<P> In the theory of plume tectonics developed during the 1990s, a modified concept of mantle convection currents is used . It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells . These ideas, which find their roots in the early 1930s with the so - called "fixistic" ideas of the European and Russian Earth Science Schools, find resonance in the modern theories which envisage hot spots or mantle plumes which remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators). Modern theories that continue building on the older mantle doming concepts and see plate movements as a secondary phenomena are beyond the scope of this page and are discussed elsewhere (for example on the Plume tectonics page). </P> <P> Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below the Earth's crust, which then provide basal friction to the lithosphere . This theory, called "surge tectonics", became quite popular in geophysics and geodynamics during the 1980s and 1990s . Recent research, based on three - dimensional computer modeling, suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere . </P> <P> Forces related to gravity are usually invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above . </P> <P> Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges . As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load . The result is a slight lateral incline with increased distance from the ridge axis . </P>

Which is not a force causing plates to move