Structure and Composition of the Earth

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LU 2 STRUCTURE AND COMPOSITION OF THE EARTH

Structure of the Earth

We think that the Earth is composed of core, mantle, crust,

hydrosphere, atmosphere. The layers from outside in are: Atmosphere (gas)

Hydrosphere

Crust (continental crust, oceanic crust): 5-70 km think

Upper mantle (rocks): down to 660 km depth

Lower mantle (rocks): from 660 km to 2890 km depth

Outer core (molten metal): from 2890 to 5150 km depth

Inner core (solid metal): from 5159 km to the center of theearth

The composition of the core and mantle are thought to be >99% of mass of Earth.

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The Evidence for the Structure of the Earth

We have only indirect evidence for the structure.

From various lines of evidence we think that the Earth is composed

of core, mantle, crust, hydrosphere and the atmosphere.

Lines of evidence that the earth is layered include:

(i) Average density

The average density of the Earth (5.5 g/cc) is much greater thanthat of the crustal rocks that we can observe, hence, somethingvery dense (such as metals) must be down there.

(ii) Moment of inertia

The moment of inertia of the earth is significantly smaller thanthat for a uniform sphere indicating that the Earth is not auniform sphere with a density of 5.5 g/cc everywhere except forvery close to the surface.

(iii) Meteorites.

iv) Seismic studies.

Seismic studies pinned down the depth of different layers of theEarth and helped constrain the composition of the layers.

Seismic velocity discontinuities owing to large differences inmaterial properties (owing to either compositional differences ordifferences in phase) are interpreted as different layers of theEarth.

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Explosive Device

Seismic Waves

Seismic waves are the waves of energy caused by the suddenbreaking of rock within the earth or an explosion. They are the energythat travels through the earth and is recorded on seismographs.

The two main types of waves are body waves and surface waves .

Body waves can travel through the earth's inner layers Surface waves can only move along the surface of the planet like

ripples on water.

Earthquakes radiate seismic energy as both body and surface waves.

Body Waves The first kind of body wave is the P wave or primary wave The second type of body wave is the S wave or secondary wave

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http://www.classzone.com/books/earth_science/terc/content/visualizations/es1009/es1009page01.cfm?chapter_no=visualization

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Velocity and density variations within Earth based onseismic observations.

The main regions of Earth and important boundaries arelabeled.

This model was developed in the early 1980's and is calledPREM for Preliminary Earth Reference Model .

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Earth Structure I: seismic evidence

From velocity structure, density structure, and existence of refracted, reflected,and converted phases at various source-receiver distances, we know the earth hasa core , a mantle , and a crust . We know the depths of the boundaries. We knowthe outer core is liquid, the other regions are solid.

Meteorites

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Meteorites are bits of the solar system that have fallen to the Earth.

Importance of meteoritesMeteorites are our only material evidence of the universe

beyond the Earth .They are surviving remnants of fragmented planetesimals: means of

ascertaining the nature of planetary interiors

undergone no change since their formation

clues to the composition of the solar system: estimate thecosmic abundance of elements

TermsMeteor: the luminous trail caused by a glowing meteoroid or glowingfragments

Meteoroids: extraterrestrial material (range from cosmic dusts toasteroids) that enters the Earths atmosphere.

Meteorites: meteoroids that reach the Earths surfaceCraters: produced by large iron meteoroid impact and explosion

Recovery of meteoritesFalls: Meteorites that have been seen to fall and subsequently

recoveredFalls give true proportions of each class of meteorites

Finds: Meteorites that are not seen to fall but are recognized asmeteoritesMost finds are iron meteorites because these can be identifiedeasily and with confidence.

Classification of Meteorites

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There are three main categories depending on their dominantcomposition.

Stones

Stones are similar to common terrestrial rocks in that theirmineral composition is dominated by silicates , by far themost prevalent rock-forming minerals on our planet.Stones are subdivided into two classes:

ChondritesChondrites get their name from the fact that they all (withsome exceptions) contain chondrules, tiny mineral spherulesmade mostly of silicates.

AchondritesAchondrites lack chondrules

IronsIrons are mostly metallic in composition; they consist of alloys of iron (Fe) and nickel (Ni) , in varying proportions.

