THE STRUCTURE AND DYNAMICS OF THE EARTH

Transcription

1 1 INTRODUCTION AND RESOURCES THE STRUCTURE AND DYNAMICS OF THE EARTH INTRODUCTION TO THE UNIT The Earth formed in the solar system about 4.5 billion years ago. Various processes and circumstances determined how the Earth evolved. The result is a unique system consisting of different subsystems or spheres. Although they are separate, the spheres interact with each other. Matter and energy are constantly being exchanged, as if the Earth were a machine. Section 1 outlines the Earth's evolution and provides a framework for explaining the nature of the geosphere. Sections 2 and 3 describe the propagation of seismic waves through the inaccessible geosphere. The geochemical and geodynamic models, based on the behaviour of those waves, are then examined. Section 4 describes the two main engines driving the internal dynamics of the Earth: internal heat and gravity. Section 5 describes the vertical movements of the lithosphere and their effects. Section 6 summarizes the scientific theories proposed about how the continents evolved. First, in 1912, Wegener proposed the continental drift theory. In 1962, Harry Hess confirmed his seafloor spreading hypothesis, which led to a new theory: plate tectonics. The theory of plate tectonics, developed in the 1960s, is the most important paradigm in earth sciences. It is a unifying theory that can explain and predict most structures and phenomena on Earth. The importance of the mobile theories that preceded the theory of plate tectonics should be pointed out. Students will then understand how science has progressed; how the relationships between science, technology and society have evolved; and how earth sciences developed. Finally, section 7 sets out the principles of the theory of plate tectonics. The main plate movements are explained in the context of the events and structures they generate. CONTENTS FIND OUT ABOUT The origin of the solar system and the Earth. The internal structure of the Earth. The geodynamic model. The internal engine of the Earth. Vertical movements of the lithosphere. Horizontal movements of the lithosphere. Plate tectonics. KNOW HOW TO Interpret remanent magnetism. Interpret bathymetric charts. Present research; Express contrast and concession. See Language focus, page 16 of this Teacher s Book. BE ABLE TO Interpret natural phenomena based on the explanations and implications of the theory of plate tectonics. Recognize how the contributions of many scientists over the course of history have led to our current knowledge of geology. Understand the value of controversies as generators of scientific development. 26 BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L.

2 SPECIAL ATTENTION Content Understanding the Earth as a dynamic system in which different spheres exchange matter and energy, influencing one another, can challenge students. This model works at scales ranging from planetary to microscopic. Countless variables such as speed, duration and intensity of processes are involved. The objects and phenomena studied in this unit (internal structure of the Earth, formation of mountains, etc.) are huge and occur over long periods of time. Direct observation and experimentation are difficult. Students must therefore understand how indirect methods of study, models and theories explain nature and processes at a planetary level. Point out that such theories and methods change as science, technology and society advance. Students should be able to differentiate between observable facts and their interpretation. Among the facts are those described by Wegener and those discovered later: seafloor topography, age of seabed, distribution of earthquakes and volcanoes, etc. The interpretation of these facts includes how they are related to establish a theory, hypothesis or model. It is also important that students understand the concept of geothermal gradient, and the ways in which heat is transmitted in the Earth: radiation, conduction and convection. The latter is particularly important in: the dynamics of fluid spheres (atmosphere and hydrosphere); the internal dynamics of the Earth; and the dynamics of the lithosphere and associated phenomena, such as recycling of oceanic crust, volcanism, movement of continents and earthquakes. Digital resources Audio tracks. See transcripts on pages 144 and 146. Reinforcement: Student audio. Unit 1. Page 10. Activity 1. Extension: Class audio. Unit 1. Plate tectonics. Page 17. LibroMedia. Unit 1. The structure and dynamics of the Earth. RECOMMENDED RESOURCES IN ENGLISH INTERNET RESOURCES WEBSITES Birth of the solar system. STEM. Animation that explains how the Earth was formed. Key words: stem, birth, solar system Dynamic Earth. Plates & boundaries. Interactives. Annenberg Learner. Animations showing the types of boundaries and their locations on a map. Includes a two-part activity: The plates and boundary challenge. Key words: dynamic earth, plates, boundaries Essentials of geology. Chapter 2: The way the Earth works: plate tectonics. Animations. A collection of short animations by W. W. Norton & Company. Topics include plate boundaries, transform faulting, subduction, hot spot volcanoes and rifting. Key words: plate tectonics, animations, norton Plate tectonics animations. USGS. Short animations from the video Secrets in Stone. Key words: usgs, plate tectonic animations Regents Earth science resources: plate tectonics HMXEarthScience. A large collection of resources with links to animations and videos. Key words: hmxearthscience, plate tectonics APPS FOR TABLETS AND SMARTPHONES Puzzling plates. Google Play and itunes. An interactive experience on plate movement, causes of earthquakes and volcanoes. Three levels each with a bonus round. For ipad and Android. The supercontinent of Pangaea. Google Play. Topics such as mantle convection, seafloor spreading, Wegener s theory and plate tectonics can be explored interactively. Quizzes to test your knowledge. For ipad and Android. DOCUMENTARY FILMS Planet Earth. PBS, Episode 1: The living machine. This award-winning documentary examines plate tectonics and geological time. The living planet. BBC, Episode 1: The building of the Earth. The forces that shaped the Earth and how continents move are examined among other topics. The miracle planet. Japan's NHK and the National Film Board of Canada (NFB), A seven-episode series about how the Earth evolved. Time machine. BBC, A documentary in three episodes narrated by David Attenborough. Millennia are compressed into milliseconds to explain how time shapes the world. BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L. 27

