Weathering and Erosion

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Weathering and Erosion

• Changing Earths Crust

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What is weathering?
Weathering is the break down of rock, either by
physical or chemical processes.
It is split into two types, mechanical weathering
and chemical weathering.
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Mechanical Weathering is the breakdown of rock by
physical means. -they are broken into smaller
pieces by another object or material, but their
chemical composition does not change.

• Causes of mechanical weathering
• Ice- the alternate freezing and thawing of a rock
  can break it apart (called frost action).
• A type of frost action (ice wedging), takes place
  when ice seeps into a crack and expands and
  contracts as it melts and freezes.
• Abrasion- the grinding away of rock by other
  rocks or rock particles.
• Abrasion can be caused by moving water carrying
  rocks or particles, wind blowing rock particles,
  or gravity as rocks fall down hill.
• Plants- the root and shoot systems of plants grow
  through and break apart rock.
• Animals- digging in the soil or sand or walking
  around can break or crack rocks and rock
  particles.

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Most rocks are mixtures of different minerals.
When interaction with the environment causes the
different minerals in the mixture to separate, we
chemical weathering has taken place.
Chemical Weathering- the breakdown of rocks as a
result of chemical reactions.

• Causes of chemical weathering
• Water- it doesnt work quickly, but constant
  exposure to water can weather rock.
• Acid precipitation- the low level of acidity
  naturally in most rain can weather rock.
• Pollution has increased the level of acidity in
  most precipitation.
• Acidic ground water- chemicals in the ground can
  cause water running through it to mix with acids,
  which can weather rock (this is how underground
  caverns are formed).
• Organisms- living things, such as plants,
  animals, algae, fungi, and bacteria release acids
  which can weather rock.
• Air- Compounds in the air (often oxygen) react
  with the metals in rock, to cause weathering.

