Weathering
Mechanical
and
Chemical
What Caused This?
What is Weathering?
• First step to forming soil and
sedimentary rock.
– Breakdown of rock into smaller pieces
– The smaller pieces do not move to a new
location, but remain next to one another until
erosion carries them away.
– Mostly occurs at the surface but can also
occur below ground
– Many types of weathering
What is Mechanical Weathering?
• Breakdown of rock into smaller pieces
without any change in the chemical
composition of its minerals.
– Sometimes called “physical” weathering
– Rock is torn apart by physical force, rather
than by chemical breakdown.
– Smaller pieces do not move to a new location,
but remain next to one another until erosion
carries them away.
Mechanical - Ice Wedging
 Ice Wedging
– Water fills joints of rocks and freezes
– Water expands 10% when it freezes, pushes
rock apart
– Repeated freeze and thaw cycles over the
years causes rock to break along joint
Mechanical - Exfoliation
• Exfoliation or unloading
– Rock breaks off into leaves or sheets along
joints which are parallel to the ground surface
– Caused by expansion of rock due to uplift and
removal of surface material that covered the
rock
Mechanical - Thermal
• Thermal expansion
– Repeated daily heating and cooling of rock
– Heat causes expansion; cooling causes
contraction
– Different minerals expand and contract at
different rates causing the rock to split
Mechanical - Biotic
• Biotic – means life
– Weathering caused by living organisms
– Plant roots most common cause, act as a
wedge and widen cracks
– Other causes of biotic weathering are digging
animals, microscopic plants and animals,
algae and fungi.
What is Chemical Weathering?
• Breakdown of rock into smaller pieces because
of change in the chemical composition of its
minerals.
– Chemical reactions break down the bonds holding
the rocks together, causing them to fall apart.
– Chemical weathering occurs in all types of rock
but smaller rocks are more susceptible because
they have a greater amount of surface area.
– Chemical weathering is more common in locations
with lots of water and heat speeds it up.
– Rock reacts with water, gases and solutions (may
be acidic); these reactions will add or remove
elements from minerals.
Chemical - Oxidation
• Oxidation - oxygen combines with other
elements in rocks to form new types of
rock
– New substances are usually much softer than
original, easier for other forces to break rock
apart
– Causes a “rusting” of the rock, often causes a
color change in the rock
Chemical - Hydrolysis
• Hydrolysis - water combines with the
substances in rocks to form new types of
substances
– New substances are softer than the original
rock types
– Most common reaction is the hydrolysis of
feldspars producing clay minerals
– e.g. feldspar
kaolinite
Chemical - Carbonation
• Carbonation – Carbon dioxide is
dissolved in water making carbonic acid
– Weak acid is formed when carbon dioxide in
the air mixes with rain. This is the same acid
found in soft drinks.
– Acid is too weak to harm plants and animals
but slowly causes feldspars and limestone to
decompose.
– Occurs when carbonic acid reacts with a rock
to form a new substance
Chemical - Biotic
• Organisms - can change the chemistry of
their environment
– Plants lower the local pH to make it more
acidic
– Bacteria act as catalysts in various
geochemical processes
Equilibrium
• The forces of weathering stabilize the build
up of new land formations that follows
volcano eruptions.