(from http://www.ngdc.noaa.gov/)
The Earth is divided into tectonic plates. The motion of these plates is dictated largely by metamorphic petrology--principally the pressure-induced transformation of relatively low density minerals into high density minerals.
(from http://www.ngdc.noaa.gov/)
The various plate-tectonic regimes of the Earth cause rocks to experience a broad range of pressures and temperatures, which leads to a broad range of metamorphic minerals and metamorphic rock types.
stable cratons (green): cratons are stable and relatively cold, with
'normal' thermal gradients of ~20 K/km.
magmatic arcs (red-orange): magmatic arcs are sites where heat is advected
to shallow levels, producing low P/T metamorphism.
crustal extension (orange): crustal extension via normal faulting leads
to advection of heat to shallow levels, followed by cooling to a normal thermal
gradient.
oceanic extension--mid-ocean ridges (red-orange): convection carries
heat to very shallow levels, where 7-km thick oceanic crust forms; hydrothermal
circulation produces low P/T metamorphism.
ophiolite soles (red): are thrust zones beneath very hot oceanic lithosphere
emplaced onto passive continental margins; in contrast to other low P/T metamorphism,
inverted metamorphic gradients form because the emplacement rate is rapid compared
to the rate at which the extreme heat is conducted away.
subduction zones (blue): rapid subduction advects cold material into
the mantle, producing high P/T metamorphism.
continent-continent collisions: rapid crustal thickening produces high temperatures at moderate temperatures, followed by cooling
(from http://www.ngdc.noaa.gov/)
ultrahigh-pressure metamorphism: small pieces of continental material subducted to as much as 200 km are eventually regurgitated, leading to ultrahigh P/T metamorphism
