Weathering Chapter 5

5.1 Weathering = breakdown of rock and minerals 2 Types of weathering Mechanical Chemical

Mechanical Weathering = physical forces breakdown rocks into smaller pieces without changing the mineral composition

3 Types Frost Wedging Unloading (exfoliation) Biological Activity

Increases the surface area of the rock

FROST WEDGING The mechanical breakup of rock caused by the freeze-thaw cycles of water in the cracks of rocks. When water freezes, its volume expands

The freeze-thaw cycles break rock into pieces Cracks are formed from water freezing, expanding, and breaking the asphalt or concrete

Sections of rock that are wedged loose may tumble into large piles called talus which typically form at the base of steep rocky cliffs Most common in mountain regions of middle latitudes

UNLOADING Exfoliation- reduced pressure on igneous rock causes it to uplift and allows slabs of outer rock to break off in layers

Biological Activity Roots Construction Animals digging into rock Plants , burrowing animals and humans affect the weathering of rocks. Roots Construction Animals digging into rock

Chemical Weathering Occurs when the chemical and mineral composition of a rock have been changed WATER is the most important agent of chemical weathering

Water absorbs gases from the atmosphere and chemically react with minerals : EX: Water + CO2 + Calcite = Carbonic Acid breaks down limestone

EX: H2O + O2 + iron + = rust EX: H2O + sulfur gas OR nitrogen gas = Acid Rain Evidence of chemical weathering

RATE OF WEATHERING Mechanical weathering affects the rate of chemical weathering Mechanical weathering breaks rocks into several pieces, which INCREASES the surface area of exposed rock, allowing faster chemical weathering to take place.

3 things that affect the rate of weathering Rock characteristics (chemical composition) Climate Surface Area (amount) of exposed rock

Chemical Composition Rock’s mineral composition affects weathering Granite is resistant to chemical weathering Marble chemically weathers very quickly Marble is formed from calcite which easily dissolves in acid

CLIMATE Affected by temperature and moisture Chemical weathering is most effective in areas with high temperatures and an abundance (lots) of moisture

CLIMATE Tropical- high temperatures + high moisture = lots of chemical weathering Polar- low temperatures + low moisture = little or no chemical weathering Deserts- high temperatures + low moisture = very slow chemical weathering

Remember…… Climate with most favorable weathering conditions has high temperatures & lots of moisture

Exposed Surface Area Cracks allow water to seep into the rock The more the crack & crevices, the greater the weathering

5.2

Soil Composition 4 major components of soil 45% Mineral Matter (mineral broken down rock) 25% Water 25 % Air 5% Organic Matter or dead stuff (decayed remains of organisms)

Soil texture is determined by particle size sand (large) silt clay (small) Loam - a mixture of all three sizes Best suited for supporting plant life

Soil Formation Parent Material Time Climate Organisms Slope

PARENT MATERIAL Source of mineral matter in the soil TOPSOIL- PARENT MATERIAL IS THE MINERAL MATTER SUBSOIL- PARENT MATERIAL HAS BEEN DEPOSITED FROM ELSWHERE AND WEATHERS

Time Important to ALL weathering the longer the soil has been forming the thicker it becomes

CLIMATE Major effect on soil formation Variations in temperature and precipitation influence rate, depth, and type of soil *change weathering in your notes to “soil” )

Hot & wet climate may produce a thick layer of chemically weathered soil Dry & cold climates may produce only a thin layer of chemically weathered soil

Influence the soil’s physical and chemical properties ORGANISMS Influence the soil’s physical and chemical properties Plants are the main source of organic matter in the soil They also aerate the soil (provide air to the soil)

SLOPE / Angle Steep slopes erode quickly and have little or no soils Flat slope has little erosion and produces quality soils.

Orientation North slopes are cooler and wetter (poorer soil) South slopes are warmer and drier (better soil) *** soil forms best in hot/moist, tropical climates ***

Soil Profile Soil profile shows all of the soil horizons in a cross section Soil varies in composition, texture, structure, & color at different depths Variations divide the soil into zones called horizons

Mature soils have three distinct soil horizons A Horizon B Horizon C Horizon

Soil Horizons A Horizon = known as topsoil consists of organic matter, insects, fungi & microorganisms B Horizon = known as subsoil Contains: fine clay particles washed out of A horizon, by water that filters through pore spaces Lower limit of most plant roots & burrowing animals

C Horizon = falls between the B horizon and unaltered parent material Contains partially weathered parent material Unaltered parent material The bedrock that has NOT gone through any weathering

Soil Erosion Loss of fertile topsoil major problem in United States We are eroding much more soil now than we have in the past

Rate of Erosion Cross out “water carries tiny pieces of _______ All the way to Rate of Erosion

Rate of Erosion Depends on soil characteristics, climate & slope, and the amount of vegetation Accelerated by human activity Removal of natural vegetation by: Farming Logging Construction

Water erodes soil faster than wind Exception with unprotected spoil. EX: 1930’s Dust Bowl

Sediment Deposition Reservoirs fill up with sediment Sediments are contaminated by pesticides and fertilizers

Controlling Erosion Planting rows of trees called windbreaks Prevents wind erosion Terracing hillsides Prevents water & wind erosion

Soil erosion is accelerated by removing protective natural vegetation Human activities such as farming, logging, and construction increase erosion greatly = soil is being eroded faster than it can be created

EX: rivers are becoming clogged with eroded sediment from farmland = interfere with river transportation, increase pesticide levels in water, fill up reservoirs, and increase flooding