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

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classroom presentations to accompany understanding earth 3rd edition

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
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.
structure of the earth1
Structure of the Earth

Study of the behavior of seismic waves tells

us about the shape and composition of the

interior of the Earth:

  • Crust: ~10–70 km, intermediate composition
  • Mantle: ~2800 km, mafic composition
  • Outer core: ~2200 km, liquid iron
  • Inner core: ~1500 km, solid iron
composition of the earth
Composition of the Earth

Seismology tells us about the density

of rocks:

  • Continental crust: ~2.8 g/cm3
  • Oceanic crust: ~3.2 g/cm3
  • Asthenosphere: ~3.3 g/cm3
  • 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
The less dense crust “floats” on the less buoyant, denser mantle




Fig. 19.6

crust as an elastic sheet
Crust as an Elastic Sheet

Continental ice loads the mantle

Ice causes isostatic subsidence

Melting of ice causes isostatic


Return to isostatic equilibrium

earth s internal heat
Earth’s internal heat
  • Original heat
  • Subsequent radioactive decay
  • Conduction
  • Convection
  • 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
use of magnetism in geology
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
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

earth s magnetic field1
Earth's magnetic field

declination: horizontal angle between magnetic N and true N

inclination: angle made with horizontal

earth s magnetic field2
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
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.

self exciting dynamo
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 dynamo1
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 dynamo2
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 dynamo3
Self-exciting dynamo

If the details seem vague, it is

because we have a poor

understanding of core dynamics.

magnetic reversals
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 reversals1
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 reversals2
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
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.
magnetic reversals3
Magnetic reversals

We can now use the magnetic

properties of a sequence of rocks to

determine their age.

the geomagnetic time scale
The GeomagneticTime 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.