WEATHERING – PHYSICAL AND
CHEMICAL
Weathering

Weathering is the wearing away of
rock and can be chemical or physical.
 Physical
weathering – the process by which rock is
broken down into smaller pieces by physical means.
 Chemical weathering - the break down of rocks by
chemical means
Physical Weathering


Process by which rocks are broken down into smaller
pieces by physical forces.
Types of Mechanical weathering
 Abrasion
 Temperature
changes (freezing and thawing)
 Ice or Frost wedging
 Plant roots
 Burrowing of animals (animal action)
Abrasion


Abrasion is weathering
by grinding action – This
is physical weathering
These mountains in Costa
Rica were mechanically
weathered by abrasion.
These mountains in Salt Lake City, Utah,
were also weathered by abrasion.
Freezing and Thawing
(temperature changes)
A rise in temperature
causes rock to expand.
A decrease causes
rocks to contract. This
repeated change
weakens rocks, causing
them to crumble.
Ice or Frost Wedging
Water enters the cracks
in rocks and freezes,
causing the cracks to
expand. This process will
eventually break even the
biggest rocks.
Plant This
Roots
happens as these small
roots grow into small cracks,
as the tree grows and the
roots get bigger, they will
eventually break the rock.
Think about how plants and
weeds will grow in the
smallest crack of a sidewalk.
Also think about seeing tree
roots that crack the sidewalk
in older neighborhoods with
large trees.
Animal Action (Burrowing)
Many dig burrows in
the ground. Examples
are squirrels, prairie
dogs, ants and
earthworms.
As animals move soil, it
exposes rocks, these
rocks then experience
weathering.
Chemical Weathering


the process that breaks down rock through chemical
changes.
The agents of chemical weathering:
 oxygen-
combines with iron to form rust (oxidation)
 Acid precipitation (rain)
 Acids in ground water
 Acids in living things
Oxygen
Iron combines
with oxygen in
the presence of
water in a
processes called
Oxidation.
 The product of
oxidation is rust.

Acid Precipitation or Rain


Compounds from burning coal, oil and gas react
chemically with water forming acids.
Acid rain causes very rapid chemical weathering.
Water

Ground Water weathers rock by dissolving it.

CaCO3 + 2CH3COOH = Ca(CH3COO)2 + H2O + CO2.

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
Read more :
http://www.ehow.com/info_7888211_vinegar-affectlimestone.html
The dissolved rock is carried by water and later
deposited.
Examples are Stalactites and stalagmites
Living Organisms

Lichens and mosses that grow on rocks produce
weak acids that chemically weather rock.
2 factors that determine the rate of
weathering:


Rock Type - Rock that is permeable (has holes in it),
weathers faster than rock that is impermeable.
- The mineral content of rock also
determines how fast a rock weathers.
Climate - Rock weathers faster in warm, humid
climates.
Erosion


Erosion is the movement
of sediment by water,
wind or ice.
In this picture taken in
Oregon, a gully has
been created by a
stream that has cut a
path through soft
sediment after a strong
rainfall.
Erosion

Erosion has caused this
bank in Squaw Creek to
be cut away by the
weathering of the river,
forming a landform that
simulates a partial cave
on the left side of this
picture.
Erosion

Here, erosion by
longshore drift, has
caused a lagoon to form
along this beach in
Costa Rica.
Deposition
Deposition is the
adding of sediment in
an area as it settles
out and forms new
landforms. In this
picture, taken in
Squaw Creek, near Sisters, Oregon, rocks have been
deposited along the bank of the river after heavy
rains.
Deposition

In this picture, also
taken in Squaw Creek,
a sandbar has been
formed by sediment
that has been washed
down the river. This
might also be
considered a spit.
Deposition

In this picture, salt has
been deposited in the
Great Salt Lake, Utah.
You can see the salt
deposits. They appear
as white areas on the
edge of the water.
Landforms

http://www.classzone.com/books/earth_science/ter
c/content/visualizations/es1205/es1205page01.cf
m?chapter_no=visualization
Erosion Control Methods:

These native plants were
left undisturbed on the
beach in Costa Rica, so
that they would hold in
the sand and prevent
erosion on the beach.
This is an example of
permanent erosion
control.
other examples of erosion control:
silt fencing
terracing
River Formation
•
A river is a natural stream of freshwater that is
larger than a brook or creek. Rivers are normally
the main channels or largest tributaries of drainage
systems. Typical rivers begin with a flow from
headwater areas made up of small tributaries, such
as springs. They then travel in meandering paths at
various speeds. Finally, they discharge or flow out
into desert basins, into major lakes, or most likely,
into oceans.
Erosion and Deposition

Meanders are formed by
the weathering of one
bank of a river and the
deposition of the
weathered sediment on
the opposite bank of the
river. This creates a
loop-like bend in a river,
such as this one. This
view was taken from an
airplane over a mountain
range in the midwest.
•changes in a meandering river
•
•
•
http://www.classzone.com/books/earth_science/terc/con
tent/visualizations/es1306/es1306page01.cfm?chapter
_no=visualization
formation of an oxbow lake:
http://www.cleo.net.uk/resources/dis
playframe.php?src=309/consultants_
resources%2F_files%2Fmeander4.swf
The world's longest river is the Nile River in Africa,
which runs 4,187 miles (6,739 kilometers) from its
source in Burundi to the Mediterranean Sea.
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Every river has a point of origin- it is the highest point
in the water basin.
Gravity- is important; rivers almost always flow
downhill watercourse.
Some rivers start from springs, especially in humid
climates. Springs occur as groundwater rises to Earth's
surface and flows away.
Other rivers originate from lakes, marshes, or runoff
from melting glaciers located high in the mountains.
Some rivers have their beginnings in huge glaciers.
(STOP) LESSON 3Steps to River Formation
1) Precipitation (rainwater or snow ) is the source of the water for
most rivers.
2)When a heavy rain falls on ground that is steeply sloped or is
already saturated with water, water runoff trickles down Earth's
surface rather than being absorbed- this is called surface
runoff.
3) After it travels a short distance, the water begins to run in small
parallel rivulets called rills.
4) As these rills pass over fine soil or silt, they begin to dig shallow
channels, called gullies. This is the first stage of erosion. A gully
only has water in it when it rains.
5) A stream is formed when gullies join together. It is a channel
along which water is continually flowing down a slope. Streams
rarely dry up.
6) A stream grows larger as it gets water from tributaries. A
Visual steps to river formation
Rills
gullies
River System
Notes quiz
1. Examples of mechanical weathering are
a)
ice wedging, animal actions, oxidation
b)
plant growth, ice wedging, abrasion
c)
freezing and thawing, burrowing, acid rain
2. Examples of chemical weathering are
• a) acid rain, oxidation, plant roots
• b) freezing and thawing, animal actions, oxidation
• c) burrowing, carbon dioxide, abrasion
3. Deposition is
a)
the movement of sediment or rock
b)
the settling out of sediment or rock
c)
the breaking down of sediment or rock
4. A meander is
• a) a small river that runs into a larger river
• b) a loop-like bend in a river
• c) a lake that has been cut off from a river
•
•
•
•
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5) Name 3 methods of erosion control
a)
b)
c)
6) What are 2 factors that affect the rate of
weathering?
a) ______________________
b) ______________________
 7) What is surface run-off?
•
•
•
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•
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8) Rills in soil run together to form __________ that
only have water in them when it rains.
9) What forms when gullies join together?
a) a rill
b) a stream
c) a tributary
10) What is the original source of water of most
rivers? (1st step in river formation)