Plate Boundary Diagrams
This diagram from the USGS shows a complete system of plate boundaries:
There are three types of tectonic plate boundaries:
Here the Earth's crust is being torn asunder by convection currents in the mantle. Magma from the mantle is injected in thin sheets as the plates separate. On land a rift valley will form, but eventually the rift will connect to the ocean and be flooded. A new ocean basin is created with the mid-ocean ridges marking the active plate boundary. Rifts areas active today include the Afar valley of East Africa and the Atlantic mid-ocean ridge.
Plates can also move past each other without creation or destruction of crust. They connect divergent and convergent boundaries to form a continuous boundary around a plate. The San Andreas fault is a transform fault.
Plates will collide. Given the two type of Earth crust (ocean and continental), there are three possibilities for convergent boundaries.
Very complex assemblages of rock can be created at oceanic-continental convergence zones. As the denser oceanic crust is trust under the lighter continental crust, sediments and seamounts are scraped off. This collection of rocks is recognizably different from the rocks created by the descending and melting oceanic crust. Magma melts some of the continental crust as it rises to form a volcanic arc on the continent. The Andes and cascade ranges are examples of these volcanic mountain ranges.
In an oceanic-oceanic plate boundary, the sediments of the subducting ocean crust are scraped off and accumulate on the end of the overriding plate. Below 50 km, the descending plate melts partially and release volatiles such as water. The magma rises and as pressure on it decreases, becomes more liquid. Subducting oceanic crust again forms a volcanic arc of islands. Examples include, the Aleutians, the Philippines and the Marianas.
- Continental-Continental convergence zones usually arise from an oceanic-continental convergence that has consumed all the oceanic crust. The lighter continental crust piles, folds and buckles. the Himalayas formed in this manner beginning about 50 million years ago.