Low-velocity zone

Velocity of seismic waves in the Earth versus depth.[1] S-waves (seismic shear waves) cannot propagate in liquids, leading to negligible velocity in the liquid outer core. The seismic velocities very near the surface (≲ 220±30 km) are markedly lower than at greater depths, demarking the LVZ.

The low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity. It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves.[2] A second low-velocity zone (not generally referred to as the LVZ, but as ULVZ) has been detected in a thin ≈50 km layer at the core-mantle boundary.[3] These LVZs may have important implications for plate tectonics and the origin of the Earth's crust.[2][3][4]

The LVZ has been interpreted to indicate the presence of a significant degree of partial melting, and alternatively as a natural consequence of a thermal boundary layer and the effects of pressure and temperature on the elastic wave velocity of mantle components in the solid state.[2] In any event, a very limited amount of melt (about 1%) is needed to produce these effects. Water in this layer can lower the melting point, and may play an important part in its composition.[4][5]

  1. ^ GR Helffrich & BJ Wood (2002). "The Earth's Mantle" (PDF). Nature. 412 (2 August). Macmillan Magazines: 501–507. doi:10.1038/35087500. PMID 11484043. S2CID 4304379.
  2. ^ a b c L Stixrude & C Lithgow-Bertolloni (2005). "Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone". Journal of Geophysical Research. 110: B03204. Bibcode:2005JGRB..11003204S. doi:10.1029/2004JB002965. hdl:2027.42/94924.
  3. ^ a b EJ Garnero, MS Thorne, A McNamara & S Rost (2007). "Chapter 6: Fine-scale ultra-low-velocity zone layering at the core-mantle boundary and superplumes". In David A Yuen; Shigenori Maruyama (eds.). Superplumes: beyond plate tectonics. Springer. p. 139. ISBN 978-1-4020-5749-6.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. ^ a b Philip Kearey; Keith A. Klepeis; Frederick J. Vine (2009). Global tectonics (3rd ed.). Wiley-Blackwell. p. 32. ISBN 978-1-4051-0777-8.
  5. ^ It is hypothesized that the absence of plate tectonics on the planet Venus is due to the absence of water in its crust and upper mantle. Cooling occurs largely through mantle plumes. See Gillian R. Foulger (2005). Plates, plumes, and paradigms; Volume 388 of Special papers. Geological Society of America. p. 857. ISBN 0-8137-2388-4.