Chapter 12Chapter 12

Weathering and erosionWeathering and erosion

Weathering ProcessWeathering Process

• Weathering changes the physical form of chemical composition of exposed rock.

• Mechanical weathering physically breaks down rock into smaller pieces– agents include ice, plants & animals, gravity,

running water, wind– Physical changes in the rock can increase

weathering• exfoliation

Mechanical weatheringMechanical weathering

• Ice wedging occurs when water gets into cracks and then freezes, causing cracks to widen– Thawing allows more water in– Rocks eventually split apart

• Organic activity of plants and animals:– Plant roots get into cracks and widen as they grow– Animals dig – expose rock to weathering

• Abrasion occurs when rocks collide with each other– Gravity, water, wind

Chemical WeatheringChemical Weathering(Decomposition)(Decomposition)

• Occurs when chemical reactions take place between rock minerals and water, CO2, O, and acids– Change the chemical composition and physical

appearance

• Hydrolysis – change in mineral composition due to chemical reaction with water– Minerals are dissolved in water and carried to lower

rock layers = leaching -> mineral ore deposits

Chemical WeatheringChemical Weathering

• Carbon dioxide dissolves in water to form carbonic acid– H2O + CO2 -> H2CO3

– Some minerals combine with H2CO3 to form new substance = carbonation

– Calcite reacts with H2CO3 to form calcium bicarbonate which dissolves easily in water -> underground caverns in limestone form this way

Chemical WeatheringChemical Weathering

• Oxidation occurs when metallic elements combine with oxygen– Iron combines with oxygen to form rust

4Fe + 3O2 -> 2Fe2O3

– Red soil in SE US

• Rain water is slightly acidic naturally but is a damaging weathering agent in industrial areas

• Some plants such as lichens and mosses produce acids that can dissolve the surface of rock and seep into cracks to break rock.

Rates of WeatheringRates of Weathering

• Major factors:– Composition– Exposure– Climate– Topography

CompositionComposition

• Sedimentary weathers fastest of 3 types– Limestone & others with calcite fastest

• carbonation

– Rate of mechanical weathering depends on what is holding sediments together

• Igneous and metamorphic– Quartz is affected least (one of hardest minerals)

ExposureExposure

• More exposure = faster weathering

• Most rock at surface are broken by fractures an joints -> split into smaller pieces– Fractures & joints increase surface area to

allow more rapid weathering

ClimateClimate• Rainfall and freezing/thawing have biggest effect• Variable weather causes ice wedging fractures

– Exposes new rock surface– Chemical weathering attacks fractured rock more quickly– Temperatures changing daily or seasonally speed up chemical

reactions

• Hot, dry climates slow weathering because of lack of water for reactions

• Weathering is very slow in cold climates – cold slows rxns

• Warm, humid climates are most damaging– Faster chemical rxn rate, moisture is damaging

Cleopatra’s NeedleCleopatra’s NeedleCleopatra's Needle, an obelisk with inscribed Egyptian hieroglyphs, remained virtually unweathered for more than 3,000 years in Egypt (A), but then began to weather rapidly when brought to New York City's Central Park in the late 1800s (B).  The difference in climate (arid versus humid) accounts for the change in weathering rates. 

Topography & ElevationTopography & Elevation

• Ice wedging increases with elevation

• Rock fragments on steep slopes are pulled down by gravity and washed along by heavy rain.

Weathering and SoilWeathering and Soil

• Weathering breaks and alters all exposed rock– Forms regolith (rock fragments) over solid

bedrock– Upper regions of regolith weather faster and

form a layer of fine particles = basic components of soil

– Soil = mixture of minerals (from rock), water, gasses, and humus (organic matter from decaying organisms)

Soil CompositionSoil Composition• Rock material in soil consists of sand, silt, & clay (in

that order from largest to smallest)• Proportions of these particles depends on the

“parent rock” = the rock it was weathered from– Rich in clay = rich in feldspar– Sandy soils = rich in quartz– Silt – gives soil a gritty feel. Often carried by river currents and

deposited on soils near river banks

• Minerals may be carried by wind or water away from the parent rock = transported soil– may have a different composition than underlying bedrock.

Soil ProfileSoil Profile

• Shows the layers of soil and the underlying bedrock

• 3 main layers = horizons– A horizon = topsoil – contains humus, other organic

matter, and living organisms– B horizon = subsoil – minerals leached from topsoil,

clay, little humus– C horizon = bottom layer – partially weathered bedrock

(will eventually weather into B then A horizon)

• Transported soils do not have horizons

Soil ProfileSoil Profile

A horizon

B horizon

C horizon

Climate Influences Soil FormationClimate Influences Soil Formation

• Humid tropical climate – chemical weathering causes thick laterites to develop– Contain iron and aluminum – do not dissolve easily– Rain leaches mineral form A & B horizons and washes away topsoil

