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Weathering, Soil, and Erosion
Chapter 5
Weathering Def: the breakup of rock due to exposure to
processes that occur at Earth’s surface 2 types:
Mechanical- the breakdown of rock that takes place when a rock is broken into smaller pieces of the same material without changing its composition
Chemical- the breakdown or decomposition of rock that takes place when minerals are changed into different substances
Mechanical Weathering
ProcessesFrost wedging
Def: a process in which water freezes in the cracks of a rock and wedges it apart
Mostly in porous rocks and in rocks with many cracks
In places where there are frequent freezes and thaws
Ice Heaving- creates potholes; ice lifts pavement up and it collapses when it thaws
Examples of Frost Wedging and Ice Heaving
Mechanical Weathering cont…
AbrasionDef: the wearing away of
rock material by grinding action
Water, wind and ice are capable of moving rocks.
Sand is a product of abrasion.
Mechanical Weathering cont…
Plants and AnimalsGrowth contributes
Roots wedged into tiny pores and crevices
Burrowing contributesDigging holes into soilBringing rock fragments to
surface
Mechanical Weathering cont…
Upward ExpansionExfoliation- the peeling of surface layers from exposed
bedrockRock is uplifted by tectonic forces (ex: granite)Overlying rock is worn away which reduces pressureUpward expansion causes the granite to break along curved
jointsLarge sheets of loosened rock break away
Upward Expansion Examples
Chemical Weathering
WaterHydrolysis- the chemical
weathering by reaction of
water with other substancesAcids: seeps into the ground
and reacts chemically with
many common mineralsCalcite
Dissolves completely Underground caverns
Chemical Weathering
cont… Acid Rain
Def: rainwater that contains unusually high amounts of acids that can be traced back to pollutants
Increases the rate of chemical weathering
Chemical Weathering cont… Oxidation
Def: chemical reaction of oxygen with other substances
Effective with minerals containing iron
Formation of rust, or iron oxides
Rates of Weathering Weathering is usually a slow process Factors that affect the rate of weathering:
Surface ExposureMore surface exposed, weathers more quickly
Composition of RockVarious rocks are affected differently by weathering
processesClimate
Warm/wet climates: both chemical and mechanicalCold/dry climates: mainly mechanical
Soil Def: loose, weathered rock and organic material in
which plants with roots can growFormed by weatheringContains air, water, organic material, mineral matterParent material
Def: material from which a soil is formedResidual soil- soil whose parent material is the bedrock
belowTransported soil- soils formed from transported materials
Deposits left by winds, rivers, and glaciers
Soil Profile Def: cross section of earth exposed by the digging Soil horizons- three distinct zones of mature soil
A-horizon- topsoilDarker, contains humus (organic material), sandy
B-horizon- subsoilRed or brown, iron oxides, clay from topsoil, calcium and
magnesium
C-horizonWeathered parent material, rock fragments
Soil Composition 3 noticeable parts: sand, silt, and clay
Affects soil’s ability to hold water and air Factors that affect composition:
TimeParent materialPlants and animalsTopographyClimate- MOST IMPORTANT!
12.3 Mass Movements and Erosion
Mass movements- the downward transportation of weathered materialsGravity causes materials to fall, slide, or move
at slow speeds to lower levelsErosion- the removal and transport of
materials by natural agents such as wind and running water
Mass Movements Talus- rock fragments that have
been weathered from a cliff and pulled down by gravity
Landslide- movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain, or cliff Steep slopes Regions near volcanoes and in
earthquake-prone regions
Mass Movements Creep- slow, imperceptible
movement of soil down a slope Causes fence posts, poles and other
objects fixed in soil to lean downhill
Slump- a block of land tilts and moves downhill along a surface that curves into the slope Tends to occur because bottom of
slope can no longer support top of slope
Mass Movements Earthflows- the downslope movement
of a mass of earth materials that have been saturated with water Slower and less fluid than a mudflow;
velocity affected by amount of water present, the composition of the soil, and the steepness of the slope
Mudflows- the downslope movement of water that contains large amounts of suspended clay and silt Rapid movement; capable of moving
rocks, boulders, trees, and houses; lahars- volcanic mudflows
Erosion Topography- uplifting and weathering/erosion are
happening at same timeMore uplifting- rugged and sharp (Himalayas)More erosion- smooth and rounded (Appalachians)
Climate- in humid areas, water is primary agent and causes rounded topography
Composition of rock- some types are more resistant than other rock typesVolcanic neck- the solidified lava filling the center vent
of an extinct volcano (picture pg 270)