Constructive plate margins are areas where new crust is formed. This type of margin develops in situations where rising and diverging mantle convection currents create forces which pull or push the plates apart. Magma is injected into the lithospheric plates along the spreading line and crystallises to form new plate material. These types of plate margin are characterised by the birth and growth of ocean basins which are formed by the divergence of the plates. Constructive margins occur both on land and under the sea.
This type of margin is developed where rising mantle convection currents occur below a continental plate. The rising hot mantle material causes the overlying continental plate to dome up and stretch. Stretching of the relatively rigid plate causes brittle fractures to develop, and the plate eventually cracks. As the mantle convection currents diverge, the plate is gradually pulled apart and the central blocks between the faults sink downwards, forming arift valley. Movement between the different fragments of crust in the rift valley causes frequent shallow focus earthquakes.
In the centre of the rift valley, magma from the underlying asthenosphere wells up to fill in the crack between the two halves of the broken plate. Rift valleys usually produce large amounts of volcanic lava, which form extensive lava plateaus and volcanic cones, such as the active Erta Ale volcano (pictured right) located in the Ethiopian part of the East African Rift Valley. The magma is produced by melting of mantle material (peridotite) in the asthenosphere directly below the rift system.
Partial melting of the asthenosphere to form large amounts of magma is largely due to the reduction of confining pressure on the upper mantle (known asmantle decompression) – this is rather like releasing the pressure in a bottle of fizzy drink by unscrewing the cap. Mantle decompression is caused by the doming and thinning of the overlying lithosphere as it is stretched. Rising convection currents of heated mantle material raises the temperature of this region, and also brings up volatiles such as carbon dioxide and sulphur dioxide into the mantle rocks below the developing rift system, which helps lower the melting point of the mantle rocks. The combination of pressure reduction, increased heat flow and the introduction of volatiles result in the partial melting of the asthenosphere to produce massive quantities ofbasaltic magmawhich segregate and collect in magma chambers below the developing spreading line.
Continental rifting eventually leads to formation of new ocean crust in the split between the two halves of continental plate. This process doesnt happen overnight – it takes millions of years! The simplest way to understand it is to break the process down into a series of stages…
Continental rifting all starts with hot, rising convection currents in the mantle. As the currents reach the upper part of the mantle, they spread outwards and away from each other (diverge). The rising mantle material is hot andbuoyant, and it causes the overlying plate to bulge upwards and stretch. At the same time, the diverging currents drag on the bottom of the plate, also causing the plate to stretch. Eventually, cracks start to appear in the plate.
The diverging mantle convection currents continue to diverge (move apart), causing the plate to stretch until it splits. Along the line of the split, huge blocks of crust start to sink downwards (subside) due to gravity. Think about what happens when you pull a blob of warm blue tak or bubble gum apart! The release of pressure on the mantle directly below the broken plate enables magma to form and well up into the cracks (faults) between the blocks. Huge amounts ofbasalt(a type of volcanic rock) can pour out onto the floor of the valley during this stage.
The plates continue to be pulled apart, and arift valleyforms in the central part, which is still subsiding. Magma from the underlying mantle feeds numerous volcanoes, and shallow earthquakes are caused by the edges of the blocks rubbing together.
At this stage, the plates are still being pulled apart by the mantle currents, but injection of magma along the central crack (rift) also pushes the plates apart. The central rift valley has now sunk down to below sea level and has been flooded with seawater to create a new ocean.
Magma injected along the central rift spills over and forms a chain of underwater mountains and volcanoes. This is now a fully functional mid-ocean spreading ridge, with new ocean crust being injected into the central rift. The ocean will continue to grow wider until the continental crust at the edges because consumed by subduction, or continental collision occurs.
Find out more about plate tectonics:
Plate tectonics make the world go round: introduction
Ocean-continent destructive margins
Ocean-ocean destructive margins (island arcs)