Weathering is the breakdown of rocks in situ under the influence of atmospheric or biological agents. Weathering is an important part of the process for the formation of many upland and highland features. Upland areas, with land over 200m above sea level, are common in the west and north of the UK where there are many examples of features created in full or part by weathering. In these areas, the upper course of rivers is marked by V-shaped valleys. These steep valleys are a product of intense vertical erosion by rivers alongside the weathering, usually by freeze-thaw, of the newly exposed channel banks and valley sides. In an area with granite, outcrops of rock called tors may form. These are the product of chemical weather of the granite while it is underground, as wells as freeze-thaw weatherising once the outcrop has been exposed by erosion.
In limestone areas, chemical weathering of the calcium carbonate rock has resulted in the creation of limestone pavement, large blocks of rock called clints separated by deep weathered grooves called grykes. In many highland areas, the mountain slopes are covered with broken fragments of rock called scree. These are the product of freeze-thaw weathering.
2. Explain why granite, which is resistant to physical erosion, may be more susceptible to chemical weathering. (6)
Granite is an igneous rock formed deep underground in ‘batholiths’ of cooling rock made up of three principal minerals: quartz, feldspar and mica. As the molten minerals cool slowly underground the minerals form crystals which interlock. This crystalline structure gives the granite good resistance to erosion. However, while the quartz mineral is resistant to chemical weathering the mica and feldspar is not. Acids found in rainwater and soils can slowly attack these two minerals to form kaolinite, which is the main component of china clay. Once this china clay is exposed at the surface it is easily eroded by rivers or surface run-off.
3. Describe and explain the formation of limestone pavement shown in figures 30 and 31. (6)
Limestone pavement is the product of the chemical weathering of calcium carbonate-composed limestone. It is common in the Yorkshire Dales National Park. Carbon dioxide in the atmosphere dissolves in cloud droplets to form a weak solution of carbonic acid. As this acidic rainwater flows over and through cracks in the limestone, the acid reacts with the calcium carbonate to form a soluble calcium bicarbonate. This is then washed away with the rainwater. Vertical joints and horizontal bedding planes are widened by this chemical weathering to leave blocks of limestone with smooth surface runnels, called clints, separated by deep grooves called grykes.
4. Describe the formation of a scree slope such as the one shown in figure 32. (4)
Scree is the product of freeze-thaw weathering. In upland areas or in colder periods where temperature fluctuated around zero, water seeps into cracks in the rock. As the water freezes, it expands by 9% creating stresses in the rock. Repeated freezing and thawing widens the cracks in the rock until it breaks into smaller pieces. On upland slopes, this can result in a layer of angular rock fragments building up at the base of the slope as freeze-thaw weathering attacks the cliffs and exposed rocks above.
5. Suggest reasons why freeze-thaw weathering was more significant at the end of the last glacial period, but chemical and biological weathering were less significant than they are today. (6)
Freeze-thaw weathering is the breakup of rocks in situ under the influence of water as it repeatedly freezes, melts and freezes again. As the water freezes, it expands by 9% creating stresses in the rock. Repeated freezing and thawing widens the cracks in the rock until it breaks into smaller pieces. On upland slopes, this can result in a layer of angular rock fragments building up at the base of the slope.
At the end of the last glaciation temperatures in the British Isles fluctuated close to zero for many centuries. This created the ideal conditions for freeze-thaw weathering as both the freezing and thawing are essential to the breakdown of the rocks. However, cold temperatures reduce the speed of chemical reactions so reducing the pace of chemical weathering, while these same cold temperatures slowed tree and plant growth which restricted the rate of biological weathering possible.
6. Draw a single diagram of Haytor (figure 33) to indicate the general features of a granite tor. Add annotations to explain the features shown. (4)
Diagram to follow
6. Draw a single diagram of Haytor (figure 33) to indicate the general features of a granite tor. Add annotations to explain the features shown. (4)
Diagram to follow
