Transcript Weathering - Department of Geology

Weathering
Chapter-2
Sedimentary Geology
Weathering
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Granites-granodiorides
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San Lorenzo or Utuado- rock composed of:
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feldspars (plagioclase 30-50%, K-feldspars 5-35%)
quartz 5-10% or 25-30% in quartz rich granites.
The sand in the rivers is composed mostly of quartz
and feldspars are rare.
Most of the others minerals are weathered and their
elements ends up forming clay minerals
Weathering
Is the simple consequence of exposing pre-existing
rocks to the conditions at the Earth’s surface: low
temperature and pressure, organic activity, and
chemically active substances such as water and
atmospheric gases.
Two types : physical and chemical weathering are
the means by which pre-existing rocks and
minerals change and come into equilibrium with the
surface environment.
Sedimentary rocks will not exist without
weathering.
Physical weathering:
disintegrating rocks into
clasts
 Four major mechanism of physical weathering:
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Freeze-thaw
Insolation
Stress release (unloading)
Organic activity
Each process is slow and do the same, it turns
solid, difficult to erode rocks, into smaller,
movable, unconsolidated rocks and minerals.
Freeze-thaw (ice-wedging)
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Active agent is water
The catalyst is temperature changes that
occurs by hours, weeks, months, etc…
At 0° C (32 ° F) water freeze, a 9-10%
volume expansion occurs.
Water freezing along cracks and fissures
Dilatación por congelación (frost
wedging)
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- el agua se expande cerca
de 9% al congelarse. Cando
agua entra en grietas en las
rocas y luego baja la
temperatura el agua se
congela en la superficie de
la grieta primero y luego en
el resto de la grieta. Al
congelarse el agua ejerce
presión suficiente para
fracturar aun mas la roca y
hasta para separar
fragmentos.
In Puerto Rico….
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A similar process occurs in PR when salts
such as halite and gypsum crystallize in
cracks and crevices. Evaporation of the
water results in growth of the crystals in
the walls of the outcrops.
Insolation
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Refers to stresses generated when
minerals are exposed to changing
temperatures and undergo differential
thermal expansion and contraction.
This process is common in zone of
extreme temperature fluctuations such as
deserts.
In wetter climates
Stress release
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Occurs when rocks buried beneath overlying
material experience high confining pressures.
When eroded, overburden is removed,
pressures drop, and the rock mass expands.
Expansion cracks or joints develops roughly
parallel to the ground surface (onion skin-like).
Organic Activity
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organism that live on or in weathering bedrock promote
physical weathering.
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plant roots seek out small pockets of soil in weathered rocks.
as plant grows, the roots lengthens and thickens.
gradually prying apart the cracks.
microscopic and megascopic organism living
wthin soil fragment them further (ex. worms).
Otros tipos de Meteorización
mecánica
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El crecimiento de raíces
de plantas en las
fracturas ejerciendo
presión en la roca.
El caminar de las
animales en la
superficie ayuda a
romper algunos
fragmentos.
Chemical weathering reactions
1)
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Some constituents dissolve complitely
 ex. minerals such as calcite and halite
they are carried away by ground water and runoff and can be
precipitated elsewhere
2) Constituents such as feldspars and micas are altered into new
minerals (mostly clay minerals)
 It involve several simultaneous chemical reactions
 hydrolysis
 hydratation
 simple solution
 oxidation-reduction
these reactions proceed better in the presence of air and water.
Simple solution
 solid
mineral + acid/water = ions in solution
 bonds between ions in rigid crystalline lattice
are broken and ions are disseminated in solution
 ex.
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SiO2 + 2H2O> H4 SiO4
quarts + water > hydrosylicic acid
Ex.
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carbonic acid contain abundant hydrogen ions.
because their small size they have a strong affinity for
anions and displace other cations in minerals
structures
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H2O + CO2 > H2CO3
Water + carbon dioxide > carbonic acid
CaCO3 + H2CO3 > Ca2+ + 2HCO3lmst.+ rainfall>dissolved Ca+d. bicarbonate
Lmst dissolve as the hydrogen ions displace calcium
ions
Halite (NaCl) is extremely soluble. When sodium and
chlorine become dispersed the water develops a salty
taste.
 NaCl + H2O > Na+ +
Cl halite + water > ions dissolved in water
 the solubility of halite is enourmous compared with the
solubility of quartz
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quartz 6ppm, halite thosands ppm
Hydratation and dehydratation
(Solid mineral + water= New
hydrated mineral; dehydratation is
the reverse)
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Some weathering processes involve the
chemical combination of pre-existing minerals
with water (hydratation); or,
the removal of water from pre-existing
minerals (dehydratation)
CaSO4 •2H2O > CaSO4 + 2H2O
 gypsum > anhydrate + water
 Fe2O3 + 3H2O > 2Fe(OH) 3
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hematite + water> limonite (oxide in soil)
Hydrolisis:
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hidrogen ion + mineral with mobile cations =
entirely dissolved minerals or partially altered
minerals in which hydrogen ions replace
mobile ions that are put into solution.
The replacement of cations in a mineral
structure by hydrogen ions in the water or in
acid.
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Mg2SiO4 + 4H+ > 2Mg2+ + H4SiO4
Olivine+from water/acid>ions in
solution+dissolved silica
2CaMgSiO6+16H+>2Ca2++2Mg2++4H4SiO
4
Pyroxene+from w/a > ions in
solution+dissolved silica
Oxidation-Reduction
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Atmospheric Oxygen gains electrons and
is reduced as mineral constituents Lose
electrons and are oxidized, producing new
“rusted’ minerals.
Oxidation is the process by which an atom
loses electrons
Reduction is the process by which an
atom or ion gains electrons
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(Fe 2+)SiO6 + O2 + H2O> 4Fe3 +(OH)3
+H4SiO4
Pyroxene+atmosphericW/A>limonite+dissolved
silica
The Controls
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climate
High temperatures-chemical weathering is
more effective in warmer climates.
 Rainfall is important because chemical
weathering requires water.
 Hydrolysis and simple solution depend on the
availability of the ion hydrogen= pH
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pH
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pH- the concentration of H+ ions in
solution.
pH lower than 7 is acid
pH higher than 7 is alkaline
Natural water have pH between 4 and 9;
rain and streams pH of 4 - 6.5
Soils pH of 4 - 5
Sea-water pH higher than 8
Eh-redox potential
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Eh - Express the potential for either
oxidation or reduction
Redox potential- measurement of the state
of oxidation of the system. The value
includes magnitude and sign (+/-) the
higher the magnitude the more likely that
the particular ion or atom will be either
oxidized or reduced.