ocean ridges

• The Moho is the greatest depth that seawater can percolate - beneath serpentinite peridotite. 
• Types of MOR spreading:

  • Symmetrical 
  • Asymmetrical 
• Symmetrical:

  • As plates separate, the asthenosphere wells up and fills the gap 
  • The melt rises and solidifies to form an ocean crust made of layers of gabbro, dolerite and basalt 
  • The melt may accumulate in one or more magma chambers on the way up 
  • Sufficient melt is produced to make simple, layered igneous ocean crust 
  • Once the crust has formed the stresses pulling the plates apart produce normal faults 
  • The structures and spreading rates of both plates are similar, new material is added equally to both sides 
• Asymmetric spreading:

  • New model that applies to slow ridges 
  • Less melt produced so thinner crust 
  • Faults are much larger and is important in plate movement 
  • Fault penetrates all the way through the lithosphere - detachment fault
  • The hanging wall gets less melt than the fast spreading ridge so its crust is thinner 
• Asymmetric spreading:

  • The footwall is made by pulling mantle material up and forms irregular, discontinuous crust or no crust 
  • If all plate separation is taken up by slip on the fault and no new material is added to the hanging wall, spreading will be 100% asymmetric 
  • Creates sea floor landforms of an ocean core complex