Transcript The Process of Weathering Rocks

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 strains the walls around the cracks and
causes cracks to deepen and widen.
• When the ice thaws, water can flow further into
the rock. Repeated freeze-thaw cycles weaken
the rocks.
• Over time, the rock breaks up along the cracks
into angular pieces.
Ice Wedging
Mechanical Weathering :
Exfoliation
• Rocks formed deep in the Earth are under a huge amount
of pressure.
• When erosion removes the overlying rock, the once
buried rocks are exposed and the pressure on them is
released.
• The outer rock parts tend to expand. The expansion sets
up stresses which cause fractures to form parallel to the
rock surface.
• Over time, sheets of rock break away from the exposed
rocks along the fractures.
Exfoliation
Mechanical Weathering:
Thermal
• Thermal weathering results from expansion or
contraction of rock, caused by extreme changes
in temperature.
• 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.
– Chemical reactions break down the bonds
holding the rocks together, causing them to fall
apart.
Chemical Weathering:
Oxidation
• Oxidation takes place when oxygen
combines with other elements in rocks to
form new types of rock.
• These new substances are usually much
softer and thus, easier for other forces to
break apart.
• 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.
– Most common reaction is the hydrolysis of
feldspars producing clay (kaolinite)
Hydrolysis
Chemical Weathering:
Carbonation
• Carbonation takes place when carbon dioxide
reacts with water or rain, forming a weak
carbonic acid.
– 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.
• The substances in rocks form new types of softer
substances.
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)