The Dynamic Earth. English edition

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3 The Dynamic Earth English edition

4 The Dynamic Earth was developed as an Advanced Integrated Science module for Dutch upper secondary students (pre-university stream, vwo), within the course known as Nature Life and Technology (NLT). In 2008, the module received certification from the national NLT Steering Commission. The module was developed for Junior College Utrecht ( ) by a team headed by Dr. M.L. Kloosterboer van Hoeve (module coordinator) with contributions from: Utrecht University, Faculty of Geosciences o Prof. dr. R. Wortel o Dr. P. Meijer o Dr. H. Paulssen o Dr. H. de Bresser o Dr. M. van Bergen o Dr. A. van den Berg o T.S. van der Voort Junior College Utrecht o Dr. M.L. Kloosterboer-van Hoeve (module coordinator) o Dr. A.E. van der Valk (curriculum coordinator) o K.J. Kieviet MSc (layout) Partner schools of JCU: o Baarnsch Lyceum: Drs. W. Theulings and Drs. I. Rijnja o Revius Lyceum Doorn: Drs. J. Hillebrand o Leidsche Rijn College: Drs. P. Duifhuis and Drs. F. Valk o Goois Lyceum Bussum: mw. M. Raaijmaakers Translation: ms. Anne Glerum and ms. Nienke Blom, with thanks to ms. Kate Smith for editing the English text. The translation was made possible by the PRIMAS Project ( and a SPRINT grant from the Faculty of Science and the Faculty of Geosciences, Utrecht University JCU/Utrecht University. For this module a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Netherlands License applies The authors copyrights remain with lie with Utrecht University and Junior College Utrecht, P.O. box , 3508TA Utrecht, The Netherlands. Altered versions of this module may only be distributed if the fact that it is an altered version is clearly stated, along with the name of the authors who made the modifications. In the development of this material, the authors have used materials of others. If possible, the source of the materials is mentioned and a similar or more open license is applicable. If material has been used in which the source is mentioned incorrectly, please contact Junior College Utrecht (jcu@uu.nl ). The module has been composed with care. Utrecht University and Junior College Utrecht do not accept any responsibility for any damage that originates from this module or the use of this module. An electronic version of this module (pdf format) can be found on Teacher materials such as a teacher guide as well as the module in MSWord format can be requested at jcu@uu.nl.

5 The Dynamic Earth Table of Contents Table of Contents Table of Contents 4 Introduction to the English edition 6 Introduction 8 An interdisciplinary course 8 Four kinds of exercises 8 Glossary 8 Chapter 1. Basic Theory Introduction How to use maps and the atlas Types of rock Dating of rock 15 Chapter 2. Plate Tectonics The observations that led to plate tectonics theory Three steps to plate tectonics Structure of the Earth The engine driving plate motion Types of motion at plate boundaries Intraplate motion Describing plate motions 35 Chapter 3. Earthquakes and tsunamis Earthquake waves The relationship between plate tectonics and earthquakes The development of an earthquake The motion of plates during an earthquake The strength (magnitude) of an earthquake The relation between earthquakes and tsunamis 62 Chapter 4. Volcanoes Volcano types and occurrences Formation of different types of igneous rocks Magma formation 74 4

6 Table of Contents The Dynamic Earth 4.4 Volcanoes on Iceland The quantity of gas emitted during the Laki eruption Acid rain in Western Europe The consequences of acid rain 85 Chapter 5. Mountain building The relation between mountain formation and plate tectonics Information about orogenesis: hidden in rocks The relation between force, stress and deformation Fast deformation (brittle behaviour) Slow deformation (ductile behaviour) What different types of deformation tell us about orogenesis. 119 Chapter 6. Convection: the Earth as an heat engine Introduction The interior structure and chemical composition of the Earth A model for the Earth s thermal state The start of convection in a viscous medium The internal temperature of the Earth Liquid magma within solid mantle rock Recent developments What have we learnt? 146 Glossary 147 5

7 The Dynamic Earth Introduction to the English edition Introduction to the English edition Junior College Utrecht, JCU, developed the original Dutch version of this module in cooperation with the Department of Earth Sciences from the Faculty of Geoscience, Utrecht University ( JCU offers an enriched science curriculum to select groups of talented and motivated secondary school students from the Utrecht region of the Netherlands. The module was tested on JCU students as well as on students from its partner schools and, as a result of evaluations, it has been revised several times. This module can be chosen as a part of the new integrated, advanced science course Nature Life and Technology (NLT) from the Dutch upper secondary curriculum. NLT was been introduced in It aims to expose students to modern science to encourage them to consider science studies as part of their future secondary education. This module aims to orientate students towards geology, geophysics and geochemistry, subjects that traditionally get little attention in the Dutch upper secondary curriculum. This module and the module the Molecules of Life have been translated into English for two reasons: - Many Dutch upper secondary schools have bilingual programs. With these English versions, the modules can be taught in the English part of those programs - Teachers and scientists from countries other than the Netherlands are interested in NLT. Using these two modules as examples, they can inform themselves about the pedagogy and methods used for these subjects. On the modules are available in pdf format on the Internet. For many parts of the module The Dynamic Earth, the context is Dutch (e.g. assignment 2.1 when did the Netherlands lie on the equator? ) Someone from abroad who wants to use (parts of) this module can adapt the context so that it is suitable for to his or her own location (under the conditions of the creative commons licence). For this, a Word version of the student material is available. In addition, teacher materials are available, including a teacher guide, outcomes of assignments and PowerPoint presentations. Send a request to jcu@uu.nl. I would like to thank Anne Glerum and Nienke Blom for translating the module, Kate Smith for editing the English text, the writing team for checking the chapters they have written, Krijn Kieviet and Saskia Klaasing for the layout of the module. Please feel free to use this lesson material in your classes. We would be very happy to hear from you and to learn about your experience of this module! Dr. Ton van der Valk Curriculum Coordinator Junior College Utrecht, Utrecht University a.e.vandervalk@uu.nl 6

