Transcript Weathering
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 cause rock to break along the 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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