Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 19 Exploring Earth’s Interior

Structure 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 Refraction Reflection Refraction and Reflection of a Beam of Light

Fig. 19.2a P-wave Shadow Zone

Fig. 19.2b S-wave Shadow Zone

Fig P-and S-wave Pathways Through Earth

Seismograph Record of P, PP, S, and Surface Waves Fig. 19.4

Changes in P-and S- wave Velocity Reveal Earth’s Internal Layers Fig. 19.5

Structure of the Earth Study of the behavior of seismic waves tells us about the shape and composition of the interior 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 Earth Seismology tells us about the density of rocks: Continental crustContinental crust: ~2.8 g/cm 3 Oceanic crustOceanic crust: ~3.2 g/cm 3 AsthenosphereAsthenosphere: ~3.3 g/cm 3

Isostasy 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 buoyant, denser mantle Fig Mohorovicic Discontinuity (Moho)

Crust as an Elastic Sheet Continental ice loads the mantle Ice causes isostatic subsidence Melting of ice causes isostatic uplift Return to isostatic equilibrium

Structure of the Crust and Upper Mantle Fig. 19.7

Earth’s internal heat Original heat Subsequent radioactive decay Conduction Convection

Fig Upper Mantle Convection as a Possible Mechanism for Plate Tectonics

Fig Seismic Tomography Scan of a Section of the Mantle Subducted slab

Fig Temperature vs. Depth

Paleomagnetism 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 Magnetic Field of the Earth

Fig Magnetic Field of a Bar Magnet

Use 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 field The 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 Earth’s Magnetic field Fig

Earth's magnetic field declination: horizontal angle between magnetic N and true N inclination: angle made with horizontal

Earth'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 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 Magnetic Field of the Earth

Self-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 dynamo It 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 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 dynamo If the details seem vague, it is because we have a poor understanding of core dynamics.

Magnetic 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 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 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 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 Recording the Magnetic Field in Newly Deposited Sediment

Lavas Recording Reversals in Earth’s Magnetic Field Fig

Magnetic reversals We can now use the magnetic properties of a sequence of rocks to determine their age.

The Geomagnetic Time 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.