Weathering and Erosion
Changing Earth’s Crust
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.
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.
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 doesn’t 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.
Weathering doesn’t 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.
Climate can also affect the rate of weathering.
Chemical reactions usually go faster in warm places.
Water is a major weathering agent.
So…rock 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
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.
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
What is organic matter’s part in forming soil?
When plants and animals die, their body’s 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
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
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 can’t 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.
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 isn’t 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.
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.
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.
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.
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 can’t 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 rock’s 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.
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
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?
The water cycle is the continuous movement of water from the surface of the earth
to the atmosphere and back,
Water on earth’s 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.
Some water molecules can get stuck in the ocean or in glaciers for years, but they
always eventually rejoin the water cycle.
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.
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 stream’s
ability to erode.
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
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.
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.
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 can’t recharge.
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 100°F.
A well is a man-made hole that
accesses the water below the water
table.
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.
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
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.
How is water wasted in industry?
How is water wasted in agriculture?
How is water wasted at home?
How can it be conserved?
Free Throw Review
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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?
Free Throw Review
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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?