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5. Seafloor Spreading - Background
After several million years of this slow journey away from the mid-ocean ridges, the oceanic crust collides with a different tectonic plate in a process known as subduction. If the oceanic crust meets continental crust, the denser oceanic crust is forced under the continental crust. If the oceanic crust meets oceanic crust, then one or the other will be forced below the other. In these subduction zones, earthquakes are common due to the build up and sudden release of energy at the junction. Also, as the oceanic crust dives down below the other plate into the mantle, the old plate melts, forming a pool of magma that is forced back up through the crust above as volcanoes on the continent or a chain of volcanic islands on one side of the subduction zone.
In some instances, some continental crust is riding along on the oceanic plate and collides into another piece of continental crust. This is currently taking place in the Himalayas and was part of the process that created California (see the background section of Making California for more details). At these continent-continent convergent boundaries, the two sections of continental crust ram into one another, causing the crust above to buckle and fold into tall mountain ranges (the Himalayas and the Sierra Nevadas).
Using these patterns, students can infer both the direction and the relative speed each tectonic plate is moving. The following table shows all the plates and their approximate direction and speed.
This table was created using the Rice University Plate Motion Calculator. For further information on the types of plate boundaries, see the background section of the Plate Patterns lesson.
The ultimate cause of sea floor spreading (and the theory of plate tectonics as a whole) is still debated in the scientific community. Some argue that it is driven by convection currents in the mantle. Others argue that the upwelling of magma and creation of new crust at mid-ocean ridges pushes the older crust out of the way (the “Ridge Push” theory). Others argue that the sinking of the old crust at subduction zones drags the oceanic crust along behind it (the “Slab Pull” theory).