8 Introduction to the English edition The Dynamic Earth 7

9 The Dynamic Earth Introduction Introduction The Dynamic Earth is a course intended for the last two years of upper secondary school education (grades 11 and 12). It is centred on the theory of plate tectonics (see Figure I, front page). Chapter 1 will provide you with all the tools earth scientists use to study the Earth. It is essential you read this chapter first. Chapter 2 explains the theory of plate tectonics and Chapter 3 focuses on an important consequence of the dynamic Earth: earthquakes. The first three chapters combine to form the basis of this course. You will have completed the course only when you have finished these chapters and one of the optional chapters (Chapter 4, 5 or 6). The other optional chapters can be used as additional course material or as project material during other courses. An interdisciplinary course So how does this course relate to your other classes? Studying the Earth is an interdisciplinary science: all the natural sciences are required to understand processes within and at the surface of the Earth. This course will build on theory from your Mathematics, Physics and Chemistry classes. The table below shows where elements of each subject are studied during the course. The red thread, plate tectonics theory, has probably already been covered in your Geography class. However, if you did not take Geography, or geology has not yet been studied within your Geography class, you can still follow this course. The glossary in the back of the reader will help you with any unfamiliar terminology. Four kinds of exercises The reader comprises four types of exercises: 1*: An exercise with one asterisk tests the knowledge that you have acquired in other courses. The exercises assume some familiarity with the subject. 2**: An exercise with two asterisks is directly related to the text preceding it. 3***: An exercise with three asterisks requires that you to apply the theory that has just been explained to other, similar cases. You will have to use what you have learned in your other courses as well as what you have just learned from this course. You should be able to apply your new knowledge to these cases. 4****: Four asterisks denote optional exercises which are more challenging. Every chapter ends with a final exercise. Here you will revisit the main questions of the chapter and, in answering them, test what you have learned. Also, you will write down any new questions that have come up while studying the chapter. The final exercises can be used in the examination as well. You can use atlases, the Internet and Google Earth to help you with the exercises. Where needed, they are denoted by A (atlas), I (Internet) or G (Google Earth). Glossary Earth scientists use many technical terms and concepts. These are underlined in the text and explained in the glossary. 8

10 Introduction The Dynamic Earth Previous This course subject matter Basic theory Plate tectonics Earthquakes Volcanoes Mountain building Structure of the Earth Classes Math, Geography Physics, Geography Physics, Geography Chemistry, Geography Math, Geography Physics Waves and vibrations Earthquakes General structure Magnetism Paleomagnetism Plate motion reconstruction Geomagnetic field Radioactive decay Dating techniques Heat source Heat source in certain layers Forces Isostasy, plate motion Earthquakes Isostasy, formation of faults Heat and phase changes Driving motion plate Magma generation Liquid and viscous layers Math and geometry Calculating plate motion Calculations faults on Chemistry Average composition Melt diagrams Phase diagrams Composition of the different layers Chemical equilibrium Types of magma, types of rock 9

11 The Dynamic Earth Chapter 1. Basic Theory Basic Theory The main question of this chapter is: How do we work with the basic tools of earth scientists: maps, rocks and dating methods? This question is addressed by answering the following section questions: How do we use maps and the atlas? (1.2) What types of rocks are recognized? (1.3) How do we date these rocks? (1.4) Objective: To obtain the skills necessary for this course. 10

