Editing Asthenosphere (section)

=== Lower boundary of asthenosphere ===
The lower boundary of the asthenosphere, the top of the tentatively defined [[Mesosphere (mantle)|mesosphere]] or mesospheric shell,<ref>{{cite book |last1=Daly |first1=Reginald Aldworth  |year=1940 |title=Strength and Structure of the Earth |publisher=Prentice-Hall }}</ref> is less well-defined, but has been placed at the base of the upper mantle.<ref>{{cite journal |last=Anderson |first=Don L. |year=1995 |title=Lithosphere, asthenosphere, and perisphere |journal=[[Reviews of Geophysics]] |lang=en |volume=33 |issue=1 |pages=125–149 |bibcode=1995RvGeo..33..125A |doi=10.1029/94RG02785 |issn=8755-1209 |url=https://authors.library.caltech.edu/34732/ }}</ref> This boundary is neither seismically sharp nor well understood{{sfn|Kearey|Klepeis|Vine|2009|p=51}} but is approximately coincident with the complex 670&nbsp;km discontinuity.<ref>{{cite book |last1=Fowler |first1=C.M.R. |last2=Fowler |first2=Connie May |year=2005 |title=The Solid Earth: An introduction to global geophysics |publisher=Cambridge University Press |isbn=978-0521893077 }}</ref> This discontinuity is generally linked to the transition from mantle rock containing [[ringwoodite]] to mantle rock containing [[bridgmanite]] and [[periclase]].<ref>{{cite journal |last1=Ito |first1=E |last2=Takahashi |first2=E |year=1989 |title=Postspinel transformations in the system Mg2SiO4-Fe2SiO4 and some geophysical implications |journal=Journal of Geophysical Research: Solid Earth |volume=94 |issue=B8 |pages=10637–10646 |bibcode=1989JGR....9410637I |doi=10.1029/jb094ib08p10637 }}</ref>