Physical Geography

Land and Water FormsWearing Down Landforms

DENUDATIONAL PROCESSES • Natural forces gradually wear down and change Earth’s topographical

features:– Weathering is the breakdown of rocks and minerals by physical pressures and

chemical reactions.– Erosion includes two processes: 1) the breakdown of rocks and minerals, but

also 2) the moving (transportation) of weathered material from its source to another location (deposition).

– Denudation refers to the laying bare or wearing down of underlying rocks by the processes of weathering and erosion.

WEATHERING • There are two types of weathering that affect landforms,

breaking rock down into fragments, or further disintegrating them into sand or soil:

• Physical weathering is the break down of rock through mechanical (physical) processes in the environment. Physical weathering may occur in the following five ways:– Frost fracture: the expansion of freezing water that causes rocks

to crack.– Heat expansion: rocks can expand and subsequently fracture.– Exfoliation: the breaking of rounded sheets of rock along curved

surfaces. Often caused by the release of internal pressure.– Plant growth: expansion of rock due to root growth.– Burrowing animals: tunnelling animals can increase of the size of

existing cracks in rock.

• Chemical weathering is the break down of rock through chemical change, usually involving the action of rainwater. Chemical weathering may occur in the following three ways:– Solution: occurs when rainwater absorbs carbon dioxide or

sulphur dioxide from the atmosphere along with organic acids from the soil, which then reacts with rock. Acids, such as these, often dissolve limestone, breaking it down completely, forming caves or caverns in rock.

– Hydrolysis: involves minerals reacting in solution (water ions replace ions of a mineral). Carbonic acid reacts with silicates to form a new, soft clay. This clay is ultimately decayed into soil or mineral deposits.

– Oxidation: occurs when metallic minerals react with oxygen in water, producing softer minerals as a result. These softer minerals weaken rock, which begins to crumble. Oxidation with iron results in yellowish- or reddish brown stains on rock, while oxidation with copper results in pale-greenish stains.

Environmental factors and weathering. For example…

Temperature change – • Where temperature change is rapid (i.e.: deserts) there is an

increase the rate of heat expansion. In tropics the opposite is true due to more consistent temperatures.• High temperatures increase the rates of chemical reaction, such as

oxidation.

Precipitation levels – • Abundant rainfall can increase frost fracture where temperatures

are low.• A lot of rain also leads to higher rates of dissolving due to solution

and higher rates of hydrolysis.

*Complete Weathering Activity Sheet*