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Composition of the Earth

Composition of the Earth. GLY 4200 Fall, 2012. Interior of the Earth. Earth’s interior is divided into zones, with differing properties and compositions Since we live on the crust, it is the most studied The core and mantle are very important in understanding the behavior of the earth.

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Composition of the Earth

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  1. Composition of the Earth GLY 4200 Fall, 2012

  2. Interior of the Earth • Earth’s interior is divided into zones, with differing properties and compositions • Since we live on the crust, it is the most studied • The core and mantle are very important in understanding the behavior of the earth

  3. Composition of the Crust – Major Elements • Earth’s crust is composed predominantly of eight elements • Figure for Si here is correct – figure 5.2 in text has a misprint • Numbers are in weight percent

  4. Abundances Measurements • We can specify abundances using differ methods • The most common are: • Weight per cent • Atom per cent • Volume percent

  5. Comparison of Methods

  6. Minor and Trace Element Definition • Minor elements have abundances between 0.1 to 1.0 weight percent • Elements with abundances less than 0.1% are called trace elements • Their abundance is usually given in parts per million (ppm) or parts per billion (ppb)

  7. Minor and Trace Elements in Crust • Only 17 elements occur with abundances of at least 200 parts per million (ppm) – in addition to those on the major element slide, these include:

  8. Ores • Many valuable elements are in the trace element range, including the gold group (Au, Ag, and Cu) and the platinum group (Pt, Pd, Ir, Os), mercury, lead, and others • Useage does not always reflect abundance – copper (55 ppm) is used more than zirconium (165 ppm) or cerium (60 ppm)

  9. Effect of Pressure • As pressure increases, minerals transform to denser structures, with atoms packed more closely together • This is seen in the mantle • The upper mantle is dominated by the mineral olivine, Mg2SiO4 • Magnesium is in VI, and Si in IV

  10. Transition Zone • In the transition zone, from about 400 to 660 kilometers below the surface, olivine transforms to denser structures • olivine (ρ = 3.22 gm/cm3) → wadsleyite (ρ = 3.47 gm/cm3) → ringwoodite (ρ = 3.55 gm/cm3)

  11. Lower Mantle • Pressures are so great that silicon becomes six coordinated (CN = VI), and some magnesium becomes eight-coordinated (perovskite structure) • Ringwoodite (ρ = 3.55 gm/cm3) → MgSiO3 (perovskite structure) and (Mg, Fe)O (magnesiowűstite - halite structure)

  12. Core • The core is divided into two regions, the liquid outer core and the solid inner core • There is a definite chemical discontinuity between the lower mantle and the outer core • The main elements in the core are an iron and nickel alloy • Increasing temperature first melts the alloy to make the outer core • Increasing pressure freezes the alloy to produce the inner core

  13. Outer Core • Ranges from 2900 to 5100 kilometers below the earth • Composition is iron with about 2% nickel • Density of 9.9 gm/cm3 is too low to be pure metal • Best estimates are that silica makes up 9-12% of the outer core

  14. Inner Core • From 5100 to 6371 kilometers below surface • 80% iron, 20% nickel alloy • Pressures reach about 3 megabars, or 300,000 megapascals • Temperature at the center is about 7600ºC

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