12 Basic Theory The Dynamic Earth 1.1 Introduction This course discusses movements within the Earth. Some simple tools that earth scientists use when studying these movements are: Maps and atlases (1.2): To maintain an overview of the Earth, and to see where specific processes take place, we make use of maps, atlases and the Internet. Section 1.2 explains how to use these tools. Rocks (1.3): Rocks contain a lot of information about the Earth. Section 1.3 discusses the three main types of rocks. Dating of rocks (1.4): Plate movements are slow. In the earth sciences we do not consider timescales of 30, 100 or even a 1000 years, but timescales of millions or billions of years. How we date rocks using such timescales is explained in section How to use maps and the atlas In order for you to study the moving Earth, figures and maps are included in this textbook. We make frequent use of the Dutch Grote Bosatlas (see e.g. For example, the notation GB 174A refers to map 174A of the 53rd edition of the Grote Bosatlas. Google Earth is a useful tool for most exercises too. The capital letters A, I and G indicate which tool to use for each exercise: the Atlas, the Internet, Google Earth or a combination of these. Exercise 1-1*: Where do earthquakes and volcanic eruptions occur? A, I Keep track of the occurrence of earthquakes and volcanic eruptions, while working on this module. Draw their locations in on a map. Some of the other chapters will have exercises where you can reuse this map, for example Exercise 3-3 and exercise 4-1. a. Print out the blank map of the world and mark the places where you know earthquakes and eruptions occur. b. Check for new earthquakes and eruptions in newspapers and at and other similar websites. On your map, mark the location, magnitude, number of casualties and, when available, the actions and precautions taken to limit the damage done by these earthquakes and eruptions. The general term for such actions and precautions is hazard management. Exercise 1-2*: Finding the locations of earthquakes, volcanoes and mountain chains with Google Earth G a. Look up the locations of the earthquakes and volcanic eruptions found in exercise 1-1 using Google Earth. b. Use GoogleEarth for any exercise when you are adding data to your map, such as exercise Types of rock Much of what we know about processes within the Earth and on its surface comes from studying rocks. There are three main types of rocks: igneous, sedimentary and metamorphic Igneous rocks Igneous rocks are formed when liquid magma or lava solidifies. When this occurs close to or at the Earth s surface, the rock formed is called volcanic or extrusive. When this process takes place much deeper within the Earth, intrusive rock is formed. Unlike the deeper intrusive igneous rock, volcanic rock forms at the Earth s surface when liquid magma cools quickly. This fast cooling means there is little time for crystal growth, resulting in fine-grained rocks with small crystals. A crystal is a homogeneous solid with smooth flat surfaces called faces. This regular morphology is the result of the regular arrangement of the atoms making up the crystal. 11

13 The Dynamic Earth Basic Theory An example of a rock that has cooled quickly is basalt. It is a black rock that has very few visible crystals. Basalt often forms at the bottom of the ocean, where the heat dissipates quickly in the cold seawater. When molten material cools slowly there is time for a greater number of crystals to form. If there is enough space, these crystals will be larger too. An example of a rock that has cooled slowly is granite, an intrusive rock that solidifies deep beneath the Earth s surface. Granite cools slowly, enabling it to form large crystals, but often there is not enough space for crystals to grow into perfect crystal shapes. Several different crystals can easily be recognized within granite: quartz (transparent), feldspar (pink) and biotite (black). Biotite crystallizes first, followed by feldspar and then quartz. Quartz grows into the space left over by the other crystals. It does not obtain the crystal shape it could grow into if more space were available. Figure 1.1: The Rock cycle, (igneous rocks), sedimentaryand metamorphic rocks. Source: commons.wikimedia.org. The characteristics of igneous rocks depend on the circumstances under which they formed. The composition of the source material of the magma is also important. You will learn more about this in Sections 4.1 and 4.2. Possible characteristics of igneous rocks are A low density, like pumice, or a high density, like basalt Large crystals, especially in intrusive rocks Can contain metal ores A smooth and glassy morphology, like volcanic glass; it solidified so quickly there was no time for crystals to form. Basalt, granite, andesite, pumice and tuff are all examples of igneous rocks Sedimentary rocks Sedimentary rock forms after the transport and deposit (sedimentation) of loose material, the result of weathering and erosion. This loose material, or sediment, can be transported and deposited by wind (Aeolian sedimentation), oceans (marine sedimentation), rivers (fluvial sedimentation) or ice (glacial sedimentation). The sediment, for example, sand, is slowly buried under new layers of loose material. The pressure of the build-up of layers compresses the material creating rock. In our example sandstone is formed. Horizontal layering and fossils are often present in sedimentary rocks. Sediment deposited on the ocean floor is usually composed of calcium carbonate particles from marine organisms and some clay; this will become limestone. Shallower waters and rivers carry more sand; therefore this is where sandstone is formed. General characteristics of sedimentary rocks are Individual grains can be distinguished, such as sandstone A coarse surface, like sandpaper Can contain fossils A dull, matte exterior Can contain layering Rocks can contain calcium carbonate that can be detected using hydrochloric acid (HCl): if you put some drops of HCl on limestone, the rock will react and start to fizz. Examples of sedimentary rocks are sandstone, limestone, shale and lignite Metamorphic rocks Metamorphic rock is formed when igneous and sedimentary rocks are exposed to high pressure and temperature conditions. As a result of this metamorphism, the rock is very compact and 12

14 Basic Theory The Dynamic Earth additional layering can form. This layering will be perpendicular to the direction of the applied pressure. This is comparable to flattening a balloon with your hands: the balloon is elongated in the vertical direction (parallel to your hands) and shortened in the horizontal direction. New minerals form during metamorphism because of the high pressure and temperature. These give the rock a shiny appearance. Metamorphism takes place deep within the Earth s crust where both the temperature and pressure are high. However high temperatures alone can also cause metamorphism. Rocks exposed to high temperatures from hot magma intruding into the Earth s crust become metamorphic. This is known as contact metamorphism. General characteristics of metamorphic rocks are Typical shimmer or shine Characteristic layering Separate grains cannot be distinguished any more. Examples of metamorphic rocks are marble (formed from limestone), slate (formed from clay), quartzite (formed from sandstone) and granite gneiss (formed from granite). Exercise 1-3***: Classifying rocks: what goes where? Your teacher will supply you with a set of rock samples. Number each of them and write down as many characteristics as you can think of, e.g. colour, density, layering, smoothness or coarseness and the presence of fossils. Separate the samples into igneous, sedimentary and metamorphic rocks and try to name each sample. You should at least be able to recognize a basalt, a granite and a sandstone sample. 13

