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Plate Tectonics

Structure of the Earth

  • At the centre of the Earth is the core
    • The inner core is made of solid iron and nickel
    • The outer core is liquid at approximately 6000°C
  • Around the core is the mantle
    • This is semi-molten rock that moves very slowly at approximately 4000°C
  • The outer layer is the crust
    • It is about 10-70km thick
    • The crust is divided into slabs that float on the mantle
    • There is oceanic crust which lies beneath oceans and continental crust which lies beneath land
      • Oceanic crust is 5-10km thick whereas continental crust is 25-70km thick
      • Oceanic crust is thin and denser whereas continental crust is thicker and less dense
      • Oceanic crust is typically composed of basalt rock whereas continental crust is typically composed of granite rock
      • Oceanic crust is younger because it is recycled
  • The lithosphere is made up of the crust and upper mantle and the asthenosphere is just below that
  • Circular movements called convection currents move tectonic plates
    • The lower parts of the mantle are sometimes hotter than the upper parts, so when the lower parts heat up, they become less dense and rise slowly
    • As they move towards the top of the mantle, they cool down, become denser and sink slowly
  • Convection currents are not strong enough, so scientists research ridge push and slab pull
    • Ridge push occurs when new crust rises, becomes warm and thin, creating a ridge; it then pushes older crust away from the ridge
    • Slab pull occurs when the old crust is cooler and thicker than the hot mantle, so it sinks into the mantle

Plate Boundaries

  • Earth’s crust is broken up into tectonic plates which meet each other at plate boundaries
  • A constructive boundary is when plates are moving apart, creating new land
    • Magma rises from the mantle to fill the gap, and cools, constructing new crust
    • The Eurasian plate and the North American plate are moving apart at the Mid-Atlantic Ridge
  • A destructive boundary is when plates are moving towards each other, with one being forced under the other
    • When an oceanic plate meets a continental plate, the denser oceanic plate is subducted into the mantle and destroyed
    • This often creates volcanoes, fold mountains and ocean trenches
    • The Pacific plate is being force under the Eurasian plate along the east coast of Japan
  • A conservative boundary is when plates are moving alongside each other
    • Either the plates are moving in the same direction at different speeds, or, the plates are moving sideways past each other
    • Crust isn’t created or destroyed
    • The Pacific plate is moving past the North American plate on the west coast of the USA, at the San Andreas fault
  • A collision boundary is when plates are moving towards each other and creating mountains
    • Both plates are continental crust, so neither plate is forced down into the mantle
    • Both plates are forced upwards and the sedimentary rock forms fold mountains
    • The Eurasian and Indian plates are colliding to from the Himalayas
  • Hotspots are areas where a magma plume rises causing an unusually large flow of heat from the mantle to the thin crust

Earthquakes

  • Earthquakes are caused by tension that builds up at plate boundaries
    • At constructive boundaries, tension builds along cracks with in the plates as they move apart
    • At destructive boundaries, tension builds up when one plate gets stuck as its subducting
    • At conservative boundaries, tension builds up when plates that are grinding past each other get stuck
    • At collision boundaries, tension builds as the plates are pushed together
  • The plates eventually jerk past each other, sending out vibrations, these are the earthquake
  • The seismic waves spread out from the focus, the point in the Earth where the earthquake starts
  • The epicentre is the pint on the Earth’s surface directly above the focus
  • The energy released by an earthquake can be measured on the moment magnitude scale (or the Richter scale that is no longer used), and the effects can be measured on the Mercalli scale from Level I-XII
  • The focus of an earthquake can be at the Earth’s surface or anywhere up to 700km below
    • Shallow focus earthquakes are caused by tectonic plates moving at, or near, the surface; they have a focus up to 70km below Earth’s surface
      • These are common and cause widespread damage
    • Deep focus earthquakes are caused by previously subducted crust heating up or decomposing; they have a focus between 70-700km below Earth’s surface
      • These are rare and cause localised damage
      • They are generally less damaging because they must travel through more rock before reaching the surface; however, the seismic waves radiate vertically, so the damage is more concentrated

Volcanoes

  • Volcanoes are found and constructive and destructive plate boundaries
    • At destructive plate boundaries, the subducted oceanic plate melts, forming a pool of magma which rises through vents in the continental plate
      • The magma erupts onto the surface, and the lava forms a volcano
    • At constructive plate boundaries, the magma rises into the gap created by the plates moving apart
  • When a volcano erupts, it emits lava and gases
    • Some emit ash which can cover land, block out the sun and form pyroclastic flows
  • At hotspots, magma plumes burn through the crust and spill over to form a shield volcano
    • Hotspots remain stationary overtime, but the crust moves above them; this can create chains of volcanic islands such as Hawaii in the middle of the Pacific plate
  • Shield volcanoes occur at hotspots and constructive plate boundaries
    • They are not very explosive and are made up only of lava
    • The lava is runny, so it flows quickly and spreads over a wide area, forming a low, gentle-sided volcano
  • Composite volcanoes occur at destructive plate boundaries
    • Subducted oceanic crust contains lots of water which can cause the subducted crust to erupt
    • The eruptions start with ashy explosions that deposit a layer of ash
    • Then, a layer of thick, viscous lava erupts that can’t flow very far
    • The alternate layers of ash and lava form steep-sided cone shapes

