Transcript Slideshow
Why is the earth's crust unstable? AQA Geography p8 Learning Objectives Understand why the Earth’s crust is unstable and the characteristics of different plate margins. Success criteria • Grade C – Describe the structure of the earth and the movement at plate boundaries • Grade B – Explain what is happening at each plate boundary and name the features that are formed. • Grade A - Explain with the use on annotated diagrams what is happening at each plate boundary and name the features that are formed. Give named examples. KWL What do you think you KNOW? What do you WANT to know? What did you LEARN? The structure of the earth •The crust - the outer layer of the earth is relatively thin •The crust is not one single piece of skin, like that of an apple. •Instead, it is split into plates of varying size and at plate margins it is liable to move. •This is because the slabs of crust float on the semi-molten upper mantle. The structure of the earth Cross section of the Earth The structure of the earth •Convection currents within the mantle determine the direction of plate movement. •Therefore, in some cases the plates are moving together and sometimes they are moving apart http://www.kidsknowit.com/educationalsongs/play-educationalsong.php?song=Our Earth Inside Out Why do the plates move? Continental drift Evidence for plate tectonics Study of fossils Similar fossils are found on different continents. This is evidence that these regions were once very close or joined together. Pattern of rocks Similar pattern of rock layers on different continents is evidence that the rocks were once close together or joined. Shapes of continents Some continents fit together like a jigsaw. Africa South America This is the way the World may look like 50 million years from now! This is the way the World may look like 150 million years from now! "Pangea Ultima" will form 250 million years in the Future The crust is in pieces Layers of the earth rap Types of plate margin Destructive plate margins Destructive plate boundary Collision plate boundary •Convection currents in the mantle cause the plates to move together. •If one plate is made from oceanic crust and the other from continental crust, the denser oceanic crust sinks under the lighter continental crust in a process known as subductlon. •Great pressure is exerted and the oceanic crust is destroyed as it melts to form magma. •If two continental plates meet each other, they collide rather than one sinking beneath the other. This collision boundary is a different type of destructive margin. Constructive plate boundary •When plates move apart, a constructive plate boundary results. •This usually happens under the oceans. •As the plates pull away from each other, cracks and fractures form between the plates where there is no solid crust. •Magma forces its way into the cracks and makes its way to the surface to form volcanoes. •In this way new land is formed as the plates gradually pull apart. Conservative plate margins Conservative plate boundary •At conservative plate margins, the plates are sliding past each other. •They are moving in a similar (though not the same) direction, at slightly different angles and speeds. •As one plate is moving faster than the other and in a slightly different direction, they tend to get stuck. •Eventually, the build-up of pressure causes them to be released. •This sudden release of pressure causes an earthquake. •At a conservative margin, crust is being neither destroyed nor made.