weathering, soil, and erosion chapter 5. weathering def: the breakup of rock due to exposure to...
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Weathering, Soil, and Erosion
Chapter 5
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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
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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
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Examples of Frost Wedging and Ice Heaving
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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.
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Mechanical Weathering cont…
Plants and AnimalsGrowth contributes
Roots wedged into tiny pores and crevices
Burrowing contributesDigging holes into soilBringing rock fragments to
surface
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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
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Upward Expansion Examples
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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
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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
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Chemical Weathering cont… Oxidation
Def: chemical reaction of oxygen with other substances
Effective with minerals containing iron
Formation of rust, or iron oxides
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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
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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
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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
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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!
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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
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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
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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
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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
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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)