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Transcript
KS4 Physics Earth’s Structure 1 of 20 32 © Boardworks Ltd 2005 2004 Contents Earth’s Structure Earth and earthquakes Seismic waves More about plate boundaries Continental drift Summary activities 1 2 of 20 32 © Boardworks Ltd 2005 2004 Structure of the Earth crust mantle inner core outer core 1 3 of 20 32 © Boardworks Ltd 2005 2004 Tectonic plates The Earth’s crust is made of sections called tectonic plates. Where two plates meet is called a plate boundary. Convection currents in the mantle plates move these tectonic plates, which can move towards each other, move away from each other and or move past each other. 1 4 of 20 32 © Boardworks Ltd 2005 2004 Plate boundaries 1 – sliding plates Two plates can slide past each other, but this doesn’t always happen smoothly. The large friction forces involved mean that strain builds up and the plates move suddenly when the strain gets too much. plate A plate B This sudden movement of plates is called an earthquake and it can be very destructive. 1 5 of 20 32 © Boardworks Ltd 2005 2004 Plate boundaries 1 – conservative boundary 1 6 of 20 32 © Boardworks Ltd 2005 2004 Why do earthquakes happen? 1 7 of 20 32 © Boardworks Ltd 2005 2004 Features of an earthquake 1 8 of 20 32 © Boardworks Ltd 2005 2004 Cross-section of the Earth – labels Which part of this cross-section of the Earth should each label point to? crust outer core mantle inner core 1 9 of 20 32 © Boardworks Ltd 2005 2004 Effect of an earthquake An earthquake has occurred in the area shown in this picture. If the plates moved in the directions indicated by the arrows, what would the location would look like after the earthquake? plate A 1 10ofof20 32 plate B © Boardworks Ltd 2005 2004 Contents Earth’s Structure Earth and earthquakes Seismic waves More about plate boundaries Continental drift Summary activities 1 11ofof20 32 © Boardworks Ltd 2005 2004 Primary and secondary waves During an earthquake there are two types of seismic waves that are released from the epicentre. These are called primary waves (P waves) and secondary waves (S waves). Primary or P waves are the faster type of seismic waves. They are longitudinal waves and when they hit the Earth’s surface they make objects and buildings vibrate vertically. P waves can travel through solids and liquids. Secondary or s-waves are the slower type of seismic waves. They are transverse waves and when they hit the Earth’s surface they make objects and buildings vibrate horizontally. S waves can only travel through solids. 1 12ofof20 32 © Boardworks Ltd 2005 2004 Effects of different seismic waves P-waves arrive first and shake buildings vertically. This causes little damage. 1 13ofof20 32 S-waves arrive next and shake buildings horizontally. This can cause a lot of damage. © Boardworks Ltd 2005 2004 Seismic waves inside the Earth Waves within a layer change direction gradually because there is a gradual change in density. S wave shadow - only P waves are detected in this region. earthquake focus At a layer boundary there is a big change in direction because there is a big change in density between layers. 1 14ofof20 32 © Boardworks Ltd 2005 2004 Words about seismic waves epicentre 1 15ofof20 32 The faster type of seismic wave, that is longitudinal and makes buildings vibrate up and down. primary waves The location where the shift in plates occurred producing seismic waves. secondary waves The slower type of seismic wave, that is transverse and makes buildings shake from side to side. © Boardworks Ltd 2005 2004 Comparing seismic waves Complete this table comparing the properties of the two types of seismic waves. P wave S wave Type of wave Speed Media wave can travel through Damage caused on Earth’s surface 1 16ofof20 32 © Boardworks Ltd 2005 2004 Questions about seismic waves mantle wave A wave B outer core 1. What type of wave is wave A? How can you tell this? 2. Explain the path of wave B in terms of density. 3. What is the S wave shadow? 1 17ofof20 32 © Boardworks Ltd 2005 2004 Contents Earth’s Structure Earth and earthquakes Seismic waves More about plate boundaries Continental drift Summary activities 1 18ofof20 32 © Boardworks Ltd 2005 2004 Plate boundaries 2 – colliding plates What happens at a plate boundary where an oceanic plate and a continental plate collide? The thinner, more dense oceanic plate is driven down into the mantle. This is known as subduction. thicker, less dense continental plate convection current thinner, more dense oceanic plate convection current melted oceanic plate 1 19ofof20 32 © Boardworks Ltd 2005 2004 Plate boundaries 2 – destructive boundary 1 20ofof20 32 © Boardworks Ltd 2005 2004 Plate boundaries 3 – plates moving apart What happens at a plate boundary when oceanic plates move apart due to convection currents in the Earth’s mantle? oceanic plate Magma moves up to the convection surface, cools current and forms new oceanic plate. 1 21ofof20 32 oceanic plate convection current This is known as sea-floor spreading. © Boardworks Ltd 2005 2004 Plate boundaries 3 – constructive boundary 1 22ofof20 32 © Boardworks Ltd 2005 2004 Words about plate boundaries subduction sea-floor spreading friction 1 23ofof20 32 The force that causes earthquakes. When an oceanic plate is pushed down into the mantle as it collides with a continental plate. When two oceanic plates move apart to create new crust. © Boardworks Ltd 2005 2004 Contents Earth’s Structure Earth and earthquakes Seismic waves More about plate boundaries Continental drift Summary activities 1 24ofof20 32 © Boardworks Ltd 2005 2004 What is continental drift theory? It is thought that all the continents were once joined together in a huge supercontinent called Pangaea. Over millions of years the continents drifted apart because the tectonic plates they were on moved apart due to convection currents in the Earth’s mantle. 1 25ofof20 32 © Boardworks Ltd 2005 2004 Continental drift theory animation 1 26ofof20 32 © Boardworks Ltd 2005 2004 Evidence for continental drift theory There are a number of pieces of evidence to support the theory of continental drift: 1. The shapes of the continents fit like a jigsaw. 2. The rock strata on separated continents are identical. continent A continent B 3. The fossil records on separated continents are identical. 1 27ofof20 32 © Boardworks Ltd 2005 2004 Contents Earth’s Structure Earth and earthquakes Seismic waves More about plate boundaries Continental drift Summary activities 1 28ofof20 32 © Boardworks Ltd 2005 2004 Glossary (1) continental drift – The theory that millions of years ago all the continents formed a huge supercontinent and have been slowly drifting apart ever since. core – The central region of the Earth below the mantle. The outer part is liquid and the inner part is solid. crust – The thin, rocky, outer layer of the Earth. earthquake – A sudden movement of the Earth’s crust, which causes vibrations that travel through the Earth. mantle – The layer of the Earth, between the crust and the core, which is mostly solid rock that can flow very slowly. P waves – These are the fastest type of seismic waves. They are longitudinal waves and can travel through solids and liquids. 1 29ofof20 32 © Boardworks Ltd 2005 2004 Glossary (2) plate boundary – An area where two tectonic plates meet. S waves – These are the slowest type of seismic waves. They are transverse waves and can only travel through solids. sea-floor spreading – This occurs under oceans where plates move apart and magma rises to the surface and cools to form new crust. seismic waves – Vibrations that travel through the Earth as a result of an earthquake. subduction – This occurs where plates move together and one of the plates is driven down into the mantle. tectonic plates – Large sections of the Earth’s crust and upper mantle that move slowly over the surface of the Earth. 1 30ofof20 32 © Boardworks Ltd 2005 2004 Anagrams 1 31ofof20 32 © Boardworks Ltd 2005 2004 Multiple-choice quiz 1 32ofof20 32 © Boardworks Ltd 2005 2004