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Weathering

Powerpoint presentation

Weathering is the breakdown of rock by physical, chemical and biological means in situ. Erosion on the other hand involves moving water, ice and wind and the broken rock is then transported away. Weathering is important as it breaks down rock and helps encourages erosion and transport. It also helps soils to form which then allow crops to grow.

Physical/ mechanical weathering: involves the disintegration of rocks without any chemical changes taking place. The result is smaller, angular pieces of rock.
There are two main types of physical weathering: freeze thaw and exfoliation.

• Freeze thaw or frost shattering involves water going into cracks and then freezing. The ice expands by 10%, sets up stresses which enlarge the crack. When the ice thaws rock fragments may fall away. This takes place over several cycles. What climates and rock types encourage freeze thaw?
• Exfoliation or onion peeling occurs when there are large fluctuations in diurnal temperature. The rock is heated up during the day and cools at night, as it is a poor conductor, the expansion and contractions only occur near the surface, and if there is moisture present then the outer layers may peel away What locations would you associated with large diurnal temperature changes?

Chemical weathering involves the decomposition of the rock into altered chemical state.
The two most important are carbonation and oxidation.

• Carbonation - solution involves weak acids dissolving the calcium carbonate (CaCO₃₎ in chalk and limestone. These acids originate from carbonic acids in rain drops
• Oxidation occurs when iron compounds react with oxygen to produce a reddish-brown coating- rust. The end product expands and thus weakens the original rock

Biological weathering involves the actions of flora and fauna. Plant roots grow into cracks and cause them to expand and rabbits can burrow into weak rocks such as sands. Also during their life cycle both plants and animals produce organic acids which can aid the decomposition of rocks

Controls of weathering

Climate:

You can see that in cold climates, as the number of frieze thaw cycles increase so freeze thaw dominates. In contrast, in humid tropical climates, chemical weathering dominates. Chemical weathering increases 2 to 3 times for every 10°C.

Geology

Rock type and structure influence the rate and type of weathering in many ways. This is because of differences in:

• chemical composition;
• joints and bedding planes;
• the nature of the cement in sedimentary rocks

For example, the calcium carbonate in limestone encourages carbonation and the joints and bedding planes in the rock helps the water to move through the rock. Fine grained chalk is very susceptible to freeze thaw as the water can enter into the porous rock, freeze and causes pieces to easily become detached.

The stalactites and stalagmites in Cheddar Caves owe their origin to carbonation and the water moving through the rock.
Granite has many pseudo joints caused as the rock cooled. The facilitates chemical weathering as the surface area of the rock increases. Granite is made of of feldspar, quartz and mica and in certain instances the feldspar is chemically weathered into kaolin. The China clay mines in Cornwall owe their existence to the chemical weathering of the feldspar

Case study Malham Tarn and Limestone

GeoActive 227

Open Google earth and 'Fly to' Malham. Move slightly north to find Malham Tarn.

Change to angle view, select Tools, then Options and change elevation exaggeration to 3. Now move around the area. Decsribe the landscape that you see

Now work through this virtual field trip through Malham Cove

Make your own notes on limestone features above and below ground, copy the karst scenery diagram.

Make a annotated field sketch of Malham Cove

Watch the videos

This may help as background as well.

Watch this animation for the weathering of limestone

Case Study: Dartmoor and granite

GeoActive 238

Open Google earth and 'Fly to'Oakhamton, Devon, move about 10 km south on to Dartmoor. Make sure that you have 3D and 3 on the elevation exaggeration. Decsribe the landscape that you see. What are the differnces in the landscape betwen Malham and Dartmoor.

Open the factsheet (It should be a fact sheet about Geology, if not type in Geology and Landforms Factsheet in the Search box) and in your own words explain how the tors are formed.

Now read this and make notes on the sections that you are not sure of.