3 1 REINFORCEMENT 1 Label the two models of the Earth: geodynamic or geochemical. Then label the layers and seismic discontinuities. The model The model 28 BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L.

4 2 REINFORCEMENT 1 Label the diagrams: D" layer or lithosphere. Then label the components. The The BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L. 29

5 3 REINFORCEMENT 1 Label the layers and describe the convection currents: ascending current or descending current. 2 Complete the sentences. Write core, mix, independent and densities. In the, the layers have different This results in the generation of convection currents because the layers cannot 30 BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L.

6 4 REINFORCEMENT 1 Write the name of each tectonic plate Plate boundary Oceanic trench Origin and direction of movement of plates Plate collision zone Speed of plate movement (cm/year) Earthquake zone Major active volcano _P15_placas_tectonicas Interpret the map key. Name the plates that move more than 5 cm per year. 3 Use the map and the key to describe your country s location: tectonic plates, earthquake zone, etc. BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L. 31

7 5 REINFORCEMENT SUMMARY Crust: Layers in the geochemical model Continental: Oceanic: Mantle: Upper mantle: Transition zone and lower mantle: Core: Outer core: Inner core: Seismic discontinuities Mohorovicic: ˇ Gutenberg: Wiechert Lehmann: Layers in the geodynamic model Lithosphere: Sublithospheric upper mantle: Mesosphere: D" layer: Core: The blocks into which the lithosphere has been fractured can be continental, or mixed. Their movements NORTH AMERICAN 1.8 Tectonic plates produce various effects: Movement of the and contraction of the On plate boundaries, 3.0 JUAN DE FUCA PACIFIC Plate boundary Oceanic trench Origin and direction of plate movement COCOS 7.2 Plate collision zone Speed of plate movement (cm/year) Earthquake zone Major active volcano and NAZCA NORTH AMERICAN CARIBBEAN SOUTH AMERICAN SCOTIA EURASIAN IRANIAN PHILIPPINE ARABIAN AFRICAN INDO-AUSTRALIAN ANTARCTIC PACIFIC activity takes place _P15_placas_tectonicas 32 BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L.

8 6 EXTENSION THE MOST TECTONICALLY ACTIVE PLACES IN THE WORLD Geological activity in the Earth's interior generates the main structures on its surface: mid-ocean ridges, mountain ranges, volcanic islands, oceans, etc. Earthquakes and volcanic eruptions are the most spectacular forms of geological activity. They are catastrophic events. They occur rapidly and their intensity, effects and periodicity have significant impacts on human beings. Tectonically active regions are found all over the world. Many are located on the boundaries between tectonic plates. However, some have developed due to other factors. Most volcanic eruptions are invisible to us because they occur on the seafloor where they generate mid-ocean ridges. In contrast, visible eruptions are often spectacular. They can be explosive or occur under glaciers as in Iceland, Kamchatka or Colombia. Volcanic eruptions are considered a natural geological risk. Measures are taken to mitigate their effects or to provide protection. Earthquakes and tsunamis are the most-feared natural disasters because they can cause deaths, injuries and damage. They are widely reported in the news. In contrast, each year thousands of smaller earthquakes go unnoticed by the general public. They can only be detected by seismographs. PROCEDURE Project objective: find out about the most tectonically active regions on Earth. Present your results as a map and as a brochure. Recommended research topics: Do research on one of the following places, in your group: Iceland, lakes in East Africa, the Red Sea, Hawaii, Yellowstone National Park, the San Andreas Fault (California), the Himalayas, the Andes, Japan, the Aleutian Islands or the Lesser Antilles. Summarize the characteristics of the place. Describe the causes of its geological activity. Find two types of pictures of the place: some showing its current relief and some showing past catastrophes. Draw a cross-section (not to scale) to show the relation between tectonic plates and the interior of the Earth. Find news articles about the catastrophes caused by earthquakes and volcanoes in the place selected. Find out about the geological risk of volcanoes and earthquakes in the place selected. Research sources: The Internet: news articles, YouTube, blogs. Website of the United States Geological Survey (USGS). Development time: three to four sessions. Organization: groups of three or four students. Presentation: Class poster: a map of the world showing the place each group studied. Each place should be labelled with information on its main characteristics and causes of geological activity. Brochure: two to four pages describing a research trip to the place studied. Include the cross-section, photos and extracts from the news articles you found. Cite your sources. BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L. 33

