unit 2: surface processes and the hydrosphere lesson 5: weathering and erosion ( heath earth science...
TRANSCRIPT
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Unit 2: Surface Processes and the Hydrosphere
Lesson 5: Weathering and Erosion
(Heath Earth Science – Pg. 130-147)
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Today’s Objectives Relate the processes associated with
weathering and erosion to the resulting features, including: Distinguish between weathering and
erosion Use examples (eg. Exfoliation) to
distinguish between mechanical and chemical weathering (including weathering by biological organisms)
Describe the origin and function of soil
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Where is this?
Shilin Stone Forest, Yunnan
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Weathering and Erosion Weathering and Erosion Songs What do you think weathering means? What do you think erosion means? Weathering:
Is the break-up of rock due to exposure to the atmosphere
Erosion: Is the removal and transport of earth materials by
natural agents (such as wind, water, ice, tides, or gravity)
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Weathering an Erosion – working together – Grand Canyon
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Weathering Why do rocks weather? Partly due to the difference between
conditions deep beneath the surface and the conditions at the surface: Deep underground, where many rocks
are formed, pressure and temperature are very high
When these rocks rise to the surface, pressures and temperatures are much less: rock expands
Water and oxygen, also not present at depth, exist on the surface: break rock down
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Types of Weathering Weathering includes many processes,
but these processes are grouped into two types of weathering: mechanical and chemical weathering
Mechanical (Physical) Weathering – Also called disintegration Takes place when rock is split or broken
into smaller pieces of the same material without changing its composition
Ex.)The breaking of a rock cliff into boulders and pebbles
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Types of Weathering Chemical Weathering –
Also called decomposition takes place when rock’s minerals are
changed into different substances Water and water vapor are important
agents of chemical weathering Ex.) Clay being formed from feldspar
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Types of Weathering Mechanical and Chemical weathering
processes are often studied separately but they seldom occur alone
Because water vapor is present almost everywhere, chemical weathering happens almost everywhere
In different parts of the world, one process may be more important than the other, based on climate
However, mechanical and chemical weathering almost always act together
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Working Together
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Practice
Topic Questions, pg. 137 #1-2
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Types of Mechanical Weathering Mechanical weathering happens in
many different ways. Frost action, wetting and drying, action of plants and animals and the loss of overlying rock and soil all play a role in mechanical weathering.
Common processes include: 1) Ice (Frost) wedging 2) Exfoliation (sheeting) 3) Root wedging
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1) Ice Wedging Water takes up about 10% more space when it
freezes This expansion puts great pressure on the walls
of it’s container Example) Put a completely full bottle of water into
your freezer and leave it there for 2 or 3 days, then check it out!
Water held in the cracks of rocks wedges the rocks apart when it freezes, a process called frost wedging, or frost action
Often occurs in areas where daytime temperature is > 0˚ C, and night temperatures are < 0˚ C (such as southern Canada)
Most damaging of all weathering processes in these areas
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1) Ice Wedging
• Ice wedging is very common on bare mountain tops and in porous rocks or rocks with cracks in them
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2) Exfoliation (sheeting) Some rock is formed far below Earth’s surface,
for example, Granite (rock that makes up Huangshan)
It is exposed when large masses of rock are lifted to form mountains and the rocks above the granite are worn away
The removal of this rock reduces the pressure on the granite, and it begins to expand
Upward expansion leads to long, curved breaks, or joints. The joints are parallel to the surface (sheet jointing)
When these sheets break away, it is called exfoliation
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2) Exfoliation (sheeting)
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3) Root Wedging Small plants or even
trees grow on rocks When they are very
small, they wedge their tiny roots into pores or crevices in the rock
As the roots grow, the rock splits apart
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Practice
Topic Questions, pg. 137 #3
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Types of Chemical Weathering Chemical weathering of rock results
mainly from the action of rainwater, oxygen, carbon dioxide, and acids of plant decay
Typical processes include: 1) Oxidation 2) Hydrolysis 3) Carbonic acid dissolving
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1) Oxidation The chemical reaction of
oxygen with other substances is called oxidation
Iron-bearing minerals are the ones most easily attacked by oxygen
Oxidation of these minerals results in rust, or iron oxides
When water is present, oxidation occurs more quickly
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2) Hydrolysis The chemical reaction of water with other
substances is called hydrolysis Common minerals that undergo hydrolysis
include feldspar, hornblende, and augite When these minerals are exposed to water,
they slowly unite with it, and form clay
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2) Hydrolysis
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3) Carbonic Acid Dissolving Carbon dioxide dissolves easily in water
forming carbonic acid This is the same acid found in soft drinks The acid dissolves out elements such as
potassium, sodium, magnesium, and calcium from minerals
When this occurs, the original mineral is changed into a clay mineral
The greatest effect is on the mineral calcite Carbonic acid completely dissolves this
mineral, leaving behind great underground caverns in limestone bedrock
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3) Carbonic Acid Dissolving
Where is this?
Yangshuo, Guangxi
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Practice
Topic Questions, pg. 137 #4
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Weathering and Erosion Video
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Differential Weathering
Some rocks are more resistant to weathering than other rocks.
This can lead to very strange looking landscapes such as in this photo or the photo from the title page
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Differential Weathering - Pedestals Where is this?
Zhangjiajie, Hunan
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Soil Weathering has attacked the rocks of Earth’s
surface since the beginning of time It has helped wear down mountains and to
shape countless landforms This weathering has led to valuable mineral
deposits and to form a priceless resource – Earth’s life supporting soil
Soil is made of loose, weathered rock and organic material in which plants with roots can grow
Without soil, there could be no life on land The rock material in soil contains three
noticeable parts: sand, silt, and clay
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Soil Content
• There are many different soil classifications, based on the amount of sand, silt, and clay particles present
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Soil Content These particles only
make up about half of the material in soils
Organic matter (decayed plant and animal material), air, and water also make up substantial portions of soil
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Soil Origins The material from which soil is formed is
called its parent material Often this material is the bedrock beneath
the soil Soil that has bedrock as its parent material is
called residual soil In some places, deposits left by winds,
rivers, and glaciers have covered over the bedrock
Soils formed from such materials are called transported soils
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Soil Origins
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Importance of Soil Soil has many important functions that permit
life on Earth: 1) Soil provides a stable medium from which
plants can grow 2) Soil provides habitat for numerous organisms 3) Soil is porous and permeable, allowing water
to reach roots beneath the surface 4) Soil acts as a filter helping to clean water
before it reaches the ocean or drinking wells 5) Organic matter in soil provides nutrients for
plants and detrivores
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Practice
Topic Questions, pg. 143 #7
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Review What is weathering?
What is erosion?
What is mechanical weathering?
What is chemical weathering?
Where does soil come from?
Why is soil important?