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WEATHERING PROCESSES

A. Introduction

Weathering important because:

Weathering: in-situ breakdown of material

Learning outcomes: you should be able to:


Photo: talus conesB. Physical Weathering

Frost action

(Talus cones, Banff National Park, Alberta. Photo: Marli Miller, University of Oregon. http://marlimillerphoto.com/talus.html)

Salt weathering

  • salt crystallization: occurs as saline solutions evaporate

  • salt crystal expansion: occurs when salt crystals get wet

  • occurrence:

    • hot and cold arid and semi-arid environments

      • capillary rise brings saline water toward surface

      • limited liquid water (either due to supply or phase) incapable of washing salts away

      • hot arid regions: large diurnal changes in temperature and relative humidity promote repeated wetting and drying

      • cold regions: cold temperatures encourage salt precipitation from solutions

    • rocky coastal areas

  • rock susceptibility to salt weathering

    • proportion of micropores

    • water absorption capacity

    • surface texture

    • presence of clay minerals

  • landforms

    • tafoni

    • honeycomb weathering

    • granular disintegration

    • spalling

Photo: honeycomb weathering
Honeycomb weathering in greywacke sandstone, Golden Gate National Recreation Area. Photo: National Park Service, http://www.nps.gov/goga/forteachers/graywacke-sandstone-faq.htm
Photo: tafoni
Photo: K. Segerstrom, USGS Photographic Library image sk000626. http://libraryphoto.cr.usgs.gov/
Photo: honeycomb weathering
Photo: R.C. Moore, USGS Photographic Library, Image mrc00098. http://libraryphoto.cr.usgs.gov/

Wetting & drying

  • susceptible soils & rocks:

    • soils with 2:1 layered clays (e.g. montmorillonite)

    • shale, clayey siltstones and sandstones, granite

  • result: spalling, granular disintegration

Photo: granular disintegration
Photo: Marli Miller, University of Oregion. Earth Science World Image Bank, photo hhrhuz, http://www.earthscienceworld.org/

Thermal expansion and contraction

  • different minerals have different coefficients of thermal expansion

    • e.g. quartz is about 3 times that of feldspar

  • effectiveness of insolation debated

  • result: spalling

Photo: spalling
Photo: J.R. Stacey, USGS Photographic Library, Image hcb00980. http://libraryphoto.cr.usgs.gov/

Pressure release

  • sheet joints and exfoliation

  • rock bursts in deep mines

Photo: sheet joints
Photo: F.E. Matthes, USGS Photographic Library, Image mfe00007. http://libraryphoto.cr.usgs.gov/
Photo: Half Dome, Yosemite
Half Dome, Yosemite. Photo: F.E. Matthes, USGS Photographic Library, Image mfe00001. http://libraryphoto.cr.usgs.gov/

Photo: sheet joints
Sheet joints, Yosemite National Park. Photo: N.K. Huber, USGS Photographic Library, Image hnk00031. http://libraryphoto.cr.usgs.gov/


C. Chemical Weathering

Introduction

Solution

  • virtually all chemical weathering involves some solution

  • Diagram: dipolar water molecule

  • solution of calcite (CaCO3) and halite (NaCl)

Diagram: calcite solution

Diagram: solution sodium   

Diagram: solution cloride

Hydrolysis

Oxidation/reduction

Cation exchange


D. Patterns to Weathering

Diagram: weathering & climateModel based on global air temperature patterns and global precipitation patterns

Amount or rate of weathering


E. Summary


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©Karen A. Lemke: klemke@uwsp.edu
Last revised October 29, 2015