Geography Physical - Tectonics

• Tectonic hazard


  A natural/geophysical event that has the potential to be of significant risk to life, livelihoods and the economy
• Tectonic hazards


  • Earthquakes
  • Volcanic activity
• Plate boundaries where tectonic hazards occur


  • Divergent
  • Convergent
  • Conservative
• Earthquakes


  • Often found along plate boundaries
  • 70% of all earthquakes are found in the Pacific Ring of Fire
  • Most powerful earthquakes are found at convergent boundaries
• Conservative plate boundaries


  • Plates slide past each other along transform faults (e.g. San Andreas Fault)
  • No volcanic activity
  • Earthquakes tend to be shallow focus and considerable magnitude
• Convergent plate boundaries


  • Continental plates collide into each other, forcing the plates upwards which buckle and form fold mountains (e.g. the Himalayas)
  • Earthquakes occur due to a build up of pressure that is released when the plates get unstuck
  • No volcanic activity
  • Frequent, shallow, high magnitude earthquakes
• Convergent plate boundaries (oceanic-continental)


  • Oceanic plate subducts under the continental plate as it is more dense and melts, generating magma that erupts explosively
  • Creates andesitic/composite volcanoes
  • Can create some of the largest earthquakes in the world
  • Island arcs are formed here
  • Ocean trenches are created by the seafloor and the lithosphere bending due to subduction of the oceanic plate
  • Tsunamis can be caused here
• The distribution of tectonic hazards is uneven - some areas are at major high risk, others are at low risk
• Fracture Zones

  • Oceanic fracture zone- belt activity through the ocean along mid ocean ridges
  • Continental fracture zone-belt activity following mountain ranges
• Volcanic hotspots


  • An area in the mantle from which heat rises as magma plumes from the core
  • High heat and low pressure at the base of the lithosphere melts rocks which rises through cracks in the crust to form volcanoes
  • As the tectonic plate moves over the stationary hotspot, the volcanoes move away and new ones are formed
  • Oceanic volcanoes cool and produce islands, atolls and seamounts -> over time this creates chains of volcanoes eg the Hawaiian Islands
• Formation of volcanoes at hotspots

  1. Heat rises from the mantle

  2. Melts rocks at base of lithosphere

  3. Magma rises through cracks in crust

  4. Forms volcanoes
• The Earth's crust is made up of tectonic plates that move over the mantle
• Volcanic hotspots are stationary while the tectonic plates move over them
• Chains of volcanoes are formed as the tectonic plate moves over the stationary hotspot
• Oceanic volcanoes cool and produce islands, atolls and seamounts
• Layers of the Earth


  • Inner Core
  • Outer Core
  • Mantle
  • Crust
• Inner Core


  Solid, mostly iron
• Outer Core


  Semi-molten, mostly iron and nickel
• Mantle


  Solid upper part, semi-molten lower part (asthenosphere)
• Crust


  Oceanic crust and continental crust
• Oceanic Crust


  Thin, very dense, composed of basalt
• Continental Crust


  Thick, less dense, composed of granite
• Benioff Zone


  • Depth of seismicity due to subduction of oceanic plates
  • Different speeds and movements of rock produce earthquakes
  • Contact between subducting plates is a thrust fault
  • Plates can lock for hundreds of years before releasing as a mega-thrust earthquake
  • This process of strains, stress & failure is the Elastic Rebound Theory
  • Locked faults can present a significant tectonic hazard
• Subduction


  Process of a plate sinking underneath another at convergent plate boundaries
• Slab Pull


  The cold, dense oceanic plate being subducted pulls itself down into the mantle
• Gravitational Sliding


  Constructive margins have elevated altitudes due to rising heat, creating a slope down which oceanic plates slide
• Continental Drift


  Alfred Wegener's theory that the continents were originally connected together
• Evidence for Continental Drift


  • Fit of South America and Africa
  • Fossils
  • Rock type
• Convection Currents


  • Radioactive decay in the core releases heat which rises through the mantle and drives convection currents
  • Convection currents act as cells which move tectonic plates
  • Tectonic plates can move as the lithosphere 'floats' above the asthenosphere
• Palaeomagnetism


  • The study of past changes in the Earth's magnetic field
  • Magnetic materials in cooling magma line up with the Earth's magnetic direction at the time
• Seafloor Spreading


  • Oceanic plates move away from each other, allowing magma to rise up and form new crust at mid-ocean ridges
  • Magnetic patterns in the seafloor can be used to date the age of the tectonic crust
• Magnetic Striping


  • Alternating bands of magnetic polarity on the seafloor, symmetrical on both sides of the mid-ocean ridges
  • Caused by magma flowing from the ridges preserving the then-current orientation of the Earth's magnetic poles
• Subduction


  Plate sinking underneath another at convergent plate boundaries
• Slab


  The oceanic plate being subducted is cold and dense - its own density pulls it down into the mantle
• Gravitational sliding


  Constructive margins have elevated altitudes due to the rising heat between them which creates a slope down which oceanic plates slide
• Earthquakes


  Sudden vibrations in the Earth's crust caused by a release in a build-up of energy between tectonic plates
• On average 10,000 people die each year because of earthquakes
• Earthquakes


  • Occur in the crust or upper mantle which ranges from the earth's surface to about 700 kilometers deep (about 500 miles)
  • Can occur at every plate boundary, but the severity differs
• Earthquake types by plate boundary


  • Divergent (weak & shallow)
  • Convergent (most powerful, destructive)
  • Convergent collision (powerful, deep)
  • Conservative (powerful, shallow)
• Tsunami


  A large ocean wave caused by a sudden movement on the ocean floor, which can be caused by earthquakes, volcanic eruptions or underwater landslides