plate_tectonics/usgs_004_convection.gif

    Caption: Convection in the asthenosphere driving plate tectonics.

    This cartoon shows "whole" mantle convection which is favored circa 2017 (see Wikipedia: Mantle convection: Types of convection).

    The alternative theory is "layered" mantle convection in which there are multiple vertical convection cells.

    A "whole" mantle convection convection convection cycle is thought to take ∼ 200 Myr (see Wikipedia: Mantle convection: Speed of convection)

    For further explication, see Plate tectonics videos below (local link / general link: plate_tectonics_videos.html):

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    Credit/Permission: U.S. Geological Survey (USGS), before or circa 2003 / Public domain since prepared by a US government employee (see Copyright and Other Rights Pertaining to U.S. Government Works).
    Image link: USGS: Some unanswered questions: Image link direct: Itself.
    Local file: local link: usgs_004_convection.html.
    File: Earth: Geology: Plate tectonics file: usgs_004_convection.html.