The Crust How does the crust move?

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1 Lesson Plan 3 The Crust Introduction Discuss with the aid of a world map Have the Earth s land areas always looked the same? Can you show any evidence if you think the position of continents has changed? Some continents look like they fit together like a jigsaw puzzle eg the east coast of South America and the west coast of Africa. What forces might cause continents to change position? Something underneath the Earth s surface must be able to carry huge parts of the Earth s crust. What do you already know about the interior of the Earth? It s hot and parts of the mantle can flow very slowly. Activities 1. To show that the crust floats on the denser, heavier layers underneath: Float a mandarin in water. The mandarin represents the Earth and should float low in the water. Peel the mandarin so that the skin is in one piece if possible. Predict which parts will float now. The skin represents the crust of the Earth and the flesh represents the core and mantle. Test your predictions The skin (crust) will float because it is lighter than the same volume of water. The flesh will sink. Point out to students that the skin is lighter and floats even though it looks more solid when compared with the flesh which is mainly liquid. Learning Intentions See that the Earth s crust is broken up into huge plates. Explore ways in which the plates could move around the Earth. Success Criteria Students can Explain what plates are Explain why they can move Explain some possible results when plates interact. Resources World map Mandarins Water containers Vocabulary tectonic plate, push, slide, grind, rub, crumple, interact, rigid 2. To show that the crust is broken into plates which can move over the softer layer underneath. Break the mandarin skin into 3 or 4 large pieces. These represent the crust of the Earth broken into plates. --The Earth s crust is broken into a number of massive, rigid slabs called tectonic plates. They move very slowly over the hot mantle below. Different plates move at different speeds and in different directions. The average speed is about as fast as your fingernails grow, 3-4 cm per year or 3-4 m per century. By holding 2 pieces of mandarin skin against the peeled mandarin work out how the plates could behave as they move around on the Earth s surface. Students should work out that the plates (sections of mandarin peel) can interact in at least 5 ways: --push into one another, resulting in both plates being forced up and crumpled. --push into one another, resulting in one plate being pushed under the other. (You may need to force this one to happen ) --slide past one another, resulting in the edge of both pieces being rubbed. --move away from one another so that they are no longer touching. --rotate like a wheel so that one grinds into the other.

2 Teacher Notes 3 & 4 The Crust and Plates As early as the 1600 s scientists noted that the shapes of continents on opposite sides of the Atlantic Ocean (South America and Africa) seemed to fit together like pieces of a jigsaw puzzle. In the following centuries scientists began to speculate that the Earth s landmasses had once been joined in a supercontinent and some force had split them apart. Curriculum Links Planet Earth and Beyond Physical World In 1912 Alfred Wegener, a German scientist, proposed the theory of continental drift which said that the continents were slowly drifting around the Earth. He used evidence of matching rock types, geological structures and fossils to show that continents had once formed a giant continent he called Pangaea. Wegener s theory was not widely accepted at the time because he could not provide a mechanism to explain how Pangaea split up to form the continents we know today. By the 1960 s there was evidence from new areas of scientific research, such as under sea exploration and the magnetism of rocks, to support Wegener s theories and explain possible mechanisms for the movement of continents. Within less than a decade a new theory called plate tectonics was widely accepted. It combined Wegener s continental drift theory with evidence from many new areas of Earth science that explained the mechanisms causing continents to move. Plate tectonic theory states that the Earth s outer shell, the lithosphere (including the crust and upper mantle), is broken up into a number of large rigid plates that slowly move about the planet. These plates are continually being formed and destroyed. Plates are separating at mid ocean ridges where new plate material (magma) is welling up from the mantle and pushing the plates apart. The other edge of the plate is pushed into or alongside a neighbouring plate or plates. When two plates collide a number of things can happen depending on which plate is heavier and the angle of collision. The heaviest plates are those made mainly of sea floor. Although the crust under the seafloor is thinner it is heavier than continental plate material because of the type of rock (basalt) that is formed at mid ocean ridges. Many plates are a mixture of oceanic and continental crust and some carry parts of more than one continent. When a heavy plate is pushed against a lighter plate, the heavy plate will be forced beneath the lighter plate and the lighter plate will ride over the top. This is called subduction. Over time the heavier plate will be forced deeper and deeper down into the mantle where it may partially melt and later rise to produce volcanoes along the edge of the subduction zone. This is the process creating the Taupo Volcanic Zone, Taranaki and the Kermadec Arc volcanoes. The reason why plates move is a good example of the Nature of Science strand of the curriculum in action. There is scientific debate over whether plate movement is caused by ridge push or slab pull. It is possible that the new plate material formed at a mid ocean ridge pushes the plate along but it is also possible that the opposite edge drags the plate along behind it as it is subducted. The movement of the mantle (convection) could be the cause of, or caused by either of these mechanisms. Both are stories that fit the current state of scientific knowledge and they will change over time. Whatever the mechanism plate movement averages 3-4 cm per year or 3-4m per century, Very slow! Science Concept PE-Earth Systems L3/4 develop an understanding of what makes up our planet PW-Physics Concepts L1/2-explore physical phenomena such as movement, forces and heat. NOS Understanding about Science Investigating in Science For a more detailed explanation of convection see: GNS Science

