Earth.Parts #19 - Origins of the continents and continental crust
Channel: Earth.Parts
Date: 2017-01-30

Where do the continents come from? Ocean crust is produced at mid-ocean ridge spreading-centers and destroyed at subduction zones, but what processes produce granitic continental crust?

Old ocean crust can subduct under young ocean crust, because even though both are basaltic and dense, younger basaltic crust is still less dense than very old, cold ocean crust. When driven together an older slab of ocean crust will be driven below the younger, where it partially melts to generate new magma.

The new magma produced by partially melting wet basalt, will be lighter and less dense than its parent rock. The daughter melt will be poorer in Mg, Ca and Fe and richer in Na, K, Si and Al, because of which minerals have the lowest melting points and form the first partial melt. 

Once melt forms it's driven upward by pressure, and it can erupt to form new rock at the surface. Magma derived from ocean crust will be more like andesite in composition, and partial melt derived from andesitic rock produces granitic rock, the primary component of continental crust. 

The South Sandwich Trench in the southern Atlantic Ocean is an example of ocean-ocean subduction, and the South Sandwich Islands that are now rising above that small subduction zone are forming rock that could form a small mass of continental crust. 

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