The earth consist of floating plates that diverge, collide or shear with each other and in doing so volcanoes, earthquakes and mountains are formed. Major plates which are composed of both oceanic and continental crusts include
Pacific Plate(oceanic crust only)
However, they are other micro plates which either constitute oceanic or continental crusts only. These plates include:
Juan de Fuca(oceanic crust)
Somali Plate (oceanic crust)
Nubian Plate (continental crust)
Divergent/Constructive Plate Boundary
Along divergent boundaries, plates move apart and magma is forced to ooze out to fill up the gap. The magma eventually solidifies and new crust is created.Divergent plate boundaries mostly occur along oceans where new sea floors are created.The Atlantic Ocean is one example where new sea floor is created along the Mid Atlantic Ridge a phenomenon known as Sea Floor Spreading. However, divergence can also occur on continents such as the the East African Rift Valley. This was formed when magma piled up near the surface but failed to burst through forcing it to create a huge dome near the surface which exerted trumendous force on the crust and pulled it apart. Along divergent boundaries, magma is extruded regularly (not violently) creating new oceanic crust. Earthquakes are less pronounced as there is no collision involved. Ridges are common which are a series of volcanic islands or solidified magma stretching along the boundary. Submarine volcanoes which are found below the ocean surface can also be present.
Convergent/Destructive Plate Boundary
Convergent boundaries involves the collision of plates (just like traffic accident) resulting in cataclysmic destruction. Convergent boundaries can occur between oceanic and continental plates two oceanic plates two continental plates
Oceanic and Continental Plate
When oceanic and continental plates collide , the dense oceanic plate is subducted beneath the less dense, buoyant continental plate. The water from the oceanic slab causes the partial melting of underground rocks which triggers magma to the surface. Continental volcanic arcs are formed at the surface e.g. Mt St Helens in West USA(N
American plate and Juan de Fuca (oceanic plate). A deep narrow depression known as an Oceanic Trench is formed by the bend of the oceanic plate, for example the Peru-Chile Trench in the west of S. America. Earthquakes are more pronounced along these boundaries due to the collision mechanism, for example the 2011 Japan earthquake (Eurasian and Pacific plate). Fold mountains can also be formed along these boundaries.
Two oceanic Plates
When two oceanic plates collide one is forced down the other triggering magma to the surface. A chain of volcanic islands are formed on the surface, e.g. Philippine Islands.
Two Continental Plates
When two continental plates collide none is subducted as the plates are composed of the same less dense rocks that are kept buoyant. Instead of sinking, the plates wedge up to form mountains. The Himalayas between India and China is a good example which was formed when a fast moving Indian sub-continent rammed into the Eurasian plate millions of years ago. Up to this day the Himalayas are growing as theIndian sub-continent continues to push into the Eurasian plate.
Shearing/ Transform Boundary
Occurs when plates slide past each other, the San Andreas Fault in California where the North American and Pacific plates are sliding together is a good example. When plates slide past one another in opposite directions an opening (divergent) is observed and when they slide together such that one is advancing into another another a collision occurs.To simplify, transform faults are associated with both divergent and convergent plate boundaries. On a diverging transform fault new seas can be created e.g. Salton Sea in California. In contrast, on a convergent transform boundary fold mountains can also be formed e.g San Gabriel in California.