Weathering and SoilChapter 7

Section 1Section 1

WeatheringWeathering

This formation is called a “tor.”

Weathering is a process that breaks down rock into smaller pieces.

Two different types of weathering:MechanicalChemical

Mechanical weathering occurs when rocks are broken apart by physical processes.Something has to hit, grind, or crack rock.Things that are responsible for mechanical weathering are:

Plants and animalsIce WedgingRain Chuck Norris

Plant roots are very strong and can push rocks apart.

Animals dig and burrow, loosening sediments and bringing them to the surface to weather more.

Tiny living organisms also help break down rock.

Ice wedging occurs in climates where water enters cracks and freezes.Water expands when it freezes, pushing rock apart.This process repeats itself, the crack widens, and

the rock eventually breaks apart.

Mechanical Weathering by RainRain drops act as miniature hammers, pummeling

the ground, breaking rock into soil.

As rocks break apart into smaller pieces, more surfaces are exposed to weathering.

The inside of the rock is now also exposed to weathering, so weathering will occur faster.

Chemical weathering occurs when chemicals break down rock or change the rock into different minerals.

These chemicals can come fromNatural acidsPlant acidsOxygen

Water can chemically react with carbon dioxide and create a weak acid called “carbonic acid”—the same acid found in many soft drinks.

This acid reacts with rock that contains calcite, like limestone, and dissolves it.

Other rocks are also affected.

Caves form when acidic groundwater dissolves underground limestone.Stalactites can form

on cave ceilings from dripping dissolved rock.

Stalagmites can form on the floor when the drips hit.

Plant roots and decaying plants give off acids that can dissolve rock.

Plants thrive on the broken down rock (new soil) and take the nutrients into their roots.

Oxidation is the process of oxygen chemically combining with other matter to change it.

Rust is caused by oxidation—oxygen combining with iron.Many rocks contain

iron and will turn

reddish through

oxidation. The

reddish material

is softer and

eventually crumbles

—like rust.

Climate is the pattern of weather in an area over many years.

Warm, wet climates cause weathering to happen more quickly.Chemical reactions happen faster with

water and heat. Cold, dry climates slow weathering down.

Soft rocks break down easier in wet climates.LimestoneMarble

Hard rocks don’t allow water to soak in as much, so they are harder to weather in wet climates.GraniteBasalt

Section 2The Nature of Soil

Five things affect soil formation:ClimateTypes of parent rockSlope of landTypes of vegetation in the areaAmount of time rock has been

weathering

Climate

Parent Rock

More on ParentRock

Slope of the Land

Type ofVegetation

The type of minerals in a rock will affect the type of vegetation that forms in an area.

The type of vegetation then affects the type of soil that will form.

Time It takes time for weathering to take place.

Time

Soil is a mixture of weathered rock, decayed organic matter, mineral bits, water, air, and living organisms.Can take thousands of years to formDecayed organic matter turns into nutrient rich

material called humus.Humus comes from the decay of a layer of material

on the surface called litter.Includes leaves, twigs and other dead organisms

http://www.ucopenaccess.org/courses/APEnvSci/course%20files/multimedia/lesson17/animations/2c_soil_formation.html

How Soil Forms• Parent rock begins to break down through

chemical and mechanical means.• Organisms begin to grow in and on rock,

further breaking it down. • As the organisms die, they add nutrients to the

soil (humus).• Rain carries dissolved nutrients down into the

soil layers (leaching).

Make-up of different layers of soilHorizon O – top organic

layer—humus.Horizon A—top layer of

soil. Fertile layer with more

humus and less rock and mineral particles than other soil horizons

http://courses.soil.ncsu.edu/resources/soil_classification_genesis/soil_formation/soil_transform.swf

Horizon B – middle soil layerContains less

humus and is lighter in color than A horizon

Minerals travel from A horizon to B horizon in a process called leaching.

Bottom layerHas very little

organic matter and is not strongly affected by leaching

Contains rock – the parent material of the soil

Soil Formation Animation

Glacial Deposits

Glaciers deposit sediments when they melt.These types of soils are extremely fertile.http://mrbdc.mnsu.edu/mnbasin/flash/glaciers/

glaciation_animation.htmlSoil from one place is moved to another by glaciers.http://www.wwnorton.com/college/geo/egeo/flash/

18_1.swfhttp://www.brainpop.com/science/earthsystem/

glaciers/

Soil TypesSoil Types Different climates yield different types of soilsPrairie soils have thick,

dark A horizons because grasses there leave lots of organic matter.