15 The Dynamic Earth Basic Theory Box: Carbon Carbon is an abundant element in the Earth. Lignite and coal are good examples of rocks containing carbon. Both form under high pressure and temperature from organic matter, the remains of plants. As a mineral, carbon is found in the form of diamond and graphite (and it can have the shape of Bucky ball, C 60, but this is not considered here because they are artificial, not natural). The crystal structures of graphite and diamond are very different (see Figure 1.2). Diamond is formed under extreme pressure in the order of 10 9 Pascal (1 Giga Pascal or 1 GPa). This is four orders of magnitude greater than normal air pressure of 10 5 Pa (1 bar or 1 atmosphere). Within the three-dimensional crystal structure of diamond, each carbon atom is connected to four other atoms. This way, diamond forms one large molecule with a continuous and stable lattice that is equally strong in every direction and contains no weak points. Atoms are connected through strong covalent bonds. The melting point of diamond is therefore very high, 3600 o C. Diamond is insoluble and very strong. All the electrons are used in forming bonds between atoms, which makes diamond an excellent insulator. In graphite, atoms form hexagonal rings. These rings are connected in the horizontal plane, forming one large molecule. The bonds between different planes are weak; they are known as vanderwaals bonds. The distance between layers is more than twice as big as the distance between atoms in the same plane. When you use a pencil or put on mascara, carbon layers slide off leaving a mark or deposit because the weak bonds between the layers have been broken. Unlike diamond, graphite has many free electrons, which makes it a good conductor. Its density is only two thirds that of diamond. At a pressure of 10,000 atmospheres, graphite can be compressed into diamond. Diamond is formed under high pressure, while graphite forms under lower pressure. This implies that at the low pressure of the Earth s surface graphite is stable, while diamond is not! At the Earth s surface diamond is described as metastable. This is comparable to super-cooled water: When liquid water is cooled carefully, it can reach a temperature of e.g. 5 o C without solidifying and turning into ice. With super-cooled water, only a small amount of movement is needed to initiate solidification, but for diamond, the energy needed for this phase change is much larger, the reason why it is hard to change its metastable state. Other minerals can also be stable or metastable. At the Earth s surface at a standard temperature of 20 o C and pressure of 1 atmosphere - sand and clay minerals such as illite and kaolinite are stable, while ruby and emerald are metastable. Figure 1.2: Carbon bonds of a) diamond and b) graphite. 14

16 Basic Theory The Dynamic Earth 1.4 Dating of rock We live in the Holocene, the youngest episode, or epoch, of Earth s history. The Holocene is part of the Cenozoic, the youngest era (see Figure 1.3). The Precambrian spans the period starting 4.6 billion years ago with the formation of the Earth, and ending 542 million years ago. Little is known of this period, which is again divided into three parts (see Figure 1-3): the Hadean, the Archean and the Proterozoic. Only a few rocks dating back to these early times have been found and little remains of the primitive life that existed at this time. After the Precambrian, the Phanerozoic began: the eon of life. The Phanerozoic is also divided into three parts: the Paleozoic, the Mesozoic and the Cenozoic (see Figure 1.3). The Paleozoic started 542 million years ago and comprises the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. During the Carboniferous period, coal deposits formed in many countries; salt deposits formed during the Permian period. The Mesozoic, starting 251 million years ago, consists of the Triassic, Jurassic and Cretaceous. This was the time of ammonites and dinosaurs. Mammals evolved during the Cenozoic, which started 65 million years ago. The Cenozoic is made up of the Tertiary and Quaternary periods, of which the Quaternary period is divided into the Pleistocene and the Holocene mentioned above. Alternating glacials (cold periods) and interglacials (warm periods) characterize the Quaternary. The Holocene corresponds to the current interglacial. Modern humans evolved approximately 200,000 years ago, during the Pleistocene. If we compare the lifetime of the Earth to a 12-hour clock where 00:00:00.00 is the time the Earth came into being, man s arrival corresponds to 11:59:58.20 a.m., or 1.8 seconds before noon (see the 12-hour clock of Figure 1.3). Hadean (time 2.05h) Archean (time 3.24 h) Proterozoic (time 5.06h) Paleozoic (time 46 min) Mesozoic (time 29 min) Cenozoic (time 10 min) Figure 1.3: The geological history of the earth shown in a 12 hour clock (source: The Precambrian is sub divided in the Hadean, the Archean and the Proterozoic. How did earth scientists come to such a timescale? The age of the Earth is determined with the help of two types of dating techniques: relative dating and absolute dating. Both are discussed below Relative dating Relative dating is based on the order of geological events. Relative dating uses e.g.: The principle of superposition: the rock layer at the bottom of a sequence is the youngest Fossils, the remains of plants and animals in rocks: fossils often characterize a certain period of time Paleomagnetic reversals: you will learn more about reversals in Chapter 2 15

