although these rests of seafloor spreading seem slow o a human time scale, they are rapid enough to have generated all of earth's ocean basins in the last 200 million years. spreading centres can also develop within a continent, in which case the land mess may split into two or more smaller continents. this is known as continental rotting. continental rifting takes place where forces pill the plate in opposite directions. as the lithosphere stretches, molten rock rises from below to upward the crust. eventually, the crust fractures to form a long trough called a rift valley. slowly, the rift valley lengthens and deepens. the east african rift valleys are examples of such features. if spreading continues, it will extend to the plate margin and split the landmess in two much like south america and africa did over 100 million years ago. at this point the rift valley becomes a long, narrow sea with an outlet to the ocean, similar to the red sea. consequently, the red sea provides earth scientists with a view of how the atlantic ocean may have looked in its infancy. continued divergence brings us back "full-circle" to the development of a spreading centre such as the present-day mid-atlantic ridge. although new lithosphere is continually being produced at divergent plate boundaries the surface area of our planet is not growing larger. to balance the amount of newly created lithosphere, older portions of oceanic lithosphere descend into the mantle along convergent plate boundaries. convergent plate margins occur where two plates are moving toward each other and the motion is accommodated by one plate sliding beneath the other. convergent plate boundaries are also called subduction zones, because they are sites where lithosphere is descending (being sub ducted) into the asthenosphere. deep-ocean trenches develop between two converging plates and indicate where the sub ducting plate begins its descent beneath the overriding plate. subduction occurs because the density of the subducting lithospheric plate is greater than that of the underlying asthenosphere. older oceanic lithosphere is cooler and denser than the underlying asthenosphere. by contrast continental lithosphere is less dense and therefore more buoyant, depth. as a consequence the sub-ducted plate is almost always oceanic. some slabs of oceanic lithosphere descend into the asthenosphere at angles of only a few degrees, whereas others plunge nearly vertically (90 degrees). the angle at which oceanic lithosphere into the asthenosphere, depends on tis density. when a s;ab of lithosphere is less dense than the underlying asthenosphere and subduction is nearly horizontal. as oceanic lithosphere ages (gets further from the spreading centre), it cools. this causes it to thicken and increase in density. once the age of oceanic lithosphere exceeds 10 million years, it is more dense than the asthenosphere and will sink given the opportunity. in parts of the western pacific some oceanic lithosphere is older than 160 million years. this is the thickest and most dense in today's oceans. the sub ducting sleds in this region typically descend into the mantle at angles approaching 90 degrees. it is important to realise that a sub ducting plate does not follow a fixed path into mantle. rather, it sinks vertically as it descends, causing the trench to retreat, or "roll back". as the sub ducting plate sinks. it creates a flow in the asthenosphere that "pulls" the overriding plate toward the retreating trench. 9image what would happen if you were sitting in a lifeboat near titanic as it sank!). although all convergent plate boundaries have many similar characteristic, they are highly variable features. nevertheless, they can be placed into one of three broad groups based on the type of crustal material involved. convergent plate boundaries form: (1) where continental lithosphere overrides oceanic