Plate Boundaries

There are three types of tectonic plate boundaries:

Divergent

• 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.

Transform

• 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.

Convergent

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, Philippines and Marianas.

Continental-continental convergence zones

• These develop from an oceanic- continental convergence that has consumed all the oceanic crust.
• The lighter continental crust piles, folds and buckles.
• Example: Himalayas formed in this manner beginning about 50 million years ago.