17 The Dynamic Earth Basic Theory The astronomical timescale: this type of dating uses small changes in the orbit of the Earth around the Sun. Mathematician Milanković calculated these changes, which can be traced back in sequential layers of rock. Exercise 1-4**: How much time is needed to deposit 300 meters of clay? Small clay sediments are usually deposited at a rate of about 1 centimetre per 1000 years. The Dutch subsurface contains a thick layer of clay part of the Rupel Formation that is in parts over 300 meters thick. a. Disregarding soil settling and drainage, how long would it take for 300m of clay to be deposited? b. If you take soil settling and drainage into consideration, would this deposition take more or less time? Hint: both settling and drainage result in denser material Absolute dating The real (numerical) age of a rock can be determined using absolute dating techniques. These techniques are based on the process of radioactive decay. Certain elements have isotopes that are unstable and start to decay after a given time. As they decay, these isotopes are converted into isotopes of another element and emit radiation. With the help of experiments, the time it takes for half of the original amount of isotope to decay have been established for every radioactive isotope. This period of time is called the half-life of an isotope. The half-life does not depend on the original amount of isotopes: a tonne of radioactive material decays to half a tonne in the same amount of time as a milligram of the same material to half a milligram. The half-life is not effected by temperature, pressure, chemical reactions or other processes. Every unstable isotope has a different half-life. If the half-life of an isotope is known and the amount of new isotope formed can be measured, the absolute age of a rock that contains both isotopes can be determined. For dating rocks older than 50,000 years, the potassium isotope 40 K is often used. Rocks containing potassium are common; the potassium fraction is made up of 7% 41 K, 0.012% 40 K and 93% 39 K. The former and latter are stable, but 40 K is radioactive. On average, 11% of 40 K decays to 40 Ar (argon) by electron capture: an electron from the K-orbit is pulled into the nucleus. Together with a proton, it an extra neutron is formed. The half-life of the 40 K isotope decaying to 40 Ar is years. So, after 1.31 billion years, half of the 40 K isotopes have decayed to 40 Ar. To use this decay reaction for dating, you must find rocks that did not contain 40 Ar when they first formed. Otherwise the original 40 Ar influences the age calculation, resulting in an incorrect age determination. Volcanic rock is a good candidate: argon gas is completely expelled from molten lava. It is only when lava solidifies that 40 Ar forms from the decay of 40 K. This 40 Ar is captured in the crystal lattices of the minerals containing potassium. As the 40 Ar/ 40 K ratio can be measured, the age of the volcanic rock can be determined. Exercise 1-5***: K/Ar dating a. Why is the K/Ar dating method of little use when dating Pleistocene deposits? b. What are the two main controls for the reliability of K/Ar dating? c. Argon is a gas and can easily escape. What will be the resulting error in your dating due to argon escape? Explain. Other decay reactions used in absolute dating are rubidium/strontium ( 87 Rb to 87 Sr decay, half-life of years) and uranium/lead ( 238 Ur to 206 Pb and 235 Ur to 207 Pb decay, half-life of years and years respectively). Exercise 1-6**: Discrepancy between different dating techniques A geologist discovers the fossils of a fish that is characteristic for the Devonian period. The fossils are found in slightly metamorphosed rock. The age determined with Ru/Sr dating is only 70 million years. How could you explain the discrepancy between relative and absolute dating? 14 C dating is used for geologically young rock, that is, rock no older than 70,000 years. This method is based on the ratio of the amount of 14 C (a radioactive isotope of carbon) compared to the amount of 12 C in material containing carbon. Plants and animals have a constant amount of both isotopes due to their continuous carbon exchange with the atmosphere. 14 C is formed as the result of the interaction of cosmic neutrons and 14 N in the atmosphere. The ratio 12 C to 14 C is about 1 to , although this is not constant through time and the ratio must be corrected for this 16

18 Basic Theory The Dynamic Earth variation. When plants and animals die, the exchange of gases with the atmosphere ceases and the amount of 14 C decreases due to radioactive decay. The half-life of 14 C is 5730 ± 40 years. Since the atmospheric isotope ratio 12 C/ 14 C is known, the age of organic material can be determined from the ratio of carbon isotopes present in a sample. Because the half-life of 14 C is relatively short, after a certain amount of time there is too little 14 C to detect, hence the maximum determinable age of 70,000 years. Carbon dating can only be used on rocks and deposits containing organic material (e.g. wood, charcoal, seeds, nuts, bones and shells) and peat Radioactive decay and age calculations Radioactive dating Scientists use radioactive carbon ( 14 C) to date biological remains because the amount of 14 C in the atmosphere has been more or less constant throughout history. As plants incorporate 14 C and 12 C through photosynthesis and animals through eating plants, the amount of 14 C within living material is in equilibrium with the atmosphere. However, when a plant or animal dies, it no longer absorbs 14 C. As 14 C decays and its concentration decreases, the 12 C/ 14 C ratio within the plant or animal changes. Therefore the quantity of 14 C found in biological remains depends on the length of time they have been buried. Simple 14 C-dating calculations The factor by which the amount of 14 C in organic material has decreased is used to determine the time passed since decay set in. This factor is easily established when understanding decay through time. The half-life is symbolized by t½. Every period of time of t½ = 5730 years results in a decay factor ½. After 5730 years the original amount of 14 C should thus be multiplied with ½, in which case only half of the amount remains. As one half-life has passed, only half of the original 14 C isotopes are left. So After a period of 5730 years the decay is (½) After a period of 3 x 5730 years the decay is (1½) x (½) x (½) = (½) 3 In general, after a period of n x 5730 years, decay is (½) n. This also applies to non-integer values of n. For example, if n = ¼, after a period of ¼ x 5730 years the decay should be (½) ¼ because four of those periods will give a decay of ½. The above can be summarized in the following equation: N(t) = N(0)*( ½) n Here N(t) is the amount of decaying isotope left at time t, while N(0) is the original amount of that isotope. The fraction ½ indicates halving the original amount n times, and n corresponds to the n from the previous example, the number of half-lives. If n = 3, three times 5730 years have passed. What percentage of the original amount of radioactive isotope remains after n = 3? Take N(0) = 1 and n = 3, this will give N(t) = 1*(½) 3. Thus N(t) = 0.125, or 12.5%. Example In a peat sample, only 10% of the original amount of 14 C is left. How old is the sample? Suppose the sample is n half-lives old. As 100% = 1, 10% = 0.1. N(0) is therefore 1 and can be left out of the equation. What remains is: n ( 1/ 2) You can find n by taking the logarithm of both sides and then using logarithmic identities. n log( 1/ 2) 0.1 log(0.1) The age of the organic material is then n times the half-life of 5730 years, in this case 19,000 years. 17