3 FAQs and links for further inquiry 3 & 4 The Crust and Plates How many plates are there? Most scientists agree that there are between 8 and 15 plates. Some large plates have areas within them that behave like a separate plate. They are often called micro plates. This makes it difficult to give a definite number. What plate is New Zealand on? New Zealand lies across the boundary of two plates. Some parts are on the western edge of the Pacific Plate and some parts are on the eastern edge of the Australian Plate. The Australian Plate is called a continental plate because a lot of it is covered by land. The Pacific Plate is mostly seafloor so it is called an oceanic plate. Oceanic plates are heavier than continental plates even though the crust they are made of is thinner. Oceanic crust is mainly basalt, a dark, dense rock formed by the magma upwelling at ocean ridges where plates grow. How does being across 2 plates affect New Zealand? The two plates are crashing together. You could say New Zealand is a geological car crash! The force of the collision causes our earthquakes and volcanoes. Plate boundaries (edges) are not straight lines so this means the collision between the same two plates can have different results at different places. This is why some areas of the collision zone in NZ have volcanoes and some areas have mountain building and earthquakes. If the plates are continually moving what did the world look like before and what will it look like in the future? For animations see: html What will New Zealand look like in the future? For an animation of NZ s future shape see Australian Plate Pacific Plate Which plate are you on? All of the North Island is on the Australian Plate and so is everything to the west of the Alpine Fault in the South Island, including Nelson and Westland. The rest of the South Island, Stewart Island and the Chatham Islands are all on the Pacific Plate. The collision zone (plate boundary) can easily be seen on the map running from north of East Cape around the east of the North Island, curving below Cook Strait, down the mountainous spine of the South Island and out to sea south of Milford Sound. Which plate is Australia on? All of Australia is on the Australian Plate. There are no plate boundaries running through Australia. Because there are no plate collisions happening in Australia they have fewer volcanoes and earthquakes.

4 Lesson Plan 4 Moving Plates Introduction Discuss How does a heater heat up a room? The air near the heater warms up and moves further out into the room. Which direction does the warmed air move in? Unless there is a draft the warm air will rise because warm air is lighter (less dense) than cold air. When air molecules are heated they move faster and further apart so the same number of air molecules will take up more space when they are warm. This is why an inflated balloon will expand on a hot day. The rising warm air will be replaced by cooler air and this creates a circular current called a convection current. Most students will realise that the top of a house or room is usually the hottest. Can you see hot air rising? You cannot easily see it but you can demonstrate that hot air is rising by making a spinner and holding it above a heater. The spinner will twirl as it is pushed by the rising air. Activities 1. To show hot air rises, make the paper spinners using the instructions provided and complete the drawing to show the air currents. 2. To show convection in liquids Heat a pot or wide beaker of water until it is almost boiling. Drop a small number of frozen peas into the centre. As they thaw the rising hot water in the middle of the pot will carry them up and over to the edge of the pot. If you increase the heat a little to get a more rapid boil, you should see peas dropping down at the edge of the pot where it is coolest and rising again as the hottest water pushes them up. This activity will also work with crushed 2 minute noodles. Learning Intentions Observe and explain convection currents in air and water Success Criteria Students can Draw or describe a convection current and relate this to plate movement. Resources Paper spinner instructions. Pot or beaker Heat source Frozen peas Vocabulary Circulate, convection, current See the youtube clip for another demonstration of convection that could also be carried out in the classroom. Discuss How are these activities related to the movement of the Earth s plates? A common explanation for the slow movement of the crustal plates is that they are being carried along by convection currents in the mantle below. The deeper and hotter mantle rocks rise slowly toward the crust and sink down again as they cool, creating a circular current. Because the mantle is so thick and slow moving, the plates can only move at about 3-4cm per year. Unlike air or water the plates do not move smoothly. Friction between the two moving surfaces means plates may become locked together for thousands of years. When the pressure becomes too great the rocks rupture and the stored energy is released by an earthquake.

5 Activity sheet 4 Name: Moving Plates Paper Spinners 1. Draw a circle on photocopy paper. A small paper plate makes a good template. 2. Decorate the circle and cut it out. 3. Beginning at the outside edge, cut a spiral around and around your circle. Keep the cut quite wide so the paper doesn t tear. 4. Make a hole in the centre of the spiral and attach a hanging thread. 5. Hold the spinner over a heater or suspend above a heat source. 6. Watch the rising hot air make your spinner twirl. 7. Draw your spinner in the space below and show the air movement from the heat source that makes it twirl. 8. Explain what happens to the air as it is heated.