Gradation and Weathering

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Gradation and Weathering
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Tectonic processes are constantly acting on the
surface of the earth in a long term and massive
way to uplift, depress and compress the earths
surface. Gradational forces are responsible for
the finer detailed shaping of the earths surface
that we see on a day-to-day basis. The hills,
valley, mountains, shorelines, etc. that result
form these gradational processes are called
landforms and the study of landforms is called
geomorphology. Gradational process are driven by
solar energy, producing wind, rain, snow, ice,
and waves, which act to break up the surface of
the earth.
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A second energy source - gravity - then acts with
the agents of gradation to move the broken up
material from higher areas to lower areas of the
earths surface. In motion the agents of
gradation have kinetic energy that can cause
further breaking up of the earths surface. The
overall tendency of gradational forces is to
lower the higher parts of the earths surface and
to fill in the lower parts to reduce and then
eliminate differences in relief. If tectonic
processes were to stop, then gradational forces
would ultimately reduce the earths surface to
level plains. This action has been called
peneplanation.
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Weathering is the first step in gradation. It is
all processes that cause rocks to decompose and
disintegrate into what is called regolith, which
can then be acted upon by flowing water, wind and
so on. There are two broad categories of
processes that act to weather rock mechanical
and chemical. Both are a function of
climate. Mechanical causes rocks to
disintegrate. Chemical causes rock to decompose.
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Mechanical Weathering Processes

• Freezing of water (apart from glaciers)
• also called frost wedging
• waters volume expands by 10 when it freezes
• exerts tremendous pressure
• especially important at a very small scale but
  also moves large boulders
• effective in locations where temperatures change
  frequently to either side of 00, eg., in
  mountains day to night

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http//www.geo.wvu.edu/wilson/Geol1/lec17/lec171.
htm
Frost wedging has fragmented this outcrop of
jointed rock occupied by an Adelie penguin
Antarctic Peninsula, Antarctica.
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Mechanical Weathering Processes

• Changes in Temperature
• occurs where rock surfaces are exposed to diurnal
  (daily) heating and cooling
• rock minerals, because they have different
  combinations of elements, have different thermal
  expansion and contraction rates
• this causes internal stresses and eventually the
  rock crumbles
• slow process
• responsible for most of the worlds sandy deserts

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www.kfshrc.edu.sa/directory_services/
html/red_sands.html
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Sun or salt spilt rock
http//geogweb.berkeley.edu/GeoImages/Wells/geomor
ph/rock1.html
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Mechanical Weathering Processes

• Salt Crystal Growth
• occurs in dry climate areas, usually in sandstone
• during drought periods, water is drawn to the
  surface by capillary action and it carries with
  it dissolved mineral salt from the rock
• at the surface the water evaporates but the salt
  precipitates
• salt crystals slowly grow, and force the rock
  grains apart
• in the arid south-west of the US huge salt caves
  formed at the base of large cliffs, later
  inhabited by the Pueblo Indians

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large sandstone cliff
rain falls
water seeps through porous rock
hard impervious layer impedes downward movement
waterdrawn to surface by capillary action
as water evaporates, salt crystals force grains
of sandstone apart
wind blows them away
cave gets bigger
Salt Crystal Growth
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volcano.und.nodak.edu/.../ north_america/Ban26.jpg
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Mechanical Weathering Processes

• Action of Plants and Animals
• plant roots help to widen cracks in the rock,
  called root wedging
• especially important is the action of tiny root
  hairs of small plants, eg., lichens, mosses
• animals act to aerate the soil down to the
  bedrock, exposing it to other mechanical and
  chemical processes
• plant mechanical action is works with plant
  chemical action

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Mechanical Weathering Processes

• Release of Pressure
• also called unloading
• when surface layers are removed newly exposed
  rock often can expand due to the release of
  overlying weight and the rock cracks
• happens often in quarries
• can create exfoliation domes where large sheets
  of rock crack and peel away, like an onion

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http//geology.about.com/library/bl/images/blexfol
iation.htm?termsexfoliation
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Mechanical weathering can affect chemical
weathering. When you break a large block of
material into smaller pieces you create more
exposed surface area for the agents of chemical
weathering to act on. The block on the left has
a volume of 1 cubic centimetre and a total
surface area of 6 square centimetres. After it
has been weathered into 8 equal blocks, the
material still only has a volume of 1 cubic
centimetre, but what is its surface area?
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Chemical Weathering Processes

• minerals in rock come in contact with CO2, H2O,
  or O2 to form new minerals that are either larger
  in volume or softer and more water soluble
• there are three basic types of chemical
  weathering processes solution, oxidation, and
  hydrolysis.

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Chemical Weathering Processes

• Solution
• dissolved carbon dioxide from atmosphere and
  organic acids from decaying plant and animal
  matter are added to rain water to form a weak
  carbonic acid
• the acid reacts on basic rocks such as limestone
• certain minerals like calcite are dissolved and
  carried away in solution

• calcium bicarbonate is water soluble and easily
  carried away

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• eventually the limestone is completely removed
  leaving caverns, sink holes and even underground
  rivers
• where limestone is widespread in a region, a
  distinctive type of typography called Karst is
  produced

http//images.google.com/images?num20hlenqKar
sttopography
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Karst Topography
www.geologyone.com/esa/geopro/ karst/illustra.jpg
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Stalactites hang from the ceiling and stalagmites
grow from the floor of Carlsbad Caverns, NM
c ceiling
g ground
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Form when the limestone is dissolved away from
below, removing the support for the surface and
it collapses in. Sinkholes are commonly
funnel-shaped and broadly open upward.
Sinkhole, Cape Breton, Canada
They may be a few metres to more than 35m in
depth, though usually ranging from 3 to 10 m.
Sinkhole sizes range from a few square metres to
several hectares in area. Streams flowing along
the surface may enter a sinkhole as a
disappearing stream and flow underground for some
distance to reappear at the surface.
http//www.uwsp.edu/geo/faculty/ritter/geog101/tex
tbook/mass_movement_weathering/karst_landscapes.ht
ml
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Chemical Weathering Processes

• Oxidation
• results from the reaction caused by iron bearing
  minerals and oxygen dissolved in water
• the iron is changed into iron hydroxide, what is
  commonly called rusting
• there is a lot of iron in crustal rocks and
  therefore this is a common form of weathering
• rock discolours to yellow-brown or reddish-brown

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Chemical Weathering Processes

• Hydrolysis
• similar to solution, this process affects mainly
  the silicate group of rock forming minerals - one
  of the most common rock forming minerals
• water in the atmosphere forms weak carbonic acid
  in the presence of carbon dioxide
• this acid then reacts with silicates to form clay
  minerals, which are washed away
• the rock is weakened and falls apart - soil
  starts to form
• in the tropics hydrolysis has created soils to
  amazing depths but which are largely infertile
  because the minerals have been washed away