Techtonic theory

• crust

  • very light because contains elements silicon, oxygen, potassium and sodium
  • varies in thickness from 6-10km beneath oceans to 30-40km below continents but can reach up to 70km
• Mantle

  • molten and semi molten rock
  • contains lighter elements such as silicon and oxygen
  • makes up 84% of earth's volume
• lithosphere

  • consists of crust and rigid upper section of mantle
  • 80-90km thick
  • divided into 7 large plates
• asthenosphere

  semi molten on which plates float
• core

  • made of iron and nickel
  • solid inner core due to pressure from other layers
• constructive plate margins

  • occurs when two plates are moving apart
  • the mantle is under pressure from the plates above and when they move apart the pressure is released at the margin
  • the mantle melts producing magma which is less dense so rises to form a volcano
  • creates two different landforms depending on where they are - ocean ridge or Rift Valley

  A) ocean ridge

  B) apart

  C) lithosphere

  D) magma

  E) constructive

  F) lava

  
• Mid-ocean ridges

  • occurs at a constructive plate boundary which plates diverge underwater
  • underwater volcanoes erupt along mid ocean ridges which can build up to above sea level
  • transform faults - regular breaks which cut across the ridges at right angles to the plate boundaries
  • e.g. Mid-Atlantic ridge

  A) lithosphere

  B) asthenosphere

  C) ocean ridge

  
• Rift valleys

  • occur at constructive plate boundaries
  • plates diverge beneath land and rising magma causes continental crust to bulge and fracture forming fault lines
  • crust between parallel faults drop down to form a rift valley
  • rift valleys are widened by magma rising from the asthenosphere which cools and solidifies to form new crust

  A) fractures

  B) apart

  C) Rift Valley

  
• conservative plate boundaries

  • occurs when plates slide past each other horizontally without subduction or destruction of crust
  • no volcanic activity but active margins with strong earthquakes
  • characterised by lateral movement, friction, stress, earthquakes, fault lines, surface structures and no volcanic activity
  • e.g. Alpine fault New Zealand

  A) conservative

  B) focus

  C) epicentre

  D) seismic waves

  
• destructive plate boundary (oceanic + continental)

  • denser oceanic crust is forced under continental
  • fold mountains made of sediment form where the plates meet
  • oceanic crust is heated by friction and contact with upper mantle
  • magma is less dense so will rise to form volcanoes
  • pressure causes earth quakes

  
• destructive plate boundary (oceanic + oceanic)

  • the plate which is faster or denser is sub ducted
  • forms a deep ocean trench
  • forms island arcs

  A) island arc

  B) trench

  
• destructive plate boundary (continental + continental)

  • neither are sub ducted as plates have lower density than asthenosphere beneath
  • forms fold mountains - Himalayas

  A) lithosphere

  B) asthenosphere

  
• sea floor spreading

  • The Mid Atlantic ridge and a similar feature in the Pacific Ocean were studied
  • implied that the earth must be getting bigger but it isn't
  • plates must be being destroyed to accommodate for the increase in size of mid ocean ridges
  • found oceanic trenches were large areas of the oceanic plate was being sub ducted
  • hot spots around the earth's core generates thermal convection currents within the asthenosphere so magma rises, spreads, cools and sinks
• continental drift theory

  • Alfred Wenger 1912
  • said 300million years ago there was one content called Pangea which split up due to plate movement
  • fossils of the same reptile was found in South America and Africa and their coastlines fit together

  
• ridge push/ gravitational sliding 


  • occurs at constructive plate boundaries
  • downward gravitational force acting on the cold, dense, depending plate as it sinks into the mantle
  • whole oceanic plate is forced down due to the negative buoyancy of the plate
  • magma rise, cools and forms new crust which heat surrounding rock which expands and rises forming a slope
  • crust cools and condenses so the denser rock moves down
  • pressure is put on plates so they move apart

  A) ridge push

  
• slab pull

  • downward gravitational force acting on the cold descending plate as it sinks into the mantle
  • whole oceanic plate is forced down due to the negative buoyancy of the plate
  • after subduction the lithosphere sinks into the mantle which pulls the rest of the plate with it

  A) ridge push

  B) slab pull

  
• ridge push/ gravitational sliding 


  • occurs at constructive plate boundaries
  • forces acting on the plate can be generated at the boundaries
  • forces push from the ridge, drag the plate down at the trenches or act alongside the plate at conservative boundaries
  • result of gravity acting down the slope
  • magma rise, cools and forms new crust which heat surrounding rock which expands and rises forming a slope
  • crust cools and condenses so the denser rock moves down
  • pressure is put on plates so they move apart

  A) ridge push

  
• deep sea trenches 


  • oceanic and continental plates meet and the denser oceanic plate is forced under by subduction
  • similar happens when two oceanic plates move towards each other
• island arcs

  • during subduction the descending plate encounters hotter surroundings and heat from friction which begins to melt the plate
  • as it is less dense it rises as plutons of magma which reaches the surface to form composite and explosive volcanoes
  • if the eruptions take place offshore a like of volcanic islands form
  • e.g. Hawaii islands
• deep sea trenches 


  • oceanic and continental plates meet and the denser oceanic plate is forced under by subduction
  • similar happens when two oceanic plates move towards each other
  • e.g. Peru Chile trench when Nazca plate is subjected under South American plate
• Young fold mountains

  • plates that form continental crust have a lower density than underlying layers so less subduction when plates meet
  • when they move together their edges and the sediment are forced up into fold mountains
• magma plumes

  • concentration of radio elements below the crust causes at hotspot to develop
  • an upwelling plume of magma rises, eating into the plate above
  • when the lava breaks through to the surface, active volcanoes form above the spot
  • basaltic lava flows slowly forming shield volcanoes
  • usually found close to plate margins but can rise within the centre of plates
  • e.g. Hawaiian islands - the hotspot is stationary so as the plate moves over a line of volcanoes is created

  A) hotspot

  B) magma plume

  C) older

  
• hotspot

  an area of volcanic activity caused by a magma plume rather than a plate boundary
• hotspot volcano chains

  as plates move over stationary plumes as series of volcanoes form
• super volcanoes

  massive magma chambers associated with plumes
• flood basalts

  large scale volcanic eruptions associated with magma plumes