Lecture 3_Structure of the Earth_2.pdf

Structure of the Earth 2 After this lecture you should be able to… Distinguish between continental drift and plate tectonics Discuss the different types of plate boundaries and the processes that happen at each Explain the relationship between pressure and the melting temperature of rock...

Structure of the Earth 2 After this lecture you should be able to… Distinguish between continental drift and plate tectonics Discuss the different types of plate boundaries and the processes that happen at each Explain the relationship between pressure and the melting temperature of rock Explain the relationship between moisture and the melting temperature of rock Explain how magma is generated at different types of plate boundaries using the above relationships Give examples of different types of plate boundaries on Earth Theory of plate tectonics (Tarbuck, Lutgens and Tasa, pages 51 to 55) Plate tectonics (Tarbuck, Lutgens and Tasa, pages 54 to 55) Three types of boundaries between plates Divergent boundaries Moves continents apart Expands oceans Creates seafloor Also called constructive margins Convergent boundaries Brings continents closer together Closes oceans and destroys oceanic lithosphere Builds mountains and islands Also called destructive margins Transform fault boundaries Two plates grind past each other without the production or destruction of lithosphere Also called conservative margins Divergent Boundaries  Plates move apart  Constructive margins  new ocean floor generated here  Results in upwelling of rock material from mantle and sea- floor spreading  E.g. Mid-Atlantic Ridge Seafloor spreading (Tarbuck, Lutgens and Tasa, pages 55 to 56) Two adjacent plates moving away from one another As a result, hot rock from the mantle underneath migrates upward to fill the void  spreads laterally , carrying the material in a conveyor-belt fashion away from the ridge which produces a new seafloor (youngest seafloor found at the ridge while older seafloor away from the ridge) Material gradually cools and Direction of plate movement produces new oceanic lithosphere Takes places at oceanic ridges  elevated areas of the seafloor that are characterised by high heat flow and volcanism Relationship between pressure and the melting temperature of rock Rock can melt at a lower temperature when it is also at a lower pressure = decompression melting  How is magma generated at divergent plate boundaries? (Tarbuck, Lutgens and Tasa, pages 121 to 122) Decompression melting  Process responsible for generating magma/molten rock at divergent boundaries Hot, solid mantle rock ascends in zones where plates have moved apart The rock is now closer to the surface  lower pressure Rock melts, because the melting point of rock is lower when pressure is reduced. Convergent Boundaries (Tarbuck, Lutgens and Tasa, page 56)  Plates move into each other (converge) North American  Lithosphere from one plate Plate descends into the mantle  also called subduction zones  Only oceanic lithosphere descends  denser than continental lithosphere  Forms deep-ocean trenches  Three collision types Oceanic and Continental Oceanic and Oceanic Continental and Continental Pacific Plate Subduction: Oceanic – Continental (Tarbuck, Lutgens and Tasa, pages 57 to 58) Mountain ranges containing numerous volcanoes develop on continental crust Plate  continental volcanic arcs Movement 4 1 3 2 Increased volcanic activity above Oceanic lithosphere subducts below descending plate continental lithosphere and eventually gets destroyed  Subduction: Oceanic-Continental The denser oceanic lithosphere subducts or descends beneath the less dense continental lithosphere forming a very deep part of the ocean known as a trench, subsequent melting of rock in the wedge above the subducting oceanic plate leads to an increase in volcanic activity and the formation of a continental volcanic arc running parallel with a trench. Subduction: Oceanic – Oceanic (Tarbuck, Lutgens and Tasa, page 58) Island arcs result from increased volcanic activity Two plates collide at a CONVERGENT boundary Increased volcanic 4 activity above 1 descending plate 3 2 One of the plates is forced downward into the Earth’s interior where it eventually melts leading to the destruction of the descending plate  Subduction: Oceanic-Oceanic The plate on the right is subducted underneath the plate on the left forming a very deep trench. As it subducts melting happens in the wedge above the subducting plate leading to an increase in volcanic activity immediately above the wedge this forms a string of volcanic islands known as a volcanic island arc which runs parallel to the trench. Relationship between moisture and the melting temperature of rock https://opentextbc.ca/physicalgeologyearle/wp-content/uploads/sites/145/2016/06/flux-decompression.png Wet rock melts at a lower temperature than dry rock Why does the subduction of cool oceanic lithosphere cause mantle rock to melt? (Tarbuck, Lutgens and Tasa, page 122) Addition of volatiles (water)  Mechanism that initiates volcanic activity at subduction zones. Oceanic crust contains a large amount of water, which is carried to great depths by the descending plate. Heat and pressure drive water from the voids in the rock. Wet rock in a high-pressure environment melts at lower temperatures than dry rock. At approx. 100 km depth melting occurs Continental – continental collisions (Tarbuck, Lutgens and Tasa, page 59) No subduction occurs. Lithosphere thickens and leads to mountain building 2 1 Direction of plate movement  Continental-continental collision Where the continental plates meet after colliding, the immense impact pushes the material upwards and mountain ranges such as the Himalayas form. Transform Fault Boundary (Tarbuck, Lutgens and Tasa, pages 60 to 61)  Plates grind (causes friction which leads to earthquakes) past each other without the production/destruction of lithosphere  E.g. San Andreas fault  earthquake prone region as a result of this grinding Transform Fault Boundary (Tarbuck, Lutgens and Tasa, pages 60 to 61) Direction of plate movement 2 1 Plates sliding past each other. Lithosphere is neither created nor destroyed, it is therefore conserved. Continental Divergence (African Rift Valley System) Arabian Plate African Plate Oceanic Divergence (Mid Atlantic Ridge) South American African Plate Plate Oceanic - Oceanic Convergence (Kuril Islands) North American Plate Pacific Plate Oceanic - Continental Convergence (Andes Mountains) South American Trench Plate (Peru-Chile Trench) Andes Nazca Plate Mountains Continental - Continental Convergence (Himalayas) Eurasian Plate Himalayas Indian Plate Where did the information in this presentation come from? Tarbuck, E.J., Lutgens, F.K. and Tasa, D. 2011 Earth: An Introduction to Physical Geology. Tenth Edition. Pearson Prentice Hall (ISBN No 978- 0-321-69903-3) Chapter 2: Plate Tectonics Chapter 4: Magma and Igneous Landforms