Tectonics🌋

• Lithosphere


  Crust and upper most mantle
• Mantle


  • Silicate rocks rich in iron and magnesium
  • Semi molten state
  • Extends to a depth of 2900km
  • High temperatures create convection currents
• the core


  Iron and nickel

  outer core- semi molten state

  inner core- solid

  6200 celcius
• Continental crust


  Composed of silica and alumina

  70km thick

  Covers 40% of the earth

  mainly granite
• oceanic crust 


  denser than continental crust

  composed of silica and magnesium

  mainly basalt

  6-10km thick
• Moho discontinuity


  Separates the crust from the mantle
• Earth's surface


  • Made up of rigid lithospheric plates
  • Seven major plates
• Convection current theory


  1. Rising limbs of convection cells move heat from the earth's core towards the surface

  2. Spreading out either side of the ridge and carrying the plates with them

  3. Layers float on a lubricated layer between the upper mantle and lithosphere (the asthenosphere)

  4. Allows the solid lithosphere to move over the upper mantle
• Plate movement (modern thinking)

  1. Molten material wells up at constructive plate margins due to thinning of the lithosphere

  2. Decrease in pressure causes partial melting of the upper mantle

  3. Lithosphere is heated, rises and becomes elevated above the sea floor to form an ocean ridge

  4. Slope down and away from the ridge is formed

  5. Fresh rock formed at the spreading centre is hot, less dense and more buoyant than rock further away from the constructive margin
• Constructive plate margins- two oceanic crusts

  1. Driven by slab pull

  2. Brings magma from the asthenosphere to the surface
• Pressure at diverging plate margins


  • Leads to doming of the earth's surface
  • Formation of a ridge (e.g. mid-atlantic ridge)
• Mid-atlantic ridge extends across Greenland and the mid Atlantic for 10,000km
• Hazards at diverging plate margins

  • Basaltic volcanoes
  • Minor, shallow earthquakes
• Two continental plates at a constructive margin

  • When two continental plates move apart, the brittle crust fractures forming parallel faults
  • Between the faults the crust slowly subsides, forming a rift valley
• Rift valley formation


  • Mountains
  • Volcanoes
  • Valley
  • Magma
• Converging (destructive) plate margins (two oceanic crusts)

  1. Two plates move apart due to slab pull

  2. Subduction occurs

  3. One plate is colder, older and more dense than the other, this plate is subducted, heated and melted under pressure

  4. The melted material rises through lines of weakness towards the surface, resulting in the formation of a chain of volcanoes and volcanic islands above the subduction zone (island arc)
• Oceanic crust


  Denser than continental crust
• Oceanic to continental (destructive)

  1. Oceanic crust is subducted below the continental plate

  2. Oceanic trench is formed
• Continental plate response (destructive)


  1. Lighter continental plate is uplifted and folded

  2. Rising magma breaks through lines of weakness to form volcanoes
• Oceanic to continental plate interaction

  • South America - Nazca plate converging with and subducting beneath the continental South American plate
• Continental to continental plate collision

  • Both plates are low density and buoyant so no subduction occurs
  • No volcanic activity
  • Shallow, hazardous earthquakes
  • Plates are heaved upwards forming complex mountain ranges
• Continental to continental plate collision

  • Himalayas - Indo-Australian plate collides with the Eurasian plate
• Conservative plate margins


  • Two plates move laterally past each other
  • No volcanic activity as there's no subduction
• Shallow earthquakes


  1. High frequency, low magnitude earthquakes happen when pressure is relatively easily released

  2. Occasional major events take place when there is a significant build up of pressure
• San Andreas fault


  • Pacific plate and North American plate meet
  • The plates are moving in the same direction, but the Pacific plate is moving faster
• Hotspots


  Small areas with an unusually high heat flow away from plate boundaries
• Oceanic hotspots


  • Plumes of magma rise from asthenosphere
• If crust is thin


  Magma escapes as a volcanic eruption
• Lava islands


  • May build up over time and form volcanoes
  • Extremely active volcanoes with frequent eruptions
• Hawaiian Islands


  • Pacific plate has been moving over a hotspot for 70 million years
  • As the plate moves, volcanoes have been carried away from the hotspot forming a chain of extinct volcanoes
• Yellowstone National Park


  • Rhyolitic super volcano
• Magnitude of earthquakes


  • Most important influence on the severity of an earthquake
  • The size or physical force of an earthquake
  • Measured by logarithmic moment magnitude (MM Scale) or Richter Scale
• Richter scale


  Measure of the magnitude of volcanic eruptions
• Volcanic Explosivity Index (VEI)

  Combines the total amount of ejected product, height of eruption cloud, and duration of main eruptive phase
• Frequency


  How often an event occurs
• Occurrence level


  e.g. 1 in 100 years
• Frequency


  Inverse relationship between frequency and magnitude
• Severe magnitude


  The less frequent it occurs
• Areas that experience frequent tectonic events are more adapted to them (monitoring, education and awareness)
• Unexpected tectonic events can be particularly devastating as the community is not prepared (e.g. 1993 Killari earthquake)