weathering soils

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Weathering and Soils

Dr. R. B. Schultz


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Weathering and Soils

Weatheringrefers to numerous processes that convert bedrock exposed at earth's surfaceto smaller pieces that can be eroded by wind, rivers, waves or glaciers, as well asprocesses that chemically attack minerals in rock to produce a variety of dissolvedsubstances.

Weathering is an important part of the rock cycle, because weathering products are thesource for sedimentary deposits and sedimentary rocks.

Rock weathering also plays a role in: 1. Soil development 2. Landslide potential 3. Changes in rock strength 4. Surface water chemistry.

This section examines the common processes that weather rock and how soils can formfrom weathered rock.

There are two (2) types of weathering:

Physical-- a physical process occurs to make the change; processes that break rock apartChemical-- chemical reactions between minerals and water/air

Physical & chemical weathering work together in concert to break down rock.

The Tetons in Jackson Hole, Wyoming are an example of how physical and chemicalweathering create the landscape that we see around us.


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Chemical Weathering Processes: HydrolysisReactions between minerals and water, e.g. feldspar weathers to clay.Products: material in solution and new mineral.Rain is naturally acidic because carbon dioxide in the atmosphere dissolves in water to

form carbonic acid.

Chemical Weathering Processes: DissolutionDissolution -- mineral dissolves completely in acidic watere.g. calcite dissolves easily in weakly acidic water

Chemical Weathering Processes: Oxidation

Mineral (usually iron- bearing) reacts with oxygen to form oxide (iron oxide) or "rust"Mafic silicates like olivine, pyroxene, and amphibole weather readily due to oxidation

Physical Weathering Processes: Freeze-thaw or frost wedgingIce grows in cracksIce volume is 9% greater than waterVolume increase exerts pressure that can widen cracks, break off pieces of rock

Produces large, angular blocks of rock called talus

Physical Weathering Processes: Pressure releasedue to unloadingRock splits due to expansion when material above or alongside it is removedCan result from natural erosion or human excavation. Fractures called joints develop


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Fractures called

joints develop in

weathered rock


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Factors that influence weathering

1. Climate -- more precipitation --> more weathering2. Organisms -- can aid in breakdown of rock, e.g. burrowing organisms, tree

roots3. Rock type -- some minerals (and therefore rocks) weather more easily thanothers

4. Time -- the longer rocks are exposed to surface weathering, the moreweathered they become

Influence of rock type on weathering

1. Olivine, pyroxene, and amphibole form at high temperature and pressure.They are very unstable at surface T & P, and weather easily.

2. Igneous rocks that contain lots of mafic silicates weather more easily thanones that contain felsic silicates.

3. Limestone is very easily weathered.4. Shales weather more easily than sandstone, which is composed mostly of

quartz.5. More water or fluid means quicker and more weathering6. Warmer temperatures speed up reactions and act as a catalyst to

weathering processes7. Smaller fragments weather more quickly than large ones because more

surface area id exposed


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SOILS

* Forms on weathered material but are distinctly different from weathered material

What is soil?*Loose mixture of weathered rock, organic matter, water, and air that is chemically and

physically different from parent material

*Soils have distinct zones that roughly parallel land surface. Call these zones horizons.

Typical soil profile

O horizon -- abundant organic matter; dark

A horizon -- contains some organic matter, minerals, but many minerals are removed fromthis layer; Lighter-colored than O (Zone of Leaching)

B horizon -- contains material removed from A horizon: clays, iron oxides, calcite; may bered, yellow, or tan (Zone of Accumulation)

C horizon -- slightly altered parent material

3 Soil Groups -- Forming in different climates (depending on precipitation)

Pedalfer -- contain Al and Fe oxides in B-horizon, no calcite. Form in humid climate wherecalcite dissolves away.

Pedocals -- contain calcite in B-horizon; form in dry climates.

Laterites -- contain Al or Fe oxides, everything else is dissolved away; forms in tropicalclimates


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Typical Soil Profile

O Horizon

A Horizon

B Horizon

C Horizon


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Typical Soil Profile

development in an

arid region

E Horizon:

Eluviation; calcite-

rich, only in aridregions.


