Weathering and Erosion

GEOGRAPHY 12

WEATHERING and EROSION NOTES

Text: Planet Earth pp 240 – 251

Landforms result from the uplift caused by tectonic forces and the processes of denudation (or LEVELLING).

The processes of denudation work to reduce land to BASE LEVEL, the lowest level to which a land surface can be eroded.

Denudation ca be divided into two categories:

1) DEGRADATION includes the processes of:

A) WEATHERING(breaking up of rock near or at earth’s surface)

B) EROSION (removal and movement/transport of rock debris)

2)AGGRADATION (building up of land surface by deposition of rock materials – this occurs when a drop in energy SLOWS the transporting agent…e.g. rivers slow down at BASE LEVEL.)

1. Weathering

reduces rocks to smaller particles

provides PARENT MATERIALS for the process of soil formation

Weathering can be divided into two types:

MECHANICAL or physical weathering is the breaking up of rocks without any change to their chemical composition

Examples of Physical/Mechanical Weathering

1. FROST SHATTERING or ICE WEDGING- water seeps into cracks and joints, freezes and splits the rock apart

2. THERMAL EXPANSION - rocks composed of different crystals contract and expand with temperature change at different rates…this causes the rocks to eventually disintegrate

3)PRESSURE RELEASE (sheeting) or pressure release is the separation of layers in granite due to the removal of overburden and the release of pressure…these features often form rounded exfoliation domes

4) Animals and plant roots can also weather rock.

5) In deserts, evaporation leaves behind salt CRYSTALS in rock pores and fissures. As these grow they can wedge rocks apart.

Chemical Weathering is the break up of rocks due to A CHANGE IN THE CHEMICAL COMPOSITION OF THE MINERALS IN WHICH MAKE THEM UP .

1. SOLUTION - rainwater, which is naturally weakly acidic, reacts with SOLUBLE minerals, dissolving them and carrying them away.

LIMESTONE, a commonly used building material is very prone to solution.

2. OXIDATION - the process of combining oxygen with a mineral.

3. HYDROLYSIS - ions of water replace ions of a mineral which produces amongst other things, clays.

The rate of weathering depends on a number of factors.

Explain how the following factors may contribute to the rate and amount of weathering:

1. Nature of bedrock – THE COMPOSITION OF IT, SIZE OF IT, JOINTS.

2. Temperature - COLD AND DRY, WARM AND MOIST, SHOWS WHETHER IT'S MORE CHEMICALLY OR PHYSICALLY ERODING.

3) Precipitation – THE AMOUNT OF RAINFALL, HOW MOIST/ DENSE THE AIR IS. HOW MUCH SNOW (IF ANY).

4) Slope/drainage – HOW MUCH WATER BUILD UP, BLOCKAGE, HOW MANY RIVERS/STREAMS TO CARRY SEDIMENTS AWAY.

We can generalize where chemical and physical and mechanical weathering are predominant:

Chemical weathering is most common in area that are …

Warm and moist, like California.

Mechanical/physical weathering is most common in area that are …

Cold and harsh, places with lots of hard rain, snow and ice.