Shaping the Land: Lecture Notes
____________________________________________________________
Introduction:
The planet we live on is always changing.  Some major forces of change are volcanoes, earthquakes, weathering and erosion. 

The terms "weathering" and "erosion" don't sound very destructive.  You don't think of weathering and erosion causing massive land changes an powerful earth movements like volcanoes and earthquakes, but they are very powerful agents of destruction.

It is the intention of this short lesson to highlight the effects of weathering and erosion on the land around us. 

Weathering, Erosion and Denudation:
**(students may have these definitions in their textbooks already)

Weathering - the process by which rocks are broken down and decomposed by the action of external forces such as wind, rani, plants, bacteria and temperature change.

Weathering affects rocks in situ, no transportation  is involved for weathering to occur.  Weathering is the first stage of a process called denudation.

Erosion - the process of wearing away rocks and land surfaces by the mechanical action of transported debris. 

The agents of transportation (wind, water and ice) are, by themselves, only capable of minute wearing action on the rocks and landscape but, when they contain particles of weathered material, they become powerful agents of erosion.

Denudation - the sum combination of the processes of weathering and erosion which results in the general lowering of the land surface.

Weathering and erosion take place constantly.

There are two main types of weathering:

1.  Mechanical - this is the breaking down of rock into smaller pieces without any chemical changes in the rock itself.  This type of action can occur in a number of ways:

    a) Frost action - water seeps into cracks and pores in a rock and freezes, expands,
       exerts pressure within the crack or pore and causes pieces of the rock to break off.

    b) Wedging of plant roots - the roots of shrubs and trees work their way into smaller
        cracks in a rock causing the rock to split and crumble.

    c) Exfoliation or onion skin weathering - caused by excessive heating by the sun during
        the day followed by rapid cooling at night causing thin sheets and flakes of rock to
        split off.

2.  Chemical weathering - process where a chemical change takes place in the rock forming new products that can be carried away more easily than the original rock.  Areas where water is present or the air is very humid are ideal locations for chemical weathering.  Chemical weathering takes place in a variety of ways:

    a) One example of chemical weathering is the formation of carbonic acid.  When
        carbon dioxide from the air dissolves in water, a weak acid, carbonic acid, is
        formed.  This acid attacks rocks such as limestones and shales forming materials
        that dissolve in water and are carried away more easily than the original rock.

    b) Lichen growth is another form of chemical weathering.  Lichen are tiny plants
        that grow on rocks and produce an acid that attacks the rock and breaks it into
        smaller pieces.  Lichens use minerals from the rock to live and grow and produce
        rock-breaking acid.

    c) Another example of chemical weathering is oxidation.  Oxygen in the air combines
        directly with many minerals and rocks, forming new materials or rocks that can
        also change into another material or type of rock which crumbles moe easily than
        the original rock.  Ex. magnetite which combines with oxygen and changes into
        Hematite, which combines with water and changes into Limonite (try to get
        examples of these rock from the science department).