Final answer:
Rock from the mantle is added at divergent boundaries where plates move away from each other, while convergent boundaries see plates moving toward one another.
Step-by-step explanation:
The rock from the mantle is added to the edges of both plates at divergent plate boundaries, where the plates move away from each other. Divergent boundaries typically involve the creation of new crust as magma pushes up from beneath the Earth's surface and solidifies at rifts, like the Mid-Atlantic Ridge.
Conversely, at convergent plate boundaries, plates move toward each other and can lead to subduction, where one plate is forced underneath another plate and melts into the mantle.