Weathering
This week we write it down if it’s in
ORANGE.
Photo: Grand Canyon, National Park Service
Wind blows
sand and other
sediment
against rocks.
Little by little,
it wears away
the rock!
This is an
example of
mechanical
weathering.
What do you think made the hole in
this rock?
Water from the
river wears
away the rock,
over thousands
of years.
This is another
example of
weathering.
Photo: Yosemite Falls, National Park Service webcam
What do you think made the notch
where the waterfall starts?
Weathering is the process that breaks
down rock and other substances on
Earth’s surface.
Part 1: Mechanical
Weathering
Mechanical Weathering
Force of Mechanical
Weathering
Abrasion
Plant Growth
Animal Actions
Freezing and
Thawing
Release of Pressure
How it Works
Here we go
again with wind
and rock
particles
sculpting rock!
Photo: Delicate Arch, Arches National Park, National Park Service.
Abrasion: Sand and other rock
particles carried by wind and water
grind away rock like sandpaper on
wood.
Perhaps you have
seen this on
sidewalks?
Photo source: Morguefile.com
Plant Growth: Roots of trees and other
plants grow in the cracks of rocks and
pry them apart.
Photo source: Morguefile.com
Animal Actions: Animals dig in the soil
and loosen and break apart rocks.
Perhaps you have
seen a frozen bottle
get cracked? Or
cracks in the road?
Why does that
happen?
Ice Wedging: When water freezes it
expands. Ice in the crack of a rock
expands and makes the crack larger.
Whiteboard Question
CHECK YOUR UNDERSTANDING!
Mechanical weathering in which rock is
worn away by the grinding action of other
rock particles is called
A. erosion.
B. abrasion.
C. ice wedging.
B. abrasion
Release of Pressure: When rocks reach
Earth’s surface, reduced pressure can
cause cracks and flakes in the rock.
Mechanical Weathering
Force of Mechanical
Weathering
How it Works
Abrasion
Sand and other rock particles carried by
wind and water grind away rock like
sandpaper on wood.
Plant Growth
Roots of trees and other plants grow in
the cracks of rocks and pry them apart.
Animal Actions
Animals dig in the soil and loosen and
break apart rocks.
Freezing and
Thawing
When water freezes it expands. Ice in the
crack of a rock expands and makes the
crack larger.
Release of Pressure
When rocks reach Earth’s surface,
reduced pressure can cause cracks and
flakes in the rock.
Whiteboard Question
CHECK YOUR UNDERSTANDING!
Ice wedging causes mechanical weathering
of rock by means of:
A. plant growth.
B. animal actions.
C. freezing and thawing of water.
C. freezing and
thawing of water
Chemical Weathering
Write it down if it’s in ORANGE.
Chemical
weathering is the
process that slowly
breaks down rock
with chemical
changes, including
changes in the
minerals.
This rusty color is caused by
oxidation in a wet climate.
Whiteboard Question
CHECK YOUR UNDERSTANDING!
What kind of weathering causes the mineral
composition of rocks to change?
A. Chemical weathering.
B. Mechanical weathering.
C. General weathering.
A. Chemical weathering
Part 2: Chemical
Weathering
Chemical Weathering
Chemical weathering is____________________________________________
_______________________________________________________________
Force of Chemical
Weathering
Water
Oxygen
Carbon Dioxide
Living Organisms
Acid Rain
How it Works
This limestone
cave was created
by water
dissolving the
stone.
Photo source: Morguefile.com
Water: Water is the most important cause of
chemical weathering. When a rock dissolves
in water, it mixes completely with the water.
What made this old truck rusty?
Photo source: Morguefile.com
Photo source: Morguefile.com
Rust happens to
old vehicles, and
to rocks.
Photo source: Morguefile.com
Oxygen: Iron combines with oxygen and
water in a process called oxidation. The
result is rust. Oxidation of a rock makes it
soft, crumbly and red or brown.
Limestone.
Carbon dioxide: Carbon dioxide in the air
dissolves in rainwater, becoming a weak acid
called carbolic acid. Acids weather rocks
such as limestone and marble.
Lichen growing
on a rock will
weather it.
Photo source: Morguefile.com
Living organisms: Plants, and other living
organisms such as lichen, release weak
acids that slowly dissolve rocks.
Acid rain: Burning
coal, oil, and gas for
energy produces
pollution. This
pollution combines
with air and water,
to create acid rain,
and fast chemical
weathering.
Photo source: Morguefile.com
Whiteboard Question
CHECK YOUR UNDERSTANDING!
A rock containing iron becomes soft and
crumbly and reddish-brown in color. It
probably has been weathered by:
A. abrasion
B. oxygen.
C. acid rain.
B. oxygen
Chemical Weathering
Chemical weathering is____________________________________________
_______________________________________________________________
Force of Chemical
Weathering
How it Works
Water
Water is the most important cause of chemical
weathering. When a rock dissolves in water, it mixes
completely with the water.
Oxygen
A rock containing iron becomes soft and crumbly and
reddish-brown in color. It probably has been
weathered by
Carbon Dioxide
Carbon dioxide in the air dissolves in rainwater,
becoming a weak acid called carbolic acid. Acids
weather rocks such as limestone and marble.
Living Organisms
Plants, and other living organisms such as lichen,
release weak acids that slowly dissolve rocks.
Acid Rain
Burning coal, oil, and gas for energy produces
pollution. This pollution combines with air and water,
to create acid rain, and fast chemical weathering.
Is it Chemical or Mechanical
Weathering?
Final Whiteboard Questions




Abrasion of a rock by blowing sand is an
mechanical
example of __________________
weathering.
Acid rain washing away the face of a statue is
chemical
an example of __________________
weathering.
True or False? Living things can cause both
chemical and mechanical weathering. True
A tree’s roots growing through cracks in a rock
mechanical
is an example of _________________
weathering.
Part 3: Rates of Weathering
What makes a rock weather fastest?
Photo source: Morguefile.com
Rate of Weathering
The most
important factors
for the rate
(speed) of
weathering are
1. rock type; and
2. climate.
Photo source: Morguefile.com
Mechanical and Chemical Weathering Work Together
Photo source: Morguefile.com
Photo source: Morguefile.com
Mechanical weathering increases the surface
area available for chemical weathering.
Increased surface area usually results in faster
chemical reactions (chemical weathering).
Mechanical and Chemical Weathering Work Together
Which rock do you think will weather faster?
A permeable rock weathers easily because
it contains many small, connected air
spaces
Whiteboard Question
CHECK YOUR UNDERSTANDING!
A permeable rock weathers easily because
it:
A. is made up of small particles.
B. contains many small, connected air
spaces.
C. is made up of many minerals.
B. contains many small,
connected air spaces.
How Does Weather Affect Weathering?
Photo source: Morguefile.com
Which of these places
would have faster
weathering of rock?
Why?
Photo source: Morguefile.com
Photo source: Morguefile.com
Rainforests
have warm,
wet weather.
Photo source: Morguefile.com
Wet climates with high temperatures
speed up both mechanical and
chemical weathering.
Photo source: Morguefile.com
Whiteboard Question
CHECK YOUR UNDERSTANDING!
Granite lasts a long time when it is used for
building in areas where the climate:
A. is cool.
B. has lots of acid rain.
C. is hot and rainy.
A. is cool
Whiteboard Question
CHECK YOUR UNDERSTANDING!
The most important factors in determining
the rate of weathering are:
A. carbon dioxide and acid rain.
B. animal actions and oxygen.
C. rock type and climate.
D. abrasion and acids from root plants.
C. rock type and climate