Chapter 6Chapter 6

Weathering & SoilWeathering & Soil

6.1 Weathering6.1 Weathering WEATHERINGWEATHERING -The breaking down of rock -The breaking down of rock

material into smaller pieces.material into smaller pieces. Two Forms of WeatheringTwo Forms of Weathering

MechanicalMechanical Some BiologicalSome Biological

ChemicalChemical Some BiologicalSome Biological

Weathering reduces mountains to hills & Weathering reduces mountains to hills & boulders to dirtboulders to dirt

Pgs. 148 - 155

Lichen on boulder

Chemical Weathering

Mechanical WeatheringMechanical Weathering Mechanical Weathering-Mechanical Weathering- The breaking down of The breaking down of

rocks without changing their chemical rocks without changing their chemical composition.composition.

Fragments formed without changing chemical Fragments formed without changing chemical compositioncomposition

Fragments have exact characteristics as Fragments have exact characteristics as parent rockparent rock

Caused by many factors:Caused by many factors: Tree RootsTree Roots Ice ExpandingIce Expanding Crystal GrowthCrystal Growth Lightning Lightning Expansion & Contraction Expansion & Contraction Heating & Cooling Heating & Cooling FrictionFriction WindWind

Plants & AnimalsPlants & Animals Roots grow into cracks in rocksRoots grow into cracks in rocks Start out thin as hair, grow larger & largeStart out thin as hair, grow larger & large Act as a wedge forcing rock pieces apartAct as a wedge forcing rock pieces apart

Ex: Sidewalks buckling due to tree rootsEx: Sidewalks buckling due to tree roots Burrowing animals increase weathering as wellBurrowing animals increase weathering as well Allow water & air to act on rocks below the Allow water & air to act on rocks below the

surfacesurface Animals also bring un-weathered materials to Animals also bring un-weathered materials to

the surface where weathering occursthe surface where weathering occurs

Ice WedgingIce Wedging Natural process of breaking rocks down Natural process of breaking rocks down

quicklyquickly Here is the process:Here is the process: Water seeps into cracks of rocksWater seeps into cracks of rocks Water expands as it freezesWater expands as it freezes Crack widensCrack widens More water able to seep in & freezeMore water able to seep in & freeze Crack widens even moreCrack widens even more The process repeats till pieces break off the The process repeats till pieces break off the

parent rockparent rock

Wind Action:Wind Action: Winds pick up dust & particlesWinds pick up dust & particles These particles act as abrasives, wearing These particles act as abrasives, wearing

away rock surfacesaway rock surfaces Equivalent to sand blastingEquivalent to sand blasting

Chemical WeatheringChemical Weathering CHEMICAL WEATHERINGCHEMICAL WEATHERING – The process when – The process when

water, air, or other chemical wear away rocks water, air, or other chemical wear away rocks and change it’s composition.and change it’s composition.

Water:Water: ““The universal solvent”The universal solvent” Oxygen & hydrogen in water reacts with Oxygen & hydrogen in water reacts with

mineralsminerals The minerals change into much different The minerals change into much different

compoundscompounds Ex: Rust- Oxidation of Iron onlyEx: Rust- Oxidation of Iron only

Acids:Acids: React with minerals, usually dissolving them over React with minerals, usually dissolving them over

timetime Acids from naturally in many ways:Acids from naturally in many ways: Carbonic Acid:Carbonic Acid: Carbon dioxide & water (Weak Carbon dioxide & water (Weak

acid) wears away limestone to make a lot of caves. acid) wears away limestone to make a lot of caves. Wears away Granite to make clay.Wears away Granite to make clay.

Sulfuric Acid:Sulfuric Acid: Sulfur and water (Strong acid) wears Sulfur and water (Strong acid) wears away marble, concreteaway marble, concrete

Some plant roots: Mosses produce Nitric Acid Some plant roots: Mosses produce Nitric Acid (Strong acid) Breaks down rocks and causes acid rain(Strong acid) Breaks down rocks and causes acid rain

Oxygen:Oxygen:

Oxidation:Oxidation: the bonding of oxygen to other the bonding of oxygen to other elements that changes the chemical elements that changes the chemical compositioncomposition

Almost all elements oxidizeAlmost all elements oxidize Iron Iron Iron Oxide (Rust) Iron Oxide (Rust) Aluminum Aluminum Aluminum Oxide white powder Aluminum Oxide white powder

on window sillson window sills Sulfur Sulfur Sulfur Dioxide- Acid Rain Most Sulfur Dioxide- Acid Rain Most

comes from power plants.comes from power plants. Copper Oxide – Turns copper green (Statue Copper Oxide – Turns copper green (Statue

of Liberty)of Liberty)

