Constructive Plate Margins
These are located over rising convection cells. The high heat flow results in the partial melting of the upper mantle. The molten rock rises up and accumulates in a magma chamber, which may push the crust upwards causing tensional rifting.
Magma rises through the crust and forces it apart
As the magma forces the original crust apart it forms new crust. The rift valley continues expanding and water collects in the valley forming a sea.
The sea floor continues to spread and the plates get bigger and bigger. This
process can be seen all over the world and produces about 17 square kilometers of new plate every year.
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images taken from www.moorlandschool.co.uk