Editing Seafloor spreading (section)

== Incipient spreading ==
[[Image:Plates tect2 en.svg|thumb|Plates in the crust of the earth, according to the [[plate tectonics]] theory]]
In the general case, seafloor spreading starts as a [[rift (geology)|rift]] in a [[Continental plate|continental land mass]], similar to the [[Red Sea]]-[[East Africa Rift]] System today.<ref>{{Cite journal|last1=Makris|first1=J.|last2=Ginzburg|first2=A.|date=1987-09-15|title=Sedimentary basins within the Dead Sea and other rift zones The Afar Depression: transition between continental rifting and sea-floor spreading|journal=Tectonophysics|volume=141|issue=1|pages=199–214|doi=10.1016/0040-1951(87)90186-7|bibcode=1987Tectp.141..199M|s2cid=129438029 }}</ref> The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see [[isostasy]]). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120-degree angles. These areas are named [[triple junction]]s and can be found in several places across the world today. The separated margins of the [[continent]]s evolve to form [[passive margin]]s. Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge.

If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift' or [[aulacogen]]. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust and upper mantle [[lithosphere]] begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea. The East African rift was thought to be a failed arm that was opening more slowly than the other two arms, but in 2005 the [[Ethiopia]]n Afar Geophysical Lithospheric Experiment<ref>{{Cite book|last1=Bastow|first1=Ian D.|last2=Keir|first2=Derek|last3=Daly|first3=Eve|date=2011-06-01|title=The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading|journal=Special Papers|language=en|volume=478|pages=51–76|doi=10.1130/2011.2478(04)|issn=0072-1077|series=Geological Society of America Special Papers|isbn=978-0-8137-2478-2|hdl=2158/1110145}}</ref> reported that in the [[Afar (region)|Afar region]], September 2005, a 60&nbsp;km fissure opened as wide as eight meters.<ref>{{Cite journal|last1=Grandin|first1=R.|last2=Socquet|first2=A.|last3=Binet|first3=R.|last4=Klinger|first4=Y.|last5=Jacques|first5=E.|last6=Chabalier|first6=J.-B. de|last7=King|first7=G. C. P.|last8=Lasserre|first8=C.|last9=Tait|first9=S.|date=2009-08-01|title=September 2005 Manda Hararo-Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data|journal=Journal of Geophysical Research|language=en|volume=114|issue=B8|pages=B08404|doi=10.1029/2008jb005843|issn=2156-2202|bibcode=2009JGRB..114.8404G|hdl=10220/8648 |url=https://hal.archives-ouvertes.fr/hal-01622558/file/2008JB005843%20%281%29.pdf|doi-access=free}}</ref> During this period of initial flooding the new sea is sensitive to changes in climate and [[eustasy]]. As a result, the new sea will evaporate (partially or completely) several times before the elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to [[petroleum]] [[geologist]]s.

Seafloor spreading can stop during the process, but if it continues to the point that the continent is completely severed, then a new [[ocean basin]] is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the [[Niger Delta]]. The Niger River has formed in the failed rift arm of the [[triple junction]].<ref>{{Cite journal|last=Burke|first=K|date=1977-05-01|title=Aulacogens and Continental Breakup|journal=Annual Review of Earth and Planetary Sciences|volume=5|issue=1|pages=371–396|doi=10.1146/annurev.ea.05.050177.002103|bibcode=1977AREPS...5..371B|issn=0084-6597}}</ref>