The Process of
Weathering Rocks
Weathering
• The breaking down of rock into smaller
pieces that remain next to each other.
• Weathering forms sediments.
• There are two types of weathering.
Two Types of Weathering
1. Mechanical weathering is the breaking down
of rock without any change in the chemical
composition of the rock.
– 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 Weathering:
Ice Wedging
• Water enters the cracks in rocks and then freezes
and expands about 10%.
• The ice puts stress on the walls around the cracks
and causes cracks to get deeper and wider.
• Water can flow deeper into the rock as the ice
thaws. As the cycle repeats, rocks grow weaker
over time.
• Along the cracks, the rock will break into angular
pieces over time.
Ice Wedging
Mechanical Weathering :
Exfoliation
• Rocks formed deep in the Earth are under a huge amount
of pressure.
• When the overlying rock is removed by erosion, pressure
is released and the once buried rocks are exposed.
• Expansion occurs along the outer parts of rocks. This
stress will cause fractures to form parallel to the rock
surface.
• Along the exposed rock fractures, sheets of rock will break
away.
Exfoliation
Mechanical Weathering:
Thermal
• Thermal weathering is a result of extreme
changes in temperature that causes the expansion
or contraction of rock.
• Common in deserts, where it is hot in the day and
cold at night; different minerals expand and
contract at different rates causing the rock to split
• The outer layers peel off into thin sheets.
Thermal
Mechanical Weathering :
Biotic (Force)
• Biotic weathering is caused by living
organisms.
• Most often, plant roots are the cause.
• They act as a wedge, widening and
extending the cracks.
• Digging animals can also cause weathering.
Biotic (Physical Force)
Two Types of Weathering
2. Chemical weathering is the breaking down of
rock into smaller pieces because of chemical
changes within the rock.
– Rock reacts with water, gases, and solutions (may
be acidic); these reactions will add or remove
elements from minerals.
– Rocks fall apart because the bonds holding them
together are broken down by chemical reactions.
Chemical Weathering:
Oxidation
• When oxygen combines with other elements
in rocks, a chemical reaction known as
oxidation occurs and new types of rock are
formed.
• The new types of rock are easier to break apart
because they are much softer than the original
substances.
• A reddish-brown coloration on the surface is
called rusting.
Oxidation
Chemical Weathering:
Hydrolysis
• Hydrolysis is the weathering reaction that
occurs when water and chemical
compounds in a rock meet.
• This results in the decomposition of the rock
surface by forming new compounds.
– The most common reaction is the hydrolysis
of feldspars producing clay (kaolinite).
Hydrolysis
Chemical Weathering:
Carbonation
• When carbon dioxide reacts with water or rain,
carbonation occurs and a weak carbonic acid is
formed.
– This is the same acid found in soda.
– The acid is too weak to harm plants and
animals, but slowly causes feldspars and
limestone to decompose.
• New types of softer substances are formed within
the rocks.
Carbonation
Chemical Weathering:
Biotic (Acid)
• Lichens and similar plants live on the
surfaces of rocks.
– Plants lower the local pH to make it more acidic.
• Their roots give off a chemical that dissolves
rocks and minerals.
Chemical Weathering:
Biotic (Acid)