Case Study: Tectonic Event, Nepal Earthquake

  • On 25 April 2015, there was an earthquake in Nepal which was 7.8 on the moment magnitude scale
    • Geo-scientists were aware of pressure building however they didn’t know when an earthquake would occur
    • The Indian and Eurasian plates move towards each other at a collision boundary and move at about 20mm a year
    • The focus was shallow at 15km below surface
    • The intensity measured IX (violent) on the Mercalli scale
    • The fault line ran underneath the densely populated capital city of Kathmandu
  • The earthquake was followed by many aftershocks, two almost equally large as the original
    • It also triggered deadly avalanches on Mount Everest
  • The mountainous terrain increased the danger as this can lead to landslides
    • Infrastructure was damaged
    • Transport facilities were disrupted
    • Many people went missing
    • It delayed the rescue of earthquake victims
  • Nepal is an LIDC meaning the consequences were more severe; buildings are poorly built, and the people received no warning
    • 8635 people were killed
    • 19 009 people were injured
    • Over 180 buildings were reduced to rubble in Kathmandu
    • Historical and religious buildings were destroyed
    • Thousands of homeless people had to sleep outside
    • Schools, health facilities and government offices had to close
    • People in Kathmandu struggled to return to their families in rural areas
    • Landslides in the mountains buried houses and cut off roads
    • $10 billion of damage was caused
    • Aftershocks destroyed weakened buildings
  • The earthquake was instantly recorded on seismometers; initially reports were focused on Mount Everest, then Kathmandu as it took longer to find out about more remote rural areas cut off by landslides
  • As an LIDC, Nepal themselves were not well-equipped to respond to the event
  • Emergency aid was donated by many countries all over the world
    • India responded with 10 tonnes of blankets, 50 tonnes of water, 22 tonnes of food, doctors and 2 tonnes of medical supplies and an engineering task force
    • NGOs such as the Red Cross and Oxfam helped to support the injured and homeless
    • Aid was hampered by infrastructure that was damaged
    • The World Food Programme provided food for 1.8million people and was a complex job due to the difficult terrain
    • Save the Children protected children from the danger of traffickers
    • The British Government pledged£33million to help Nepal, £10million of which for primary healthcare and rebuilding hospitals
  • The response was very rapid however in more isolated areas, the supplies took up to a month to arrive
  • Long term response was not as effective
    • Poor policing meant that children were in danger of being abducted by traffickers
    • Nepal was given little monetary aid as international aid organisations feared that the government embezzle the money due to corruption
    • Three years after the earthquake thousands were still living in temporary homes
    • A “cash for work” scheme offering loans to locals to rebuild businesses did have some success

Technological Developments

  • Technological developments can have a positive impact on mitigation, such as building design, prediction and early warning systems in areas prone to tectonic hazard
  • People can prepare for earthquakes
    • Educational leaflets or programmes
    • Drills
    • Emergency kits
    • Training search and rescue teams
  • Earthquakes cannot be reliably predicted in advance, but networks of seismometers and lasers can be used to monitor earth movements in areas at risk of earthquakes
    • These can give small, but vital amount of warning before an earthquake occurs
  • Early warning systems mean that warnings can be communicated quickly and automatically to people and control systems using the internet, SMS networks and sirens
  • The warning can be useful for people to protect themselves
    • People can get undercover before the shaking starts
    • People can stop if they’re performing dangerous or delicate jobs
    • Utilities can be shut off to prevent leaks and fires
    • Trains can start slowing down, making derailment less likely
  • Volcanic eruptions can be predicted if the volcano is well monitored; this enables people to evacuate in time
  • Remotely operated seismometers, lasers and other sensors can detect indications that and eruption is likely such as tiny earthquakes, escaping gas and changes in the volcano’s shape
  • Volcanoes are also monitored during their eruptions helping to plan the appropriate response
    • Evacuation zones can be extended if the eruption becomes more violent
    • Ash clouds can be tracked so air traffic can be diverted or cancelled
    • If ash and poisonous gas spreads, authorities can warn people to put on gas masks
  • Industrial buildings can be designed to withstand earthquakes
    • Damper in the roof acts as a pendulum to reduce sway
    • Cross bracing stops floors collapsing
    • Steel frames to sway during earth movements
    • Rubber shock-absorbers in the deep foundations to absorb earth tremors
    • Automatic window shutters over strong safety glass to prevent falling glass
  • Simple buildings can also be designed to withstand earthquakes
    • Lightweight thatched roof prevents injuries during collapse
    • Cross-braced wood or bamboo frame makes the building more flexible when being shaken
    • Steel rod foundations allow the buildings to move with the earth where the ground is shaking
    • Walls of mud and straw packed between wooden slats offer reinforcement and are subject to smaller forces