19 The Dynamic Earth Basic Theory General equation for radioactive decay (here we will use exponential functions) N(0) is the original amount of radioactive isotope; N(t) is the amount of isotope after time t; t ½ is the half-life of the isotope; and n is the number of passed half-lives, n = t t 12. If, for example, three times 5730 years have passed and the half-life is 5730 years, then for this t = 17,190 years and t½ = 5730 years, n = 3. This gives: t t Nt () N(0) 1/2 12 You know ½ = e -ln2 ; you can check this on your calculator. This allows you to express the formula above as an exponential function. Usually not the half-life, but the decay constant λ is used in such a formulation: ln 2 t Therefore, the general equation for radioactive decay is written as: 1/2 Nt () N(0) e t Exercise 1-7***: Calculations on radioactive decay a. Derive the last equation yourself. Differentiate it to obtain an equation for the rate of change of amount of isotope, N (t). b. Show N (t)/n(t) = -λ by using substitution. c. Describe the meaning of λ: λ is the ratio between. Hint: Look at the equation in b and the meaning of the derivative of N(t). In addition, you can now do the optional exercise 1-1 you can find at the end of this chapter The dating of rock and the geological timescale All absolute and relative dating techniques have the same limitation: the older the tested material, the less accurate the results. Therefore as many different techniques as possible are applied to the same rock. Unfortunately only certain types of rocks are appropriate for a certain techniques: fossils are only found in sedimentary rock, 40 K dating can only be done on igneous rock and 14 C dating only on organic material. Absolute and relative dating enables scientists to establish extensive and detailed geological timescales. The most commonly used timescale is from the International Commission on Stratigraphy (see Simple representations of this timescale can be found in Figure 1.4 and in the atlas, map GB 192E. 18

20 Basic Theory The Dynamic Earth Figure 1.4: Geological timescale. The top of the first column corresponds to the entire second column and the top of the second column corresponds to the third column. Source: Exercise 1-8*: Mass extinctions and the geological timescale Several times during the geological past, many (sometimes over 70%) of the species present became extinct within a short period of time. These events are called mass extinctions. The four most important mass extinctions occurred 444 million years ago, 416 million years ago, 251 million years ago and 65 million years ago. a. In what periods of the timescale of Figure 1-4 did these events occur? b. What is it about the timescale that makes these extinctions so obvious? Final exercise Ch1. Answer the section questions and the main question a. Answer the three section questions as well as the main question from the beginning of this chapter. b. If you find you have new questions after reading this chapter, write them down. Optional exercise 1-9****: Determining the age of ice (Based on a question in the Dutch final exam Physics, havo 1998-I. Question a, b and c lie outside the scope of this course, but are essential to the question as a whole.) Greenland. A group of scientists are investigating the ice at the North Pole. They drill a hole in the ice over 3 km deep, retrieving ice cores of 2.5 m in length and 84 cm 2 in cross section. By investigating the ice cores, knowledge of the past - for example of the average temperature on Earth during the formation of the ice - is gained. Past temperatures are known from the concentration of two particular oxygen isotopes. The age of the ice can be obtained by dating ash layers within the ice with C-14 dating. The concentration of C-14 in the ashes is compared to the normal concentration of C-14 in the atmosphere. 19

21 The Dynamic Earth Basic Theory After: Mens en Wetenschap, July 1994 The temperature of an ice core is -4 C. a. Calculate the mass of the ice core. Hint: You can look up the density of ice, and calculate the volume of the core. Oxygen isotopes are mentioned in the article. b. Name one difference and one similarity on a molecular level between the two different oxygen isotopes. C-14 dating is based on the decay of the radioactive isotope 14 C. c. Write down the decay reaction of 14 C. With time, the concentration of 14 C in the ashes found in the ice cores decreases due to radioactive decay. Measurements show that the present concentration of 14 C in the ashes is 25% of the normal concentration in the atmosphere. d. Calculate the age of the ashes. 20

