Chapters 7-9
Weathering
Erosion
Deposition
Chapter 7: Weathering and Soil
• Weathering: surface processes that work to break down
rock
– Weathering breaks rock into smaller and smaller
pieces (sediment) such as sand, silt and clay (these
terms are used to describe the sizes of the particles).
Sand grains are larger than silt and silt is larger than
clay.
– Over millions of years, weathering has changed
Earth’s surfaces and this process continues today
• Two types of weathering:
• 1) Mechanical weathering
• 2) Chemical weathering
1) Mechanical weathering: breaks apart rocks without
changing their chemical composition (without changing
the type of rock)
– IE – growing plants, burrowing animals, expanding
ice, mineral crystal growth, lightning and expansion
and contraction due to heat or cold
• These processes produce enough force to break apart
rocks
• More surface area is then produced for additional
weathering (pg. 184)
• Usually occurs in cold climates…freezing…
2) Chemical weathering: the reaction of water,
air, and other substances with the minerals in
rocks which changes the chemical composition of
the rocks
– water (H & O) react with rocks
– acids – CO from air mixes with water and
forms carbonic acid (Soft drinks…)
– Carbonic acid dissolves minerals in
limestone…this can form caves…
– Acids from plants and rotting plants can
dissolve minerals in rocks also…this
weakens and then will cause the rocks to
break…
– Oxidation: Oxygen combined with water….
causes iron to rust…rocks that contain iron
then turn reddish in color…
– Usually occurs in warm, wet climates…
– Dry climates slow down chemical
weathering…IE…?
Weathering’s Effect on Climate
• Climate: pattern of weather that occurs in a particular area over
many years
• Climate affects soil temperature and moisture and also affects
the rate of mechanical and chemical weathering.
• In cold climates, where freezing and thawing are frequent,
mechanical and chemical weathering rapidly breaks down
rock through the process of ice wedging.
• Chemical weathering is more rapid in warm, wet climates.
• High temperatures tend to increase the rate of chemical
reactions.
• Chemical weathering tends to occur quickly in tropical
areas.
• Lack of moisture in deserts and low temperatures in
polar regions slow down chemical weathering.
Soil Erosion
• Soil erodes when it moves from the place where it originally forms.
• Erosion occurs as water flows over Earth’s surface or when wind
picks up and transports sediment.
• More severe on steep slopes or where there is little vegetation.
• Under normal conditions, a balance between soil production and soil
erosion often is maintained = soil forms at about the same rate as it
erodes.
• Humans sometimes cause erosion to occur faster than new soil can
form.
• Agriculture cultivation: nutrients taken from topsoil, quality of soil
reduced so extra fertilizers need to be added.
• Forest Harvesting: when trees are cut down, soil is exposed and
erosion increases.
• Overgrazing: in some areas of the world, sheep and cattle graze
on grasses until almost no ground cover is left to protect the soil.
• Excess Sediment: could be deposited into streams which could fill
up.
Preventing Soil Erosion
• Manage Crops
• No-till Farming: farmers leave plant
stalks in the field over the winter
months. At the next planting they seed
crops without destroying these stalks
and plowing the soil.
•Provides cover for the soil year
round which reduces water runoff
and soil erosion.
• Reduce Erosion on Slopes
• Contour Farming: planting along the
natural contours of the land…slows
water down the slopes.
Flatten the soil in your plate.
Find 3 ways to move the soil from one side of the
plate to the other.
You may not touch the soil with your hands! But,
you can touch the plate.
Do not spill!
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Erosion: process that wears away surface material and moves
them from one place to another
•
major causes: gravity, glaciers, wind, water
•
they all need energy for motion!
Deposition: the dropping of sediments that occur when an
agent of erosion looses its energy of motion and is no longer
able to carry the load
•
drop when energy of motion decreases!
What causes erosion and deposition?
- Gravity! What is gravity? The force of attraction that
exists between all objects
- Since Earth is so large, all objects are attracted to it.
- The Earth is the source of all our gravity.
- Any loose material will be pulled down a slope (hill), due to
gravity.
- When gravity alone causes material to slide down a slope this is
called mass movement.
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Examples of mass movement:
1)
Slump: mass movement that happens
when loose rock layers slip down a
slope.
•
When a slope becomes too steep,
the base material no longer can
support the rock and sediment
above it.
•
The soil and rock slip downslope as
one large mass or break into several
sections.
2) Creep: sediments slowly inch their way
down a hill…most common in freezing
areas.
•
Ex: Leaning trees and fence posts
that lean downhill.
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3)
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Rockfalls (tumble) and Rock Slides:
when large rocks break loose and
tumble down a hill…can be very
destructive!
•
Rockfalls - As rocks fall and
tumble through the air, they crash
into other rocks and knock them
loose. More and more rocks
break loose and tumble to the
bottom.
•
Rock Slides – occur when layers
of rocks – usually steep layers –
slip down a slope suddenly.
Happen most after heavy rain or
earthquakes.
4) Mudflows: a thick mixture of
sediment and water flowing down a
slope…dry areas with thick layers of
sediment that suddenly get really wet
from rain…
All mass movements…
•
•
•
Occur on slopes…
Occur more often after a rain…
Increase if vegetation has been removed…
How can you prevent these problems?
•
Strengthen slopes with vegetation…roots hold
soil together
•
Walls made of concrete…
We can try all we want to avoid erosion and deposition
problems, but in the long run…gravity will still be too
powerful!
IE – video of mudslide…
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Erosion by Glaciers!
•
Glacier – a large mass of ice and snow moving on land under its own weight.
•
As glaciers pass over land, they erode it, changing features on the
surface.
•
They then carry eroded material along and deposit it somewhere else.
•
Change large areas of the earth’s surface.
Plucking – process that adds gravel, sand and boulders to a glacier’s bottom and
sides as water freezes and thaws, breaking off pieces of surrounding rock.
•
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Transporting and Scouring – as glaciers
move forward over land, they can
transport huge volumes of sediment and
rock. The rock leaves marks behind as it
moves.
• Plucked rock fragments and sand at its
base scour and scrape the soil and
bedrock like sandpaper against wood,
eroding the ground below even more.
•
When bedrock is gouged deeply
by rock fragments being dragged
along, marks are left behind.
•
Deep, long parallel scars
are called grooves.
•
Shallow marks are called
striations.
•
These indicate the
direction in which the
glacier moved.
•
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Ice Depositing Sediment…
When glaciers begin to melt, they are unable to carry much
sediment and that sediment drops, or is deposited, on the land.
• When a glacier melts and begins to shrink back, it is said to
retreat.
• As it retreats, a jumble of boulders, sand, clay and silt is left
behind.
• The mixture of different sized sediments is called till.
•
Till deposits can cover huge areas of land.
•
Thousands of years ago, huge ice sheets in the northern US
left enough till behind to fill valleys completely and make
these areas appear flat.
•
Ex: areas of Montana, Iowa, Ohio, Illinois, Indiana and
New England.
•
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Moraine Deposits –
large ridges of rocks and
soil deposited by a
glacier when it stops
moving forward.
• Outwash Deposits –
material deposited by
meltwater from a
glacier.
• Eskers – a winding
ridge of sand and gravel
left behind when a
glacier melts.
• Pg. 217
•
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Wind!
Wind erosion: air moves and picks up particles and deposits them in
other places
•
Air differs from other erosional forces because it can’t normally
pick up heavy objects, but it can deposit sediment over a large
area
•
Wind erodes Earth’s surface by deflation and abrasion…
•
Deflation: wind blows across loose sediment and removes small
particles and leaves behind heavy material…big rocks…
•
Abrasion: windblown sediment hit rocks and surface gets worn away –
occurs everywhere…but mostly in deserts, beaches & plowed fields
•
WHY? – fewer plants to hold sediment
•
Sandstorms - sand is normally too heavy for wind…wind blows
forcefully & sand bounces & hits other grains causing them to rise
into air (low cloud just above ground…)
•
Dust storms - dry soil picked up by wind and blown high into
atmosphere because they weigh less than sand…carries particles
far distances…
•
•
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•
IE – 1930…dust picked up in Kansas was found in New England…
important part of soil erosion…
Reduce wind Erosion:
•
Erosion is most common in areas with no plants…
•
Stop erosion?…PLANTS!
•
Vegetation will reduce erosion…
•
Roots hold soil in place
•
Windbreaks with trees – slow down wind…
•
Stop energy of motion…
•
IE – row of cottonwood trees reduced wind to 66% of normal
speed
But…strong wind with dry soil…nothing will stop erosion by wind!
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Deposition by wind: sediments are eventually
deposited in new areas…
Creates a variety of land forms…
Loess: wind deposits of fine- grained
sediment…packed together creates a thick
unlayered yellowed- brown colored deposit
What happens when wind blows sediment
against an obstacle? = dune!
Dunes: created when sediment settles behind
an obstacle
Dunes can be moved as sand is lifted and
deposited downwind…(picture )
Overall…Erosion and Deposition constantly
shape and reshape the land!
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Chapter 9: Water Erosion
Surface Water
Runoff – water that doesn't soak into the ground or
evaporate but instead flows across Earth's surface.
 Factors affecting runoff:
 Amount of rain
 Length of time rain falls
 Ex: light rain falling over several hours will have
time to soak into the ground while heavy rain
falling within an hour will run off because it
cannot soak in fast enough.
 Steepness of slope
 Vegetation

