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


Oxidation


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



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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



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CarbonCycle andRock Cycle

Carbon Cycle and Rock Cycle



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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


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

1 - weathering

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