At divergent plate margins, plates are moving apart. In the present day mid-Atlantic, the edges of two oceanic plates are involved.

When South America separated from Africa 180 million years ago, the divergent plate margin was in the centre of the super continent Gondwanaland. This supercontinent split along a major rift system, probably similar to the one now seen in East Africa. Widespread extrusions of plateau basalts accompanied the split. The rift valley widened and subsided below sea level, allowing the ocean to flood in. As the separation continued, the continental crust stretched and became thinner, leaving wide continental slopes on the edge of each section of plate. The thinned continental crust finally split completely and the gap was filled with basalt, forming the beginnings of the oceanic crust. The one plate had become two. Sea floor spreading started as new crust was generated at the MOR, giving the alternate name of constructive plate margn.
At all mature divergent plate boundaries, oceanic plates move apart and create new crust.

Examples of Divergent Plate Boundaries
Divergent plate boundaries are locations where plates are moving away from one another.
Rising convection currents in the mantle push up and move along the bottom og the lithosphere, flowing sideways beneath it. This lateral flow causes the lithosphere plate above it to be dragged along in the direction of the flow. In the area where the convection current is moving up, the overlying plate is stretched thin, breaks and pulls apart, forming a MOR. Extensional forces stretch the lithosphere and produce a deep axial rift.

When the plates start to move apart, pressure is reduced, allowing the peridotite mantle material below to partially melt. The magma accumulates below the ridge and some finds its way up through feeder dolerite dykes until it reaches the surface or sea floor and becomes a fissure lava flow.

The Mid-Atlantic Ridge is an example. For most of its 7000km length, it is a submarine feature. However, Iceland is above sea level, probably because there is a hot spot under Iceland as well as the ridge.

The East African Rift Valley may become a new ocean in time. The northern part of this rift system, in the Red Sea between North Africa and Arabia, is now a divergent plate margin where new basaltic ocean crust is being formed. This split started in the Eocene about 40 Ma and accelerated during the Oliogocene about 23 Ma. The sea is still widening and it is likely that it will become an ocean in about 50 Ma.

Evidence of Divergent Plate Boundaries

dating of the basalts, sediments and magnetic anomalies on the ocean floor clearly shows that the age of the crust increases with distance from the MOR. The two plates on either side, which carry the oceanic crust, must therefore be moving apart. The centre of spreading is the axial rift where tension forms a rift valley
The MORs are among the largest mountain ranges in the world. They are made of mafic igneous material derived from the ultramafic magma of the upper mantle.
Heat flow is higher than average across the ridge, peaking at the median rift, confirming that hot magma is rising beneath it. This upwelling of magma and expansion of the hot rock may help to increase the height of the idge, where there are eruptions of basalt.
As the magma moves, it makes harmonic tremors, which are detected as shallow-focus earthquakes
Fissure volcanic eruptions of mafic lava often form basalt pillows
Shallow focus earthquakes also occur due to displacement along transform faults

	Erstellt von siobhan.quirk vor etwa 11 Jahre

Undergraduate Geology - Part 1 (Continental Drift) Notiz am Divergent Plate Margins, erstellt von siobhan.quirk am 14/05/2013.