Gradation and Mechanical
Weathering
Ms. Inden
Geography 12
Gradation
http://myweb.cwpost.liu.edu/vdivener/notes/stre
ams_basic.htm
What is Gradation?
• All activities of the earth's surface that build
up some areas and wear down others
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Rivers
Waves and currents
Glaciers
Wind
Biotic – roots, animals
Mass Wasting (gravity)
What is Base Level
http://geography.sierra.cc.ca.us/Booth/Physical/chp15_gradation/denud
ation.htm
• Gradational process
are trying to erode
the surface down
to base level
• Plate tectonics keep
the earth from
becoming
completely
smooth.
Wear down, fill in!
• Gradational
processes are
constantly trying
to wear down
high points on the
earth and to fill in
low points – like a
grader moving
along a gravel
road
• Wear down, fill
in!
Weathering
• Weathering is the
breaking up of
rock into smaller
pieces (sometimes
called regolith)
and/or changing
the rock
chemically
http://www.goodearthgraphics.com/virtcave/staltite/staltite.html
– Mechanical
weathering
– Chemical
weathering
– Biotic weathering
Erosion
• Weathering (decomposition of rocks) takes
place without movement (in situ), by heat,
water, ice, or pressure, and chemical
reactions
• Erosion is weathering and transportation
together (water, wind, ice and gravity)
Mechanical Weathering
Frost Shattering
Corrasion or Abrasion
Hydraulic Action
Exfolation
Insolation
Frost Shattering
Talus or
Scree Slope
-eroded rock
deposited at the
bottom of the
slope
• Frost shattering/frost wedging/frost action
– Water seeps into rock cracks
– Freezing water expands 9%
– Forces rocks to break apart
Freeze and thaw; repeat
Located on mountain tops in England,
dating back to the last ice age. Rock
breaks up in situ, forming blockfields or
blockslopes
– Most common in
temperate climates
where freezing occurs
overnight and thawing
occurs during the day
through much of the
year
– Not as common in the
arctic where it stays
frozen months at a
time – no freeze/thaw,
freeze/thaw
Link to blog showing frost
shattering
• http://kespilotgeography.blogspot.com/20
06/10/frost-shattering.html
Abrasion or corrasion
• Breaking down and grinding away of
material by collisions of moving particles
– Waves rubbing rocks against each other
– Rivers tumbling rocks along stream beds
– Glaciers scraping rocks along the ground or
bedrock
– Wind picking up sand and sandblasting
landforms
Abrasion or corrasion
http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/imag
e_preview.html?image
• Rocks and
sand
pounding
against the
cliffs have
created this
coastal arch
(and the sand
on the beach)
on the Dorset
Coast in
England
Glacial Abrasion
• Glacial striations or
scratches
• Glacier drags rocks
and gravel along the
surface of the bedrock
Hydraulic Action (water)
• Water currents rush into cracks
• Air is compressed as the water forces itself
in
• Water leaves as the wave recedes
• Air releases with explosive force
• Cracks widen
• Explosive action increases as the crack
widens
High water level
Chemical, biological and
Note the
spring –
moss in
March!
Hydraulic Action
• this can be as high as
6 tonnes/cm3 – the
force of a bulldozer
•
Exfoliation
• When a granite or
other igneous intrusion
is exposed through
erosion, the pressure
comes off and the rock
falls apart – it peels off
like layers of an onion.
Creates an exfoliation
dome.
• Happens in the
Canadian Shield,
although this one is in
Georgia
Onion Skin Weathering
• http://www.geointeractive.co.uk/contribut
ion/ppfiles/Onion%20skin%20weathering.
ppt#256,1,Onion skin weathering
• Insolation – caused by extremes of hot and
cold over a 24 hour period
Silt – created by mechanical
weathering
• Rock flour, stone dust
• Glaciers, sandblasting, river and wave
action
• Aeolian (created by wind) deposition of
silt called loess
Often all these processes are at
work at once
• http://www.bbc.co.uk/schools/gcsebitesize
/geography/coastal/coastalprocessesrev3.sh
tml
• Example – water eroding a cliff
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Hydraulic action
Chemical weathering
Attrition
Abrasion or corrasion