Stony-ironsStony-irons are combinations of both ; they contain silicateand metallic phases in approximately equal amounts.

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Meteorite Types

IronPrimarily iron and nickel

Stony IronMixtures of iron and stony material

ChondriteBy far the largest number of meteorites fallinto this class; similar in composition to themantles and crusts of the terrestrialplanets

AchondriteSimilar to terrestrial basalts ; themeteorites believed to have originated onthe Moon and Mars are achondrites

http://dept.physics.upenn.edu/nineplanets/meteorites.html

Evidence from meteorites

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http://www.alaska.net/~meteor/type.htmhttp://dept.physics.upenn.edu/nineplanets/luna.htmlhttp://dept.physics.upenn.edu/nineplanets/mars.htmlhttp://www.alaska.net/~meteor/type.htmhttp://dept.physics.upenn.edu/nineplanets/luna.htmlhttp://dept.physics.upenn.edu/nineplanets/mars.html


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Their nature provides evidence concerning nucleosynthesis

SoThe presence of three distinct types of meteorites stones, stony

irons and irons inevitably led people to believe that meteoritesoriginated from some planetary body which had fractionated into ametal-rich core with a silicate envelope.

They also provide evidence regarding the

1. Formation age of the solar system: About 4.56 Ga

2. Elemental and nuclide abundance in the solar system

3. Origin and early evolution of the solar system and the planets,the possible triggering event for the collapse of the solar nebulaand formation of the solar system

4. The evolution of the parent body of meteorites, asteroids, Mars

(5. Rocks do fall from the sky)

Distribution of the Elements in the Earth

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Composition of the different layers

Layer CompositionCORE(0-3483 km).

Total mass of the metal core is about 32% of the Earths mass.

The core is made of siderophile elements, anddepleted in lithophile and volatile elements.Inner core (0-1220 km).Made of Fe-Ni solid metal plus some minoramount unknown elements.Outer core (1220-3483 km).Made of Fe-Ni molten metal plus asubstantial amount (5-10%) of unknown lightelements (O, Si, C, S, H, etc.)

MANTLE

(~3650 - ~6350 km) Lower mantle (~3650 - 5710 km)Made of silicate and oxide minerals (One estimation: 80% MgSiO 3 perovskite, 15%oxides (Mg,Fe)O, and 5% CaSiO 3 perovskite).Upper mantle (5710 - ~6350km.)Made of peridotite (a rock made of 70%olivine, 25% pyroxenes and 5% other phases).

Crust(5 to 70 km thick)

Oceanic crust is different from the continentalcrust in both thickness and composition.Oceanic crust is thin (6 km) and is made of basalt.Continental crust is thick (30-70 km) and ismade of granite, andesite and basalt (alsosedimentary rock cover).The continental crust is highly complex.Composition of the crustal rocks can be directlydetermined.

Oceans Not a continuous layer (covers 2/3 of the earthssurface).

Atmosphere The atmosphere gradually thins into solar systembackground space.

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Concentrations of elements in the bulk earth and each layer

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Estimated bulk earth composition (Zindler and Hart, 1986)Component mass % Element mass%SiO 2 31.5 Fe 32Fe ~23.0 O 31MgO 25.9 Mg 16FeO ~11.8 Si 15Al2O 3 2.78 Ni 1.8CaO 2.2 Ca 1.6Ni 1.7 Al 1.5Cr 2 O 3 0.32Na 2 O 0.23

Estimated composition of the earths core (Zindler and Hart, 1986)

Outer core Inner coreComponent mass% Component mass%

Fe 89 Fe 93.8Ni 5.4 Ni 5.6Co 0.25 Co 0.26Others 5.35

Estimated composition of Earths mantle and crust

Component Mantle+crust Oceanic crust Continental crustSiO 2 46 50 57.3MgO 38 11 5.3FeO 8.0 8.5 9.1Al2O 3 4.1 16 15.9CaO 3.2 11 7.4Cr 2O 3 0.47 0.04 0.02Na 2O 0.33 2.1 3.1NiO 0.28 0.01 0.01TiO 2 0.18 0.9 0.9MnO 0.13 0.16 0.2K2O 0.032 0.10 2.2

Dry atmosphere composition Ocean water composition

Component volume % Component mass %N2 78.084 H 2O 96.5

O 2 20.948 Cl - 1.89Ar 0.934 Na + 1.05CO2 0.035 SO 4 2- 0.26Ne 0.00182 Mg 2+ 0.13He 0.00052 Ca 2+ 0.040CH4 0.00014 K + 0.037Kr 0.000114N2O 0.00005

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H2 0.00005Xe 0.0000087O 3 0.000007

Average abundances in a very inhomogeneous Earth

The estimates made for the distribution in the whole earth (the core,the mantle, the oceanic crust, the continental crust, the sea and theatmosphere) and the crust alone are as follows.

