Download weathering-introduction jan2005

Introduction
Weathering is the decomposition and disintegration of rocks and
minerals at the earth's surface. Weathering results in the formation of
sediment that is carried away by erosion to form sedimentary rocks,
and regolith that remains in place to form soils. Weathering can be
subdivided into two types: physical (mechanical) and chemical.
Physical (Mechanical) Weathering
Physical weathering represents the disintegration of rocks and
minerals into smaller pieces. Physical weathering can be further
subdivided into pressure release and ice wedging.
Pressure Release
Rocks below the earth surface support the weight of the overlying column
of rock. Erosion strips away this overlying rock and decreases pressure on
buried rocks. All rocks are slightly elastic, so the buried rocks respond to
the reduction of pressure by expanding upwards. This results in the
formation of pressure release fractures (cracks) that form parallel to the
surface. With continued erosion, these rocks are exposed on the surface
and slabs of rock break off along the pressure release fractures. This
weathering creates bare rock surfaces that may be more resistant than
surrounding rocks. These features are termed exfoliation domes; the
slabs of rock that break off are termed exfoliation sheets.
Independence Rock, Wyoming; an example of an exfoliation dome. Note
bare rock and rounded surfaces. Image courtesy of Dr. Annabelle Foos.
Click here to see another example of an exfoliation dome, Half Dome,
Yosemite National Park, California.
Ice Wedging
Ice wedging occurs as the result of the expansion (9%) of water as it is
converted to ice. Cracks filled with water are forced further apart when it
freezes. This process requires a range of temperatures to generate
alternating freeze - thaw activity.
Both pressure release and ice wedging break up rocks and minerals,
creating more surface area for chemical weathering.
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Chemical Weathering
The decomposition of rock by the chemical breakdown of minerals.
Results in a change in rock composition; typically replacing strong
minerals with weaker minerals, thus hastening the break down of the
rock. The three common chemical reactions associated with chemical
weathering are dissolution, hydrolysis, and oxidation.
Dissolution
Dissolution occurs when rocks and/or minerals are dissolved by water.
The dissolved material is transported away leaving a space in the rock.
One consequence of this process is the formation of caves in limestone
areas.
rain + carbon dioxide (from air)  carbonic acid (reacts with rocks)
H2O + CO2  H2CO3
The details of marble statues and
tombstones will be gradually worn
away by dissolution. Image on right
courtesy of Dr. Annabelle Foos.
Carbon dioxide (CO2) from the air is dissolved in rainwater to create a
weak acid, carbonic acid (H2CO3), that preferentially dissolves certain
rocks and minerals, e.g. limestone, marble. All rain is mildly acidic
(average pH ~5.6, compared with neutral fluids [pH 7] and highly
acidic [pH 1] substances).
Caves form when dissolution occurs along a series of fractures in
limestone to create a larger opening. Water passing through the rock
enlarges the cave and associated re-precipitation can form a variety of
features. The dissolved limestone is transported through the cave and
may be precipitated to form new features such as stalagtites that grow
downward from the cave ceiling and stalagmites that grow up from the
floor. If they meet they form a compound cave formation such as a
column.
Crystal Ball Cave, Utah. Features hanging down from the ceiling are stalactites. Pillar-like
features on the cave floor are stalagmites. Both stalagmites and stalactites are formed from
the precipitation of limestone within the cave. Image courtesy of Dr. Ira Sasowsky.
The Virtual Cave website contains images and descriptions of a variety
of cave formations. You can review pictures of Lechuguilla Cave in
Carlsbad Caverns National Park and effectively take your own virtual
tour of the cave.
Not all the products of dissolution are below ground. Sinkholes form
at the surface from the collapse of the roof of an underlying cavern or
by dissolution of rock along a series of fracture surfaces.
Image on right
shows a sinkhole,
(Watlings Blue Hole)
in the Bahamas.
Road at bottom left
of image for scale.
Image courtesy of
Dr. Ira Sasowsky.
