geography hazards

• oceanic crust


  is the part of the earths lithosphere that surface in ocean basin. this is thinner between 6-8km thick
• continental crust


  is the layer which form the continents and the areas of shallow seabed's close to the shores, known as continental shelves on average round 36km thick.
• what are plate boundaries

  alfred wegners theory that all the continents were joined together, africa and south america were joined together. he called this pangaea.
• convection currents

  1. the molten mantle below the crust is heated by the core. this makes it become less heavy than its surroundings and rises.
  2. as it reaches the crust above, it spreads out carrying the plates above with it
  3. as the molten then cools, its gradually sinks back down to be re-heated

  
• destructive boundary 


  south America plate

  continental crust

  1. two plates pushed together and the heavier oceanic crust gets surrounded
  2. the oceanic plate sticks and locks as its tries to slide under the continental plate
  3. heat from friction and the mantle starts to melt the subducted plate

  
• constructive boundarys

  1. two plates move apart from each other
  2. a gap appears and magma rises to fill the gap
  3. the solidifies to create new rocks on the sea beds
  4. overtime the layers of the rock build up and break through the surface of the ocean
• how are oceanic trenches formed

  formed where the ocean crust bend downards and slides back into the mantle on the destructive plate margains. oceanic trenches are long, narrow, deep holes in the seabed, deeper than 6km
• rift valleys formation 


  iceland sits astride the mid-atlantic ridge on a volcanic hotspot. iceland is gradually being torn into two and this can be clearly seen in thingvellir national park.

  long fissure opens in the surface. it will widen the land at the center and collapse. this forms a rift valley