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Gradation and Weathering
Tectonic processes are constantly acting on the surface of the
earth in a long term and massive way to uplift, depress and
compress the earth’s surface.
Gradational forces are responsible for the finer detailed
shaping of the earth’s 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.
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 earth’s surface.
In motion the agents of gradation have kinetic energy that can
cause further breaking up of the earth’s surface.
The overall tendency of gradational forces is to lower the higher
parts of the earth’s 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 earth’s surface to level plains.
This action has been called peneplanation.
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.
Mechanical Weathering Processes
Freezing of water (apart from glaciers)
•also called frost wedging
•water’s 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
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.
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 world’s sandy deserts
Mechanical Weathering Processes
Water
• moving water
• Rain
• waves
• Rivers
• runoff
www.kfshrc.edu.sa/directory_services/ html/red_sands.html
Sun or salt spilt rock
http://geogweb.berkeley.edu/GeoImages/Wells/
geomorph/rock1.html
Mechanical Weathering Processes
The Wind
•This is self explanatory
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
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
impermeable layer
Salt Crystal Growth
volcano.und.nodak.edu/.../ north_america/Ban26.jpg
Mechanical Weathering Processes
Action of Plants and Animals
•plant roots help to widen cracks in the rock
•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
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
Exfoliation Domes
As erosion removes rock
material from the surface
of the land, the underlying rocks
are under less pressure.
As the pressure is lowered
on them, the rocks expand
upwards, creating fractures.
Slabs of rock then 'peel off'
along the fractures, creating
domical hills.
Famous 'Half Dome Mountain'
is an example of an
'exfoliation dome.'
http://geology.about.com/library/bl/images/blexfoliation.htm?terms=exfoliation
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.
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
CaCO2 + H2CO3
Ca(HCO3) 2
calcite
calcium bicarbonate
carbonic acid
•calcium bicarbonate is water soluble and easily carried away
•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?num=20&hl=en&q
=Karst+topography
Karst Topography
www.geologyone.com/esa/geopro/ karst/illustra.jpg
A Sink Hole!
Go to the site below to
see two animations that
illustrate
Karst Topography and
the creation of a Sink
Hole.
http://www.watersheds.org/blue/earth/hotearth.htm
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
4FeO
iron oxide
+
3O2
2Fe2O3
oxygen
iron hydroxide
Chemical Weathering Processes
Hydrolysis
•similar to solution (which is mostly underground), this
process occurs more on the surface and 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 (in rain) then reacts with silicates to form clay like
minerals, which are washed away
•the rock is weakened and falls apart or is rounded
•This is most common on buildings and grave stones.
The End!