Hydrolysis
Carbonic acid ionizes (breaks down) into two ions, hydrogen (H+) and
bicarbonate (HCO3-1).
H2CO3  H+ + HCO3
The free hydrogen ions may alter mineral composition
by replacing other ions in a mineral’s atomic structure;
this reaction is termed hydrolysis. Hydrolysis occurs
when minerals react with water to form other products.
Feldspar, the most common mineral in rocks on the
earth's surface, reacts with water to form a secondary
mineral such as kaolinite (a type of clay) and additional
ions that are dissolved in water. The weaker clay is
readily worn away by physical weathering.
feldspar + hydrogen ions + water  clay + dissolved ions
4KAlSi3O8 + 4H+ + 2H2O  Al4Si4O10(OH)8 + 4K+ + 8SiO2
Oxidation
Oxygen, the second most common element in the air we breathe, reacts
with iron in minerals to form iron oxide minerals, e.g. hematite (rust). As
many minerals contain iron, it is not unusual to see red-colored rocks like
the example from Arches National Park shown here.
Delicate Arch,
Arches National
Park, Utah.
Physical weathering breaks rocks down into smaller pieces thus
increasing the surface area over which chemical weathering can occur
(see diagram). All forms of chemical weathering tend to promote the
decomposition of minerals in rocks to a less resistant form. Working in
concert, physical and chemical weathering can reduce a once resistant
rock to nothing (dissolved limestone) or to easily eroded weaker
materials (clays).
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Weathering Rates
Rock type, structure, and climate primarily control weathering
rates. Rocks composed of minerals that are relatively unaffected by
chemical weathering will be the most resistant to weathering. For
example, quartz is unaffected by dissolution, hydrolysis and oxidation,
therefore, rocks composed almost exclusively of the mineral quartz are
more resistant than other common rock types. Rocks such as
sandstone and quartzite often form resistant ridges separating valleys
formed in weaker rocks. Sand on a beach is made up almost
exclusively of quartz grains as chemical weathering alters less
resistant feldspar minerals and the resulting clays are deposited
elsewhere by streams or shoreline currents.
Rock structures such as fractures represent natural surfaces for
physical and chemical weathering processes. Consequently, rocks that
contain abundant fractures are typically weathered more rapidly than
equivalent unfractured rocks.
Weathering has been concentrated along vertical fractures separating these panels
of sandstone at Arches National Park, Utah.
A temperature range that varies between freezing and thawing
conditions and plentiful supply of water are essential for physical
weathering. Warm temperatures and a plentiful supply of water are
necessary for chemical weathering. Areas with little water or low
temperatures have relatively slow rates of chemical weathering.
How would climate
influence weathering
rates at locations on the
map?
1. Tropical rainforest (equatorial regions including South America,
Africa, Indonesia, southeast Asia): chemical weathering rates
would be rapid as these regions have both high temperatures and
plenty of rainfall.
2. Hot desert (subtropical regions including North Africa [Sahara],
southwest South America [Atacama], southwest Africa [Namib],
Asia [Gobi], southwestern U.S., central Australia): plenty of heat
but insufficient water to cause significant physical and/or chemical
weathering.
3. Temperate mountains (Rocky Mountains, Sierra Nevada
Mountains, Alps, Andes Mountains): insufficient temperatures for
rapid chemical weathering but elevations contribute to freeze-thaw
cycles necessary for ice wedging.
4. Polar Regions (Alaska, Antarctica, Siberia): too much cold
weather to permit thawing. Water in solid form (ice) unable to
react with rock.
Inscription carved in granite on Independence Rock, Wyoming. Settlers travelling on the
Oregon Trail aimed to arrive at the landmark before July 4 to ensure sufficient time to
cross the mountains before winter. The inscription is was carved by someone with the
surname Fox and is dated June 29, (18)52. The phrase CIN. Ohio can be seen in the top
right corner. The inscription is still preserved due to the dry climate of the region.