Weathering and Erosion
Definition: weathering
Weathering is the
physical or chemical
alteration of rocks
near or at the Earth’s
surface
As a rule: ROCKS are
NOT HAPPY BEING
EXPOSED TO THE
AIR!
Types of weathering:
Physical weathering: when a rock is broken down
into smaller parts by being acted on physically.
There are several types of physical weathering:
1.
2.
3.
Frost wedging (aka frost action)
Root wedging
Abrasion (aka sandblasting)
Frost Wedging
Frost wedging occurs
as a result of a
special property of
water. Nearly all
matter contracts or
shrinks when it is
cooled – water,
however EXPANDS!
• Definition: Frost wedging is physical weathering
where rock is split apart by the expansion of
water freezing in small cracks.
Some examples of frost wedging:
Root Wedging
Plants and trees send out roots to
seek water and nutrients from the
ground. These roots can move
into small spaces and cracks in
their search.
When roots end up in cracks in
rocks, they eventually grow larger
and can split the rock apart.
Definition: root wedging is physical weathering
caused by plant and tree roots splitting rock
apart.
Abrasion (including sand-blasting)
Wind and water can
carry tiny particles
of debris – rock and
dust.
When these particles
are washed against
or blown past rock,
it can wear the rock
down like sanding
wood.
• Definition: abrasion is physical weathering where small
particles moving in wind or water abrade rock, smoothing it
out.
Some rocks are abraded by windblown sand in arid regions.
Abrasion also happens at beaches and in rivers and
streams – here the particles doing the weathering
are carried by the water.
Types of weathering:
Chemical weathering: when a rock is broken
down into smaller parts by having its
chemical structure changed.
There are several types of chemical weathering. We will be most
concerned wtih:
1.
2.
Oxidation
Dissolution
Oxidation:
When oxygen is added
to the chemical
structure of a mineral
or rock, it changes the
physical structure of
the mineral or rock.
This is best seen in
rust.
Definition: oxidation is chemical weathering where
oxygen is added to other elements.
A chemical formula change might
look like this:
Iron + oxygen = iron oxide or….
Fe + O -> Fe2O3
Iron oxide is rust. Notice that while
iron is hard and strong, rust is not
– it easily breaks apart…
(Chemical weathering)
Dissolution
Many materials are
dissolved by water.
From salt and sugar
to carbon dioxide and
other gases, water
dissolves many
things.
Acids can also dissolve
materials.
Definition: dissolution is chemical weathering where
rock is dissolved by water or acid.
Factors that increase weathering:
• Exposure
• Particle size
• Mineral composition
• Climate
Why????? Think about it!
Meet with a partner, and talk about why each of
these things would increase weathering. Then
find another two people and share your ideas.
We’ll talk about them!