Constructive (divergent) plate boundaries
Where two tectonic plates move apart below an ocean, the gap between them fills with molten magma, and the lava that is released constructs new crust. The Mid-Atlantic Ridge is the longest conservative plate margin in the world. It extends the length of the North and South Atlantic oceans where the North and South American plates are moving away from the African and Eurasian plates. The ridge contains many undersea volcanoes and experiences minor earthquake activity. Volcanic activity and earthquakes are relatively gentle compared to other plate margins. Iceland was formed due to the separation of the North American and Eurasian plate.
Where two plates containing continental landmass move apart a rift valley is formed. A rift valley is a lowland region that forms where Earth’s tectonic plates move apart or rift. The most well-known example of this is the Rift Valley in south-east Africa.
Destructive (convergent) plate boundaries
Destructive plate boundaries occur where dense, oceanic crust subduct lighter, continental crust forming a subduction zone. As the oceanic crust sinks below the continental crust, it melts, forming magma. Where magma rises to the surface, it forms explosive composite volcanoes. The Andes mountain range that runs the length of the west coast of South America was formed due to the subduction of the oceanic Nazca Plate below the continental South American Plate.
Volcanoes may form a line of volcanic islands called island arcs where this process occurs within oceans. The islands of the West Indies in the Caribbean were formed in this way.
Mighty earthquakes often occur along destructive plate margins. The fifth-largest recorded earthquake that affected Japan in 2011 occurred along the destructive plate boundary where the Pacific Plate subducts the Eurasian Plate.
Conservative (passive) plate boundaries
Conservative plate boundaries occur where two tectonic plates are moving past each other, either in opposite directions or in the same direction but at different speeds. Plates do not slide past each other smoothly. Friction causes them to lock together. Over time, pressure builds up until the plates tear apart along a fault line. Earthquakes along conservative boundaries can be very powerful, such as the Haiti earthquake in January 2010.
Collision plate boundaries
Collision plate boundaries occur where two continental plates move towards (converge) towards each other. The continental plates are not dense enough to sink into the mantle. Instead, as the plates collide, they cause the bedrock to fold, forming fold mountains such as the Alps and the Himalayas. Powerful earthquakes occur along collision plate boundaries such as the one that affected Nepal in 2015.