There are three types of tectonic plate boundaries:
- Earth's crust is being torn asunder.
- 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.
- Example: Afar valley of East Africa and the Atlantic mid-ocean ridge.
- Plates move past each other without creation or destruction of crust.
- Connect divergent and convergent boundaries to form a continuous boundary around a plate.
- Example: San Andreas fault.
Plates will collide. Given the two type of Earth crust (ocean and continental), there are three possibilities for convergent boundaries.
Oceanic-continental convergence zones
- Very complex assemblages of rock created.
- Denser oceanic crust is thrust under the lighter continental crust.
- Sediments and seamounts are scraped off.
- These rocks are 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.
- Examples: Andes and Cascade ranges.
Oceanic-oceanic convergence zones
- 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: the Aleutians, Izu-Bonin-Mariana Arc.