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 until erosion carries them away
– 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 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 sheets along joints which
are parallel to the surface
– Caused by expansion of rock due to uplift and
removal of surface material that originally
buried 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 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
– Rock reacts with water, gases and solutions
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 - Carbonation
• Carbonation – Carbon dioxide (CO2) is
dissolved in water making carbonic acid
– Weak acid is formed when CO2 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