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Inplate tectonics, adivergent boundary ordivergent plate boundary (also known as aconstructive boundary or anextensional boundary) is a linear feature that exists between twotectonic plates that are moving away from each other. Divergent boundaries withincontinents initially producerifts, which eventually becomerift valleys. Most active divergent plate boundaries occur betweenoceanic plates and exist asmid-oceanic ridges.[1][2]
Current research indicates that complexconvection within theEarth's mantle allows material to rise to the base of thelithosphere beneath each divergent plate boundary.[3][failed verification]This supplies the area with huge amounts of heat and a reduction in pressure that meltsrock from theasthenosphere (orupper mantle) beneath the rift area, forming largeflood basalt or lava flows. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other.
Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary. Because of this, rocks closest to a boundary are younger than rocks further away on the same plate.
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At divergent boundaries, two plates move away from each other and the space that this creates is filled with new crustal material sourced from moltenmagma that forms below. The origin of new divergent boundaries attriple junctions is sometimes thought to be associated with the phenomenon known ashotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface, and thekinetic energy is thought to be sufficient to break apart the lithosphere.
Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including theMid-Atlantic Ridge and theEast Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East AfricanGreat Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massivetransform faults occur. These are thefracture zones, many bearing names, that are a major source ofsubmarine earthquakes. A seafloor map will show a rather strange pattern of blocky structures that are separated bylinear features perpendicular to the ridge axis. If one views the seafloor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper... (from thermal contraction andsubsidence).
It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. Airbornegeomagnetic surveys showed a strange pattern of symmetricalmagnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studyingpolar reversals and the link was made byLawrence W. Morley,Frederick John Vine andDrummond Hoyle Matthews in theMorley–Vine–Matthews hypothesis. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.
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