Editing Seafloor spreading (section)

==Significance==
Seafloor spreading helps explain [[continental drift]] in the theory of [[plate tectonics]]. When oceanic plates [[Divergent boundary|diverge]], tensional stress causes fractures to occur in the [[lithosphere]]. The motivating force for seafloor spreading ridges is tectonic plate [[slab pull]] at [[Subduction|subduction zones]], rather than magma pressure, although there is typically significant magma activity at spreading ridges.<ref>{{Cite journal|last1=Tan|first1=Yen Joe|last2=Tolstoy|first2=Maya|author-link2=Maya Tolstoy |last3=Waldhauser|first3=Felix|last4=Wilcock|first4=William S. D.|title=Dynamics of a seafloor-spreading episode at the East Pacific Rise|journal=Nature|volume=540|issue=7632|pages=261–265|doi=10.1038/nature20116|pmid=27842380|year=2016|bibcode=2016Natur.540..261T|s2cid=205251567}}</ref> Plates that are not subducting are driven by gravity sliding off the elevated mid-ocean ridges a process called [[ridge push]].<ref name=":2" /> At a spreading center, [[Basalt|basaltic magma]] rises up the fractures and cools on the ocean floor to form new [[seabed]]. [[Hydrothermal vent]]s are common at spreading centers. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone.

''Spreading rate'' is the rate at which an ocean basin widens due to seafloor spreading. (The rate at which new oceanic lithosphere is added to each tectonic plate on either side of a mid-ocean ridge is the ''spreading half-rate'' and is equal to half of the spreading rate). Spreading rates determine if the ridge is fast, intermediate, or slow. As a general rule, fast ridges have spreading (opening) rates of more than 90&nbsp;mm/year. Intermediate ridges have a spreading rate of 40–90&nbsp;mm/year while slow spreading ridges have a rate less than 40&nbsp;mm/year.<ref>{{Citation|last=Macdonald|first=Ken C.|chapter=Mid-Ocean Ridge Tectonics, Volcanism, and Geomorphology|date=2019|encyclopedia=Encyclopedia of Ocean Sciences|pages=405–419|publisher=Elsevier|language=en|doi=10.1016/b978-0-12-409548-9.11065-6|isbn=9780128130827}}</ref><ref name=":1">{{Cite journal|last=Macdonald|first=K. C.|date=1982|title=Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone|journal=Annual Review of Earth and Planetary Sciences|volume=10|issue=1|pages=155–190|doi=10.1146/annurev.ea.10.050182.001103|bibcode=1982AREPS..10..155M}}</ref><ref>{{Cite book|title=Mid-ocean ridges|last=Searle|first=Roger|publisher=Cambridge|year=2013|isbn=9781107017528|location=New York|oclc=842323181}}</ref>{{rp|2}}  The highest known rate was over 200 mm/yr during the [[Miocene]] on the [[East Pacific Rise]].<ref>{{Cite journal|last=Wilson|first=Douglas S.|date=1996-10-15|title=Fastest known spreading on the Miocene Cocos-Pacific Plate Boundary|journal=Geophysical Research Letters|language=en|volume=23|issue=21|pages=3003–3006|doi=10.1029/96GL02893|bibcode=1996GeoRL..23.3003W}}</ref> 

In the 1960s, the past record of [[geomagnetic reversal]]s of Earth's magnetic field was noticed by observing magnetic stripe "anomalies" on the ocean floor.<ref>{{cite journal|author=Vine, F. J.|last2=Matthews|first2=D. H.|year=1963|title=Magnetic Anomalies Over Oceanic Ridges|journal=Nature|volume=199|issue=4897|pages=947–949|bibcode=1963Natur.199..947V|doi=10.1038/199947a0|s2cid=4296143}}</ref><ref>{{Cite journal|last=Vine|first=F. J.|date=1966-12-16|title=Spreading of the Ocean Floor: New Evidence|journal=Science|language=en|volume=154|issue=3755|pages=1405–1415|doi=10.1126/science.154.3755.1405|pmid=17821553|bibcode=1966Sci...154.1405V|s2cid=44362406|issn=0036-8075}}</ref> This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred from data gathered with a [[magnetometer]] towed on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. By identifying a reversal with a known age and measuring the distance of that reversal from the spreading center, the spreading half-rate could be computed.  
[[File:Polarityshift.gif|alt=|thumb|magnetic stripes formed during seafloor spreading]]
In some locations spreading rates have been found to be asymmetric; the half rates differ on each side of the ridge crest by about five percent.<ref>{{Cite journal|last1=Weissel|first1=Jeffrey K.|last2=Hayes|first2=Dennis E.|date=1971|title=Asymmetric Seafloor Spreading south of Australia|journal=Nature|language=en|volume=231|issue=5304|pages=518–522|doi=10.1038/231518a0|bibcode=1971Natur.231..518W|s2cid=4171566|issn=1476-4687}}</ref><ref name=":3">{{Cite journal|last1=Müller|first1=R. Dietmar|last2=Sdrolias|first2=Maria|last3=Gaina|first3=Carmen|last4=Roest|first4=Walter R.|date=2008|title=Age, spreading rates, and spreading asymmetry of the world's ocean crust: DIGITAL MODELS OF THE WORLD'S OCEAN CRUST|journal=Geochemistry, Geophysics, Geosystems|language=en|volume=9|issue=4|pages=n/a|doi=10.1029/2007GC001743|bibcode=2008GGG.....9.4006M |s2cid=15960331|url=https://archimer.ifremer.fr/doc/00000/3900/|doi-access=free}}</ref> This is thought due to temperature gradients in the asthenosphere from [[mantle plume]]s near the spreading center.<ref name=":3" />