Summary the major terrestrial factories reworking matter

Several major processes are involved in the formation and evolution of the principal terrestrial reservoirs and the exchange of matter between them. All these processes are direct or indirect consequences of mantle convection.

Processes involving the partial melting of silicate rocks play a key role in the evolution of matter in the Earth. Among them mid-ocean-ridge magmatism is by far the most productive factory, reworking fertile peridotitic mantle rocks and generating the relatively short-lived and small but important basaltic oceanic crust and the complementary depleted suboceanic lithospheric mantle (Chapter 24). Another process, plume-related melting and mantle fractionation, appears to be less prolific at present. However, plume magmas, gases and rocks, brought up from the deep Earth, can yield unique evidence on the composition and structure of the Earth's interior. Moreover, plume magmatism could have been of greater importance in the geological past. Both processes have in common that melting results from the decompression of ascending, hot and relatively dry mantle materials. Magma degassing and hydrothermal processes operating on the oceanic plates change the composition of the terrestrial atmo-hydrosphere.

While ridge and plume magmatism originate from upwelling mantle matter, the sinking of oceanic plates back into the mantle, subduction, serves as an opposite, downwelling, branch of the major terrestrial matter-evolving conveyor. Dehydration of the "wet" subducting ocean crust slab plays a role in subduction-related magmatism and the associated fractionation. Ascending magmas erupt at arc volcanoes and add matter to the continents.

Heat, supplied by arc and intraplate mantle melts or resulting from continental collisions, can cause metamorphism and intracrustal melting; magmatic fractiona-tion can then lead to the formation of the most evolved magmatic rocks, granites (Chapter 26).

The above summary shows that magmatism plays an extraordinary role in generating compositional diversity within and on the Earth; therefore its various manifestations are discussed in some detail in Chapters 24 and 25 and Section 26.7.

Volcanic activity on the continental margins and within the continents, the postglaciation uplift of crustal blocks and, in particular, continental collisions generate mountains and mountain chains, which are particularly susceptible to weathering and erosion. The resulting sedimentary rocks constitute a distinct small but important reservoir (Section 26.3). Chemical compounds and organic debris that originated mainly in oceans, seas and lakes also contribute to this reservoir. The accumulation of sediments onto oceanic crust, followed by their subduction into the mantle, can replenish the mantle with volatile and incompatible elements.

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