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In this chapter you will answer... ?
What is the difference between weathering and erosion? When does freeze-thaw weathering happen? Which rocks are weathered by carbonation? Where does exfoliation occur? Why is chemical weathering faster in the tropics?
8.1 Weathering
Weathering breaks rocks into smaller pieces. It is the effect of rainfall and temperature on rocks. Weathering occurs in situ. This means the rocks stay in the same place and are not moved. This is different from erosion. Erosion is when rocks are moved around or hit by something moving so that they break into smaller pieces.
8.2 Types of weathering
Rocks can be weathered in three ways: Physical (or mechanical) weathering causes rocks to disintegrate. This means the rocks fall apart into smaller pieces. Chemical weathering^ causes rocks to decompose. This means the minerals that make up the rock are changed by a chemical reaction. Biological weathering^ is when plants cause rocks to break up.
KEY WORD
Weathering is the disintegration and decomposition of rocks in situ. Changing temperature, rainfall and rock type have an important influence on the type of weathering occurring
KEY WORDS
In situ staying in the same place Disintegration breaking into smaller pieces Decomposition changing the chemicals which make up a rock
8.1 Weathering in situ compared to erosion by moving agents
Exercise
1 Using the information above, copy and complete the
following table in your notebook. Noun Verb disintegration to compose decomposition integration to erode exfoliation to fluctuate
8 Weathering
8 ^ Weathering
8.3 Physical weathering
Physical weathering causes rocks to disintegrate in situ. This means the rocks break up. They form smaller pieces of rock with sharp edges. Physical weathering happens when there are changes in temperature over a short period of time. The temperature needs to be fluctuating. Two types of physical weathering are freeze-thaw weathering and exfoliation.
8.4 Freeze-thaw weathering
Freezing is when water becomes ice. This happens at a temperature of 0°C. Water expands when it becomes ice, taking up more space. Thawing is when ice turns to water. This happens when the temperature rises above 0°C. Freeze-thaw weathering occurs when the temperature keeps fluctuating above and below 0°C. When the temperature drops below 0°C water in a crack in a rock will freeze. The ice thaws during the day when the temperatures rise. The water freezes when the temperature drops again at night and the ice widens the crack even more. This is freeze-thaw weathering.
8.5 Exfoliation
Exfoliation is when pieces of the outer layer of rock breaks away. Exfoliation happens in places where there is a very big difference in temperature between the night and day. This is most common in deserts. During the day in deserts the temperature may rise to over 40°C. At night the temperature may drop to below 5°C. During the day the heat causes the outer layers of the rocks to expand. At night the cold temperature causes the outer layers of the rocks to get smaller and they contract. This makes it weaker until it breaks up.
8.6 Chemical weathering
The composition of a rock is the chemicals or minerals that it is made from. Chemical weathering causes rocks to decompose. This means the composition of the rocks is changed, because chemical reactions have occurred. Chemical weathering usually needs water from rainfall, and warm temperatures. Carbonation and oxidation are types of chemical weathering.
8.7 Carbonation
Carbonation is the chemical weathering of chalk and limestone rocks by rainfall.
Exercise
2 Copy and complete the following sentences using words from the box below.
3 Put the sentences above in the correct order to describe the process
of freeze-thaw weathering. (Hint: start with c and finish with f )
a As the water freezes it and this makes the wider. b This freezing and makes the cracks in the rock wider and the rock becomes. c When temperatures above and below 0°C then -thaw weathering may occur. d The ice the crack even more.
e When the drops below 0°C __________ in cracks in the rock will freeze. f Eventually the falls apart. g During the next the water in the crack when the temperature below 0°C. h When the temperature rises again the ice and water fills the cracks.
fluctuate temperature melts water thawing widens freeze expands drops rock crack weaker freezes night
8 ^ Weathering
short time. Rainfall is not always necessary. Freeze-thaw weathering occurs most rapidly where temperatures fluctuate just above and below 0°C over a short time. Rainwater is
needed. Exfoliation happens most rapidly where there are large changes in temperature between night and day. It does not need rainfall.
Exercise
5 Are the following statements true or false? Copy the correct statements into your
notebook. a Chemical weathering occurs fastest where temperatures rapidly fluctuate. b Chemical weathering occurs fastest in warm, wet places. c Chemical weathering does not happen where it is cold and wet. d Tropical areas such as the rainforest will experience the most chemical weathering.
e Limited physical weathering occurs in tropical areas where there is little change in temperature. f Physical weathering occurs quickly in deserts. g Rainfall is needed for weathering to occur.
Language
g^
g^ Verbs in formal and informal English
In English there is often more than one way to express the action of a verb. Phrasal verbs are used frequently in informal (everyday) conversations, while one word verbs tend to be used in formal or technical language. Informal the match was put off until the following Saturday because the pitch was flooded. Formal The match was postponed…. (prefix post- after) [Hint: the one word verbs often have prefixes that make their meaning more precise so it is helpful to know the meanings of the prefixes.] Prefixes: com- bringing or putting together de- undoes the action of the stem verb dis- moving away (opposite meaning to the stem verb) ex- moving out of/away
8.11 Rock type and mineral composition
Certain rock types are made up of different minerals. This is the mineral composition of the rock. Certain rock types are more affected by certain types of weathering. Limestone is composed of calcium carbonate. This means that it can be weathered by carbonation. However granite is not affected by carbonation as it does not contain calcium carbonate. Rocks containing iron minerals will be weathered by oxidation.
