Constructive Margins

  • Oceanic ridges form the longest continuous uplifted feature on the surface of the Earth, having a total combined length of over 60,000km.
  • Where two plates pull apart there is a weaker zone in the crust and increase in heat near the surface.
  • The hotter, expanded crust forms a ridge. The central part of the ridge may feature a central valley where a section of crust has subsided into the magma below.
  • The split in the crust provides low pressure zone where more liquid lavas can erupt to form submarine volcanoes.
  • If these eruptions persist, volcanoes may develop until they reach the surface; islands can be formed in this way. Examples include Iceland and the island of Surtsey.
  • As crust is pushed away from the heat source at the mid-ocean ridge, it cools, contracts and sinks towards the deeper regions, where it becomes covered in fine sediments.