Chapeter 2 Flashcards
Weathering
In-situ breakdown and alteration of earth forming materials (physical or chemical)
Why does weathering happen
Rocks are formed within the crust: higher P,T
Unstable under surface conditions
Products of weathering are more stable
Controls on weathering rate
Mineralogy
Petrology
Climate
Vegetation
Main control on weathering
Climate
Most active part of the weathering zone
soils
Active process on soil weathering
Organic accumulation and decay Oxidation Reduction Ground water movement Bioturbation Microbial activity
Soil O
Organics
Soil A
Surface soil layer (top soil)
Soil E
Zone of eluvation
Soil B
Zone of Accumulation
Soil C
Realtively unalterd parent material
Types of Physical weathering
Unloading Thermal expansion Frost action Wetting and drying Salt crystal growth Root action
Unloading
(pressure release)
Erosion removes overburden–> rocks expand
Rocks split into sheets parallel to the topography
Thermal expansion
Rocks and minerals expand/ contract in respoince to heating and cooling
Need frequent, extreme fluctuations
Frost action
Repeated formation and melting of ice in pore
Water expands 9% upon freezinf
Wetting and drying
Materials swell when wet and shrink. crack when dry
Most effective in clay-rich rocks and sediment
Salt crystal growth
Precipitation of salt crystals in rock pores/ cracks
Typical of dry climate and coasts
Root action
Plant roots grow into joints, force rock apart
Chemical weathering
AKA Decomposition
Secondary minerals are produced (clays)
Types of Chemical Weathering
Hydrolysis
Oxidation
Solution
Carbonation
Hydrolysis
Breakdown of water into H+ and OH- ions H ions replace metal cations in rock Freed cations form soluble oxides Accelerated if there is CO2 in the water Dominant process in igneous rocks
Oxidation
Addition of oxygen to minerals- forms oxides
Most effective in aerated zone: presence of air and water
Solution
Dissociation of minerals in water Ions lost to ground water Most effective in acidic water Forms karst like features Salts, gypsum, carbonate are especially prone
Carbonation
So;ution of minerals in carbonic acid
CO2 from atmosphere, decaying organic matter
Prevalent in limestone bedrock, dolomite and marble
Weathering landscapes
High rates of weathering no other dominant processes
Types of weathering land scapes
Felsenmeer
Karst
Felsenmeer
Block feild topography
Dominated by frost action
Alpine areas, artic, antartic
karst
Landscapes formed by carbonation
Limestone or dolomite bedrock
Humid clomate
Sediment
Fragments of rock that have moved from their orignal position
Requires weathering
Three phases to sediment movement
Erosion
Transport
Deposition
Slopes may be
Weathering limited
Erosion limited
Equilibrium
Erosional slope is
Convex upper slope , Possible ‘ free face’
Transport slope
Straight mid slope
Depositional
Concave lower slope
Rain splash
Impact of raindrops—> particles ‘ jump away
Effective in areas with sparse vegetation cover
Main outcome: pores sealed by fine material—> more surface run off’
Sheet erosion
Result of sheet flow: Overland flow as a shallow layer
removes particles in thin layers
Rilling
Sheet flow concentrates into rivulets
Cut small parallel channels called rills
Gully erosion
rills may join to form gullies: deeper v shaped channels carved by concentrated run off
causes for more intence run off
land clearing, roads
Causes for less permeable soil
Land clearing farm equipment, ranging live stock
Causes for steeper slopes
Roads
