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The rock cycle



Different paths and successions characterize the crustal histories of Earth materials from different geologic environments. The cycle has been repeated in variable sequence, depending on local geologic circumstances, since formation of the planet.

So, what is a rock cycle?

The rock cycle is the extensive reworking of Earth’s materials. The rock cycle relates the history of formation of various units to one another, to their immediate and ultimate sources and to the continuing compositional differentiation and growth of the crust. This chemical differentiation involves crystal-melt fractionation (including partial fusion) of the uppermost mantle and lowermost crust, weathering, differential solution, mechanical and chemical transport, sedimentation and metamorphic fluid-rock chemical exchange. The following steps in the rock cycle:

1. igneous rocks crystallize at considerable depths or on the surface

2. many are eventually uplifted, weathered and eroded

3. the products of erosion are bodily transported

4. after mixing with contrasting materials, deposited as sediments

5. then lithified during and after burial

6. may be metamorphosed or even partially remelted near the base of the continental crust (the products of these changes are metamorphic and secondary plutonic igneous rocks)

Both sedimentary and metamorphic rocks are derived ultimately from igneous parents – the primary sources of crustal crust. Both sedimentary and metamorphic rocks may be subjected to the changes described above:

1. erosion and deposition as new sediments

2. granulation and recrystallization to produce metamorphic rocks

3. partial fusion giving rise to igneous rocks and residual, refractory to metamorphic.

This complex sequence of lithologic addition and reworking is repeated over and over in different orders as dictated by geologic circumstances. The process results in the increasing heterogeneity and compositional differentiation of the crust, as well as in the gradual increase in the total volume of continental mass. The entire cycle is intimately related to the deformational process. The rocks and minerals that constitute the dynamic Earth are constantly involved in the processes of breakdown and formation anew.

(Robert Lauterbach “The World of Geology -The Earth then and now” 1983 Leipzig)





Дата публикования: 2014-11-03; Прочитано: 606 | Нарушение авторского права страницы | Мы поможем в написании вашей работы!



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