210 Plate tectonics Chapter 12 aE a raced The theory of plate tectonics tells us that the continents on Earth were once connected, forming a ‘supercontinent’ called Pangaea. All scientific theories require evidence to support them - this is a vital part of the scientific method. In this chapter we look at evidence for plate tectonics, including fossils of animals and plants that are now extinct but which must have lived on Pangaea before the tectonic plates moved apart. We also look at the ideas that explain how tectonic plates move. You will learn about: * The evidence that supports the theory of plate tectonics The reason why tectonic plates move in the ways observed You will build your skills in: Describing and using models Describing how models can be supported or changed based on new evidence Evaluating issues using scientific understanding Discussing the global enviro: nmental impacts of some uses of science Describing how unexpected results from scientific investigations can improve understandingfat) 1 ce eo) Co You will learn: * To explain the evidence for tectonic Evidence for plate plate movement including coastal shapes, volcano and earthquakes, fossil tectonics records and the position of magnetic materials To understand that models and analogies can change according to scientific evidence To describe the uses, strengths and limitations of models and analogies To evaluate how the prediction is supported by the evidence collected To describe trends and patterns shown ina set of results, identifying, and explaining, any anomalous results present To describe examples where scientific understanding has been improved by the unexpected outcomes of scientific enquiries. To use scientific knowledge to make predictions Starting point BUMS Rr el heed |The Earth's crust is divided into a number of | Describe a model and its strengths and {tectonic plates that ‘float’ on the mantle weaknesses ee Wegener suggested that the Earth's continents | Describe how scientific hypotheses canbe lare moving apart | supported or contradicted by evidence = Earthquakes and volcanoes occur most often at | Describe trends and patterns in results the boundaries between tectonic plates | | Some rocks that form over long periods of time | Produce layers that help us determine when ‘they formed edstetatante veer ta ec 'Fyou have ever completed a jigsaw puzzle, you will know that the shapes of two pieces have to match exactly for them to fit together neatly (figure 12.1). Two pieces that do ot match will not fit together.12.1 Pieces in a jigsaw puzzle have to match exactly to fit together. ne Scientists observing the Earth's continents noticed that coastline: outside edge sometimes, the coastlines of two separate continents appear of a continent, where to match very well. This is particularly apparent when we rock meets the ocean look at South America and Africa (Figure 12.2). Pangaea: supercontinent on Earth that broke apart about 175 million years ago. supercontinent: area of land where a number of continents were joined together. a : b 7 12.2 @ Map showing South America and Africa when joined. b The coastlines of modern-day continents of South America and Africa. It is more than the shapes of continents that fit together. Over millions of years, rocks form in layers. At coasts, waves in the sea cause these rock layers to be exposed (figure 12.3). Scientists have compared the rock layers in coasts that are thousands of kilometres apart, such as on the eastern side of South America and the western side of Africa, and found 423 Layers of rack on a coastline, that the layers match in many places. The ‘jigsaw puzzle’ shapes that match up and the rock layers that match up form strong evidence that the coasts of continents were once joined together, and that these continents must have moved apart. The joined continents formed what scientists call a supercontinent, which they have named Pangaea. Making links so You should have learned about how different types of rocks and rock layers are formed. a) Describe briefly how sedimentary rocks form. b) Suggest how sedimentary rocks naturally form clear layers. 212 Plate tectonics‘ Sa eC ORsar ttt work in small groups. Your teacher will hand you an outline map of the world as it appears today. 1 Cut out the shapes of the main continents. 2 Experiment with arrangements of the continents to make pairs of continents fit together. Start with South America and Africa. 3 Investigate whether it is possible to make all the continents fit together to suggest what Pangaea looked like. ‘Al Which continents appear to fit together well? A2 Some coasts of continents fit together less well. Suggest what might have happened to the ‘missing’ land that linked these continents together. 