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Chapter+7+Interior+Modifications - Interior Modications 1...

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Interior Modifications 1 Chapter Seven Interior Modifications After the planets formed from planetesimals they underwent profound internal changes that created their primary internal structure. The grand scheme of planetary differentiation can be summarized as a progressive stratification of the planet, where dense materials sink to the interior and light materials rise to the surface. Core separation is a consequence of the immiscibility of metallic and silicate liquids and the much higher density of metal, leading to metallic core underlying silicate mantle. At the very high temperatures of the earth’s interior, the solid silicate mantle convects. The crust forms by melting of the mantle at shallow depths. The melt is lighter and buoyantly rises to the surface. Melts of the mantle form the mafic rocks of the ocean crust. Further igneous processing leads to the creation of the felsic rocks of the continent. Both oceanic and continental crust are lighter than the underlying mantle and float on top of it. The lower density and greater thickness of the continental crust causes the continents to float at higher levels than the ocean crust. The outermost layers of the earth, the liquid ocean and gaseous atmosphere, formed largely by degassing of the mantle, but were also likely influenced by the continuing influx of comets. Evidence from radionuclides shows that core and atmosphere formation happened in the earliest few tens of millions of years of earth history. The crust that we see today formed much later. The ocean floors are continually being created and destroyed and are presently all younger than 160Ma. The continents preserve a longer record, back to about 4000Ma. As the earliest crust on the earth is not preserved anywhere, its composition and origin are not subject to direct observation. Inferences about earliest Earth history come from study of other solar system objects where much older rocks have been preserved. INTRODUCTION Segregation of planets from the solar nebula explains features such as the bulk density, loss of volatile elements and the preponderance of the big four planet forming elements in terrestrial planets. Planets today, however, are not homogenous mixtures of matter. They have structure and have been differentiated into layers with distinct compositions. This is readily apparent from the meteorites, many of which provide us with samples from the interiors of disrupted parent bodies circling around the solar system. As we saw in Chapter 4, some of these non-chondritic meteorites are Fe-rich metals, some are mixtures of metal and rocks, and others are volcanic rocks that reflect partial melting of planetary interiors. Processes of melting and separation of metal and silicate within planetary bodies clearly operated in early solar system history, and therefore must have affected the earth and moon as well. The meteorites thus provide clues as to what may have happened within the earth.
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