Effects of Climate on WeatheringEffects of Climate on Weathering

Mechanical & Chemical weathering occur Mechanical & Chemical weathering occur everywhereeverywhere

Climate plays a big part in how quickly they Climate plays a big part in how quickly they occuroccur

Climate:Climate: weather patterns for a given area weather patterns for a given area over long periods of timeover long periods of time

Weathering by climate:Weathering by climate: Cold climates:Cold climates: Freezing & thawing cycles Freezing & thawing cycles

allow ice wedging to break down rocks quicklyallow ice wedging to break down rocks quickly Warm, wet climates:Warm, wet climates: Chemical weathering Chemical weathering

acts quicklyacts quickly Dry Climates:Dry Climates: Wind action prevalent Wind action prevalent

6.2 Soil6.2 Soil Formation of Soil:Formation of Soil: Rocks breakdown into tiny fragments = Rocks breakdown into tiny fragments = DIRTDIRT Dirt is not SoilDirt is not Soil Soil:Soil: weathered rock material weathered rock material with organic with organic

matter mixed inmatter mixed in Soil Formation Process:Soil Formation Process: Rocks break down into dirtRocks break down into dirt Dirt gets some organic material added Dirt gets some organic material added

(animal waste, plant material, etc.)(animal waste, plant material, etc.) Small grasses & weeds begin to growSmall grasses & weeds begin to grow Insects, worms, small animals move in Insects, worms, small animals move in

(adding more organic matter)(adding more organic matter) Larger plants begin to replace grasses & Larger plants begin to replace grasses &

weedsweeds

Pgs. 156 - 163

Soil Maturity:Soil Maturity: As organic material gets mixed into soil it As organic material gets mixed into soil it

gets darkergets darker Mature soil is called HumusMature soil is called Humus Humus:Humus: soil rich in decaying plant matter & soil rich in decaying plant matter &

other organic matterother organic matter Humus provides all nutrients required by Humus provides all nutrients required by

plantsplants Nitrogen – Phosphorous – Potassium & Nitrogen – Phosphorous – Potassium &

WaterWater Depending on climate, soil formation takes Depending on climate, soil formation takes

a few months or thousands of yearsa few months or thousands of years Soil thickness ranges from a few Soil thickness ranges from a few

centimeters to 60 mcentimeters to 60 m

Soil Profile:Soil Profile: The visible difference in soil layers from the The visible difference in soil layers from the

surface to bedrocksurface to bedrock Each layer is called a HorizonEach layer is called a Horizon Usually there are 3 horizons (Labeled A, B, C)Usually there are 3 horizons (Labeled A, B, C) Horizon A: Top soil layerHorizon A: Top soil layer Mostly Humus & small mineral fragmentsMostly Humus & small mineral fragments Horizon B: Layer below Horizon AHorizon B: Layer below Horizon A Minimal organic matter (humus) Minimal organic matter (humus) Leaching-Leaching- removal of soil by water removal of soil by water Larger dirt fragmentsLarger dirt fragments Horizon C: Layer below Horizon BHorizon C: Layer below Horizon B Almost totally dirt & larger rock fragmentsAlmost totally dirt & larger rock fragments

Types of SoilTypes of Soil

Texture of soil is based on amounts of sand, Texture of soil is based on amounts of sand, silt, & claysilt, & clay

Depth of horizons in soil profiles also Depth of horizons in soil profiles also importantimportant

The US can be broken into 9 general types The US can be broken into 9 general types of Soilof Soil ArcticArctic Desert Desert Glacial Glacial

MountainMountain PrairiePrairie River River

TemperateTemperate Tropical Tropical Wetlands Wetlands

6.3 Land Use & Soil Loss6.3 Land Use & Soil Loss Agriculture:Agriculture: Soil as an important Soil as an important

ResourceResource Many resources are directly tied to Many resources are directly tied to

the soilthe soil Food, paper products, natural fibersFood, paper products, natural fibers Vegetables, grains, cotton Vegetables, grains, cotton plants plants Cattle, sheep, pigs Cattle, sheep, pigs feed off the feed off the

landland When vegetation is removed… Soil When vegetation is removed… Soil

directly exposed to weatherdirectly exposed to weather Erosion occursErosion occurs Nutrients are lostNutrients are lost

