Lecture 3 understanding plate tectonics


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Lecture 3 understanding plate tectonics

  1. 1. Understanding PlateTectonicsA2
  2. 2. The History• Alfred Wegener 1915• First came up with the theory that the earth’s plates were slowly moving in relation to each other (Continental Drift)• It was not until the 1960’s and Harry Hess who proposed that it was caused by sea floor spreading at mid ocean ridges• This was where the modern history of plate tectonics begins
  3. 3. Why do we have tectonic Plates• The earth has 7 large plates• 7 smaller ones• And dozens of micro plates• The earth’s surface is broken up into plates because the heat generated in the earths core by radioactive decay drives convection currents in the outer core and mantle.• These slowly tear the earth apart and are responsible for movement• This movement will continue as long as heat is generated at the core
  4. 4. All Major and Minor Plates
  5. 5. How do the move?• Plates come in two types Oceanic and continental• Oceanic Plates – Are young, thin and dense. – They are formed on the sea floor spreading at constructive plate boundary ridges and destroyed at Subduction zones – Most are under 150 million years old and made of dense Basalt (6-8km thick)
  6. 6. Oceanic Plates
  7. 7. Continental Plates• Are ancient thick and less dense• They are over 4 billion yrs old and are not being formed today• They are made of less dense Granite and are 30-60km thick
  8. 8. Convection• At the rising limbs of convection cells, heat from the core moves towards the earth’s surface then spreads to either side.• It is this spreading motion that splits the plates and drags them apart.• Plates move on a layer between the upper mantle and lithosphere. The asthenosphere acts as a type of lubrication• The plate motion depends however on two factors: – The weight of the cold plates at the subduction zones pulling the plate downwards – Gravitational sliding force between ‘High’ and ‘Low’ Trenches
  9. 9. Convection CurrentsLayer Physical StateLithosphere Solid, rigidEarth’s Crust)Asthenosphere Solid, plastic, (partially molten approx 4%)Mantle Solid, plasticOuter core LiquidInner Core Solid (Iron and Nickel)
  10. 10. Convection Currents
  11. 11. Hazards• Most occur where the plates meet. Boundary Hazards• There are rare ‘Intra-plate earthquakes’ and ‘Mid-plate volcanic hotspots’• There many different tectonic ‘settings’ which produce various hazards
  12. 12. SettingsSETTING MOTION HAZARDS EXAMPLECONSTRUCTIVE 2 Oceanic plates moving Basaltic volcanoes and Mid-Atlantic ridgePLATE BOUNDARY apart minor , shallow (Iceland) mostly earthquakes submerged) 2 continental plates Basaltic splatter cone African Rift Valley and moving apart volcanoes Mnt. NiyragongoDESTRUCTIVE PLATE 2 oceanic plates in Island arc explosive Soufrierre Hills onBOUNDARY collision Andesite eruptions and Montserrat EQ’s 2 continental plates in Major, shallow Himalayan orogenic belt collision earthquakes along thrust faults Oceanic and continental Explosive, andesitic Andes mountain chain plate collision eruptions and major EQTRANSFORM Plates sliding past one Major shallow San Andreas faultBOUNDARY another earthquakesHOTSPOTS Oceanic Basaltic shield Hawaiian Island chain volcanoes, minor earthquakes Continental Colossal Rhyolitic mega- Yellowstone eruptions ‘supervolcano’ USA
  13. 13. Research Task• What caused and started the following earthquakes:• San Francisco 1906• Great Kanto (Tokyo) 1923• Chile 1960• Mexico City 1985• Izmit Turkey 1999• Kashmir 2005
  14. 14. Volcanoes• Why aren’t they all the same?• We have discussed the reasons for why not all volcanoes are the same – Basaltic – basic magma – Andesitic – Intermediate – Rhyolitic – Acidic magma
  15. 15. Basaltic• Very hot iron rich silica poor• Low gas content and very hot runny lava (Melted ice cream)• Can erupt almost continuously• Not very explosive
  16. 16. Andesitic• In the middle between rhyolitic and basaltic• Sticky can take decades or centuries between eruptions• Can be very explosive
  17. 17. Rhyolitic• High silica content low temperature and high gas content therefore combustible• Erupt rarely• Can be devastating
  18. 18. The Richter Scale• Developed in 1935 to measure magnitude of earthquakes• Today the Moment Magnitude Scale MMS id more commonly used and is very similar• Most earthquakes over 6.5 on the Richter scale generate interest as at this magnitude they will cause some, if not significant damage• However this does not tell the whole story
  19. 19. Earthquake Depth• Shallow surfaced EQ’s (<70km) intermediate (70-300km) Deep focused (>300km)• Shallow ones are the most destructive as less energy is lost travelling to the surface• Especially important in the Benioff Zone of subducting plates
  20. 20. The Benioff Zone• This is the active seismic zone on a subduction plate• In a subduction zone the earthquake foci normally plots along a dipping plane at an angle of 33 to 60 degrees and this plane is called a Benioff zone• It is named after Hugo Benioff, a US seismologist who first described this feature• The Benioff zone extends to a depth of about 700 km
  21. 21. Physical Nature of the Ground• If the ground consists of loose sediment then liquefaction can occur• In high mountain areas such as the Himalayan fold mountains landslides can have devastating effect.
  22. 22. Finally• After looking at these your research should take into account the ‘Settings’ of the events as this will help you understand the impacts that it has on humans
  23. 23. References• Leeds University Lecture Notes