3 Compare your group's ideas with those of other groups. Have you all had the same ideas? Evidence for plate tectonics 213Volcanoes and eta te EL Cry Stage 7 Chapter 10 described how earthquakes are sudden Movements of the Earth's crust when forces build up over long periods of time, and that volcanoes appear at places where the Earth is thinner and weaker, which allows molten rock to break through the surface. Over hundreds of years, scientists observed that earthquakes and volcanoes happen much more often in particular Parts of the Earth’s surface. For example, there are many volcanoes and frequent earthquakes along the western coast of South America, Figure 12.4 shows where volcanoes have been observed around the world. 12.4 A map of the world showing volcanoes (red triangles). Some of this evidence was produced unexpectedly. In the 1960s, a network of detectors that could record small vibrations in the Earth’s crust was placed around the world. These detectors were intended to monitor the testing of nuclear bombs, which caused particular patterns of vibrations. But they also detected vibrations due to earthquakes and volcanoes. These detectors produced the first detailed map of regions where most earthquakes and volcanoes occurred naturally, and helped produce the first map of Earth’s tectonic plate boundaries. The western coast of South America forms the edge of a tectonic plate. This plate is moving towards and pushing against a tectonic plate under the Pacific Ocean, which is being pushed below the continent of South America, Large forces build up at these plate boundaries, causing earthquakes and volcanoes. 214 Plate tectonicsinfact nearly all volcanoes and most large earthquakes are found along plate boundaries. This Provides strong evidence for the movement of tectonic plates. in Stage 7, you may have learned about plate boundaries. They are important in linking the evidence of volcanoes and earthquakes to the theory of plate tectonics. a) Describe how fold mountains may also occur at a plate : | | Making links b) Explain why it is easy to predict the places on Earth where earthquakes are likely to occur, but not easy to predict when they will occur. Be — plate boundaries {East African Rift System 125 aA map of East Africa with an area marked that is called the East African Rift System. b One ‘part of this area, The East African Rift System is an area where scientists think that a tectonic plate is Splitting into two new plates, forming a new plate boundary. 1 The plate appears to be breaking into two pieces that are moving apart. Predict what ‘types of observable events are likely to happen in this area. 42 Suggest why deep valleys and lakes are forming along the plate boundary as it develops. 43 Describe the timescale over which the new plates will move apart. 7Look at figure 12.4. Describe three areas where large numbers of volcanoes are found. 'b) Suggest why these areas have large numbers of _ volcanoes. ‘ © Kceland is an island in the North Atlantic Ocean. Iceland sits on the boundary of two tectonic plates that are moving apart. Explain why there are a large number of volcanoes in Iceland, Cs context: Making use of plate tectonics Iceland is an island that has formed as a result of the molten rock that has broken through the Earth's crust at a plate boundary where the plates are moving apart. The crust is quite thin under Iceland and there are many active volcanoes. This produces hazards for the people of Iceland; sometimes people have to be evacuated from areas where a volcano is active. However, people in Iceland have found many uses for the ‘hot rocks’ found underneath the island. Water that circulates through the hot rocks is heated naturally and contains many minerals that are believed to be good for human skin. There are natural outdoor baths that people use, such as the Blue Lagoon (figure 12.6a). Another use is to provide energy. By drilling down and placing water pipes through the hot rocks, energy is transferred to the water which can then be used to heat buildings. Water that is heated so that it becomes steam can be used to turn generators to produce electricity (figure 12.6). Iceland leads the world in this geothermal energy technology, which is a renewable resource and a clean source of electricity. 126 a The Blue Lagoon, where people bathe in the hot, mineral-rich water. 1, Evaluate whether geothermal technology could bi around the world instead of fossil fuel BA geothermal power plant. @ used to generat ann Is Such as gas oF oil, Generate electricity all You should consider the advantages and disadvanta . impacts of the technology in your answer, ges, and potential environmental= fossils form over many millions of years when the remains of dead animals and plants are preserved in layers of sedimentary rock. One very important fact to remember is that the fossil forms at the place the animal or plant died, When we find a fossil within layers of rock on a hillside, we can be fairly certain that the living thing was alive near the area where the fossil is found. Changes in rock layers, or earthquakes, or the action of glaciers may move rocks small distances, but not over hundreds of