The mid-oceanic ridges underling the hot asthenosphere which creates a depression that causes the melting of the peridotites at shallow depths. As they rise that results in mafic magma and this only takes place at divergent boundaries.
Here the plates sperate and move away, causing magma below to rise and ill spaces of low pressure. The rocks then cool and new crust is firmed. The mafic rocks crystallize at higher temperatures and pressure.