8.12 Rock type and lines of weakness
Lines of weakness are cracks in rocks which are attacked by weathering. Water and air can enter these cracks and break down the rock by physical or chemical weathering. A rock with
lots of lines of weakness will be more easily weathered. Chalk and limestone are sedimentary rocks. They are formed in layers. Each layer is separated by a bedding plane. These are horizontal lines of weakness in the rock. There are also vertical lines of weakness called joints. Rain water flows through limestone through the joints and bedding planes. The rock is weathered by carbonation. Limestone caves are formed by carbonation. Granite is an igneous rock. It is formed when magma slowly cools as it rises towards the surface of the earth. As it cools horizontal bedding planes and vertical joints form. These are lines of weakness in the rock. Granite is weathered to form tors.
Weathering ^8
Exercise
6 Copy the table below into your notebook and match the phrasal verb
with the correct verb in the following table. Phrasal verb One word verb To work together To destabilise To break up To exfoliate To change the make up To dissolve To make up To combine To make unstable To expand To lose a layer To decompose To absorb into liquid To disintegrate To spread out To compose
Exercise
7 Copy the sentences below into your notebook and put the appropriate
verbs from the table above into the correct form. a Freeze-thaw weathering the cracks as water becomes ice. b The calcium carbonate in limestone is by the carbonic acid in rainwater. c When a plant dies it producing chemical reactions in the surrounding rock. d Granite rocks in climates which fluctuate rapidly above and below 0°C. e Physical, chemical and biological weathering often to weaken sedimentary rock. f Sedimentary rock is of layers of sediment, while igneous rock is of cooled magma from the earth’s mantle. g Rocks when they have been weakened through physical weathering.
Exercise
8 Look at the following photos and match them to the descriptions below.
a Enchanted Rock in Texas is a granite dome. Cracks in the granite have allowed rainwater to enter and weaken the
rock. Vegetation has also contributed to the weathering. The outer layer of granite has been separated from the lower layers.
Weathering ^8
8.4 Weathering process C
Plant grows in a crack in a rock face
Plant roots seek ground-water through cracks in rock
The plant dies and decays
8.5 Weathering process D
37°C 2°C^ 37°C 2°C^ 37°C 2°C
11 Study figure 8.2 and answer the questions below in your notebook.
a Where is the rainwater going? b What happens when temperatures fall below 0°C? c Why has the crack widened? d What effect will this have on the rock? e Which process does this flow chart illustrate?
12 Study figure 8.3 and answer the questions below in your notebook.
a What kind of weathering is this? b How does this kind of weathering affect the rock? c Which climates speed up this kind of weathering?
13 Study figure 8.4 and answer the questions below in your notebook.
a What happens to the crack as the roots get bigger? b What kind of weathering is this? c When the plant dies the roots decay and chemicals are produced. What kind of weathering does this process cause?
14 Study figure 8.5 and answer the questions below in your notebook.
a Are the changes in temperature gradual or rapid? b In which regions do these fluctuating temperatures often occur? c Where are the points of weakness in the rock? d What is this weathering process called? e Which types of rock are affected by this process?
8 ^ Weathering
Exercise
15 Fill in the missing words in the following sentences and copy into your notebook.
a Carbonic acid is present in. b Carbonation causes limestone to and be washed away. c Caves form in by carbonation. d Freeze-thaw is a type of weathering. e When the top layer of rock becomes detached from lower layers it is called. f lines of weakness in rocks are called bedding planes.
g Joints are the cracks in rocks. h The reaction of oxygen with minerals in rocks causes. i Granite is an rock which is not weathered by carbonation. j Plant roots grow down through cracks in rocks to find. , thus widening the cracks and causing biological weathering. k dead plants produce acids which cause chemical weathering.
Exercise
16 Choose the correct options form the statements below. Copy the correct sentences
in your notebook.
Extension Internet search: Find out about… Weathering Desert rock formations Exfoliation
a Weathering is different from erosion because it occurs now and again/ between moving objects/in situ. b Freeze-thaw weathering needs fluctuating/constant/low temperatures. c Exfoliation is a process similar to peeling a banana/an onion/an orange. d Plants cause only biological/only physical/both biological and chemical weathering.
e Physical weathering affects only sedimentary/sedimentary and igneous/only igneous rock types. f Oxidation is a chemical weathering process which makes rocks appear red/ black/white.
Explain the role of a) rainfall and b) temperature fluctuation in the weathering process. Why is granite a more popular surface for kitchen work surfaces than limestone? How do plants contribute to the weathering process?
Explain the role of a) rainfall and b) temperature fluctuation in the weathering process. Why is granite a more popular surface for kitchen work surfaces than limestone? How do plants contribute to the weathering process?
Talking points
Think about the questions from the start of the chapter. Can you answer these now? What is the difference between weathering and erosion? When does freeze-thaw weathering happen? Which rocks are weathered by carbonation? Where does exfoliation occur? Why is chemical weathering faster in the tropics?
