Classroom presentations to accompany Understanding Earth , 3rd edition

Classroom presentations Classroom presentations to accompany to accompany

Understanding EarthUnderstanding Earth, 3rd edition, 3rd edition

prepared byPeter Copeland and William Dupré

University of Houston

Chapter 19Chapter 19Exploring Earth’s Interior

Exploring Earth’s Exploring Earth’s InteriorInterior

Structure of the EarthStructure of the Earth• Seismic 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.

Fig. 19.1

Refraction

Reflection

Refraction and

Reflection of a

Beam of Light

Fig. 19.2a

P-wave P-wave Shadow Shadow

ZoneZone

Fig. 19.2b

S-wave S-wave Shadow Shadow

ZoneZone

Fig. 19.3

P-and S-wave Pathways Through P-and S-wave Pathways Through EarthEarth

Seismograph Record of Seismograph Record of P, PP, S, and Surface WavesP, PP, S, and Surface Waves

Fig. 19.4

Changes Changes in P-and S- in P-and S-

wave wave Velocity Velocity Reveal Reveal Earth’s Earth’s Internal Internal LayersLayers

Fig. 19.5

Structure of the EarthStructure of the EarthStudy of the behavior of seismic waves tellsus about the shape and composition of theinterior of the Earth:

• CrustCrust: ~10–70 km, intermediate composition

• MantleMantle: ~2800 km, mafic composition

• Outer coreOuter core: ~2200 km, liquid iron

• Inner coreInner core: ~1500 km, solid iron

Composition of the EarthComposition of the EarthSeismology tells us about the density

of rocks:

• Continental crustContinental crust: ~2.8 g/cm3

• Oceanic crustOceanic crust: ~3.2 g/cm3

• AsthenosphereAsthenosphere: ~3.3 g/cm3

IsostasyIsostasy

• Buoyancy of low-density rock masses “floating on” high-density rocks; accounts for “roots” of mountain belts

• First noted during a survey of India

• Himalayas seemed to affect plumb

• Two hypotheses: Pratt and Airy

The less dense crust “floats” on The less dense crust “floats” on the less buoyant, denser mantlethe less buoyant, denser mantle

Fig. 19.6

MohorovicicDiscontinuity

(Moho)

Crust as an Elastic SheetCrust as an Elastic SheetContinental ice loads the mantle

Ice causes isostatic subsidence

Melting of ice causes isostatic uplift

Return to isostatic equilibrium

Structure Structure of the of the

Crust and Crust and Upper Upper MantleMantle

Fig. 19.7

Earth’s internal heatEarth’s internal heat

• Original heat

• Subsequent radioactive decay

• Conduction

• Convection

Fig. 19.8

Upper Mantle Convection as a Upper Mantle Convection as a Possible Mechanism for Plate Possible Mechanism for Plate

TectonicsTectonics

Fig. 19.9

Seismic Tomography Scan of a Seismic Tomography Scan of a Section of the MantleSection of the Mantle

Subducted slab

Fig. 19.10

Temperature Temperature vsvs. Depth. Depth

PaleomagnetismPaleomagnetism• Use of the Earth's magnetic field to

investigate past plate motions

• Permanent record of the direction of the Earth’s magnetic field at the time the rock was formed

• May not be the same as the present magnetic field

Fig. 19.11

Magnetic Magnetic Field of Field of

the Earththe Earth

Fig. 19.11

Magnetic Magnetic Field of a Field of a

Bar Bar MagnetMagnet

Use of magnetism in geologyUse of magnetism in geology

Elements 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 fieldEarth'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 Evidence of a Possible Reversal of the Earth’s Magnetic fieldof the Earth’s Magnetic field

Fig. 19.12

Earth's magnetic fieldEarth's magnetic field

declination: horizontal angle between magnetic N and true N

inclination: angle made with horizontal

Earth's magnetic fieldEarth's magnetic field

It 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 fieldThe Earth's magnetic field

Since the geothermal gradient in the Earth is ≈ 25°C/km, nothing can be permanently magnetized below about 30 km.

Another explanation is needed.

Fig. 19.11

Magnetic Magnetic Field of the Field of the

EarthEarth

Self-exciting dynamoSelf-exciting dynamo

A 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 dynamoSelf-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 field—a self-exciting dynamo!

Self-exciting dynamoSelf-exciting dynamo

The 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 dynamoSelf-exciting dynamo

If the details seem vague, it is

because we have a poor

understanding of core dynamics.

Magnetic reversalsMagnetic reversals• The 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 reversalsMagnetic reversals

• A 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 reversalsMagnetic reversals

• Discovered 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 theorySelf-reversal theory

• First suggested that it was the rocks that had changed, not the magnetic field

• By dating the age of the rocks (usually by K–Ar) it has been shown that all rocks of a particular age have the same magnetic signature.

Fig. 19.13

Recording the Magnetic Field in Recording the Magnetic Field in Newly Deposited SedimentNewly Deposited Sediment

Lavas Recording Reversals in Lavas Recording Reversals in Earth’s Magnetic FieldEarth’s Magnetic Field

Fig. 19.14

Magnetic reversalsMagnetic reversals

We can now use the magnetic

properties of a sequence of rocks to

determine their age.

The GeomagneticThe GeomagneticTime ScaleTime Scale

Based 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.