Wearing Down the Earth

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Wearing Down the Earth
Tectonic Processes - build up the Earths
crust Gradational Processes - wear down the
Earths crust
These two processes working together are trying
to achieve a hypothetical state where the land is
flat and smooth BASE LEVEL
However, this will never happen and it is the
reason why we have such a unique and varied
landscape.
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Three Processes of Gradation

• 1. WEATHERING first stage where rock is broken
  down
• Regolith known as rock which is broken down to
  form weathered material.
• 2. EROSION when weathered materials are moved
• 3. DEPOSITION when eroded material settles
  in a new location

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Two types of WEATHERING
I MECHANICAL physical wearing down of rock
without any change in composition -mostly in
cool, dry environments

• Frost Shattering polar regions and temperate
  climates
• - moisture seeps into cracks in
  rock, freezes, ice expands and widens
  crack, eventually breaking or splitting the
  rock apart.

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Frost Wedging
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b) Thermal Expansion rapid change in
temperature causes the rock to split
apart -often in desert regions where the
temperature drops overnight and warms
rapidly in the day
c) Exfoliation/Sheeting rocks within the
Earths crust were formed under pressure of
the rocks lying above them. When these rocks
are exposed, the pressure that held the rock
together is gone, so it falls apart. As a
result, the layers of the rock peel off.
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Independence Rock, Wyoming an example of an
exfoliation dome. Note bare rock and rounded
surfaces. Image courtesy of Dr. Annabelle Foos.
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• d) Plant/Animal action roots of plants may
  break up rock as they grow and spread,
  increasing pressure and widening cracks.
• -ants, earthworms and groundhogs loosen
  soil by burrowing. This allows water and
  air to penetrate more deeply and come more
  directly in contact with solid rock.
• -humans play a part too by excavating
  roads, digging mines an cultivating land.

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II Chemical Weathering weathering which changes
rock composition and decomposes rock- common in
warm, humid climates
a) Dissolution/Solution when dissolved carbon
dioxide from the atmosphere is dissolved in
water, it forms an acid that acts upon the rock
to chemically change it. Calcite carbonic acid
Calcium bicarbonate - The new compound is
carried away by water leaving the broken up
pieces of rock.
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Limestone and marble contain calcite and are
soluble in acidic water Marble tombstones and
carvings are particularly susceptible to chemical
weathering by dissolution. Note that the urn and
tops of ledges are heavily weathered, but the
inscriptions are somewhat sheltered and remain
legible.
Photo taken in an above-ground cemetery in New
Orleans
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Karst topography forms on limestone terrain and
is characterized by caves/caverns, sinkholes,
disappearing streams, springs
General view of karst topography, St. Paul Group,
Chambersburg Limestone. Pennsylvania, north of
Clear Spring, MD. Note small closed depressions.
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b) Oxidation Iron Water RUST -Oxygen
dissolved in water results in iron oxide. Then
it is dissolved in water and carried
away. -this is very common because many rocks
contain iron and thus is a common cause of
weathering. -it is easy to see because rocks
are discoloured yellowish-brown to rust red.
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Oxidation turned these rocks in Nevada's Valley
of Fire red.
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c) Hydrolysis when carbonic acid acts upon
silicates -ions of water replace ions of the
silicate so the rock falls apart. -water
soluble minerals are carried away but the clay
minerals are left behind.
Feldspar alters to clay (kaolinite) plus
dissolved materials (ions)
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Factors Influencing Weathering

1. Climate
2. Type of rock
3. Slope and topographical relief
4. Time

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Climatic influences on Reactions
Climatic influences on Reactions

Most chemical
processes occur
more rapidly at higher
temperatures and in
the presence of
liquids.

Chemical weathering
is more effective in
tropics than in arctic
or arid regions.
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Features of Weathering

• MAST WASTING removal of weathered material by
  the force of gravity.
• -either single particles or huge masses of rock
  may move anywhere from a few millimetres to
  thousands of metres.
• -Factors that influence the degree of mass
  wasting are slope, particle shape and size,
  depth of regolith, vegetative cover, tectonic
  stability and moisture content.

Slump, Route I-675 south of Atlanta, Georgia
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LANDSLIDES are a type of mass movement which
involve rapid movement of rock
particles -usually occur in mountains or steep
slope regions -sometimes triggered by
earthquakes -weathered debris is accumulated at
the base and is called talus or scree.
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EARTHFLOWS slow mass movements that usually
occur on steeply sloping, grass-covered
hills -caused by heavy saturation of ground by
rainstorms or spring thaw. -regolith can no
longer hold the weight of the slope. Mudflows
rapid earthflows -great velocity and density can
transport large objects for great
distances. Solufluction high latitude and high
altitude areas -permafrost is present and when
the surface layers melt in the summer they
become saturated, resulting in the melted
surface to slide over the frozen subsurface.
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USGS, Photograph by D. R. Crandell, 1980. A house
damaged by a mudflow (lahar) along the Toutle
River about 25 miles west-northwest of Mount St.
Helens
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CREEP slow mass movement of the surface of the
regolith -slope is less steep -produces no
obvious landforms but the cumulative effect of
the surface material may alter the land
substantially over a period of time. Examples
trees with curved trunks, roads or railways moved
out of alignment, gravestones tilted
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By A. McFadden
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