Hotspots and the Case for a High Viscosity Lower Mantle

  • Mark A. Richards
Part of the NATO ASI Series book series (ASIC, volume 334)


There are two obvious forms of convection in the Earth’s mantle: Plate-scale flow with upwellings at mid-ocean ridges and downwellings at subduction zones, and narrow upwelling plumes from the deep mantle which give rise to volcanic hotspots. Hotspots have relative motions which are at least an order of magnitude less than relative plate motions, and these two styles of convection appear to be decoupled on timescales of ~100–200 m.y. Also, hotspot basalts have trace element and isotopic signatures of mantle source regions which have been isolated from the upper mantle mid-ocean ridge source for a significant fraction of Earth history. These observations are compatible with whole mantle convection models which include a viscosity increase with depth. Lower mantle strain (mixing) rates and horizontal motions at, e.g., the core-mantle boundary are sufficiently reduced if lower mantle viscosity is about 1–2 orders of magnitude greater than that of the upper mantle. Such models also explain the depth distribution and focal mechanisms of deep earthquakes in subducted slabs as well as the long-wavelength geoid highs over subduction zones. Relatively isoviscous, chemically stratified convection models can probably satisfy the constraints from hotspot fixity and geochemistry. However, none of these observations has been shown to be compatible with models of whole-mantle convection which do not include a substantial increase in viscosity with depth.


Subduction Zone Mantle Plume Plate Motion Lower Mantle Mantle Convection 
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Copyright information

© Springer Science+Business Media Dordrecht 1991

Authors and Affiliations

  • Mark A. Richards
    • 1
  1. 1.Department of Geology and GeophysicsUniversity of CaliforniaBerkeleyUSA

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