Tectonics

• Earth structure:

  inner core: a ball made of solid iron and nickel, hottest part of the earth with temps of up to 5,500

  outer core: liquid, also made of iron and nickel, also very hot

  mantle: semi-molten rock (magma) which moves very slowly, widest section with diameter of 2900km

  crust: outer layer of the earth, 10-70km thick

  2 different types of crust:

  continental crust: thicker + less dense, carries land

  oceanic crust: thinner + more dense, carries water
• Convection currents + plate movement

  • the earths crust is broken up into tectonic plates
  • they move due to convection currents in the mantle
  • heat from the core makes magma in the mantle rise towards the crust
  • As the hot current nears the crust, it begins to cool and sink back towards the core
  • as the magma sinks it drags the plates across the surface of the earth
• Destructive plate boundaries:

  • where a continental plate meets an oceanic plate
  • oceanic plate descends under the continental plate because its denser(subduction)
  • creates volcanoes and oceans trenches
  • the oceanic plate melts, is now hot magma
  • the magma escapes under extreme heat and pressure through cracks in the rock + rises through the continental plate
  • if it reaches the surface, the liquid rock forms a volcanoe
  • eruption can be violent, with lots of steam, gas and ash


  .
• constructive

  • where tectonic plates pull apart due to convection currents
  • magma rises from the mantle to fill the gap and cools creating new crust
  • oceanic: forms underwater volcanoes and mid-ocean ridges
  • The magma can escape easily at the surface, so the volcano doesn’t erupt with much force
  • continental: the partially melted mantle rises in the gap
  • it heats the plates above causing them to bulge and rise
  • as the plates stretch, they fracture along fault lines creating rift valleys
• conservative

  • where 2 plates slide past each other along a fault
  • moving in the same direction at different speeds
  • as plates move, friction occurs and they become stuck
  • pressure and stress builds up in the crust
  • when the pressure is suddenly released, waves of energy move through the crust causing an earthquake
  • no volcanos
• collision

  • both plates made of continental crust and move towards each other
  • no subduction
  • both plates are folded and forced upwards, forming fold mountains
  • earthquakes occur
• What drives plate movement?

  • the earths core acts as a heat source, as radioactive material decays the heat it emits is transferred to the outer core and to the mantle
  • the magma in the lower mantle are heated, becoming less dense than the surrounding rock and it rises towards the Asthenosphere and the crust
  • as it reaches the crust above it, it cools becoming more dense and sinks down to become reheated (convection currents)
• ridge-push, slab-pull

  • where there are cracks, fault lines or weaknesses in the crust, magma from the mantel erupts to form new crust, pushing the tectonic plate away (ridge-push)
  • At the same time, the far edge of that same tectonic plate (which is older and more dense) is being subducted down into the mantle, which pulls the rest of the plate with it due to gravity (slab pull)
• earthquakes

  • tectonic plates jerk past each other sending out shock waves (vibrations), aka the earthwuake
  • shock waves spread out from the focus: the point in the Earth where the earthquake starts
  • near the focus, the waves are stronger and cause more damage
  • epicentre: the point on the Earth’s surface straight above the focus
  • Earthquakes are measured using the moment magnitude scale: measures the energy released by an earthquake
  • or the Mercalli scale: measures the effects
• Shallow focus earthquakes:

  • caused by tectonic plates moving at or near the surface
  • have a focus between 0km and 70km below the Earth’s surface
• deep focus earthquakes

  • caused by crust that has previously been subducted into the mantle moving towards the centre of the earth, heating up or decomposing
  • they have a focus between 70km and 700km below the Earth’s surface
• Hotspots

  Can be found in oceanic or continental crust, and near or far from plate boundaries
• Hotspots remain stationary over time, but the crust moves above them
• Hotspot

  A bit of the Earth’s crust that is hotter than normal
• Hotspot

  • Hawaii is a chain of volcanic islands in the middle of the pacific plate
• Magma can break through the crust

  Eruption and a volcano forms
• Hotspots


  Areas of intense volcanic activity that aren’t at any plate boundaries
• Formation of hotspots

  A plume of hot magma from the mantle moves towards the surface, causing an unusually large flow of heat from the mantle to the crust
• composite volcanoes, e.g Mount Fuji in Japan

  • destructive plate boundaries
  • subducted oceanic crust contains lots of water
  • the water can cause the subducted crust to erupt
  • Made of alternating layers of lava and ash
  • Eruptions are a pyroclastic flow: a mixture of hot steam, ash, rock and dust
  • cannot flow far, forming a steep-sided cone
• Shield volcanoes

  • constructive plate boundaries/hotspots
  • not very explosive, as only made up of lava
  • frequent but gentle eruptions
  • runny lave, flows quickly and spreads over a wide are
  • forms a low, gentle-sided volcanoe
  • e.g Mauna Loa on the Hawaiian islands
• earthquake mitigation

  protection

  • earthquake proof buildings can be designed
  • they are often expensive due to the high level technology required
  • even simple buildings can be made safer in a cheap way
• protections

  • damper: in roof, acts like a pendulum to reduce sway, e.g the Taipei 101 in japan has a mass damper
  • cross bracing: stops floors collapsing
  • shock absorbers
  • strong safety glass, prevents windows breaking
  • gas pipes automatically shut off
  • very deep foundation
  • simple buildings:
  • lightweight thatch roof
  • cross-braced wood/bamboo frame
  • walls of mud + straw packed between wooden slats
  • simple steel rod foundations
  • concrete ring ties the walls to the foundations
• prediction

  • predicting time, date + locations of an earthquake is difficult as there is little warning
  • seisometes + computer modelling are used to monitor earthquake prone areas
  • has been no successful prediction yet
• earthquake early warning systems

  • eew systems use earthquake science + the tech of monitoring systems to alert and send messages to devices and people. when shaking waves generated by an earthquake are expected to arrive at their location
  • the seconds of advance warning can allow people + systems to take actions to protect life + property from destructive shaking
• preparation

  • places prone to earthquakes hold regular earthquake drills so everyone knows what to do in the event of an earthquake
  • people are encouraged to keep provisions of basic necessities + first aid équipement in an accessible location, in case water, food + medical kits become scarce