9 7 EXTENSION SCALE DRAWING OF THE INTERNAL STRUCTURE OF THE EARTH PROJECT OBJECTIVES Learn how to draw diagrams using a linear scale. Estimate the dimensions of the main layers of the Earth in the geochemical and geodynamic models. Develop presentation skills: plan a layout; insert a title and indicate the linear scale. Label layers and seismic discontinuities. Learn how to use unconventional resources to draw arcs. DATA For all of the measurements, the surface of the Earth is considered to be at an elevation of 0. Radius of the Earth: km Lower limits of the layers in the geochemical model: crust: 35 km (average) upper mantle: 670 km lower mantle: km outer core: km. Lower limits of the layers in the geodynamic model: lithosphere: km (average) sublithospheric upper mantle or asthenosphere: 350 km mesosphere: km D" layer: km core: to the centre of the Earth. Geochemical model Geodynamic model ACTIVITIES 1 Use an easily accessible linear scale. For example, a notebook with squared paper. The length of one square could equal 100 km. Measure and mark the radius of the Earth on the paper. Then, draw an arc, like a wedge of cheese, and divide it into two parts: left for the layers of the geochemical model; and right for the layers of the geodynamic model. Separate the right models with a line representing the radius of the Earth. Draw the layers and colour them in as you wish. 2 Use a scale of 1: In other words, 1 cm on the diagram equals 10 km on the Earth. How large would the model be? Would it fit on one page in your notebook? 34 BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L.

10 8 EXTENSION TECTONIC S AND OCEANIC TRENCHES THE PRINCIPLE TECTONIC S The Eurasian plate. It includes the seafloor east of the Mid-Atlantic Ridge, Europe and most of Asia to the Japanese archipelago. In the Atlantic Ocean, it shares a divergent plate boundary with the North American plate. The southern side is colliding with the African plate. This formed the Alps. The eastern side is colliding with the Pacific and Philippine plates. The Pacific plate. An enormous oceanic plate that shares convergent boundaries with other plates in the Pacific Ring of Fire. The Indo-Australian plate. It includes India, Australia and the surrounding oceans. The collision of this plate and the Eurasian Plate formed the Himalayas. The South American plate. A large plate with a convergent western boundary that generates intense seismic and volcanic activity. The Nazca plate. An oceanic plate that collided with the South American plate to form the Andes. The Philippine plate. It is surrounded by convergent boundaries associated with subduction zones. Oceanic trenches and island arcs are present. The North American plate. Associated with the famous San Andreas Fault (California), this transform fault is part of the Pacific Ring of Fire. The African plate. A mixed plate. In the north, it collided with the Eurasian plate to form the Mediterranean Sea and the Alps. In the south, a rift is forming that will divide Africa into two parts. The Arabian plate. This plate is rifting or separating from Africa along its western boundary. The Red Sea and the Gulf of Aden are forming as a result. DIMENSIONS OF OCEANIC TRENCHES Trench Depth Width Length (km) (km) (km) Aleutian Japan Java Kuril Mariana Middle America Peru Chile Philippine Puerto Rico South Sandwich Tonga Trench ACTIVITIES 1 Locate each of the trenches above on a world map. 2 Do research to answer the following questions: a. In which ocean are most of the trenches located? b. Are the trenches located in the middle of the ocean or close to continents or islands? Why do you think this is? c. How do oceanic trenches form? Write a short explanation. Include a diagram. 3 Find out what types of organisms live in these oceanic trenches. BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L. 35

11 1 ASSESSMENT 1 Briefly explain the origin of the different layers of the Earth. 2 Interpret the graph below and label the discontinuities shown. Upper mantle Lower mantle Outer core Inner core Seismic wave velocity (km/s) S-waves P-waves Kilometres 3 Wegener presented various types of evidence to support his theory of continental drift. a. Describe two of those types of evidence. b. Why was Wegener's theory of continental drift rejected? 4 Think about the seafloor. a. Which technological advances made it possible to explore the seafloor? b. Describe two characteristics of mid-ocean ridges that support Hess's seafloor spreading hypothesis. 36 BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L.

12 5 The Atlantic Ocean is widening at an average rate of 1 4 cm/year, depending on the region. The Pacific Ocean, on the other hand, is shrinking. Explain why this is happening. 6 Define the following terms: Lithosphere: Tectonic plate: Collisional orogen: Transform fault: Isostasy: 7 What is the origin of the internal heat of the Earth? 8 Label the main tectonic plates on the map. What type of lithosphere makes up the Pacific, African and Eurasian plates? A B A A. B. D F C. D. C E E. F. 9 The Scandinavian Peninsula is rising at a rate of 1 cm/year. Propose a hypothesis to explain this phenomenon. 10 List the main geological processes caused by convection currents in the mantle. BIOLOGY AND GEOLOGY 4. Photocopiable material 2017 Santillana Educación, S. L. 37