Weathering and Mass Weathering and Mass WastingWasting

Chapter 10

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

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

Denudation ProcessesDenudation Processes

• Weathering—Rock disintegrating in place• Mass wasting—Eroded rock moving

downslope• Erosion—Extensive removal of rock material,

generally transported long distances

The Grand CanyonThe Grand Canyon

WeatheringWeathering

• 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

• Changing bedrock into regolith

Weathering of BedrockWeathering of Bedrock► Wherever bedrock is Wherever bedrock is

exposed to the natural exposed to the natural elements, it weatherselements, it weathers

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

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

Mechanical WeatheringProcesses

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

Frost Wedging

Most effective where there is repeated freeze and thaw, as in arctic or tundra environments

Frost Wedging

Salt Wedging

• Most effective in coastal environments and semi-arid environments

Honeycomb,or “tafoni”, Salt Point, Sonoma Coast

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Unloading or Pressure-release jointing

• Rock brought near the surface relieves confining pressure and allows the rock to expand slightly, forming cracks

Thermal Expansion and Contraction

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

Thermal Expansion and Contraction

Biologic WeatheringGrowth of plant roots, burrowing animals loosen and break apart rocks

Chemical Weathering

• Rocks forming at depth are stable under those conditions

• Once rocks are exposed to surface conditions many of the minerals become less stable and may undergo a slow chemical change that weakens their internal structures

Chemical Weathering

• Decomposition of rock thorough the chemical alteration of its minerals– 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 is most effective in warm, moist climates

Chemical Weathering:The Influence of Temperature and

Precipitation

Oxidation• Oxygen dissolved in soil water or ground water can

bond with the chemical elements of the minerals to form new minerals

• Iron (Fe) turning to rust (Fe2O3) in the presence of oxygen and water

Hydrolysis and HydrationHydrolysis--Water combines with some minerals, breaking down the rocks into new mineral compounds

granite: fedspar turns to clays + quartz sandcontributes to spheriodal weathering

Hydrolysis and HydrationHydration--Water molecules become part of the chemical composition of the rock (no breakdown). Mineral alteration and expansion results in 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

• In urban areas with pollution from sulfur and nitrogen oxides, these gasses can mix with atmospheric water, forming acid precipitation

• Dissolves limestone and marble and other types of building stones

Organic AcidsDecaying vegetation mixes w/ water to form soil water w/ complex organic acids that can react to dissolve or chemically alter minerals

Mass WastingMass 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

Angle of ReposeThe steepest angle that loose fragments can lie without movement if undisturbed

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

• 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

Solifluction LobesEmphasized by Animal Trails

Soil Creep

Induced Mass Wasting

Mass wasting caused by human activity