How is continental crust formed?

Continental crust is pretty resilient. It has a lower overall density so it's pretty hard to subduct continental crust. Places where two pieces of continental crust collide (e.g. Himalayas) can create so much stacking of rock that there can be partial melt of continental crustal material near the bottom. Depending on how you want to define "destroying continental crust" that is one of the only few I'm familiar with.

Oceanic crust is constantly being recycled via subduction zones. Because of this, the oldest oceanic crust is only about 200 million years old. By comparing the ages of the oldest continental rocks, the Acasta Gneiss, at about 4 billions years, you can see continental crust stays around for quite a while.

Continental crust is typically formed where you have a subducting oceanic plate beneath continental crust. As the oceanic plate subducts, it reaches a depth where water molecules contained in the minerals of the plate become unstable and are released. This addition of water into the mantle causes partial melting. The mantle is just as hard as any rock on the earth's surface, but adding water causes melting. Looking at the graph on the right side, you can see a black dot. Before water is added, this P/T condition results in solid rock. Adding water depresses this line to the left and now the rock is liquid even though the P/T conditions haven't changed. This is kind of analogous to adding salt to ice to make it melt. This partial melt is enriched in SiO2 which will erupt on the surface in the form of rhyolite or ash deposits, or crystallize at some depth and form granite (like the Sierra Nevada Batholith). The magmatism associated with subduction is referred to arc magmatism. Further tectonic forces suture these little pieces of continental crust onto an already present continental crust.

Further reading if you're interested: (I know they're wikipedia articles but they give a decent overview)

http://en.wikipedia.org/wiki/Craton

http://en.wikipedia.org/wiki/Continental_crust

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