WEATHERING and MASS WASTING

Introduction

    As soon as bare rock is exposed to the atmosphere:

1 - Weathering: action of rain, wind and temperature changes to break down rock       - vital part of soil formation (see Class 18),       - here simply treated as creating material available for removal            (lowering mountains) 2 - Mass Wasting: universal down slope motion under gravity            (water may help by lubricating particles and decreasing friction)     This exposes new rock for weathering, and transports material for erosion. 3 - Erosion: wearing down of land surface and removal of material:     by various agents - running water, moving ice, wind, waves     separated this way because each agent gives distinctive landforms

N.B. The various terms and processes overlap somewhat. We are not concerned (in this case) with exact definitions, but with processes and results.

1 - Weathering

Two general types: a - Physical     Simple breakdown, usually by creation of physical stress: NO chemical change,     Dominant in dry climates, important in cold ones.     Usually gives jagged topography         Examples: Freeze/thaw action, action of growing roots, fire-induced expansion b - Chemical     Breakdown involving chemical change, usually making rock easier to erode     Usually requires (rain) water, so dominant in wet climates     Commonly gives smooth topography     Commonly soil formation is associated with this - weathering goes on 'under' soil         Examples: Hydrolysis (Water + organic acids)   North Carolina conditions     In general throughout state, chemical weathering dominates  In certain areas there are special conditions which dominate:

Limestone (calcium & magnesium carbonates):     Chemical weathering - extensive action since rock dissolved by water giving a variety of landscape features. In NC- small areas exposed in mountains (e.g. Linville Caverns, Coves of Great Smokies), giving many 'blind' valeys.  In other states (esp. Kentucky), larger areas exposed, giving Blue Grass region and Karst topography. (text Fig 13-19, p.390)   Physical weathering - limestone very resistant to physical weathering. Creates upstanding areas in deserts (shonw as resistant rock of Grand Canyon).

Granite: commonly creates steep slopes, with little soil or vegetation, leading to the exposure of bare rock to physical weathering.  Exfoliation is a common result.  This occurs, for example, in the mountains around Highlands (Macon County) - none of my pictures are currently available  (see - text Fig 13-11, p. 384 - NOT in NC!)

2 - Mass Wasting

    Removal of weathered material from slope

Types         Rockfalls - where physical weathering dominates.           Elsewhere soil & vegetation roots act as binding agents         Series of types: soil creep, slow, common             landslide: fast, unusual

    Slope Base        

        Initial collection at slope base

            Talus slopes in New England (ice action)

            Alluvial fans in deserts (rivers remove, then deposit physically weathered particles)

            Some colluvium  in NC mountains, development of thick (wet) soils at slope base

        Need for removal from slope base, or weathering stops

        Ready removal by running water in NC

    Human actions

            Weathering/mass wasting are natural processes

            Rates depend on slope steepness, rock type, soil & vegetation cover, climate type

                   -  Slopes come into rough general equilibrium with these factors

                 Human activity upsets this equilibrium:

                     steepen slope (e.g. remove base debris, create road cut)

                     change vegetation (e.g. remove forest & create landscaping with grass)

                     alter climate (water regime) (e.g. irrigate or drain - this especially on Coastal Plain)

                 Result: change in rate, and possibly type, of mass wasting

                       increasing possibility of all types, notably landslides

3 - Types of erosion

    Different Agents produce different types of features:

        all take weathered material and move and modify it

        creating both erosional and depositional landforms

    Running water: river action creating valleys  (Chapter 14)

        dominates NC landscape, gets most attention here - Classes 6 & 7

    Waves: coastal NC landscapes (Chapter 16)

        restricted to coast, but vital in that region - Class 8

    Glacial: Ice action gives 'glacial' landforms (Chapter 17)

        little impact for NC

        some frost action in mountains, but no real glacial landscape created, 

        (frost action important in Ice Ages, but now no real remnants)

    Eolian: wind actions works best in vegetation-free areas (Chapter 15)

         - typical of deserts, little NC impact

        In NC topsoil can be removed in dry periods, especially in spring before planting

Return to Course Schedule