Classroom presentations to accompany Understanding Earth, 3rd editionprepared byPeter Copeland and William DuprUniversity of HoustonChapter 19Exploring Earths Interior

Exploring Earths Interior

Structure of the EarthSeismic velocity depends on the composition of material and pressure.We can use the behavior of seismic waves to tell us about the interior of the Earth.When waves move from one material to another they change speed and direction.

Refraction and Reflection of a Beam of LightFig. 19.1RefractionReflection

Fig. 19.2aP-wave Shadow Zone

S-wave Shadow ZoneFig. 19.2b

P-and S-wave Pathways Through EarthFig. 19.3

Seismograph Record of P, PP, S, and Surface WavesFig. 19.4

Changes in P-and S- wave Velocity Reveal Earths Internal LayersFig. 19.5

Structure of the EarthStudy of the behavior of seismic waves tellsus about the shape and composition of theinterior of the Earth:Crust: ~1070 km, intermediate compositionMantle: ~2800 km, mafic compositionOuter core: ~2200 km, liquid ironInner core: ~1500 km, solid iron

Composition of the EarthSeismology tells us about the densityof rocks:Continental crust: ~2.8 g/cm3Oceanic crust: ~3.2 g/cm3Asthenosphere: ~3.3 g/cm3

IsostasyBuoyancy of low-density rock masses floating on high-density rocks; accounts for roots of mountain beltsFirst noted during a survey of IndiaHimalayas seemed to affect plumbTwo hypotheses: Pratt and Airy

The less dense crust floats on the less buoyant, denser mantleFig. 19.6MohorovicicDiscontinuity(Moho)

Crust as an Elastic SheetContinental ice loads the mantleIce causes isostatic subsidenceMelting of ice causes isostatic upliftReturn to isostatic equilibrium

Structure of the Crust and Upper MantleFig. 19.7

Earths internal heatOriginal heatSubsequent radioactive decayConductionConvection

Upper Mantle Convection as a Possible Mechanism for Plate TectonicsFig. 19.8

Seismic Tomography Scan of a Section of the MantleFig. 19.9Subducted slab

Temperature vs. DepthFig. 19.10

PaleomagnetismUse of the Earth's magnetic field to investigate past plate motionsPermanent record of the direction of the Earths magnetic field at the time the rock was formedMay not be the same as the present magnetic field

Fig. 19.11Magnetic Field of the Earth

Magnetic Field of a Bar MagnetFig. 19.11

Use of magnetism in geologyElements that have unpaired electrons (e.g., Fe, Mn, Cr, Co) are effected by a magnetic field. If a mineral containing these minerals cools below its Currie temperature in the presence of a magnetic field, the minerals align in the direction of the north pole (also true for sediments).

Earth's magnetic fieldThe Earth behaves as a magnet whose poles are nearly coincident with the spin axis (i.e., the geographic poles).Magnetic lines of force emanate from the magnetic poles such that a freely suspended magnet is inclined upward in the southern hemisphere, horizontal at the equator, and downward in the northern hemisphere

Evidence of a Possible Reversal of the Earths Magnetic fieldFig. 19.12

Earth's magnetic fielddeclination: horizontal angle between magnetic N and true Ninclination: angle made with horizontal

Earth's magnetic fieldIt was first thought that the Earth's magnetic field was caused by a large, permanently magnetized material deep in the Earth's interior. In 1900, Pierre Currie recognized that permanent magnetism is lost from magnetizable materials at temperatures from 500 to 700 C (Currie point).

The Earth's magnetic fieldSince the geothermal gradient in the Earth is 25C/km, nothing can be permanently magnetized below about 30 km.Another explanation is needed.

Fig. 19.11Magnetic Field of the Earth

Self-exciting dynamoA dynamo produces electric current by moving a conductor in a magnetic field and vise versa. (i.e., an electric current in a conductor produces a magnetic field.

Self-exciting dynamoIt is believed that the outer core is in convective motion (because it is liquid and in a temperature gradient).A "stray" magnetic field (probably from the Sun) interacts with the moving iron in the core to produce an electric current that is moving about the Earth's spin axis yielding a magnetic fielda self-exciting dynamo!

Self-exciting dynamoThe theory has this going for it: It is plausible. It predicts that the magnetic and geographic poles should be nearly coincident. The polarity is arbitrary. The magnetic poles move slowly.

Self-exciting dynamoIf the details seem vague, it isbecause we have a poorunderstanding of core dynamics.

Magnetic reversalsThe polarity of the Earth's magnetic field has changed thousands of times in the Phanerozoic (the last reversal was about 700,000 years ago).These reversals appear to be abrupt (probably last 1000 years or so).

Magnetic reversalsA period of time in which magnetism is dominantly of one polarity is called a magnetic epoch. We call north polarity normal and south polarity reversed.

Magnetic reversalsDiscovered by looking at magnetic signature of the seafloor as well as young (0-2 Ma) lavas in France, Iceland, Oregon and Japan.When first reported, these data were viewed with great skepticism

Self-reversal theoryFirst suggested that it was the rocks that had changed, not the magnetic fieldBy dating the age of the rocks (usually by KAr) it has been shown that all rocks of a particular age have the same magnetic signature.

Recording the Magnetic Field in Newly Deposited SedimentFig. 19.13

Lavas Recording Reversals in Earths Magnetic FieldFig. 19.14

Magnetic reversalsWe can now use the magneticproperties of a sequence of rocks todetermine their age.

The GeomagneticTime ScaleBased on determining the magnetic characteristics of rocks of known age (from both the oceans and the continents).We have a good record of geomagnetic reversals back to about 60 Ma.