22 Plate Tectonics Chapter 2. The Dynamic Earth Plate Tectonics The main question of this chapter is: Which theory lies behind the concept of plate tectonics, what drives plate tectonics and to what extent do we notice plate motion in daily life? This question is addressed by answering the following section questions: Which observations advanced the theory of plate tectonics? (2.1) Which three steps led to the theory of plate tectonics? (2.2) What does the interior structure of the Earth look like? (2.3) What causes plates to move? (2.4) What types of motion occur along plate boundaries? (2.5) Are movements only noted along plate boundaries? (2.6) How do we describe plate motion? (2.7) Objective: To describe the current distribution and motion of plates by making use of the theoretical knowledge you gained from Mathematics and Physics classes. 21

23 The Dynamic Earth Plate Tectonics 2.1 The observations that led to plate tectonics theory Consider capturing the Earth on film from a satellite: in a day several things would clearly change. For example: different places would experience day or night, cloud coverage or perhaps tornados. But the spacing of the continents would not appear to change. However, if you were to continue recording the Earth for, say, a century, you would see even the continents move about. Continents do not move fast, they move at about the same speed as your fingernails grow. Exercise 2-1*: When did The Netherlands lie at the equator? Your fingernails grow at about 2 to 3 mm per month, knowing this a. How long would it take for a part of a continent to move from the equator to the current position of The Netherlands? From Section Metamorphic rocks, you know that extensive coal deposits occur in The Netherlands. Since coal forms from tropical swamp vegetation, the deposits imply that The Netherlands used to lie at the equator. b. Assuming plates move with a speed of 2 to 3 mm per month, how old are the Dutch coal deposits and during which geological period did they form? c. Research shows that coal in The Netherlands formed during the Carboniferous period (named after the coal, which is typical for sediments of this period). Is this dating consistent with question b? If not, how do you explain the discrepancy? How long have we known that parts of the Earth s crust move? Before plate tectonics theory, scientists assumed only vertical crustal movements could take place. Their assumptions were based on the observation that the surface of the Earth is divided into oceans and continents. Both are relatively flat, but they are significantly offset in elevation. The bottom of the ocean is about 3 km deep, while the continents are 1 to 2 km high. Weathering and erosion should have evened out this difference in elevation by now. As the difference still exists, scientists thought that vertical movements must have preserved it. They expected the lighter continental rock to rise higher and higher with time, while the heavier oceanic rocks were supposed to sink. However, some scientists suggested that horizontal movements occurred in addition to vertical motion. Explorers in the 15 th and 16 th century, trying to map the Earth, noticed something peculiar about the coastline of the Atlantic Ocean: the east coast of North and South America looked like a reverse cut-out of the west coast of Africa and Europe. They surmised that America and Africa were connected in the past. From 1900, more and more observations were made that supported this connection: The same fossil species occurred on both continents (Figure 2.1) These organisms could not have crossed today s vast oceans, so the continents must have been connected some time in the past. There were traces of large ice sheets covering several continents. Such vast glaciations could only have occurred if all the ice-covered parts were situated at the pole at the same time. The same rock assemblages were found on both sides of the Atlantic. Most likely, they formed in one place before the continents split up. It was Alfred Wegener, a German meteorologist, who greatly improved our understanding of plate motion. He wrote the book The Origin of the Continents and Oceans (1925), which formed the basis of the modern view on plate motion. He suggested that all continents were connected in the past, forming the supercontinent Pangaea, but then drifted apart during the Tertiary period. Wegener called his idea the continental drift hypothesis. 22

24 Plate Tectonics The Dynamic Earth Figure 2.1: Because the same fossil species occurs on different continents, it is evident that the continents were juxtaposed at some time in the past. (Source: Exercise 2-2**: Wegener s arguments for continental drift. A The fact that the continents fit together almost perfectly, and other observations such as the distribution of fossils and the large 250-million-year-old ice sheets, fuelled Wegener s ideas about plate motion. a. Use the atlas to name the geological period during which the large ice sheets formed. b. Figure 2.2 shows where Wegener found traces of ice covering the continents in the past. What two conclusions can you draw based on Figure 2.2 about the previous connection between the ice-covered continents and their location? Wegener s hypothesis left several issues unexplained: Figure 2.2: Distribution of traces of glacial cover in 250-million-year-old sediments. (Veenvliet, 1986) Which part of the crust actually moves? The continents themselves, the oceans or something deeper? What mechanism is strong enough to cause crustal movements? It wasn t until 1960 that science made it possible to resolve these issues. Exercise 2-3**: Questioning Wegener s continental drift hypothesis a. Think of a mechanism that could explain continents drifting apart. b. Write down some questions you would ask Wegener to decide whether or not you agree with his theory. 23

25 The Dynamic Earth Plate Tectonics In Geography classes nowadays, you can learn that the continental and oceanic plates on the Earth s surface move with respect to each other and that the interior of the Earth moves as well. These ideas form the paradigm of plate tectonics, which has been the leading paradigm in the earth sciences since the late 1960s. A paradigm is a coherent set of models and theories providing a certain mindset with which to analyse reality. However, not everything about plate tectonics is known yet. Much uncertainty still exists about the processes active within the interior of the Earth that drive plate tectonics. The steps needed to get from Wegener s continental drift hypothesis to the paradigm of plate tectonics are explained in the next section. Plate tectonics considers the movement of plates. Plates are composed of continental and/or oceanic crust and the upper part of the mantle. You will learn more about the structure of plates in Section Three steps to plate tectonics Determining the oceans bathymetry: Where does new material form? The first studies that provided insight into what causes the Earth to move came from a record of the bathymetry (relief) of the ocean floors. Mid-twentieth century equipment allowed for even the deepest parts of the oceans to be mapped. These measurements showed that in the middle of oceans there were shallower areas. These areas formed ridges over the whole length of the oceans (see Figure 2.3). Further research demonstrated these Mid-Ocean Ridges (MORs) were made of active underwater volcanoes. Researchers concluded that it is at MORs where new material rises up from the depths. Figure 2.3: Topography and bathymetry of the Earth. 24