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The different parts are estimated to have the following values (allanalyses in weight %).

Continent Crust Ocean Crust Mantle Core

O 45.50 44.00 44.23

Si 26.80 23.27 21.16

Al 8.40 8.21 1.86

Fe 7.06 7.86 6.26 70.00

Ca 5.30 9.00 2.34

Mg 3.20 4.62 23.66

Na 2.30 1.94 0.21

K 0.90 0.25 0.03

Ti 0.50 0.85 0.05

Ni 0.00 0.00 0.20 30.00

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The elemental composition for the solid crust, including thehydrosphere and atmosphere, are as follows.

The Earths Crust

Crust is very enriched in some elements (such as K) compared to themantle.

The crust can be grossly subdivided into sediments, granite (uppercrust) and basalt (lower crust). More detailed estimates of crustcomposition have a problem lower crust poorly known.

Dominant element in crust is oxygen (c. 47% by weight), then silicon(c. 28% by weight). The crust is enriched in incompatible elements(elements which preferentially partition into a melt phase) suggestsigneous differentiation.

Relative abundances of elements in the Earths Crust

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Oceans and Atmosphere

These are composed of originally liquid and gaseous materialseparated from the main mass of the Earth during its early history,subsequently altered by chemical weathering, biological activity,

igneous eruptions, human activity. Both are homogeneous comparedto bulk Earth or just crust.

Seawater ~1. Constant concentrations of main elements (Cl, Na, Mg, S, Ca, K).

2. Elements involved in biological activity (C, O, N, P) vary inconcentration with depth because photosynthesis only occurs intop few metres of ocean.

3. Large variations in concentration of dissolved gases (N 2 , O 2 , CO 2etc).

4. Only 15 elements present at > 1ppm, including H & O.

Residence time = the average time that one molecule of a particularcompound (or one atom of a particular element) spends in thereservoir of interest. Combine with incoming and outgoing fluxes(mass transported /unit time) to identify major factors controllingocean chemistry.1. Oceanic residence times vary from 10 2 to 10 8 years, i.e. short c.f.

age of Earth!!!2.Atmosphere is similar reservoir with residence times < 1 day to >

1 m. y.3. Molecules with short residence times (H 2O, NH 3 , SO 2 , ) show

variable concentration during recent Earth history;4. Others (N 2 ~ 78.1%, O 2 ~ 20.95%) have had ~ constant

concentration.

Early atmosphere very different (anaerobic or anoxic = no freeoxygen / low oxygen concentration), predominantly NH 3 , H 2 , H 2O,CH 4 , CO (not CO 2). Major change in late Precambrian, whenphotosynthesis created lots of O 2 , this also strongly affected seawatercomposition.

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References

Brownlow, A.H., 1979, Geochemistry, Prentice Hall.Krauskopf, K.B. & Bird, D.K., 1995, Introduction to Geochemistry,

WCB McGraw Hill

Mason, B. and Moore, C.B., 1989, Prinsip-Prinsip Geokimia, DBP.The Evolution of the Universe - Scientific American, October 1994The Earth's Elements - Scientific American, October 1994The Evolution of the Earth - Scientifi c American, October 1994
http://www.academics.bhc.edu/GEOL101/Reading/Universe.htmhttp://www.academics.bhc.edu/GEOL101/Reading/Elements.htmhttp://www.academics.bhc.edu/GEOL101/Reading/Earth.htmhttp://www.academics.bhc.edu/GEOL101/Reading/Universe.htmhttp://www.academics.bhc.edu/GEOL101/Reading/Elements.htmhttp://www.academics.bhc.edu/GEOL101/Reading/Earth.htm

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Structure and Composition of the Earth