Transcript Blog version of weathering-and-erosion

Weathering and Erosion
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Weathering
The process by which
rock materials are
broken down by the
action of physical or
chemical processes.
Mechanical weathering
The breakdown of rock into
smaller pieces by physical
means.
(Ex. Ice Wedging and Abrasion )
.
Agents (Causes) of mechanical
: include
weathering
ice, wind, water, gravity, plants, and even
animals
Frost action
The alternate freezing and thawing of soil
and rock is a form of mechanical
weathering.
One type of frost action, ice wedging.
Ice wedging starts when water seeps into cracks, it
freezes during cold weather causing the crack
to expand and widen and eventually the rock
breaks apart.
Ice wedging
1. Water seeps into cracks during warm
weather.
2. Then temperatures drop.
3. The water freezes and expands.
4. Next the ice pushes against the sides of
the crack.
5. Then the crack widens.
Abrasion
The grinding and wearing
away of rock surfaces
through the mechanical
action of other rock or sand
particles.
Agents of mechanical weathering
(abrasion)
Wind
Water
Gravity
Water
When rocks and pebbles roll along the
bottom of swiftly flowing rivers, they bump
into and scrape against each other.
The weathering that occurs eventually
causes these rocks to become rounded
and smooth.
Wind
When wind blows sand and silt
against exposed rock, the sand
eventually wears away the rock’s
surface. This is what
sandblasting can do to a rock.
Gravity
Abrasion occurs when rocks fall on one
another. The force rocks exert on each
other as they tumble down a mountainside
causes abrasion. (rockslide/rock fall or
landslide)
Plants
• Plants roots go into existing cracks in
rocks.
• As the plant grows, the force of the
expanding root becomes so strong that the
crack widens.
• Eventually, the entire rock can split apart,
as shown in pictures below
Animals
Animals burrow (dig) through the soil and
move soil particles around. This exposes
rock to air and water and the rate of
weathering increases. (Both Mechanical
and Chemical weathering)
Almost any animal that burrows causes
mechanical weathering. Ants, earthworms,
mice, coyotes, and rabbits are just some
of the animals that contribute to
weathering.
Moles, insects, earthworms, gophers,
Chemical Weathering
The process that breaks down rocks as
a result of chemical reactions is called
chemical weathering.
Common agents of chemical weathering are
water, weak acids, and air.
Acid Precipitation
Rain, sleet, or snow, that contains a high
concentration of acids
Acid precipitation can be caused by air
pollution from the burning of fossil fuels,
such as coal and oil.
Sulfur oxides, nitrogen oxides, and carbon
oxides combine with water in the
atmosphere and form weak acids, which
then fall back to the ground in rain and
snow. (Burning of fossil fuels)
Acid in groundwater has weathered limestone to
form Rusty’s Cave in Dade County, Georgia.
Acids in Living Things
• Another source of acids that cause
weathering might surprise you. Lichens
produce acids that can slowly break down
rock.
Air
This car is undergoing
chemical weathering due
to the air. The oxygen in
the air is reacting with the
iron in the car, causing the
car to rust. Water speeds
up the process. But the
iron would rust even if no
water were present.
Scientists call this process
oxidation.
Oxidation is a chemical reaction in which
an element, such as iron, combines with
oxygen to form an oxide. This common
form of chemical weathering is what
causes rust.
Section Summary
• Ice wedging is a form of mechanical
weathering in which water seeps into rock
cracks and then freezes and expands.
• Wind, water, and gravity cause
mechanical weathering by abrasion.
• Animals and plants cause mechanical
weathering by turning the soil and
breaking apart rocks.
• Water, acids, and air chemically weather
rock by weakening the bonds between
mineral grains of the rock.
1. In your own words, write a definition for
each of the following terms: weathering,
mechanical weathering, abrasion,
chemical weathering and acid
precipitation.
2. Which of the following things cannot
cause mechanical weathering?
a. Water
b. Acid
c. Wind
d. animals
3. List three things that cause
chemical weathering of
rocks.
4. Describe three ways
abrasion occurs in nature.
5. Describe
the similarity in
the ways tree roots and ice
mechanically weather rock.
6. Describe five sources
of chemical weathering.
7. Making Inferences
Why does acid precipitation
weather rocks faster than
normal precipitation?
8. Making Comparisons
Compare the weather
processes that affect a rock on
top of a mountain and a rock
buried beneath the ground.
Differential Weathering
Differential weathering is a process by
which softer, less weather resistant rocks
wear away and leave harder, more
weather resistant rocks.
About 50 million years ago, a mass of molten rock
cooled and hardened underground to form
igneous rock.
The surrounding rock was softer than the igneous rock. As
the rocks were weathered for millions of years, the softer
rock was completely worn away. The harder, more resistant
rock of the tower is all that remains.
The Shape of Rocks
Weathering takes place on the outer surface of
rocks. Therefore, the more surface area that is
exposed to weathering, the faster the rock will
be worn down.
A large rock has a large surface area. But a large
rock also has a large volume. Because of the
large rock’s volume, the large rock will take a
long time to wear down.
If a large rock is broken into smaller
fragments, weathering of the rock
happens much more quickly. The rate
of weathering increases because a
smaller rock has more surface area to
volume than a larger rock has. So,
more of a smaller rock is exposed to
the weathering process.
Question 4
How does an increase in
surface area affect the rate
of weathering?
Weathering and Climate
Climate is the average weather condition in
an area over a long period of time.
Question 5
Why would a mailbox in a
warm, humid climate
experience a higher rate of
weathering than a mailbox in a
cold, dry climate?
Temperature and Water
The rate of chemical weathering is faster in
warm, humid climates. The rusty mailbox
has experienced a type of chemical
weathering called oxidation. Oxidation, like
other chemical reactions, happens at a
faster rate when temperatures are higher
and when water is present.
Temperature and Water
Water increases the rate of mechanical
weathering.
The freezing of water that seeps into the cracks of
rocks is the process of ice wedging. Ice wedging
causes rocks to break apart. Over time, this form
of weathering can break down even the hardest
rocks into soil.
Temperature and Water
Temperature is another major factor in
mechanical weathering. The more
often temperatures cause freezing
and thawing, the more often ice
wedging takes place. Therefore,
climatic regions that experience
frequent freezes and thaws have a
greater rate of mechanical
weathering.
Question 6
Why do mountaintops
weather faster than rocks
at sea level?
Gravity
Gravity affects weathering.
The steepness of mountain slopes increases
the effects of mechanical and chemical
weathering.
Steep slopes cause rainwater to quickly run
off the sides of mountains. The rainwater
carries the sediment down the mountain’s
slope. This continual removal of sediment
exposes fresh rock surfaces to the effects
of weathering.
Gravity
New rock surfaces are exposed to
weathering when gravity causes
rocks to fall away from the sides of
mountains. The increased surface
area means weathering happens at a
faster rate.
The effects of
weathering can
make mountain
tops pointed or
rounded.
The Appalachian
mountains which
are older and once
taller than the
Rockies are now
shorter due to
mechanical
weathering
Section Summary
Hard rocks weather more slowly than softer
rocks.
The more surface area of a rock that is
exposed to weathering, the faster the rock
will be worn down.
Chemical weathering occurs faster in warm,
humid climates.
Weathering occurs faster at high elevations
because of an increase in ice, rain, and
wind.