Transcript Chapter 7: Weathering & Soil

CHAPTER 7: WEATHERING & SOIL
Aim: How does weathering change
Earth’s surface?
Weathering
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Is surface processes that work to breakdown rocks
into smaller pieces
Smaller loose pieces are sediments
Gravel, Sand, silt and clay describe sizes of
sediments
Size order: gravel, sand, silt, clay (largest to smallest)
Wears mountains down to hills
Produces strange rock formations
2 Types of weathering:
Mechanical Weathering
Chemical Weathering
Aim: How do mechanical weathering
and chemical weathering differ?
Mechanical Weathering
Occurs when rocks are broken apart by physical
processes
 Chemical makeup of the rock stays the same
 2 ways this can happen:
1. Plants and Animals
 Water and nutrients that collect in the cracks or
rocks enable plants to grow
 As the roots grow they enlarge the cracks
 Ex: tripped on crack in the sidewalk near a tree
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Burrowing animals loosen sediments and push them
to the surface
 Sediments reach the surface and other weathering
processes occur
2. Ice Wedging
 Occurs in temperate and cold climates
 Water enters cracks in rocks and freezes
 Pressure builds up in the cracks causing them to
expand and break apart the rock
 The ice melts and this happens all over again
 The process of freezing and thawing occurs over
and over, especially in mountains
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Ice wedging wears mountain peaks
 It can also break up roads and highways
 Mechanical weathering reduces rocks to smaller
pieces giving the rock more surface area
 As the surface area increases more rock is
exposed to water and oxygen which aids in
chemical weathering
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Chemical Weathering
Chemical reactions dissolve the minerals in rocks or
change them to different minerals
 Changes the chemical composition of the rock
 Naturally occurring acids, such as carbonic acid,
react with calcite in limestone
 The acid weathers away the limestone to form
caves
 Kaolinite clay is created when acids react with
feldspar in granite
 Clay is an end product in weathering
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Plant acids can dissolve minerals in rocks and
breaks the rocks into smaller pieces
 This weathering also enables nutrients to be
available for plants
 Oxygen also causes weathering
 Oxidation occurs when some materials are
exposed to oxygen and water
 Ex: rust
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Effects of Climate on Weathering
Chemical weathering is more rapid in warm,
moist climates
 Mechanical weathering is more common in dry
climates and in cold climates
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Aim: How does soil form?
Formation of Soil
Is a mixture of weathered rock, decayed organic
matter, mineral fragments, water and air
 Can take thousands of years to form
 Climate, slope, types of rock, types of vegetation
and length of time of weathering affect rock
formation
 There are different kinds of soil all over the world
based on these factors
 4 steps
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Weathering of rocks
Natural acids in rainwater weather the surface of
exposed bedrock
Water can freeze in cracks to break them apart
Plant Growth
Plants begin growing in cracks of rocks
As they grow they continue the process of
breaking down rocks
Then a thin layer of soil begins to form
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Living Organsims
Insects & worms live underground with the plant
roots
Their wastes and dead material add organic
matter to the soil
Organic Matter
as organic matter increases, the bedrock
continues to breakdown
This causes the soil layer to thicken
Rich topsoil supports trees and plants with large
root systems
Aim: What are the characteristics of
soil?
Composition of Soil
Most organic matter in soil comes from plants
 Animals and microorganisms remains also add
organic matter to the soil
 Decayed organic matter over time turns into a
dark-colored material called humus
 Humus is a source of nutrients for plants
 Animals burrowing in the ground mix the humus
with fragments of rocks
 Good quality soil have equal amounts of humus
and weathered rock material
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Layers of Soil
Layers of soil are called horizons
 All soil horizons form a soil profile
 Most soils have 3 horizons
1. A Horizon
 top layer-AKA topsoil
 In a forest it is covered with litter
 Litter helps prevent erosion and evaporation of
water from the soil
 Topsoil has more humus and fewer rock and
mineral fragments than the other horizons
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Is dark and fertile
2. B Horizon
 is below the A horizon
 Lighter in color (less organic matter)
 Less fertile
 Leaching moves minerals that have been
dissolved in water from A horizon down into the B
horizon
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C Horizon
Bottom layer
Contains partially weathered rock
Thickest horizon
Not affected by leaching and doesn’t have much
organic material
Coarser sediments
Most like the parent material, which is rock
Aim: What determines the different
types of soil?
Types of Soil
Many different types of soil exist
 Climate affects soil
 Desert soil has a small amount of organic matter
and is thin
 Prairies have thick, dark A horizon soil due to the
organic matter from the large amount of grasses
present
 Forest soil has a thinner A horizon than prairies
because trees and leaves don’t supply as much
organic matter as grasses do
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The type of rocks soil comes from affects the type
of soil
 Sandstone turns into a sandy soil
 Clay soil comes from basalt
 Type of vegetation that grows in an affects soil
formation
 Slope of the land affects soil development
 The soil on the slope is poorly developed
 Soil in a valley is rich in organic matter
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Aim: How has human activity affected
Earth’s soil?
Soil Erosion
Soil erodes when it is moved from the place where it
formed
 Water and wind move sediments and cause erosion
 More severe on steep slopes
 Humans cause erosion to form faster by removing
vegetation (ground cover) which helps increase soil
erosion
 Serious problem for agriculture
 Soil loses nutrients, which plants need to grow
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When soil erodes farmers compensate for the
nutrient lose by buying fertilizer
 Forest harvesting also causes soil erosion
 Removing forests creates severe problems for the
environment, especially rain forests
 Forests are cleared for lumber, farming and
grazing
 Overgrazing also increases soil erosion
 Sheep and cattle are grazed on land until no
ground cover remains
 Without protection from plants that soil is carried
away by wind and water
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Too much soil erosion, sediments can damage the
environment
This can cause streams to be covered, eggs from
organisms die, or reduce downstream water quality
Aim: How can humans prevent soil
erosion?
Preventing Soil Erosion
Plant shelter belts of trees-this blocks the strength
of the wind
 No-till farming-farmers do not plow the fields
 This practice provides cover for the soil all year
round and reduces water runoff and soil erosion
 Contour farming used on slopes-this slows the flow
of water down the slope and reduces soil erosion
 Terracing-leveled areas are built on slopes (looks
like steps)
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This practice reduces runoff by creating flat areas
and shorter sections of slope