( A is very thin)– Dense vegetation continuously adds organic material to topsoil to

support plants

• Desert climate – chemical weathering is slow/ mechanical weathering forms a thin soil mostly regolith

• Arctic – chemical weathering is slow/ mechanical weathering forms a thin soil mostly regolith

Climate Influences Soil FormationClimate Influences Soil Formation

• Temperate climate – 2 main soil types:– Pedalfer

• Forms with more than 65 cm of rain annually• Contains clay, quartz & iron compounds• Gulf Coast States & States East of Mississippi

– Pedocal• Forms with less than 65 cm of rain annually• Contains much calcium carbonate – combines with

soil H to make soil less acidic and very fertile• SW States and States West of Mississippi

Topography Affects Soil FormationTopography Affects Soil Formation

• Rainwater washes soil from steep slopes causing topsoil to be thicker at the bottom than on the slope

• Topsoil left on slopes is often too thin and dry to support plant growth does not accumulate organic matter = poor for cultivation

• Lowlands that hold water have thick, wet soils with much organic matter to form humus = good for cultivation

ErosionErosion

• Soil erosion occurs about as slowly as residual soil forms

• Unwise use of land and unusual climate conditions accelerate erosion– Unwise farming and ranching

• Clearing forests & overgrazing• Expose soils for increased erosion

– Gullying = water runs through furrows in plowed land washing away soil until furrows become deep gullies

GullyingGullying

ErosionErosion

• Sheet erosion strips away layers of soil to expose subsoil– Continuous rainfall evenly washes away

topsoil which may end up clogging streams or changing their course

– In dry periods wind carries away loose soil as clouds of dust and drifting sand

ErosionErosion

• Constant erosion reduces fertility by removing the A horizon and exposing the B horizon– B horizon resists plant growth has no

protection– All soil layers could be removed within a few

years

Rapid Erosion Can Be Prevented Rapid Erosion Can Be Prevented by Soil Conservationby Soil Conservation

• Cover crops can protect soil

• Some crop planting methods can reduce erosion– Contour plowing– Strip-cropping– Terracing– Crop rotation

Contour PlowingContour Plowing

• Plowing follows the shape of the land and prevents water from running directly down slope

Strip-CroppingStrip-Cropping

• Crops are planted in alternating bands to protect the soil by absorbing and holding water.– Ex. Corn bands alternating with alfalfa (cover crop)– When combined with contour plowing soil erosion can be

reduced 75%

TerracingTerracing

• Cutting step-like ridges that follow the contour of the land– Slows the downslope movement of water

Crop RotationCrop Rotation

• Farmers alternate crops every year to stop erosion early and repair the damage already caused by erosion

Urban ConservationUrban Conservation

• In urban areas clearing vegetation and removing soil to build houses, roads, etc contributes to erosion thereby making conservation measures necessary.

Mass MovementMass Movement

• Gravity causes rock fragments to move downhill = mass movement

• Rocks break into fragments which accumulate at the base of slopes in piles called talus

Rapid Mass MovementsRapid Mass Movements

• Rockfall – the fall of rock from a steep cliff is the most rapid mass movement

• Landslide – masses of loose rock and soil move downslope– May be triggered by heavy rains, spring thaw,

volcanic eruptions and earthquakes

Rapid Mass MovementsRapid Mass Movements

• Mudflows - masses of mud flow downslope and spread out in a fan shape at the bottom– Occur in dry mountainous regions

during heavy rains

• Slump – large block of soil and rock becomes unstable and moves downhill in one piece

Slow Mass MovementsSlow Mass Movements

• Solifluction – in regions where soil is permanently frozen, thawing of the topsoil makes it muddy and it slowly flows downslope.

• Creep – extremely slow downhill movement of weathered material– Most effective mass movement– Usually undetected

LandformsLandforms

• Shaped by weathering and erosion• Result from two opposing forces which

bend, break and lift the crust then wear it down– Mountains - steep landforms of high elevation

uplifted by tectonic forces– Plains – flat landforms usually near sea level– Plateaus – broad, flat landforms at high

elevations

MountainsMountains

• Youthful – still being uplifted– Rugged shape, sharp peaks &

deep, narrow valleys

• Mature mountains – no longer rising– Rounded peaks and gentle slopes

• Peneplain – old stage, almost featureless– Usually low rolling hills

PeneplainPeneplain

– Knobs of hard rock that resist weathering and protrude above peneplain = monadnocks

– May be mistaken for depositional plain but underlying rock is folded and tilted

PlateausPlateaus• Young plateaus have deep

stream valleys separating broad, flat regions

• Mature plateaus are eroded into rugged hills & valleys

• As plateaus age they may form smaller, table-like areas = mesas

• Mesas weather into small, narrow topped formations - buttes

LandformsLandforms

• In dry regions landforms have steep walls and flat tops.

• In humid climates they are more rounded due to increased weathering

• Minor landforms = hills, valleys and dunes