How is mafic magma generated at divergent plate boundaries?
By decompression melting. As the plates move apart, mantle rises to take their place. Also, there is less pressure on that piece of mantle from overlying rock. This causes the mantle to melt. When mantle melts, the composition of the melt is basalt (mafic magma). You may be used to thinking of melting as happening when something is heated. In the Earth, this is not an important method for melting rock. Decompression melting (decreasing the pressure on rock) is a more important melting mechanism. A second important melting mechanism, that happens at subduction zones, is addition of water to the mantle (the water-soaked oceanic plate introduces water to mantle as it is subducted).