Pgs. 164 - 165

Plowing disrupts the soil:Plowing disrupts the soil: Every year the Earth’s Every year the Earth’s

population increases by at least population increases by at least 93 million people93 million people

Farmers plow more fields to Farmers plow more fields to meet then increasing demand meet then increasing demand for foodfor food

Plowing is the mechanical Plowing is the mechanical turning & loosening of soilturning & loosening of soil

Turns previous vegetation Turns previous vegetation under, exposes underlying soilunder, exposes underlying soil

2 points of View:2 points of View: Soil loss is severe in tropical regionsSoil loss is severe in tropical regions Rains wash loose soil into the oceanRains wash loose soil into the ocean Each year 1000’s of square km of rainforests are Each year 1000’s of square km of rainforests are

cleared for farming & grazing cleared for farming & grazing Tropical soil is very thin = nutrients used up Tropical soil is very thin = nutrients used up

quicklyquickly After a few years the soil is no longer fertile After a few years the soil is no longer fertile

new fields need to be clearednew fields need to be cleared Abandoned fields become desert like areas Abandoned fields become desert like areas

(Begins desertification)(Begins desertification) Grasslands near deserts are normally grazing Grasslands near deserts are normally grazing

areasareas Livestock eat every bit of vegetationLivestock eat every bit of vegetation Increases wind erosionIncreases wind erosion Desert grows larger =Desert grows larger = Desertification Desertification

Farmers Work to Minimize Farmers Work to Minimize Soil Loss:Soil Loss:

Along with using contour Along with using contour plowing, crop rotation, plowing, crop rotation, terracing, shelter belts, etc. terracing, shelter belts, etc. farmers use other methods farmers use other methods to help protect the soilto help protect the soil

Chisel Plowing:Chisel Plowing: Instead of turning over a Instead of turning over a

complete field, chisel plows complete field, chisel plows cut small furrows between cut small furrows between previously planted and previously planted and harvested cropsharvested crops

Previous crop roots hold Previous crop roots hold soil, decaying plants fertilize soil, decaying plants fertilize the soilthe soil

Newly loosened area is Newly loosened area is plantedplanted

No Till FarmingNo Till Farming:: Special planter used to cut a Special planter used to cut a

very small area for seeds to very small area for seeds to be plantedbe planted

Rest of field is not disturbedRest of field is not disturbed Cover cropping:Cover cropping: Farmers plant two crops Farmers plant two crops

that can grow together, that can grow together, harvest one & leave the harvest one & leave the other to continue growingother to continue growing

Ex: Grains with GrassesEx: Grains with Grasses Grains get combined off in Grains get combined off in

early summer,early summer, Grasses continue to grow Grasses continue to grow

and can be cut as hayand can be cut as hay

Cover Cropping with Grapes and Cover Cropping with Grapes and BeansBeans

Historic Tragedy: The Dust BowlHistoric Tragedy: The Dust Bowl People moved into the Great Plains during People moved into the Great Plains during

the 1800’sthe 1800’s Plowed under the native drought resistant Plowed under the native drought resistant

grassesgrasses Began farming large areasBegan farming large areas Corn & Wheat were planted; Cattle & Sheep Corn & Wheat were planted; Cattle & Sheep

were grazedwere grazed As the area being farmed continually As the area being farmed continually

increased, the natural grasses with their increased, the natural grasses with their extensive root systems became increasingly extensive root systems became increasingly scarce, and erosion increased drasticallyscarce, and erosion increased drastically

In 1931, a drought hit the Great Plains RegionIn 1931, a drought hit the Great Plains Region Lasted several years, Crops were not able to Lasted several years, Crops were not able to

withstand the droughtwithstand the drought Soil became totally unprotected, Wind began Soil became totally unprotected, Wind began

blowing the dry topsoil awayblowing the dry topsoil away Dust storms ragedDust storms raged Soil was blown into the ocean, or piled up Soil was blown into the ocean, or piled up

against homesagainst homes More than 8 cm of topsoil was lostMore than 8 cm of topsoil was lost Many farmer went bankrupt and People moved Many farmer went bankrupt and People moved

elsewhereelsewhere 1935: Federal government began regulating 1935: Federal government began regulating

AgricultureAgriculture Crop rotation, contour plowing & terracing were Crop rotation, contour plowing & terracing were

requiredrequired In areas able to support trees, wind breaks were In areas able to support trees, wind breaks were

plantedplanted Over time, the fertility of the plains was restoredOver time, the fertility of the plains was restored

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