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