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The Nature of Weathering
• Weathering is the physical and/or chemical alteration of rocks and minerals where the lithosphere, hydrosphere, atmosphere, and biosphere meet
Weathering
• Chemical weathering– The decomposition of rocks and minerals as
chemical reactions alter them into new minerals stable at the Earth’s surface
• Physical weathering– The disintegration or disaggregation of rocks
by physically breaking them apart
Physical Weathering
• Physical breakage of rocks into smaller pieces– Ice Wedging– Sheeting
Ice Wedging
• Freeze - Thaw cycles are effective at breaking apart rocks– Water expands when it freezes
• Volume increases by 9%
– The stress of expansion breaks the rock– Ice melts and the water percolates deeper
into the newly expanded cracks
Fig. 10.2a. Ice wedging
Sheeting
• Release of confining pressure on rocks formed deep within the Earth
• Development of fractures and joints caused by expansion
• Rocks break along fractures and joints
Fig. 10.4. Sheeting in granite
Other Forms of Physical Weathering
• Heat– Heat causes rocks (most solids) to expand– Rocks are poor conductors of heat– Outer layer of rock that expands breaks off
(spall)
• Crystal Growth– Minerals precipitate along fractures– Similar to ice wedging
Other Forms of Physical Weathering
• Root Growth– Roots may exert enormous forces in growing– Root tips pressures may exceed 10,000 kg per
square meter– Seeds gather in cracks in rock and germinate– Growing plant and roots slowly wedge rock
apart
Chemical Weathering
• Minerals are destroyed or altered by chemical reactions– Dissolution– Hydrolysis– Oxidation
Dissolution
• Some minerals are soluble in water– e.g., Halite - NaCl– Minerals dissolve into constituent ions– Ions removed with water by leaching– Solubility of compound controls
leachability
Acid Hydrolysis
• CO2 mixes with water to produce carbonic acid, H2CO3
• Decaying organic matter produces acid• Anthropogenic sources of acid
– Acid rain
Acid Hydrolysis
• H+ attacks minerals by replacing other ions in the mineral structure
• Promotes dissolution– Calcite hydrolysis by carbonic acid solution
CaCO3 + H2CO3 = Ca+2 + 2HCO3-
Acid Hydrolysis
• New “secondary” minerals may be created by this process– H+ ion replaces the K+ ion in the feldspar
structure– K+ ion goes into the water solution– Kaolinite, a clay mineral, formed
2KAlSi3O8 + 2H2CO3 + 9H2O =2K+ + 2HCO3
- + 4H4SiO4 + Al2Si2O5(OH)4
Oxidation
• Valence state increases– Often associated with free O2 in the
environment
• Iron is usually found as the Fe+2 ion in silicate minerals
• Exposed to the atmosphere it will oxidize to the Fe+3 ion
Oxidation
• Change in valence state disrupts crystal structure
• Oxidation works in combination with hydrolysis and dissolution
2FeSiO4 + 4H2O + O2 = 2 Fe2O3 + 2H4SiO4
Trends in Chemical Weathering
• Alkali and alkaline earth elements removed into solution
• Al and Si are enriched in secondary minerals
• Fe is enriched in insoluble ferric oxides• Increases in T and moisture increase
weathering rates
Weathering of Rocks
• Relative stability of minerals varies widely
• Minerals composition is primary control
• Rock texture influences role of water in weathering
Fig. 10.8. Relative stability of minerals
Soil
• Soil is earth material that has been modified or acted upon by physical, chemical, and biological processes so that it is capable of sustaining rooted plant growth
• Soils contain the mineral matter remaining from, and created by physical and chemical weathering of rocks, plus organic matter from biological processes
Soil
– Soil horizons: Soils develop from the top down, and each successive horizon has a unique set of physical and chemical properties forming a soil profile
Soil Horizons• A Horizon
– A dark gray to black horizon, rich in organic matter. Leaching carries dissolved ions and fine particles
• B Horizon– Accumulates leached material from above– Enriched in clay minerals and oxides
• C Horizon– Parent rock in various stages of weathering
Fig. 10.14Common soil profile
Climate & Weathering
• Climatic conditions strongly influence weathering reactions– Amount of rainfall
• Most reactions need water
– Average temperature• Increase of 10oC doubles reaction rate
Rates of Weathering
• Rates of weathering are linked to climate zones– Human structures are useful gages for
measuring rates– Thickness of soil profile is controlled by
weathering rates
Fig 10.19. Climate and weathering
Mass Movement
• Mass movement is the movement of regolith and masses of rock downhill
• Gravity is the force which drives mass-wasting
The Role of Gravity
• Shear stress is the force of gravity acting on an object on a slope– Gravity always pulls straight down,
perpendicular to the horizon– On a slope objects move downhill at
some angle
Fig. 11.1. Forces acting on a rock on a hillslope.
Resistance to Movement
• Cohesive strength is the resistance of an object to move downhill - an accumulation of forces– Friction– Cohesion, stickiness of particles– Other forces holding a particle in place
(plant roots, cementation)
The Role of Water
• Water may act to increase or decrease cohesive strength– Dry regolith or soil has little or no
cohesion– Damp regolith or soil is sticky– Saturated regolith or soil flows easily
Types of Mass Movement
1. Creep• Extremely slow movement of soil and
regolith - 1 to 10 mm/yr• Combination of factors cause creep
– Heaving of soil - expansion & contraction is the primary cause
• Wet-dry cycles; Freeze-thaw cycles• Other factors: Plant root growth; Burrowing
animals; streams, earthquakes, human activity
Solifluction
• Creep that occurs in permafrost– Melting of soil water occurs from the
surface down– Permafrost prevents downward
percolation– Surface soils become saturated and begin
to flow
2. Debris Flows
• Mixtures of water, mud and rock– Liquified soil flows downhill
• Up to 50 km/hr
– Water lubricates mass of soil and rock– Large boulders, building etc. may be
carried by viscous fluid
3. Landslides
• Mass movement along well defined slippage plane
• Landslide block moves as a single or group of units
• Rock type, orientation and water content influence events
Vaiont Landslide Disaster
• Worst dam disaster in history– Killed ~ 2600 people
• 240,000,000 m3 of rock involved in landslide
• Filling of dam caused change in subsurface hydrogeology
• Water lubricated bedding planes
Fig. 11.3. Vaiontdam disaster
4. Land Subsidence
• Downward movement of earth materials
• Movement is essentially vertical• Primary force is gravity• Other geologic processes make
subsidence possible
Land Subsidence
• Open space in the subsurface needs to be created– Groundwater forms caves– Buried glacial ice– Draining or over-pumping of
groundwater