When two tectonic plates spread apart, it is considered to be constructive due to the fact that new crust is generated by the process. In this type of divergent plate boundary, magma rises up and eventually hardens to fill the gap created by the plates pulling apart.
Most divergent plate boundaries occur in the ocean, as a result of two oceanic plates separating from one another. When the magma rises to the surface to create new crust, it works to replace the crust that is destroyed at convergent plate boundaries where an oceanic plate collides with a continental plate. At this type of convergent boundary, the crust in the oceanic plate slides underneath the continental plate and is eventually melted into magma as it moves deeper into the Earth.
Although convergent oceanic boundaries may be constructive, the effect of two continental plates separating creates a rift valley that eventually fills with water, usually creating inland seas. This happens because the land between the two plates continues to sink deeper as the plates pull apart. One example of this is the East Africa Rift Valley, which is being created as the eastern coast of north Africa, which is part of the Indian plate, gradually separates from the African plate that contains the rest of the continent.