Metamorphism of Sea Floor
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. 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 ultrahigh-pressure metamorphism: small pieces of continental material subducted to as much as 200 km are eventually regurgitated, leading to ultrahigh P/T metamorphism Most of the oceans have a common structure, created by common physical phenomena, mainly from