1 Chapter 5: “Weathering, Soil, and Mass Movements” 5.1: “Weathering”
1 - Weathering
Transcript of 1 - Weathering
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The Production of Sediment
Chapters 1, 3
Contents
Weathering Physical, chemical, biogeochemical
processes
Rates
Products
Carbon cycle and global change
Erosion/Soils Sediment Texture
Weathering
General process by which rocks arebroken down at the earths surface
Produces clays, soils, dissolvedsubstances in rivers
Operates on igneous, sedimentary &metamorphic rocks
2 types: Chemical & Physical (usuallywork together)
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Weathering
Physical weathering
Solid rock affected by physical processes
that do not change its chemicalcomposition
Weathering
Physical weathering
Frost wedging
Thermal effects (fires, daily/annual temperature)
Salt/mineral crystallization in fractures (arid regions)
Wetting and drying
Release of overburden pressure/unloading (uplift of crust)
Organisms (incl. humans)
Grinding and smashing action of river and ocean water
Abrasive action of wind-blown sand
Plucking by glaciers
Press and Siever, 2001
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Weathering
Chemical weathering
Changes that alter both chemical andmineralogical composition of rocks
Water (and dissolved gasses) plays a keyrole
Boggs
Chemical Weathering
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Hydrolysis
Press and Siever, 2001
Oxidation
Press and Siever, 2001
Boggs
Hey! Its Bowens reaction series!
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What happens whengranite is weathered?http://www.gpc.peachnet.edu/~pgore/geology/historical_lab/weathering.php
Unweathered granitecontains these minerals:
Na plagioclase
feldspar (white) K feldspar (pink, but
may be white inother granites)
Quartz (gray)
Small amounts ofbiotite and/oramphibole (black)
and sometimesmuscovite (notshown)
What happens when granite isweathered?
Here is what will happen to each of the mineralconstituents in a granite under warm, humid
weathering conditions: The feldspars will undergo hydrolysis to form kaolinite
(clay) and Na and K ions
The sodium and potassium ions will be removed throughleaching and will be carried in solution in running water
The biotite and/or amphibole will undergo hydrolysis toform clay, and oxidation to form iron oxides.
The quartz (and muscovite, if present) will remain as
residual minerals because they are very resistant toweathering.
Weathering
Chemical and physical weathering work together
Press and Siever, 2001
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Weathering
Biogeochemical weathering
Boggs doesnt really emphasize the role of
(micro)organisms in promoting weathering
New field
Barker and Bamfield 1998
Weathering Products
Weathering generates three products:
Source-rock residues: chemically resistantminerals and rock fragments
Secondary minerals formed in situ
Soluble constituents
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Weathering Products
Grain size of particles governed by:
the original grain size of the parent rock
and
the completeness of the weatheringprocess
E.g., granites yield sand-sized quartzcrystals whereas siltstones yield rockfragments
Weathering Products
Boggs 2001
Weathering
Rates depend on a variety of factors
Press and Siever, 2001
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CarbonCycle andRock Cycle
Carbon Cycle and Rock Cycle
Press and Siever, 2001
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Erosion
The set of processes that loosen andmove soil and rock downhill or
downwind
See next chapter for more detail
Moves weathered material from earthssurface exposes fresh rock toweathering
Erosion
Agents of erosion (some): gravity (mass-wasting)
flowing water
wind
ice (glaciers)
waves along coasts
Erosion is a natural process which can be
accelerated by human activities Associated with, but different to weathering
Weathering vsErosion
Weathering:
General process bywhich rocks arebroken down at theearths surface
Erosion:
The set of processesthat loosen and movesoil and rock downhill,downwind ordowncurrent
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Soil
Solid weathering products that are notremoved by erosion can form soil abovebedrock
Eroded material may accumulateelsewhere to form soils
Soil
Unconsolidated deposits that support plantlife (geologists definition)
Composed of:
Fragments of bedrock (boulders -> silt in size)
Clay minerals (from weathering)
Dead organic material (roots, leaf litter,organisms)
In the absence of plant life, it is calledregolith
E.g., Moon, Mars
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Soil
Takes time to develop
Factors: climate
topography
parent material
organisms
time and geologic history
surface water and groundwater
Press and Siever, 2001
Sediment Texture
Small-scale features that arise from thesize, shape and orientation of individualsediment grains
Sediment texture reflects nature ofsediment transport and depositionalprocesses
Useful for interpreting depositionalenvironment
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Sediment Texture
Primary interest:
Grain size (average, sorting, etc.)
Particle shape (form, roundness, surfacetexture)
Fabric (grain orientation, inter-particlecontacts)
These variables help to determine bulkdensity, porosity and permeability
Udden-Wentworth scale
= -log2dExample 1: What is phi value of .125 mm?
= -log20.125= 3
Example 2: What is mm size of 1.5 ?1.5 = -log2d
2-1.5 = d= 0.35mm
Phi scaleSediment Texture
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Sorting of consolidated deposits is typically
evaluated visually (see above)
Sorting of unconsolidated deposits is
defined using statistical analyses of grain-
size data
Other Factors
Roundess (angular->well rounded)
Sphericity/shape (disk, sphere, etc.)
Surface texture (SEM images)
Fabric (orientation, packing)
Sediment Texture
Grain Fabric
Imbrication
OR Rapid
deposition
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Summary
Weathering - General process by whichrocks are broken down at the earths
surface Physical, chemical, biological aspects
Products include solid particles anddissolved substances
Rates variable
Plays role in global carbon cycle
Summary
Erosion - The set of processes thatloosen and move soil and rock downhill,downwind or downcurrent
Variety of processes
Soil unconsolidated materials
Can accumulate in situabove weathered
rocks or form from eroded/transportedmaterial
Summary
Sediment textural parameters arestudied to help determine environmentof deposition
Also influence properties such as porosity,permeability
Phi scale & Udden-Wentworth scaleused to describe particle sizes