Tectonics

• Plate tectonics


  A complex jigsaw of the earth with 7 major tectonic plates and microplates moving in different directions at different speeds
• Plate tectonic theory is still just a theory and cannot be confirmed as the most accurate explanation
• Geologists study seismic waves to identify a low velocity zone (LVZ) which is essentially the asthenosphere, a weak layer that allows plates to slide
• How tectonic plates move

  1. Convection currents carry heat from the core towards the mantle

  2. Upwelling at mid-ocean ridges pushes solid lithosphere along

  3. Slab pull causes old plates to sink into the mantle
• Seismic tomography shows that subduction can be flat or steep, and detached slabs have also been found
• A large proportion of the world's earthquakes and volcanoes are found at locations of seafloor spreading or subduction zones
• Mantle plumes


  Concentrated columns of rising heat that can cause isolated areas of vulcanism
• Plate tectonic components


  • Crust
  • Tectonic plates
  • Oceanic crust
  • Continental crust
  • Oceanic trenches
  • Crescentic bars, salt marshes
• Continental drift ideas floated around in the early 20th century but had no real focus until Alfred Wegener in 1912
• Evidence for continental drift

  • Geologic
  • Biological
  • Paleomagnetism
  • Jigsaw fit
  • Sea-floor spreading
• Geologic evidence


  • Fit of continents like South America and Africa
  • Glaciation evidence in Southern Africa, Australia, India
  • Mountain chains and rock sequences in North East Canada and North Scotland
• Biological evidence


  • Similar fossils of marine shellfish in Australia and India
  • Similar reptile fossils in South America and South Africa
  • Fossil similarities in rocks younger than Carboniferous period in Australia and India
• Paleomagnetism


  Measuring variations in the Earth's magnetic field recorded in igneous rocks, showing polarity changes every 400,000-500,000 years
• Sea-floor spreading


  The lateral movement of new oceanic crust away from a mid-ocean ridge, with older rock further away and newer rock closer to the ridge
• Biological evidence is the most important piece of evidence for continental drift
• Asthenosphere 


  The layer in the earths mantle below the lithosphere. The high temperatures cause the rocks to soften and become viscous meaning that they can easily deform
• Benioff zone 


  Boundary between a subducting ocean plate and the over riding continental plate at a destructive boundary
• caldera 


  Large scale volcanic crater formed as a result of an explosive eruption which emptied the magma chamber causing the volcano sides sides to subside
• Black smoker


  Hydrothermal vent on the ocean floor at constructive or destructive plate boundaries. The water carries high amounts of metal sulphides
• Continental drift 


  The theory that the continents are mobile and have moved across the earths surface through geological time
• Disaster 


  Natural hazards that result in major loss of life , injury and economic damage
• dyke


  A vertical or near vertical minor intrusion of magma through surrounding older rocks
• Earthquake focus


  Point in the crust where rocks fracture, releasing energy
• Effusive eruption 


  A gentle, free-flowing basic eruption of lava eg basalt
• Épicentre 


  Point at surface directly above an earthquake focus
• Explosive eruption 


  A violent eruption owing to the build up of pressure within a volcano, due to viscous magma eg andesite preventing the escape of gases
• Fault scarp


  A cliff or escarpment formed directly by rocks being displaced either side of a fault
• Flood basalt 


  A large area of basaltic lava erupted over along period from multiple vents
• Graben 


  The downfaulted section of a rift valley
• Hot spot 


  An area of intense volcanic activity where a mantle plume reaches the earths surface causing eruptions, located away from plate boundaries eg Hawaï
• Hyrdrothermal vent 


  Very hot water springs, rich in dissolved minerals, flowing from the ocean floor at mid ocean ridged or hotspots
• Island arc


  Chain of volcanic islands formed alone a subduction zone
• Jokulhaulps


  Extreme glaical meltwater outbursts caused by geothermal or volcanic activity Beneath glaciers
• Liquefaction 


  The process by which sediments and soils lose their mechanical strength from a sudden loss of cohesion. The material is temporarily transformed into a fluid as the result of being violently shaken during an earthquake
• Mid-ocean ridge 


  The boundary between two diverging oceanic plates. It consists of two parallel chains of submarine mountains separated by a graben and offset in place by transform faults
• Moho 


  Short for mohorivic discontinuit, the boundary between the crust and mantle. It lies at c35 km beneath the continents and the 10-15km beneath the ocean floor
• Modified mercalli scale 


  a subjective measure of earthquake intensity using factors such as what was felt by people and the type and scale of damage to buildings and infrastructure
• Moment magnitude scale 


  A measure of earthquake strength suing the amount of physical movement caused by a quake
• Ocean trench 


  narrow deep depressions on the ocean floor adjacent to a subduction zone
• Paléomagnétisme 


  Traces of changes in the earths magnetic field in the alignment of magnetic minerals in sedimentary and igneous rocks