Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth


diagram of Juan de Fuca plate sinking below the North America plate at the Cascadia subduction zone, courtesy USGSSubduction occurs when oceanic plates collide with continental plates, and the subducting tectonic plate slides beneath the more buoyant plate, moving down into the mantle at submarine trenches.

Subduction typically proceeds at rates of mere centimeters per year, but it is associated with intense, deep-seated seismic activity and volcanic activity (volcanic island arcs). ◙ subduction zone magmas

diagram of ocean cycle with new oceanic rocks formed where mantle convection cells rise beneath mid-oceanic ridges and consumed where oceanic plates subduct at ocean trenchesWherever continental and oceanic plates collide, the denser oceanic plate subducts underneath the less dense continental plate, sinking toward the asthenosphere where the oceanic crust is consumed.

Any crustal rocks that are too buoyant to sink pile up as nappes or in orogenic zones.

Subduction zones are typically arcuate, with the convex arc directed toward the oceanic plate. More steeply inclined subduction plates create less pronounced arcs than do shallow subduction plates. Back-arc basins develop in the concave area between the subduction zone and the continent.

Model of a subduction zone with oceanic slab sinking beneath continental mantle wedge. Chains of volcanoes are associated with magmatism above the subducting slab. At subduction zones, the oceanic lithosphere, layers of marine sediments, and trapped water are recycled into the deep mantle. When the oceanic plate sinks to about 100 km in depth, peridotite is metamorphosed to eclogite, becoming denser and sinking farther into the mantle.

Diagram based on computer models of isotherms associated with an idealized subduction zone.Magmatism begins at depths of 100 km or deeper in the Wadati-Benioff zone, which is typically 100-300 km inland from the subduction trench.

Island arcs contribute about 25% of the total volume of magma produced each year on Earth (~30-35 km³), which is much less than the volume produced at mid-oceanic ridges. At the spreading centers of the mid-oceanic ridge systems, convection currents within the mantle carry basaltic magmas to the surface and regenerate oceanic crust. Island arc volcanic lavas originate in a variety of magmas.

subduction zone magmas

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