Answer:
Magma fills those area forming a new crust eventually.
Explanation:
Divergent plate boundary is where tectonic plates move apart. The divergent boundary occur between oceanic crusts or continental crusts but it is more prevalent in the oceanic crust.
The oceanic crust where most divergent boundary occurs introduce aesthenosphere material to the sea floor causing a formation of new crust.
When plates moves apart, through the oceanic ridges molten magma are released to form a new crust. The magma creeps through cracks and fissures and eventually get exposed to the surface. The rapid cooling of this magma forms mostly extrusive igneous rocks.
Rift are linear area formed as result of tectonic plates moving apart. Rifts are a kind of valley form when their is extensional forces. As the plates rift apart the valley are formed , aesthenosphere materials like molten magma fills in the rift to form a new crust after rapid cooling.
The pictures above show divergent boundaries and how rift are formed in a divergent boundary.