Water Erosion
Rill erosion – begins when a small stream
forms during a heavy rain.
 Water moving down the same path creates
a groove called a channel.
 Gully erosion – when a rill channel becomes
broader and deeper.
 Sheet erosion – when water erodes without
being in a channel. This occurs when water
that is flowing in sheets picks up and carries
away sediments.
 Ex: how water flows off the hood of a car.
 Stream erosion – takes place when water
continues to flow along a low place it has
formed.
 As water moves along it picks up
sediments from the bottom and sides of the
channel...this process makes the stream
deeper and wider.

Too Much Water
 FLOODS!!!!
 Water needs to go somewhere when
there's too much of it.
 Dams and levees are built in an
attempt to prevent flooding.
Deposition of Surface Water
 Some stream sediment is carried only a
short distance...sediment is often
deposited within the stream channel itself.
 Deltas – sediment that is deposited as
water empties into an ocean or lake that
forms a triangular, or fan-shaped deposit.
 Alluvial fan – when the river waters
empty into a mountain valley onto an open
plain.
Create a Study Guide!
Using key terms and main ideas from
chapters 7, 8 and 9 create a study guide AND
answer key (on separate sheets of paper).

10 fill in the blank questions
 5 multiple choice questions
 5 true or false questions


Ex: ______ is a mixture of weathered rock,
organic matter, mineral fragments, water and
air.
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REVIEW!!
• Fold a piece of
computer paper in
quarters.
• Using both the front
and back, complete
one bullet pointed
topic in each box.
• Add some color!
• Keep this! You get
to use it on the
quiz!
• Illustrate a slump. Include a caption.
• Illustrate a creep. Include a caption.
• Illustrate a rockfall/rockslide.
Include a caption.
• Illustrate agriculture cultivation.
Include a caption.
• Create a Venn Diagram comparing
mechanical and chemical
weathering. Include examples for
each.
• List and define the 4 types of water
erosion.
• List 6 ways to prevent all kinds of
erosion.
• Illustrate deflation vs. abrasion (so
divide your last box into two)