External Forces and Limestone Environment

PROCESSES OF WEATHERING

Natural Systems

PURPOSE:

1. Define the term the term weathering

2. Identify the three categories that processes of weathering is placed in.

3. Differentiate between the processes of weather

Denudation is the long-term sum of processes that cause the wearing away of the earth’s surface leading to a reduction in elevation and relief of landforms and landscapes.

Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth’s atmospheric , biota and waters. Weathering occurs in situ, or "with no movement“

Erosion refers to the processes which break up, transport and deposit weathered materials and rocks. Its agents includes : water, ice, snow, wind, waves and gravity.

Mass Wasting is the movement of weathered materials (with or without water or ice) downslope by the force of gravity

KEY TERMS

WEATHERING PROCESSES

The processes of weathering are facilitated by two important elements of climate: temperature and rainfall

The processes of weathering may be placed into three categories:1. Physical / Mechanical Weathering 2. Chemical Weathering 3. Biotic / Biological Weathering

The form of weathering results in the break-up of rock into small pieces with no change in the chemical structure and physical appearance of the rocks. This type of weathering is most common in cold and dry climates. Physical weathering can occur due to temperature, pressure, frost etc.

Temperature Changes Frost actionPressure Release

Physical/Mechanical Weathering Processes

Temperature changes

Rocks expand and contract in response to changes in temperature. Temperature changes are an important mechanism in deserts, where there is a large diurnal (daily) temperature range, hot in the day and cold at night. The repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. The process of peeling off is also called exfoliation. The weathered fragments stay just where they are broken up. They may be moved later by gravity as mass wasting or transported by agent of erosion.

Frost action

Frost action is the name for several processes where ice is present. Severe frost action produces huge piles of rock fragments called scree which may be located at the foot of mountain areas or along slopes. Frost weathering is common in mountain areas where the temperature is around the freezing point of water.

Certain frost-susceptible soils expand upon freezing as a result of water traveling via capillary action to grow ice lenses near the freezing front. This same phenomenon occurs within pore spaces of rocks. The ice accumulations grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. It is caused by the approximately 10% (9.87) expansion of ice when water freezes, which can place considerable stress on anything containing the water as it freezes.

A rock in Abisko, Sweden fractured along existing joints possibly by frost weathering or thermal stress

Pressure Release Pressure release could have caused the exfoliated granite sheets shown in the picture.When rocks form under great pressure (volcanic rocks for example) are exposed on the surface, the release of the pressure of overlying layers causes them to expand, crack and breakup. Pressure release forms areas of large broken blocks of rock. Intrusive igneous rocks (e.g. granite) are formed deep beneath the Earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures, a process known as exfoliation. Exfoliation due to pressure release is also known as "sheeting".

Chemical Weathering Processes

Chemical weathering changes the chemical structure and physical appearance of the rock. The rock decomposes. It occurs rapidly in hot , wet climates such as the Caribbean. Limestone which is made of calcium carbonate is especially vulnerable to the chemical weathering process known as carbonation.

Carbonation

Rainwater absorbs carbon dioxide in the atmosphere to produce a weak carbonic acid. This acid acts on the calcium carbonate in limestone to form calcium bicarbonate. Calcium bicarbonate is dissolved by water. This results in pits and holes common in limestone areas and known as karst landforms.The reactions as follows:

Weathering effect of acid rain on statues

Oxidation

This is the process of oxygen being added to a compound. Rocks rich in iron react with oxygen in water to form iron oxide as in the rusting of iron. The effect is a thin soft crust on the outer part of the rock that is easily broken down.

Solution

This occurs in rocks containing salts such as sodium and potassium that are soluble in water. Rainwater falling on these rocks will eventually dissolve them.

Biotic/ Biological Weathering Process

Biotic weathering is the result of the physical or chemical effects of plants and animals on a rock; they may cause disintegration or decomposition of rock.

Plant roots

Plant roots may get into cracks and spaces of a rock and break it apart as they grow. This is a physical process, leading to disintegration of rocks.

Decayed Vegetation

These may release organic acids which chemically alter rocks

Biological weathering of lava by lichen, La Palma.

Activity 1. Answer the following questions

1. Explain how rocks are affected by (i) ONE physical weathering process(ii) ONE biotic/ biological weathering process

2.Give two characteristics of Limestone

3. Differentiate between chemical weathering and physical weathering

4. Explain two ways in which weathering is affected by climate

5. State clearly the meaning of the terms- ‘weathering’ and ‘erosion’