on steep hillsides – Carried away as debris flows or mudflows during heavy rains

• Removal of material supporting the base of a slope

• The wetting of weathered rock material and soil from pipe breakage, lawn watering, etc. causing slippage

• Debris removal by heavy rains after fire may also remove stabilizing vegetation

Induced Mass Wasting:Construction of the Panama Canal