Dynamic Rock Cycle

ACTIVITY 2: Erosion and transport

Demonstration 2: Rock Resistance

Purpose:
To show that rocks erode at different rates and that this can explain the formation of uplands and coastal headlands by the more resistant rock types.

Notes:
The apparatus and materials required are: expendable rock specimens such as crumbly sandstone (sedimentary), limestone (sedimentary), granite (igneous), basalt (igneous). Four pieces (each of about 50g) of each rock are needed. An alternative is to use locally available rocks of variable resistance. At least 4 plastic containers with wide necks and tops; a mechanic's file; emery paper; goggles; balance; tray; some dilute hydrochloric acid (0.5 M) in a dropper bottle. Caution: corrosive.

These activities are done 'dry', to speed up the process and reduce mess. Use three or four different types of rock such as crumbly sandstone, chalk, granite, slate. Some specimens may be destroyed by the activity. The noisy bottle shaking need take only 10 seconds. The debris produced by each rock is easier to compare if each type of rock is shaken in a separate container. The provision of a balance allows quantitative comparisons to be made. By using rocks from the local area you can gain insight into the development of the landscape in your home region.

Carry out a risk assessment and wear safety goggles when using acids.

Development of knowledge and understanding:
Through these activities, pupils come to realise that some rocks resist erosion more than others. Some rocks are attacked faster by physical activity, others by chemical processes. The more resistant rocks will then form upland areas and coastal headlands, whereas the more easily eroded rocks form valleys and bays.

The eroded materials are transported away by gravity, water, wind or ice and will eventually become deposited in an accumulation of sediment which may become a sedimentary rock.

Acidic waters slowly dissolve calcium carbonate-rich rocks, increasing the calcium and hydrogen carbonate ion content of the water which then flows away. This reaction of acids with rocks is weathering not erosion, because no solid material is carried away.

Learning objective:
To investigate the relative rates of erosion of rocks by attrition and by chemical action.

There are many different ways in which rocks are weathered and eroded in the natural world. It is not always possible to imitate these different ways in the laboratory. We do not have nearly so much time available either! However, we can try.

The outlines of some investigations. Before your group does these investigations, think how you can improve them from these outlines to get the most useful information. How can you set up some sort of control, so that you can compare the results with the original rock samples?

Investigation A: Test how much each rock is affected by being bashed against rocks of the same type and other rocks. Use a plastic bottle.

Investigation B: Test how much each rock can resist being worn away, using a file or emery paper.

Investigation C: Test to see which rocks are attacked by acid. Caution: corrosive.

1. Which rocks did you find were the most resistant?
2. Where on Earth might these processes be occurring naturally?
3. How could these simulations of natural processes be made more realistic?
4. Which rocks are most likely to form hills or headlands in coastal areas?

See ESTA’s “Moulding Earth’s Surface - Landshaping”, for a pupil exercise in prediction, where the results of the above investigations are related to landscapes.

Back to top