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Weathering doesnt always occur at the same
rate. -dynamite will break up rock a lot faster
than a quiet stream.
The cause of the weathering (acid rain, abrasion,
etc.) affects the rate of weathering, but there
are other factors.
Different types of rock resist weathering better
than others. This is called differential
weathering. -soft rocks weather faster than
resistant rocks. -this accounts for some of the
unusual shapes large rock structures take.
The shape and size of the rock also affects the
rate of weathering. -Rocks with a large surface
area come in contact with the weathering agent
more than those with a small surface area, so
they weather faster.
e.g. A pile of gravel has more surface area than
a boulder with the same volume.
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Climate can also affect the rate of weathering.
Chemical reactions usually go faster in warm
places. Water is a major weathering
agent. Sorock weathers faster in warm, wet
places or places where the temperature changes
drastically from hot to cold.
The peaks of mountains weather quickly because
they are exposed to more wind, water, and
temperature changes than the bases. -Water
running quickly down a steep mountain also
weathers more aggressively than a slow stream
running through a field.
Rocky Mountains
Appalachian Mountains
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What happens to rock as it weathers down to
smaller and smaller pieces?
-It mixes with decaying organic matter, water,
and air to form soil.
Is all soil the same?
No, soil can be made of different size rock
fragments, different minerals, different amounts
and types of organic matter, and have varying
amounts of air and water.
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Different size and type rock particles
-Large rock particles are called sand -Medium
sized particles are called silt -And small are
called clay (2/1,000ths of a mm)
The type of rock and minerals in soil depend on
the rock from which the soil weathered
The rock that the soil came from is called the
parent rock
Most of the time, soil is made from the
weathering of the layer of rock beneath the soil,
called bedrock.
Soil texture describes what rock particles are
part of the soil and Soil structure tells us
how they are arranged within the soil
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What is organic matters part in forming soil?
When plants and animals die, their bodys decay
and become part of the soil. -the part of the
soil made from organic material is called Humus
Many of the nutrients that living things need,
and that make soil fertile, are only there
because it was returned after other living things
died. e.g. Nitrogen which makes
proteins Fertilizing soil is adding more of these
nutrients
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Soil Horizons and Health
Since most organic matter is deposited above
ground and most of the rock being weathered is
well below ground, soil is usually layered.
The layers of soil are called horizons.
O- organic matter A- top soil, most nutrients B-
mix of soil and weathered rocks, some leached
nutrients C- mostly weathered rock R- bedrock is
beneath
As water runs down through the layers of the soil
it drags nutrients from the upper layers down,
taking them away from plants that need them and
usually depositing them in ground water. -This
is called leaching
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Some soils are healthier than others.
How healthy they are depends on the amount of
water available, amount of organic material
available, temperature, and pH (how acid or basic
the soil is).
-Too much water means too much leaching, too
little water means no chemical weathering of
bedrock or water for living things
-Too many living things means that there is a lot
of competition for resources, too few and there
are not enough resources in the soil.
-Plants cant grow or process nutrients in soil
that is too acidic or basic
-At cold temperatures, decomposition takes place
very slowly so there is little to no organic
material in the soil
The healthiest soils on earth are in Temperate
zones.
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Why is soil important?
It provides nutrients and holds water for plants
and provides a home for many animals.
How can soil be damaged?
1)When it is overused it can lose its nutrients
and become infertile. 2)When it isnt protected
by plant roots, it can be washed away by wind,
water, or gravity (erosion).
How can soil be conserved?
1)Soil can be protected by rotating crops to
always keep fields active and restore
nutrients 2) Crops can be arranged in a way to
limit erosion.
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What is erosion?
Erosion is the transport of rock or soil, usually
by wind or water (including ice).
Rock particles are not carried forever. They are
eventually dumped (deposited) somewhere else.
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How can liquid water erode the earth?
The water on earth is constantly moving, whether
in rivers, streams, the ocean, or underground.
When it does it collides with rocks and rock
particles it moves them.
Streams and rivers- depending on fast the water
is moving, they can either pull earth from the
banks and grow wider or from beneath them and dig
down.
Oceans- the energy in waves, undertow, and
currents pickup and carry sand and small
particles and deposit them somewhere else.
This can cause beaches to change and sand bars
(and after enough time islands or peninsulas) to
form.
If the area is made of harder rock, you will have
cliffs instead of sandy beaches. However, the
energy in the water eventually breaks these too.
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How can ice move rock?
We know that ice is a major agent of mechanical
weathering, but it can also move rock.
Parts of the earth are covered in large sheets of
ice called glaciers -how much is covered in
glaciers has changes periodically through
geologic time. -as the earth cools and warms
the total area covered by glaciers grows or
shrinks.
Glaciers grow where it is cold enough to snow,
but never warm enough for it to melt. It piles
up, gets thicker, and the pressure of its own
weight makes it denser. Eventually, the pressure
is great enough to melt of crystallize the bottom
layer. Gravity can make the whole glacier slide
on this liquid or crystalline layer.
As these giants sheets of ice grow, shrink, and
slide they take large chunks of the earth with
them.
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How can wind and gravity cause erosion?
Wind can move small rock particles, but it can
also cause parts of larger rocks weakened by
weathering to break apart and move.
Particles are deposited when the wind stops
blowing or when the particle hits something that
the wind cant move, like a whole sand dune, a
rock, or tree. -the sand dunes of deserts are
constantly moving and changing because of wind
erosion.
When a rocks hold is weakened by weathering,
wind, water, or some other force gravity can
cause it to start moving downhill. -landslides,
mudslides (including lahars), and the slow creep
of soil and rock particles are all examples of
gravity driven erosion.
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Color Quiz
Which of the following is not a major agent of
erosion? Red Acid precipitation Black Water
What is a slowly moving, giant sheet of ice
called? Red Glacier Black Iceberg
Erosion can create islands, T/F. Red
False Black True
What are the piles of sand created by wind
called? Red Sand Castles Black Sand Dunes
Gravity is a major agent of erosion and always
works completely independent of other agents.
T/F. Red False Black True
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Water can cause weathering and erosion and is an
intricate part of healthy soil, but how is it
valuable as an independent resource?
The cells of all living organisms are filled
mostly with water. It is also a key component in
Photosynthesis and other organic processes.
Water is constantly moving, but how?
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The water cycle is the continuous movement of
water from the surface of the earth to the
atmosphere and back,
Water on earths surface evaporates (turns into
water vapor) and moves up into the atmosphere.
When it cools, it condenses (turns back into
liquid water) and sticks to dust particles in the
air, forming clouds.
When there is too much water in the air for the
clouds to hold (saturation), precipitation (rain,
snow, sleet, or hail) falls.
On the ground, water either percolates/infiltrates
(seeps into the soil and becomes part of the
groundwater) or it runs-off (flows in rivers and
streams towards the ocean).
Any time that it is on the surface it has the
potential to evaporate again.
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Some water molecules can get stuck in the ocean
or in glaciers for years, but they always
eventually rejoin the water cycle.
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All of the water on earth is linked, constantly
flowing from one body to the next.
Rivers and streams (runoff) that run into the
same body (an ocean, lake, or larger river)
together form a river system.
The streams or rivers giving water to larger
rivers or lakes are called tributaries.
All of the land contributing water to a
particular river system is called a watershed.
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A channel is the path a particular stream
follows. -how a channel looks depends on how
the stream is eroding the land around it.
How much a stream erodes a landscape depends on
its gradient, discharge, and load.
Gradient- how steep the area a river flows
through is. This makes the water move faster or
slower. Discharge- how much water is in a stream.
This depends on the amount of rain or water from
tributaries. Load- how much sediment and rock a
stream carries. The faster a stream moves and
the more water it carries, the bigger its load
can be.
A large gradient, large discharge, and heavy load
increase a streams ability to erode.
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The properties of the stream and the earth around
it decide the course a stream will take.
-Erosion power of the stream, coupled with the
differential weathering of the rock around it,
decide how the stream/river will look.
How rivers are described (Youthful, mature, old,
or rejuvenated) is based on their appearance not
their actual age.
Y
M
O
R
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Eventually, a river deposits its load.
The different types of deposits are characterized
by where they are compared to the river.
Placer deposit- heavier sediment deposited at a
slow point in the river. Deltas- area at the
mouth of a river where it meets the ocean or
lake, slows down, floods the surrounding area,
and deposits a lot of its load. Alluvial Fans-
When a river or stream flows onto flat land, it
slows down, temporarily floods the area, and
deposits its load there on land. Flood Plains-
When a river rises it can flood the surrounding
area. When it recedes it leaves behind some of
its load.
Flood plains are usually very fertile because of
these deposits, but the actual floods can be very
dangerous.
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How does water underground contribute to erosion
and the water cycle?
Water underground is called groundwater.
Any water that percolates or infiltrates the soil
becomes part of the ground water, where it either
sticks to soil particles or continues to move
downward.
Soil that cannot hold anymore water is called
saturated.
The boundary between the soil that is saturated
and that is unsaturated is called the water table.
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Layers of rock that allow water to move through
them are called aquifers.
The best aquifers are porous (empty space between
rock particles) and permeable (allow movement).
Aquifers get their water from precipitation that
infiltrates the soil. -If soil is impermeable,
aquifers cant recharge.
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What are springs?
A spring is any place where the water table
reaches the surface of the ground, allowing it to
flow out.
One type of spring is an artesian spring, in
which an aquifer, sandwiched between impermeable
layers of rock finds it way through a crack to
the surface.
Usually springs are cold, but if the aquifer runs
deep in the earth or near a magma chamber the
water can heat up well over 100F.
A well is a man-made hole that accesses the water
below the water table.
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Ground water carries with it any pollutants or
chemicals that it picks up from the rock
particles it flows through.
This increases its ability to weather rock.
When especially acidic ground water finds its way
into soft rock, it can weather the area
extensively, creating underground caves called
caverns.
Sinkholes appear when these underground hollows
collapse.
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There is less freshwater available to us than you
think.
Of all the water on earth, 2.5 is freshwater
68.6 of the freshwater is frozen in icecaps and
glaciers. 30.1 of the freshwater is
underground. 1.3 is on the surface as
atmospheric vapor, ice, lakes, rivers, and
streams.
Of the 1.3 on the surface, only 20.6 is in
lakes, rivers, and streams
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How much of that freshwater is clean enough (low
level of natural and man-made pollutants) to
drink?
Basically none, and we share that water with all
of the terrestrial (live on land) living things
on earth.
Most pollution does not have a single source, but
anything dumped onto the ground (even some things
put into the air) find their way into the water
cycle.
We have to clean all of our water, for the most
part this takes place at sewage treatment plants.
There are natural systems that help clean water
though.
The forests around most reservoirs are protected
because their soil helps to clean the water
before it enters the reservoir.
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How is water wasted in industry? How is water
wasted in agriculture? How is water wasted at
home?
How can it be conserved?
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Free Throw Review

