Mechanical
and
Chemical
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http://en.wikipedia.org/wiki/File:KharazaArch.jpg
• 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
 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.
 Ice Wedging
 Water fills joints of rocks and freezes.
 Water expands 10% when it freezes, pushing
rock apart.
 Repeated freeze and thaw cycles over the
years causes rock to break along joint.
http://upload.wikimedia.org/wikipedia/commons/9/90/Permafrost_pattern.jpg
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
http://commons.wikimedia.org/wiki/File:Exfoliation_and_mass
_movement.jpg
 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.
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• Biotic – means life
– Weathering caused by living organisms
– Plant roots act as a wedge and widen cracks.
– Other causes of biotic weathering:
• burrowing animals
• microscopic plants
• animals
• algae
• fungi
http://commons.wikimedia.org/wiki/File:Wedge_Rock_Turkey_Run_SP,_IN.jpg
• 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.
 Oxidation - oxygen combines with other
elements in rocks to form new types of
rock
 New substances due to oxidation:
– rock softer than original
– easier for other forces to break rock apart
– “rusting” of the rock
– color change in the
rock
http://commons.wikimedia.org/wiki/File:Sandsteinformation_in_garden_of_
the_gods.jpg
• Carbonation – Carbon dioxide (CO2) is
dissolved in water making carbonic acid
– ACIDS
– Weak acid is formed when CO2 in the air mixes with
rain.
– Acid is too weak to harm plants and animals but
slowly causes feldspars and limestone to
decompose.
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