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