26 Plate Tectonics The Dynamic Earth Exercise 2-4*: Topography of the ocean floors A Recife in Brazil and Mount Cameroon in Africa lie more or less on the same latitude at both sides of the Atlantic Ocean. a. Draw a depth profile starting from Recife to Mount Cameroon. Ascension s highest volcano reaches up to 859 m. Use the atlas. b. What is the difference in elevation between the highest and lowest point of this profile? Paleomagnetism: How do we prove plates move? Scientists wondered if the formation of new material at MORs could have anything to do with the plates drifting apart. If so, the crust on both sides of a MOR should increase in age the further it gets from the ridge. Paleomagnetism, i.e. the rock record of the past magnetic fields of the Earth (paleo is the Greek word for past), enables the dating of the crust The Earth s magnetic field A wonder of such nature I experienced as a child of 4 or 5 years, when my father showed me a compass. That this needle behaved in such a determined way did not at all fit into the nature of events which could find a place in the unconscious world of concepts. I can still remember - or at least believe I can remember - that this experience made a deep and lasting impression upon me. Something deeply hidden had to be behind things... Albert Einstein The outer core of our planet consists of liquid iron and nickel (see Chapter 6), and acts as a large magnet with north and south magnetic poles. The Earth s magnetic field resembles that of a bar magnet positioned within the Earth (see Figure 2.4: The Earth as a large bar magnet. (Source: Figure 2.4: The Earth as a large bar magnet. (Source: Physics class taught you how to work with magnetism. You will now use that knowledge to measure the magnetic field in the place where you live (Exercise 2-5). To practise working with magnetism, you can also complete Optional Exercise 2-1 (at the end of Chapter 2). 25

27 The Dynamic Earth Plate Tectonics Exercise 2-5**: Determining a magnetic field (including a practical) You will need: coiled wire, a magnetic needle and a variable power source. In this practical we will determine the magnitude and direction of the Earth's magnetic field strength in your hometown. This is called the magnetic induction B, being a vector i.e. having a magnitude and a direction. 1) Take a coil with loops that weave through a sheet of plastic and position it horizontally. The coil needs to be about 25 cm long with 40 loops. 2) Place a magnetic needle within the coil. Make sure the magnet can spin freely in the horizontal plane. 3) Align the middle line of the coil with the magnetic needle (see Figure 2.5) Connect the coil to the power source in series with a variable resistor and an electric current meter. Varying the strength of the current will show that at an electrical current strength of 90 ma the needle does not maintain a fixed position, but will spin freely. a. Calculate the magnetic induction B generated by the coil. Use B coil N I L 0 with B the magnetic induction, μ 0 the permeability of free space of 4π 10-7, N the number of loops on the coil, I the current strength and L the length of the coil. b. Why does the needle not maintain a fixed position at 90 ma? c. Copy Figure 2.6 and mark the direction of the current in the coil. The dark part of the compass needle in Figure 2.6 denotes the north magnetic pole. Take a magnetic needle that can spin freely in the vertical plane (Figure 2.7). d. Measure the angle between the needle and the horizontal plane. e. Use this angle to calculate the direction and magnitude of the magnetic induction of the Earth's magnetic field (B tot ). Hint: Use the cosine of the angle found and study Figure 2.7. Figure 2.6 Heart line Figure 2.7 Figure

28 Plate Tectonics The Dynamic Earth The magnetic poles of the Earth s magnetic field lie close to the geographic poles. The geographic poles, at 90 north and south of the equator, are the two locations where the Earth s rotation axis meets the surface. The location of the magnetic poles varies slightly on a yearly basis (figure 2.8), but these variations are not big enough to influence, for example, the calculations you did to discover the magnetic field for your hometown. Figure 2.8: The movement of the magnetic north pole. (Source: Besides small yearly variations, the magnetic poles also experience variation on a larger scale: every ten to one hundred thousand years the poles reverse. During such a reversal the north geographical pole changes from south magnetic pole to north magnetic pole or vice versa (Figure 2.9). Scientists do not know why magnetic reversal occurs; most likely it is related to processes within the Earth s core and mantle. Because we do not know why reversals occur, it is not possible to predict the next reversal. Figure 2.9: Reversals of the Earth s magnetic field. In the present situation (normal), the north geographic pole coincides with the south magnetic pole. Rocks contain a record of magnetic reversals: the magnetic minerals locked inside them are aligned with the direction of the ambient geomagnetic field at the time the rock solidified or was deposited. This is called paleomagnetism. Earth scientists can read these directions and are able to determine whether the rocks were formed during a period of normal (as it is now) or reversed (opposite to now) polarity. Geomagnetic reversals, which occur either side of periods of normal or reversed polarity, can be dated using techniques such as radioactive dating or by referring to fossils. With the age of reversals known, a paleomagnetic timescale can be computed (see Figure 2.10). 27