Weathering, Erosion and Transport

Weathering, Erosion, and Transportation

• Rocks exposed at Earth’s surface are constantly changed

by water, air, temperature variations and other factors

• Weathering is the group of destructive processes that

change physical and chemical character of rocks at or

near Earth’s surface

• Erosion is physical picking up of rock particles by water,

ice, or wind

• Transportation is the movement of eroded particles by

water, ice, or wind

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 and Earth Systems

• Hydrosphere – Water is necessary for chemical weathering

– Oxygen dissolved in water oxidizes iron and other metals in rocks

– Carbon dioxide dissolved in water creates carbonic acid

• Primary cause of chemical weathering

– Running water loosens and abrades particles

– Glacial ice removes and abrades particles

– Freeze/thaw cycling mechanically weathers

• Biosphere – Plant root growth widens cracks

– Animal movement and human activity mechanically weather

– Plant roots decaying organic matter in soils pump carbon dioxide into

the soil producing acids that dissolve the rock

Products of Weathering

• Lithic (Rock) Fragments(granite, basalt, schist, etc.)

• Dissolved Ions(Calcium, Potassium, Sodium, etc.)

• Rust Minerals (Hematite, Goethite, etc.)

• Clay Minerals(Bentonite, Montmorillonite, etc.)

• Residual Minerals(Quartz, Orthoclase, Muscovite, etc.)

Two Types of Weathering

• Chemical weathering– The decomposition of rocks and minerals as

chemical reactions alter them into new minerals stable at the Earth’s surface

• Physical (or mechanical) weathering– The disintegration or disaggregation of rocks by

physically breaking them apart

Weathering

• Physical and chemical weathering are two distinct processes

• Usually work in tandem

• Chemical weathering is more significant in warm wet low land environments; Physical weathering is more important in in cold areas and high elevations

Physical Weathering

Physical Weathering

• Frost action– Mechanic effect of freezing (and

expanding) water on rocks

• Pressure release– Removal of overlying rock allows

expansion and fracturing

• Plant growth– Growing roots widen fractures

• Burrowing animals

• Thermal cycling– Large temperature changes fracture rocks

by repeated expansion and contraction

But mostly physical weathering is a matter of things just

falling down. So in a sense, gravity, is the primary cause of

physical weathering.

Physical Weathering in cold high altitude

environments

Physical Weathering by running water

More on that later

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

Sheeting in granite

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

Frost/freeze or Ice wedging

Geometry of Weathering

• Spheroidal weathering

–Corners tend to be rounded during weathering

–Decomposition is most rapid at corners

–Rock’s shape approaches sphere

– Further weathering reduces size

Spheroidal weathering in granites

Spheroidal weathering in granites

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

Geometry of Weathering

• Fractures in rock form from the reduction in load (pressure)

–Generally form in groups

• Parallel joints

• Intersecting joints

–Cut large blocks into smaller blocks

Geometry of Weathering

• Surface Area is increased by fracturing

– The increase in surface area, increases the rate of weathering

• Both physical and chemical

– Surface area increases exponentially

y = 3e0.6931x

0

10

20

30

40

50

60

0 2 4 6

Series1

Expon.

(Series1)

Chemical Weathering

Chemical Weathering

• Minerals are destroyed or altered by chemical reactions

–Dissolution

–Hydrolysis

–Oxidation

Chemical Weathering

• Oxidation

– Chemically active oxygen from atmosphere

– Iron oxides are common result

• Soil and sedimentary rocks often stained with

iron oxides

• Acid dissolution

– Hydrogen cations replace others in minerals

– Carbonic acid from atmospheric CO2 dissolved

in water

– Sulfuric, hydrofluoric acids emitted by volcanic

eruptions

– Some minerals, such as calcite, may be totally

dissolved

– Human activity, such as mining and burning of

fossil fuels, produces acids

Chemical Weathering

• Feldspars

– Most common minerals in crust

– Slightly acidic rain water attacks feldspar

– Clay minerals produced

• K+, Na+, Ca++ ions released into water

• Other minerals

– Ferromagnesian minerals

• Clays, iron oxides, Mg++ ions produced

– More complex silicate bonds lead to lower weathering susceptibility

• Olivine most susceptible, quartz least

• Warm, wet climatic conditions maximize weathering

Chemical Weathering

• Most igneous and metamorphic rocks and minerals are formed at high temperatures and pressures– They are in a state of equilibrium at the

Temperature (T) and Pressure (P) of formation

– At the Earth’s surface, rocks and minerals are subject to chemical weathering

– Secondary minerals formed at the T and P common to the Earth’s surface

Chemical Weathering (cont)

• Sedimentary Rocks: –Limestones and Dolomites are formed in

the ocean and are easily dissolved by water, especially if it is acidic

–Evaporites (Halite, Gypsum and Anhydrite) are precipitated from seawater and easily dissolved in water even if it is not acidic

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

• Roots pump CO2 into the soil producing very high concentrations of carbonic acid

• Anthropogenic sources of acid (CO2 and SO2)– 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

• Warm wet climates increase chemical weathering rates

Weathering of Rocks

• Relative stability of minerals varies widely

• Minerals composition is primary control

• Rock texture influences role of water in weathering

Relative stability of minerals

Stability of minerals at the Earth’s surface is predicted by Bowen’s reaction series in Reverse, i.e.,

Quartz is most stable and Olivine is least.

Inorganic Carbon CycleWhat controls CO2 concentrations on geologic timescales

– Carbon dioxide present as trace

gas in atmosphere (380ppm)

– Combines with water to form

carbonic acid (H2CO3)

– Weathers rocks and provides

CaCO3 to marine animals and

plants so they can make shells.

– Returns to the mantle during

subduction

– Released back to atmosphere by

volcanic eruptions

On geologic timescales volcanism controls CO2 concentrations.

The negative feedback mechanism on this is the rate of weathering which

increases because of warmer climates due to higher CO2 concentrations.

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

Climate and weathering

From Sediment to Sedimentary Rock

• Transportation– Movement of sediment away from its source, typically by

water, wind, or ice

– Rounding of particles occurs due to abrasion during transport

– Sorting occurs as sediment is separated according to grain size

by transport agents, especially running water

– Sediment size decreases with increased transport distance

Distinguishing Characteristics of Clastic Sediments (cont.):Sorting - Well-sorted sediment indicates prolonged reworking by wind or water; poorly sorted

sediment may indicate rapid deposition, or deposition by ice or mass movement.

Angularity/Roundness and Shape – Well rounded sediment also indicate prolonged reworking

by transporting agent; the shape of grains often indicates the transport system, but also may

be related to the type of mineral or rock fragment