Editing Lithosphere (section)

==== Oceanic lithosphere ====
{{Further|Oceanic crust}}
Oceanic lithosphere consists mainly of [[mafic]] crust and [[ultramafic rock|ultramafic]] mantle ([[peridotite]]) and is denser than continental lithosphere. Young oceanic lithosphere, found at [[mid-ocean ridge]]s, is no thicker than the crust, but oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. The oldest oceanic lithosphere is typically about {{convert|140|km}} thick.<ref name="lithothick" /> This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. In fact, oceanic lithosphere is a thermal boundary layer for the [[mantle convection|convection]]<ref>Donald L. Turcotte, Gerald Schubert, Geodynamics. Cambridge University Press, 25 mar 2002 – 456</ref> in the mantle.  The thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time.{{citation needed|date=May 2023}}
<math display="block"> h \, \sim \, 2\, \sqrt{ \kappa t } </math>

Here, <math>h</math> is the thickness of the oceanic mantle lithosphere, <math>\kappa</math> is the [[thermal diffusivity]] (approximately {{cvt|{{#expr: 10 ^ -6 }}|m2/s|disp=or}}) for silicate rocks, and <math>t</math> is the age of the given part of the lithosphere. The age is often equal to L/V, where L is the distance from the spreading centre of mid-ocean ridge, and V is velocity of the lithospheric plate.<ref>{{cite book|last1=Stein|first1=Seth|last2=Stein|first2=Carol A.|chapter=Thermo-Mechanical Evolution of Oceanic Lithosphere: Implications for the Subduction Process and Deep Earthquakes|title=Subduction: Top to Bottom|series=Geophysical Monograph Series|date=1996|volume=96|pages=1–17|doi=10.1029/GM096p0001|bibcode=1996GMS....96....1S|isbn=9781118664575}}</ref>

Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. While chemically differentiated oceanic crust is lighter than asthenosphere, [[Thermal expansion|thermal contraction]] of the mantle lithosphere makes it more dense than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at [[subduction]] zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old.<ref name="Composition and development of the">{{cite journal|doi=10.1038/274544a0|title=Composition and development of the continental tectosphere|date=1978|last1=Jordan|first1=Thomas H.|authorlink1=Thomas H. Jordan|journal=[[Nature (journal)|Nature]]|volume=274|issue=5671|pages=544–548|bibcode=1978Natur.274..544J|s2cid=4286280}}</ref><ref name="Doilithos">{{cite journal|doi=10.1016/j.lithos.2009.04.028|title=Ultradeep continental roots and their oceanic remnants: A solution to the geochemical "mantle reservoir" problem?|date=2009|last1=O'Reilly|first1=Suzanne Y.|last2=Zhang|first2=Ming|last3=Griffin|first3=William L.|last4=Begg|first4=Graham|last5=Hronsky|first5=Jon|journal=[[Lithos (journal)|Lithos]]|volume=112|pages=1043–1054|bibcode=2009Litho.112.1043O}}</ref>

===== Subducted lithosphere =====
{{Further|Subduction}}
Geophysical studies in the early 21st century posit that large pieces of the lithosphere have been subducted into the mantle as deep as {{convert|2900|km}} to near the core-mantle boundary,<ref>{{cite journal|doi=10.1016/j.epsl.2004.09.015|title=Derivation of Large Igneous Provinces of the past 200 million years from long-term heterogeneities in the deep mantle|date=2004|last1=Burke|first1=Kevin|last2=Torsvik|first2=Trond H.|journal=[[Earth and Planetary Science Letters]]|volume=227|issue=3–4|pages=531|bibcode=2004E&PSL.227..531B}}</ref> while others "float" in the upper mantle.<ref>{{cite journal|doi=10.1016/S0012-821X(04)00070-6|title=4-D evolution of SE Asia's mantle from geological reconstructions and seismic tomography|date=2004|last1=Replumaz|first1=Anne|last2=Kárason|first2=Hrafnkell|last3=Van Der Hilst|first3=Rob D.|last4=Besse|first4=Jean|last5=Tapponnier|first5=Paul|journal=[[Earth and Planetary Science Letters]]|volume=221|issue=1–4|pages=103–115|bibcode=2004E&PSL.221..103R|s2cid=128974520}}</ref><ref>{{cite journal|doi=10.1029/2007GC001806|title=A new global model for P wave speed variations in Earth's mantle|date=2008|last1=Li|first1=Chang|last2=Van Der Hilst|first2=Robert D.|last3=Engdahl|first3=E. Robert|last4=Burdick|first4=Scott|journal=[[Geochemistry, Geophysics, Geosystems]]|volume=9|issue=5|pages=n/a|bibcode=2008GGG.....9.5018L|doi-access=free}}</ref> Yet others stick down into the mantle as far as {{convert|400|km}} but remain "attached" to the continental plate above,<ref name="Doilithos" /> similar to the extent of the old concept of "tectosphere" revisited by Jordan in 1988.<ref>{{cite journal|doi=10.1093/petrology/Special_Volume.1.11|title=Structure and formation of the continental tectosphere|journal=[[Journal of Petrology]]|volume=29|issue=1|pages=11–37|year=1988|last1=Jordan|first1=T. H.|bibcode=1988JPet...29S..11J}}</ref> Subducting lithosphere remains rigid (as demonstrated by deep [[earthquake]]s along [[Wadati–Benioff zone]]) to a depth of about {{convert|600|km}}.<ref>{{cite journal|last1=Frolich|first1=C.|year=1989|title=The Nature of Deep Focus Earthquakes|journal=Annual Review of Earth and Planetary Sciences|volume=17|pages=227–254|doi=10.1146/annurev.ea.17.050189.001303|bibcode=1989AREPS..17..227F}}</ref><!--Past {{convert|600|km}}, they really aren't lithosphere any more, but I haven't a source that actually spells that out. Leave for reader to infer. -->