View source for Divergent boundary

{{short description|Linear feature that exists between two tectonic plates that are moving away from each other}}
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[[File:Continental-continental constructive plate boundary.svg|thumb|upright=1.4|Continental-continental divergent/constructive boundary]]
[[File:Mid-ocean ridge cut away view.png|thumb|upright=1.35|Oceanic divergent boundary: mid-ocean ridge (cross-section/cut-away view)]]
In [[plate tectonics]], a '''divergent boundary''' or '''divergent plate boundary''' (also known as a '''constructive boundary''' or an '''extensional boundary''') is a linear feature that exists between two [[List of tectonic plates|tectonic plates]] that are moving away from each other. Divergent boundaries within [[continent]]s initially produce [[rift (geology)|rifts]], which eventually become [[rift valley]]s.  Most active divergent plate boundaries occur between [[Oceanic crust|oceanic plate]]s and exist as [[mid-oceanic ridge]]s.<ref>{{Cite book | doi=10.1029/GM071p0183| chapter=Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges| title=Mantle Flow and Melt Generation at Mid-Ocean Ridges| pages=183–280| series=Geophysical Monograph Series| year=2013| last1=Langmuir| first1=Charles H.| last2=Klein| first2=Emily M.| last3=Plank| first3=Terry| isbn=9781118663875| hdl=10161/8316}}</ref><ref>{{Cite journal |doi = 10.1029/91JB02508|title = Mid-ocean ridge magma chambers|journal = Journal of Geophysical Research|volume = 97|issue = B1|pages = 197|year = 1992|last1 = Sinton|first1 = John M.|last2 = Detrick|first2 = Robert S.|bibcode = 1992JGR....97..197S}}</ref>

Current research indicates that complex [[convection]] within the [[Earth's mantle]] allows material to rise to the base of the [[lithosphere]] beneath each divergent plate boundary.<ref>{{cite journal | author1 = Toshiro Tanimoto |author2= Thorne Lay |authorlink2=Thorne Lay| date = November 7, 2000 | title = Mantle dynamics and seismic tomography | journal =Proc. Natl. Acad. Sci. U.S.A. | volume =97 | issue =23 | pages = 12409–10 | doi = 10.1073/pnas.210382197 | pmid = 11035784 | pmc = 34063 | bibcode=2000PNAS...9712409T|doi-access= free }}</ref>{{Failed verification|date=September 2024}}
This supplies the area with huge amounts of heat and a reduction in pressure that melts [[Rock (geology)|rock]] from the [[asthenosphere]] (or [[upper mantle (Earth)|upper mantle]]) beneath the rift area, forming large [[flood 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.

== Description ==
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[[Image: Bridge across continents iceland.jpg|thumb|Bridge across the [[Álfagjá]] [[rift valley]] in southwest [[Iceland]], that is part of the boundary between the Eurasian and North American continental tectonic plates.]]

At divergent boundaries, two plates move away from each other and the space that this creates is filled with new crustal material sourced from molten [[magma]] that forms below. The origin of new divergent boundaries at [[triple junction]]s is sometimes thought to be associated with the phenomenon known as [[hotspot (geology)|hotspots]]. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface, and the [[kinetic 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 the [[Mid-Atlantic Ridge]] and the [[East Pacific Rise]], and in the continental lithosphere by rift valleys such as the famous East African [[East African Rift|Great 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, massive [[transform fault]]s occur. These are the [[fracture zone]]s, many bearing names, that are a major source of [[submarine earthquake]]s. A seafloor map will show a rather strange pattern of blocky structures that are separated by [[:File: World Distribution of Mid-Oceanic Ridges.gif|linear 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 and [[subsidence]]).

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. Airborne [[Earth's magnetic field|geomagnetic]] surveys showed a strange pattern of symmetrical [[magnetic reversal]]s 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 studying [[Geomagnetic reversal|polar reversal]]s and the link was made by [[Lawrence Morley|Lawrence W. Morley]], [[Fred Vine|Frederick John Vine]] and [[Drummond Matthews|Drummond Hoyle Matthews]] in the [[Morley–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.

==Examples==
[[File:Tectonic plates (2022).svg|thumb|upright=1.8|Map of Earth's principal plates (divergent boundaries shown as red or pink lines)]]
*[[Mid-Atlantic Ridge]] 
*[[Red Sea Rift]]
*[[Baikal Rift Zone]] – incipient plate boundary
*[[East African Rift]] – incipient plate boundary
*[[East Pacific Rise]]
*[[Gakkel Ridge]]
*[[Cocos–Nazca spreading centre]]
*[[Explorer Ridge]]
*[[Juan de Fuca Ridge]]
*[[Pacific-Antarctic Ridge]]
*[[Southeast Indian Ridge]]

==Other plate boundary types==
*[[Convergent boundary]]
*[[Transform boundary]]

==See also==
*{{annotated link|Seafloor spreading}}
*{{annotated link|Continental drift}}
*{{annotated link|Subduction|Subduction zone}}

==References==
{{Reflist}}

{{physical oceanography|expanded=other}}

[[Category:Plate tectonics]]