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CHAPTER 7: WEATHERING & SOIL
Aim: How does weathering change Earth’s surface?
Weathering 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:1. Mechanical Weathering2. 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
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
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
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
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
Effects of Climate on Weathering
Chemical weathering is more rapid in warm, moist climates
Mechanical weathering is important in dry climates and in cold climates
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
1. Weathering of rocks Natural acids in rainwater weather the
surface of exposed bedrock Water can freeze in cracks to break
them apart2. 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
3. Living Organsims Insects & worms live underground with
the plant roots Their wastes and dead material add
organic matter to the soil4. 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
Layers of Soil Layers of soil are called horizons All soil horizons form a soil profile Most soils have 3 horizons1. 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
Is dark and fertile2. 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
3. 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
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
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
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
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)
This practice reduces runoff by creating flat areas and shorter sections of slope
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