kilometres. links You may have learned about how fossils are formed, a) Describe the process that causes fossils to form. b) Explain why fossils are mostly found in sedimentary rocks. Figure 12.7a shows the fossil of a plant, a tree fern called Glossopteris. Figure 12.7b is an image showing what scientists think Glossopteris looked like when it was alive. This plant is now extinct - there have been no living examples of it for many millions of years. Key terms a b fossils: remains of dead organisms from many millions of years ago that have solidified due to the pressure of sedimentary rocks forming above them Glossopteris: plant that lived on Pangaea but became extinct over 175 million years ago; fossils found today ; provide evidence for plate 12.7 a Glossopteris fern fossil. _b What the tree fern looked like movement. eB when it was alive Glossopteris is interesting because fossils of it have been found in South America, Africa, southern Asia, Antal and Australia. The only sensible explanation for ese places must have been joined together at the time Glossopteris ferns lived on Earth. Figure 12.8 is a map of 'ow scientists think some of the Earth’s tectonic plates were once joined together. The shaded bands show where fossils °f different species of animals and plants have been found.fossil evidence of the Triassic land reptile Lystrosaurus ynognathus, 2 fossils of the fern Triassic land reptile Glossopteris, found approximately fossil remains of the in all of the southem 3m long freshwater reptile continents, show that Mesosaurus they were once joined 12.8 Map showing how scientists think tectonic plates were once joined together and the areas where fossils of different living things have been found. Key terms Cynognathus: reptile that lived on Pangaea but became extinct over 175 million years ago; fossils found today provide evidence for plate movement. Lystrosaurus: reptile that lived on Pangaea but became extinct over 175 million years ago; fossils found today provide evidence for plate movement. Mesosaurus: reptile that lived in fresh water on and near Pangaea but became extinct over 175 million years ago; fossils found today provide evidence for plate movement.i the table lists some species of animals ai asl me two different continents. s and plants that have been found as fossils sod Habitat? on which CuO fem land South Glossopteris “America, ; “Africa, Asia, years ago fi Antarctica ; and Australia Al Complete the table in Worksheet 12.1d by researching information and deciding whether each type of fossil can or cannot be used as evidence of plate tectonics. The table includes an empty row for you to use to insert your answer for question A3. A2 Record your reason for each decision in the last column of the table. 43 Research a different species of organism that has been found as a fossil and can be used to support plate tectonics. Add this to the final, empty row of the table and describe the reason you found for it, providing evidence.CTC at icy Making links You should have learned in Stage 8 Chapter 10 about mnsnatic fields and how the Earth produces a magnetic field. a) Describe briefly how you could show the magnetic field lines around a bar magnet using iron filings. b) Name the piece of equipment you could use to demonstrate that the Earth has a magnetic field. Scientists investigating the seabed of the Atlantic Ocean discovered something quite unexpected. The rocks under the centre of the ocean displayed ‘striped’ magnetic patterns. The explanation for this provided more evidence for plate movement. When molten rock breaks through a weak spot in the seabed, it rapidly cools to form a type of rock called basalt. Basalt contains quantities of minerals that can be magnetised. The core of the Earth acts as a strong magnet, causing the rapidly cooling rock under the ocean to produce a magnetic pattern that aligns with the magnetic field of Earth. Scientists found that there were ‘stripes’ of rock in the seabed that ran parallel to the plate boundary under the Atlantic Ocean. One stripe would have a magnetic alignment in one direction, and the next stripe would have a magnetic alignment in the opposite direction. The explanation for this is that the Earth's magnetic field reverses direction from time to time, so that a layer of rock that forms after a magnetic field reversal contains atoms that line up in the opposite direction. By investigating the stripes of rock on the seabed, scientists determined that there had to be a plate boundary under the ocean, where the plates were moving apart. Fresh molten rock rises into the gap between the plates and cools. The solid rock moves away from the boundary as the tectonic plate moves. When the Earth's magnetic field reverses, the next stripe of rock forms at the plate boundary. Figure 12.9 illustrates this process. 