booklet 3 - tectonics

• continental crust: thickness - 30 - 70 km

  age - over 1500 million years

  density - 2.6 (lighter)

  composition - granite: silicon, aluminium, oxygen
• oceanic crust: thickness - 6-10 km

  age - less than 200 million years

  density - 3.0 (heavier)

  composition - basalt: silicon, magnesium, oxygen
• solid core: iron + nickel, is solid despite temps of 3700 degrees due to the intense pressure
• outer core


  under slightly less pressure than solid core
• mantle: zones of molten silicates + other minerals, molten so moves due to convection currents, 2900 km thick

  - asthenosphere

  - lithosphere
• asthenosphere: - upper part of mantle

  - approx 80km deep

  - rocks kept in semi molten state

  - where plates float and move

  --- hotter + flows
• lithosphere: crust and rigid upper section of mantle

  - approx 80-90km thick

  - divided into 7 very larger plates + many smaller ones

  --- cooler and more brittle
• plate margins: edges of plates where 2 plates either slide over eachother, move apart or collide
• convection currents: heat currents in magma that move the crust slowly
• the crust


  two types - oceanic + continental
• importance of core as an energy source: - cores internal heat = main course of earths activity

  - heat may be primeval (retained by balls of gas + dust from which earth evolved)

  - greatest source of heat energy within earth derives from radioactivity - natural radio decay
• Wegener's theory: belief that the plates move

  - idea of one super continent - Pangaea

  - fossils for animals where found far apart

  - rock structures were the same in diff continents

  - jigsaw fit
• plate movement: scientists believed that not just convection currents caused plates to move
• ridge push theory: Hot magma is forced up from the asthenosphere, solidifies to form new oceanic crust, pushing oceanic plates apart at mid-ocean ridges, causing sea floor spreading.
• slab pull theory: at subduction zones, where one plate is pulled down into the mantle.

  - emerged as more dominant theory for explaining movement
• evidence for sea floor spreading - palaeomagnetism: - Harry Hess found youngest rocks were in the middle (Iceland) and oldest nearest USA and Caribbean

  - rate of spreading is estimate 5cm a year - confirmed by palaeomagnetism

  - every 400,000 years, earths magnetic field switches polarity - magnetic north and south swap