Earth's Internal Structure: Earth Magnetism (2.3.4)

Question 1

What is the Earth’s dipole magnetic field?

Answer 1

• Earth possesses magnetic field approximating to dipole magnet.
• Magnetic force field a vector quantity with magnitude and direction.
• Location of Magnetic North Pole wanders with time, not same ass True North Pole.

Question 2

What is declination and inclination?

Answer 2

Declination: horizontal direction (compass bearing).

Inclination: vertical direction/dip (not measured on compass).

Question 3

Inclination of magnetic field is a function of latitude on Earth. Tangent (magnetic included) = ?x tangent (latitude).

Answer 3

2x

Question 4

What is intensity?

Answer 4

Earth’s magnetic field intensity (strength) varies across planet.

Question 5

What is the geodynamo theory?

Answer 5

• Self-exciting dynamo: proposed mechanism by which Earth generates magnetic field.
• Electrical currents flowing in slowly moving molten iron generates magnetic field.
• Shows how rotating, convecting, electrically conducting fluid maintains magnetic field over geological time scales.
• Implication of theory is that Earth’s magnetic field is dynamic and changes through time.

Question 6

Convection cells carrying heat out core are?

Answer 6

Unstable.

Question 7

Why is the iron composition of the core important?

Answer 7

• Required for magnetic field.
• Density and seismic velocity consistent.
• High enough cosmic abundance.

Question 8

What are magnetic reversals?

Answer 8

• Complex fluid dynamics of convecting metal outer core leads to instabilities in magnetic field.
• May ‘flip’ field direction.
• Settles into new reversed state for some time, then repeats.

Question 9

What is remanent magnetism/palaeomagnetism?

Answer 9

Field direction may be recorded by rocks. Magnetic minerals in rock respond to Earth’s magnetic field.

Question 10

What are magnetic anomalies?

Answer 10

• Local measurement of magnetic field intensity may be stronger/weaker than expected regional field due to deep Earth.
• Caused by local remanent magnetisation of nearby rocks in crust.
• Positive anomaly if magnetic field of rocks is in same direction as current field.

Question 11

What are the magnetic reversals in rock/time period?

Answer 11

• Each layer preserves record of magnetic field.
• Radiometric dating used to determine rock age.
• Palaeomagnetic measurements reveal Earth’s magnetic field changed direction in geologic past.
• Present magnetic field termed Normal while reversed is Reversed.