There are three types of plate boundary (or margin): constructive, destructive and passive.
These arise where two plates move away from each other, and new crust is created at the boundary. They are mainly found between oceanic plates, and are consequently underwater features. Rift valleys may initially develop, but molten rock from the mantle (magma) rises to fill any possible gaps. Constructive margins are often marked by ocean ridges (e.g. the Mid Atlantic Ridge, the East Pacific Rise). The rising magma forms submarine volcanoes, which in time may grow above sea level (e.g. Iceland, Tristan da Cunha and Ascension Island on the Mid Atlantic Ridge, and Easter Island on the East Pacific Rise).
Different rates of latitudinal movement along the boundary cause transform faults to develop as the magma cools – these lie perpendicular (at a right angle) to the plate boundary. Of the annual volume of lava2 ejected onto the Earth’s surface, 73 per cent is found on mid-ocean ridges, and approximately one-third of the lava ejected onto the Earth’s surface during the past 500 years is found in Iceland. The Atlantic Ocean formed as the continent of Laurasia split in two, and the Atlantic is continuing to widen by approximately 2–5 cm per year. Very rarely, constructive margins can occur on land, and it is thought that this is happening in East Africa at the Great African Rift Valley System. Extending for 4,000 km from the Red Sea to Mozambique, its width varies from 10 to 50 km, and at points its sides reach over 600 m in height. Where the land has dropped sufficiently, the sea has invaded. – it has been suggested that the Red Sea is the beginnings of a newly forming ocean. Associated volcanoes include Mount Kilimanjaro and Mount Kenya to the east and Ruwenzori to the west.
These occur where two plates move towards each other, and one is forced below the other into the mantle. The Pacific Ocean is virtually surrounded by destructive plate margins with their associated features, and its perimeter has become known as the Pacific Ring of Fire. The features present at destructive margins will depend upon what types of plates are converging.
When oceanic crust meets continental crust:
- The thinner, denser oceanic crust is forced to dip downwards at an angle and sink into the subduction zone beneath the thicker, lighter and more buoyant continental crust.
- A deep-sea trench forms at the plate margin as subduction takes place. These form the deepest areas on the planet.
- As the oceanic crust descends, the edge of the continental crust may crumple to form fold mountains, which run in chains parallel to the boundary (e.g. the Andes).
- Sediments collecting in the deep-sea trench may also be pushed up to form fold mountains.
- As the oceanic crust descends into the hot mantle, additional heat generated by friction helps the plate to melt, usually at a depth of 400–600 km below the surface.
- As it is less dense than the mantle, the newly forme magma will tend to rise to the Earth’s surface, where it may form volcanoes.
- However, as the rising magma at destructive margins is very acidic, it may solidify before it reaches the surface and form a batholith at the base of the mountain chain .
- As the oceanic plate descends, shallow earthquakes occur where the crust is stretched as it dips beneath the surface. Deeper earthquakes arise from increases in friction and pressure may be released as earthquakes.
- As the oceanic plate descends, increased stresses may trigger earthquakes: shallow earthquakes occur where the crust is stretched as it dips beneath the surface, and deeper earthquakes occur due to increases in friction and pressure as the plate subducts.
- The area in the subduction zone where most earthquakes take place is known as the Benioff zone.
- The depth of the deeper earthquakes may also provide an indication as to the angle of subduction, where gentler angles of subduction give rise to shallower earthquakes.
- If subduction occurs offshore, island arcs may form (e.g. Japan, the West Indies).
When oceanic crust meets oceanic crust:
- Where two oceanic plates collide, either one may be subducted.
- Similar features arise as those where an oceanic plate meets a continental plate.
When continental crust meets continental crust (note that this is very rare):
- Because continental crust cannot sink, the edges of the two plates and the intervening sediments are crumpled to form very deep-rooted fold mountains.
- The zone marking the boundary of the two colliding plates is known as the suture line.
- These boundaries mark the site at which the Earth’s crust is at its thickest. For example, the Indo-Australian Plate is moving northeastwards and is crashing into the rigid Eurasian Plate, creating the Himalayas.
- Uplift is a continuous process (it is happening right now); however, weathering and erosion of the mountain tops means that the actual height of the mountains is not as great as the rate of uplift would suggest.
- Sediments which form part of the Himalayas were once underlying the Tethys Sea, which existed at the time of the Pangean supercontinent.
These occur where two plates slide past each other and crust is neither created nor destroyed. The boundary between the two plates is characterized by pronounced transform faults, which lie parallel to the plate boundary. As the plates slide past each other, friction builds up and causes the plates to stick, and release is in the form of earthquakes. An excellent example of a passive margin is the San Andreas Fault (one of several hundred known faults) in California, which marks a junction between the North American and the Pacific Plates.
Although both plates are moving in a northwesterly direction, the Pacific Plate moves at a faster rate than the North American late (6 cm per year, compared with just 1 cm per year), creating the illusion that the plates are moving in opposite directions.