Temperate forests, like those in the eastern US have thinner A horizons than prairies.

Deserts usually contain little organic matter and upper horizons are thinner than in wetter regions.

Other FactorsType of parent material will dictate what types of soils

come from it. Basalt will yield clay soils.Sandstone yields sandy soils.

Time is a factor. The longer weathering has been going on, the more soil

will have developed in the area.The slope of the land is important in how soils develop.

Slopes often lose their soil due to gravity.Soil is often deposited thickly at the bottom of a slope.

This soil in bottomlands is often moister, too, due to water settling at the bottom of a slope due to gravity.

Deposits at the base of hills and mountainsCalled

alluvial fans

Section 3Section 3

Soil ErosionSoil Erosion

So, What’s the Big Deal About Soil?

Soil is important because we grow our crops in soil.

We eat the crops.We eat animals that ate the crops.If soil is gone or damaged, we don’t get as much

food from the soil.

ErosionErosion

http://www.youtube.com/watch?v=w7yis7nlczs (Dust Bowl Days)

Causes of Soil ErosionAgriculture Cultivation

Farming methods in the past contributed to soil erosion.

Areas with lost soil need more chemical fertilizer, which contributes to soil damage.

Agriculture CultivationFarming methods in the past contributed to soil

erosion.Areas with lost soil need more chemical fertilizer,

which contributes to soil damage.

Forest HarvestingWhen trees are removed, soil is exposed to

weathering and erosion.Large amounts of soil can be lost if new trees

or other vegetation aren’t planted in place of the harvested trees.

Tropical rainforests are often cleared by burning.

Farming the land then increases chances for soil loss.

Forest HarvestingWhen trees are removed, soil is exposed to

weathering and erosion.Large amounts of soil can be lost if new trees

or other vegetation aren’t planted in place of the harvested trees.

Tropical rainforests are often cleared by burning.

Farming the land then increases chances for soil loss.

Causes of Soil ErosionCauses of Soil Erosion

OvergrazingIf livestock are allowed to graze too long in

the same pasture, there are no more grass roots to hold the soil in place.

Soil erosion is more likely to take place.

Causes of Soil ErosionCauses of Soil Erosion

Effects of Soil ErosionEffects of Soil Erosion

Eroded land can erode even further.Eroded sediments can clog streams.

Eroded land can erode even further.Eroded sediments can clog streams.

Effects of ErosionEffects of ErosionThe effects of strip mining can be

mountaintop erosion.

The effects of strip mining can be mountaintop erosion.

Preventing Soil ErosionPreventing Soil ErosionFarmers can use proper grazing methods.Farmers can use terracing.

Terracing stair-steps the land so that water does not flow straight downhill.

Farmers can use contour farming.Farming around hills instead of up or down them.The furrows slow down the water as it flows

downhill.Farmers can use no-till or low-till farming

methods.Land is not tilled or tilled very little so that root

systems are left in place.

Preventing Soil ErosionConstruction workers can

keep areas moist to prevent soil from blowing away.

Plants can be planted immediately after construction is complete.

Topsoil treatments can be put in place to hold soil down until plants grow.

Rocks and rip-rap can hold hillsides in place.

Construction workers can keep areas moist to prevent soil from blowing away.

Plants can be planted immediately after construction is complete.

Topsoil treatments can be put in place to hold soil down until plants grow.

Rocks and rip-rap can hold hillsides in place.

Lay down straw to hold soil in place until plants grow.

Preventing Soil Erosion• Farmers and

others can plant shelter belts (wind breaks) of trees and bushes around fields vulnerable to erosion

• Shelter belts slow down the effects of the wind.

• Farmers and others can plant shelter belts (wind breaks) of trees and bushes around fields vulnerable to erosion

• Shelter belts slow down the effects of the wind.

Terraces

Contour FarmingContour FarmingFarming around hillsFarming around hills

Replant trees and grassReplant trees and grassRoots hold soil in place.Roots hold soil in place.

Without Erosion Control

Erosion Control