Qu:Why do hazards and processes differ on plate boundaries? Aim: To D & E key characteristics of destructive and collision margins.

Transcription

1 Qu:Why do hazards and processes differ on plate boundaries? Aim: To D & E key characteristics of destructive and collision margins. Erte Ale crater, Ethiopia This volcano is in Africa's Great Rift Valley. 1 - Describe the tectonic setting. What is happening in this region? 2- Outline reasons for the effusive lava that these volcanoes produce. EXT: How might the shape of these volcanoes be influenced by the lava? 1

2 When plates collide or converge Get used to the idea that Destructive margins and Convergent margins are largely the same thing. Plates are pushing towards each other. Constructive/divergent margins are about plate/rock creation. All new crust will eventually be destroyed by being carried/drifted to a destructive boundary. 2

3 Convergent/Destructive Margin. Oceanic/continental plates. Task: Dotted around the room are a series of questions and answers on the same sheet. Someone will have the start question and it will be indicated. When the music starts you will pass them around. When it stops whoever is holding the Q&A's is in the game. We will play it twice. Those without paper will queue at the board to indicate/annotate the diagram below as each question and answer is read out. Slab Pull 3

4 Oceanic trench Crumple zone Continental Plate Oceanic Plate Fold mountains subducted 4

5 TASK: 1 - Stick a copy of the diagram and your copy of the Q&A next to each other in your notes. Add any details you wish to the diagram. 2 - Explain why some earthquakes occur away from the benioff zone, within the continental crust. Ext: Given it's proximity to the ocean explain what other tectonic hazards could be generated. 70mph Convergent/Collision plate margin Continental/Continental 70mph What happens when continental plates collide? The Matrix Reloaded Truck Collusion_5.avi 5

6 Magma which cooled before it could ever reach the surface. Convergent/Collision plate margin Continental/Continental Mountain root 800 miles earthquake focus Using your knowledge of this and previous lessons... a Copy neatly and precisely the diagram above. b Why does neither plate subduct the other? c Why are Mountains formed? d Earthquakes are concentrated on the plate boundary but are also found in 'intra plate' locations.why is this? e Account for the fact that there are no volcanoes on this type of boundary. f Name an area where this is happening including plates involved. EXT: What will happen if the mountain root melts and falls off? 6

7 Plenary Explain the widely distributed INTRA-PLATE earthquakes through the central Eurasian plate. 7

8 Resource slides 8

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10 Destructive/Convergent Margin Oceanic/Continental Crumple zone/fold mountains Oceanic trench Benioff zone Convection current defelected downward Slab Pull Subduction Zone Convection current defelected downward Destructive/Convergent Margin Oceanic/Continental Crumple zone/fold mountains Oceanic trench Benioff zone Convection current defelected downward Slab Pull Subduction Zone Convection current defelected downward 10

11 The subducting plate cools the mantle and adds to the densityof the asthenosphere. This also blocks the horizontal direction of the current and speeds up the process of convection. What is the Benioff zone? This is an area of large and frequent earthquakes along the length of the subducting plate, between the trench and subduction zone. Why are these earthquakes so frequent and large? Friction with the continental plate is constantly allowing pressure to build up and before being released. This generates the earthquakes. They can occur near the surface (shallow focus) or in the subduction zone (deep focus). How is the oceanic trench formed? As the oceanic plate is subducted, gravity helps to further pull the plate down and speeds up the motion. This is called slab pull. This motion also pulls the front of the continental plate downwards to create a characteristic deepening or trench along the seabed for the entire length of the margin. Why does the oceanic plate always sink or subduct below the continental plate? Being formed at a constructive or divergent margin means that oceanic crust is sourced by convection from very close to the outer core. Therefore it contains a high amount of dense material such as iron and nickle. When it meets continental rock it descends under the less dense material. Why are there mountains lose to the oceanic trench? As the plates collide the continental plate is slowed and crumples up as it is held up by the subducting oceanic plate whilst the rest of the plate continues to move at the same speed away from the boundary. This concertina effect piles and folds rock into hills and mountains. There are volcanoes among the mountains, Why are they not exactly on the plate margin? As the oceanic plate subducts it melts and breaks up as it travels deeper into the mantle. The melted rock and gases produced are under great pressures. Consequently they rise directly upwards over 1000's of years to an area of lower pressure - the surface. However the oceanic plate has also travelled a long way horizontally as well as vertically by the time it breaks up so volcanoes appear set back from the plate margin and ocean trench. Why are volcanoes on destructive margins erupt more violently but less frequently than those on constructive margins? 11

12 This is to do with gas content. Sea water makes it's way into the subduction zone. Along with melting oceanic crust this creates a gassy, slow moving and relatively cool mixture of magma. As it rises it can cool in vents in the overlying crust and block movement below. This allows pressure and gases to build up. When this gives way the results are dramatic. Globally, where are the best examples of oceanic/continental destructive margins. The Pacific plate is subducting under the South American plate and displays classic examples of an oceanic trench, mountains (The Andes) and violently erupting volcanoes. What significance does the deflected convection current have? 12

13 Attachments The_Matrix_Reloaded_Truck_Collusion_5.avi