220 —_—Plate tectonics Key terms basalt: a type of rock that contains quantities of minerals that can be magnetised deduce: arrive at a logical conclusion based on available information magnetic alignment ‘occurs where rocks that are magnetised produce magnetic fields that line up with the Earth’s magnetic field. magnetic field reversal Process where the direction of Earth’s magnetic field changes to be in the opposite direction.mid-Atlantic ridge direction of magnetic field in rocks agen late mantle sea floor spreading at the mid-Atlantic ridge 129 Magnetic stripes in rocks produced at a plate boundary. Scientists can estimate the ages of rock layers by investigating the direction in which different layers of rock align magnetically. By estimating the amount of time that passes between reversals of the Earth’s magnetic field, scientists can count the stripes between the plate boundary and the coastline of a continent and deduce how long ago the continents were linked. | We do not understand fully what happens during a magnetic field reversal, as the process of changing can take thousands of years and the time interval between changes varies from a few thousand years to over a million years. Scientists have used a wide range of evidence to conclude that the Earth’s magnetic field has reversed 182 times in the last 83 million years. The average time between reversals is about 450 000 years. Par ame BEC OC rte tC) In this activity you will investigate how to model Earth's magnetic field and predict how humans and animals may be affected by the next reversal in the direction of the field. 1 Use a compass to observe the direction of the Earth's magnetic field. 2 Use a bar magnet with the compass to model the Earth's magnetic field. Check that the bar magnet is placed so that the compass points in the same direction as in step 1. 3 Inyour group, decide how you could model a full reversal in the Earth’s magnetic field. Predict how you need to move the bar magnet to model this. 4 Test your prediction by making the move. Has it produced the correct result? If not, how do you need to change the placement of the bar magnet to make the field reversed? 5 There is more than one way to move the bar magnet to model a field reversal. Suggest asecond way to do this and test the outcome. ' A1 Which pole of the magnet did you need to bring closer to the compass to make it model the Earth’s magnetic field? A2 Suggest what magnetic pole is near Earth's geographic north pole. Explain your answer, Evidence for plate tectonics 22%i id this th A3_ Write down your method for modelling a field reversal. Did this produce the planned effect? ‘A4 Write down an alternative method for modelling a field reversal. Did this produce the planned effect? AS A number of animals are known to be able to detect he Earth's amie: fic ang use it to navigate. Research these animals and suggest the effect reversal would have on them. AG Describe the effects a magnetic field reversal could have on human activities. 222 Plate tectonics| | |v The scientific theory of plate is supported by a number types evidence. a = | y The shapes of the coastlines of | continents that are far apart suggest i that these continents were once joined, | together. v Rock layers in coastlines that are now thousands of kilometres apart match up, suggesting these coastlines were once joined together. v Earthquakes and volcanoes occur much more frequently in areas that are boundaries between two tectonic plates, suggesting that the plates are moving. _- Fossils of particular species of extinct animals and plants that could not cross ‘oceans have been found in different continents, suggesting thatthe continents we! Ghee jolted tSgecee v The Earth’s magnetic field reverses from time to time, causing different Check your skills progress: * I can describe the evidence for plate tectonics including matching coastlines | and rock layers, patterns of earthquakes | and volcanoes, fossils and magnetic alignment of rocks moving outwards from a plate boundary. I can describe how some of this evidence was found unexpectedly. Ican use models and analogies such as jigsaw puzzles to describe how matching coastlines provide evidence for plate tectonics.er Cun Peed You will learn: E 1 eames * To explain how convection currents cause tectonic pl. xplaining plate pier noel * To describe the model of movement plate tectonics and some of its strengths and weaknesses Starting point Ree aeons Serio ckce The Earth's crust is divided into a number of | Use models and analogies to describe the theory tectonic plates that ‘float’ on the mantle _ | of plate tectonics ee ee The theory of plate tectonics is supported Describe patterns in results by evidence including matching coastlines, earthquakes and volcanoes, fossils and magnetic patterns in rock Convection currents Particles in liquids that are heated rise upwards, and Pe ee particles that are cooling fall downwards. These movements mantle: deep layer are called convection currents and are best observed in an molten rock undern ‘experiment. “the Earth's crust. The diagram