plate tectonics

• Sea-floor spreading and paleomagnetism would suggest that the earth would be getting bigger
• Structure of the Earth

  A) Asthenosphere

  B) Mantle

  
• Different Plate Boundaries

  A) Constuctive

  B) conservative

  C) Destructive

  
• The rate of spreading was estimated at about 5cm a year, which was confirmed by the studies of the Earth’s magnetic field in rocks – palaeomagnetism.
• Every 400,000 years or so, the Earth’s magnetic field switches polarity, causing the magnetic north and south poles to swap. 
• Continental plates (over 1500 million years old) are permanent and may extend far beyond the margins of current land masses. They will not sink into the asthenosphere because of their relatively low density
• Denser oceanic plates (less than 200 million years old) are continually being formed at mid-ocean ridges and destroyed at deep ocean trenches – hence their relatively young age. 
• Convection Cells

  Radioactive decay within the core of the Earth generates exceptional temperatures. Hot Spots around the core heat the lower mantle, creating convection currents, which rise towards the surface before spreading in the asthenosphere, then cooling and sinking again.
• Why does the mantle material (magma) begin to rise?

  It increases in temp and decreases in density and so rises up above the denser material that surrounds it
• What is upwelling?


  The process of magma rising up through the mantle and asthenosphere towards the lithosphere.
• What is frictional dragging?


  Friction of a fluid against the surface of an object that is moving through it. In this instance, friction against the semi-molten magma in asthenosphere against the plates in the lithosphere.
• Why does the mantle material (magma) begin to sink? 


  As the magma gets closer to the lithosphere it begins to cool. As it cools, its density increases again causing it to sink back down into the lower mantle (downwelling).
• Ridge Push – As the oceanic plate (sima) moves away from a mid-ocean ridge, cooling begins to take place. This causes the plate to thicken and the boundary between the lithosphere and the asthenosphere gets deeper.