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):
php require("/home/jeffery/public_html/astro/earth/geology/plate_tectonics/plate_tectonics_videos.html");?>
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).
EOF
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.