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Railsback's Some Fundamentals of
Mineralogy and Geochemistry
Plate tectonics:
convergent, and
transform plate
Plate tectonic theory envisions Earth's
surface as consisting of plates of rigid
lithosphere (the crust and uppermost mantle)
moving over, and locally sinking into, a ductile
asthenosphere (the rest of the mantle). Those
plates move relative to each other (in fact, the
defining characteristic of a plate is that all of it
moves together). Motion of plates relative to
each other is thus critical to the theory, and
there are three possible motions: moving apart
(divergence), moving together (convergence),
and moving past each other (at transform
The panel at the top here shows the three
kinds of boundaries and relates them to generation
and destruction of lithosphere. It also shows the
most likely or most dominant reason that plates
move: the cooling, and therefore sinking, of oceanic
lithosphere. The lower panel, less critical to understanding the whole theory, shows two possible kinds
of transform boundaries, one offsetting a divergent
boundary (a mid-ocean ridge) and one offsetting a
convergent plate boundary (a subduction-zone
LBR PlateTectonicBoundaries01.odg 2002 assembled 1/2012