you will draw at the end of Activity 12.5 shows how particles in a liquid move due to the heating and cooling in convection currents. TORS radeon eC R Caco a Cy Scientists believe that under the Earth’s crust is a very deep layer called the mantle. The rock in the mantle is not a runny liquid like water, it is more like a sticky, stretchy substance that can flow, but only very slowly. At the centre of Earth is a hot, metal core containing iron and nickel. This is a powerful source of heat that causes convection currents to form in the mantle outside the core. This results in the surface of the mantle moving. We can think of tectonic plates in the Earth’s crust as ‘floating’ on the surface of the mantle (figure 12.10). As the mantle moves, it pulls the tectonic plates with it. 224 ‘Plate tectonicsmid-ocean ridge 12.10 Convection currents move tectonic plates, There are nine major tectonic plates that form most of the cust of Earth. There is a plate boundary between each pair of plates. You will remember from Stage 7 Chapter 10 that there are different types of plate boundary (figure 12.11): Where two plates are moving apart, molten rock rises to fill the gap between the plates, which cools to form new rock attached to the plates (figure 12.1 1a). Where two plates push together, they can either both push upwards to form fold mountains, or one plate rises above the other, forming volcanoes and causing earthquakes (figures 12.11b, c). Where two plates slide past each other, earthquakes occur as a result of the friction forces between the rock layers as they rub together (figure 12.11d).plates moving apart ined ‘one plate pushes towards and rises above another plate is mantle ‘earthquakes plates sliding along each other 12.11 Types of boundaries between tectonic plates. 226 _—~Plate tectonics* Ican make predictions based on my knowledge of convection currents and plate tectonics. | * I can carry out practical work safely. |End of chapter review Quick questions 1. Which of the following can be used as evidence for plate tectonics? (a) fossils (b) rock layers on coastlines (2 magnetic materials in rocks (d) all three of the above 2. There are several ways in which thermal energy can be moved in and between substances. Which of the following explains how tectonic plates move? (a) radiation (b) conduction (9 convection (d) all three of the above 3. Which of the following types of animals have fossils that can provide evidence of the movement of tectonic plates? (a) sea fish (b) birds (Q freshwater fish (d) mammals that swim in the sea 4. Which type of plate boundary is usually found under oceans? (a) where two plates push together so that one goes beneath the other (b) where two plates push together so that both rise upwards (©) where two plates move sideways past each other (d) where two plates move apart 5. Name the supercontinent from which the continents we see today formed. (a) Pangaea (b) Oceania (9 Antarctica (d) Africa6. (a) Name two extinct species of livin. tectonics. ig things that provide evidence for plate (b) Outline briefly how these living things provide evidence for plate tectonics. 2 Scientists can explain how tectonic plates move because of heat produced from inside the Earth. (a) Describe how heat rising from the Earth’s core through the mantle causes molten rock to move. (b) Figure 12.12 shows two plates that are moving apart. Draw arrows to show how the mantle moves under these plates. 12.12 Connect your understanding 8. Iceland is located on a plate boundary. (a) Describe evidence that shows how we can be sure that Iceland is on a plate boundary. (b) Is Iceland likely to be a good location to find fossil evidence for plate tectonics? (0) Give two reasons for your answer to part (b). 9. A scientist suggests that the continent containing Australia was once joined to Antarctica as part of a supercontinent. (a) Examine the shapes of Australia and Antarctica and say whether you agree with this suggestion. (b) Explain your answer. (©) Describe three other types of evidence you would look for to support your answer.ectonics. has a magnetic field. 10. Magnetic fields can provide evidence for plate t (a) Describe how you could show that the Earth (b) We know that the direction of the Earth’s magnetic field reverses once every 400 000 to 500 000 years. Explain how we know this. (0 Describe how the Earth's magnetic field affects basalt rocks that form from molten rock. (d) Use your answers to (a), (b) and (c) to explain how magnetic fields can provide evidence for plate tectonics. 11. The coasts of continents can provide evidence for plate tectonics. (a) Describe how the shapes of coasts can provide this evidence. (b) Describe how the rocks found on coasts can provide this evidence. (0) Suggest why sedimentary rocks are more likely to provide the evidence you described in (b). Challenge question (d) Explain how sedimentary rocks on coasts may contain another type of evidence for plate tectonics.