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Key Terminology

Physical weathering Chemical weathering

Hydrolysis DissolutionOxidation Freeze-thaw/Frost wedging

Pressure Release Joints

Mineral stability Soil horizon

Soil profile O HorizonA Horizon B Horizon

C Horizon Zone of leaching

Zone of accumulation Pedalfer

Pedocal Laterite

E Horizon Zone of eluviation


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National Cooperative Soil Survey

National Cooperative Soil Survey provides a dynamic resource of soils information for a wide range ofneeds.

National Landslide Information Center (USGS)The National Landslide Information Center (NLIC) is responsible for distributing information about

landslides to the lay public, researchers, planners, and local, state, and federal agencies.

Planet Earth (Discovery.com)The Discovery Planet Earth site features a variety of interesting information, reports, and links on Earth

science and related subjects.

Recent Landslide Events (USGS)A listing with descriptions and explanations of recent landslide events from the United States GeologicalSurvey (USGS).

Soil Erosion Research LaboratoryThe United States Department of Agriculture's (USDA) National Soil Erosion Laboratory home page.

Soil Profile Photo GallerySoil profile photographs from the United States Department of Agriculture.

Soil Science JournalSoils Online

This soil science online journal page offers many links to related Web sites.Soils and Substrates: World-Wide Web Virtual Library

An informative site that includes a comprehensive listing of soils related WWW sites.

Soils and Soil Science LinksSoils and soil science links from National Soil Survey Center, Iowa State University.

Soils LinksThis Central Michigan University site maintained by Dr. Mark Francek lists several good links to Web sites

containing information relative to topics discussed in the chapter.

Pertinent Web Sites
http://www.statlab.iastate.edu/soils/nssc/http://landslides.usgs.gov/html_files/nlicsun.shtmlhttp://www.discovery.com/guides/earth/earth.htmlhttp://landslides.usgs.gov/html_files/landslides/newsinfo.shtmlhttp://topsoil.nserl.purdue.edu/nserlweb/http://www.statlab.iastate.edu/soils/photogal/http://www.hintze-online.com/sos/soils-online.htmlhttp://www.hintze-online.com/sos/soils-online.htmlhttp://www.hintze-online.com/sos/soils-online.htmlhttp://www.metla.fi/info/vlib/soils/http://www.statlab.iastate.edu/soils/nssc/other.htmlhttp://personal.cmich.edu/~franc1m/soil.htmhttp://personal.cmich.edu/~franc1m/soil.htmhttp://www.statlab.iastate.edu/soils/nssc/other.htmlhttp://www.metla.fi/info/vlib/soils/http://www.metla.fi/info/vlib/soils/http://www.metla.fi/info/vlib/soils/http://www.hintze-online.com/sos/soils-online.htmlhttp://www.hintze-online.com/sos/soils-online.htmlhttp://www.hintze-online.com/sos/soils-online.htmlhttp://www.statlab.iastate.edu/soils/photogal/http://topsoil.nserl.purdue.edu/nserlweb/http://landslides.usgs.gov/html_files/landslides/newsinfo.shtmlhttp://www.discovery.com/guides/earth/earth.htmlhttp://landslides.usgs.gov/html_files/nlicsun.shtmlhttp://www.statlab.iastate.edu/soils/nssc/


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Soil Science Society of America

The Soil Science Society of America (SSSA) is the professional home for over 6,000 professionalsthroughout the world dedicated to the advancement of soil science. The primary purpose of the

Society is to advance the discipline and practice of soil science by acquiring and disseminating

information about soils in relation to crop production, environmental quality, ecosystem sustainability,

bioremediation, waste management and recycling, and wise land use.

Soil Web Sites

Links to soil related sites from the National Soil Survey Center (NSSC-USDA).

U.S. Department of Agriculture

Weathering and Soils Links (Houghton Mifflin)

Links to several weathering and soils sites, including class lecture notes, arranged by topic.

Weathering ImagesSeveral good photographs of various weathering phenomena hosted by Duke University.
http://www.soils.org/http://www.statlab.iastate.edu/soils/nssc/other.htmlhttp://www.usda.gov/http://www.geologylink.com/toc/chap5.htmlhttp://www.geo.duke.edu/geo41/wea.htmhttp://www.geo.duke.edu/geo41/wea.htmhttp://www.geologylink.com/toc/chap5.htmlhttp://www.usda.gov/http://www.statlab.iastate.edu/soils/nssc/other.htmlhttp://www.soils.org/

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