Pacific ocean - South America - Atlantic ocean west to east cross section of the earth showing the lithosphere and asthenosphere. Arrows indicate flow in the rock. The change of the plastic asthenosphere to brittle lithosphere is the result of cooling which causes a change in rigidity. Magma (white blob) generated by decompression melting  below the ridge produces basaltic magma that extrudes and intrudes to accumulate as the oceanic crust that is carried away from the ridge.  Where the lithosphere subducts (goes down into the mantle), heating melting of the crust carried down into the mantle  produces andesitic magma that intrudes and extrudes to add to the continental crust

 

 

 

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