• In which stage of the water cycle does water
  change from a gas to a liquid form?
• Which type of weathering breaks rock into smaller
  pieces but does not change it chemically?
• Which agent of erosion creates sand dunes in
  deserts?
• What is the decayed organic matter in soil
  called?
• What is the rock from which soil has weathered
  called?
• How fast a river flows, how much water it
  carries, and how much sediment it carries,
  together determine how ____________ it can be.
• Name one agent of chemical weathering?
• Name one agent of mechanical weathering?
• What features can be created when acidic
  groundwater find soft-rock underground?
• What type of deposit appears where a river or
  stream merges with a larger body of water?
• Which type of glacier can cover vast stretches of
  land and blanket large areas of a continent?

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Free Throw Review

• Name one technique used by farmers to protect
  soil.
• As water infiltrates soil, it pulls nutrients
  down from the upper layers, what is this process
  called?
• What was the agent of erosion that created the
  Grand Canyon?
• Which agent of erosion causes rock falls?
• Which type of mechanical weathering involves
  rocks or rock particles colliding with each
  other?
• What natural soil disaster hit the central plains
  in the 1930s?
• What is a place where an eroding agent drops its
  load (its rock particles) called?
• What is a place where ground water naturally
  finds its way to the surface called?
• What is the boundary between the soil saturated
  with water and the soil unsaturated with water
  called?