Earths magnetic field

Question 1

magnetic iron binding minerals

Answer 1

magnetite (Fe3O4) and pyrrhotite (Fe1-xO)

Question 2

magnetic field

Answer 2

• the earths magnetic field originates from liquid outer core (this acts as a geodynamo generating electric currents)
• align with the earths field
• used in navigation (lodestone)

Question 3

Seismology

Answer 3

reveals a liquid outer core made mainly of iron

Question 4

Geodynamo

Answer 4

• electrical current causes a magnetic field
• outer core convection combined with the earths spin creates helical flow and sustains magnetic field and electrical currents

Question 5

disruptions to the magnetic field

Answer 5

Magnetic field reversals
Magnetic storms
- can disrupt field strength (can cause power supple problems)
- can disrupt global navigation satellite system (GPS)
- cause aurora (northern lights)
- normally short lived. sun spot may cause increase in magnetic storms

Question 6

Palaeomagnetism

Answer 6

Remanent magnetism in ancient rocks recording the direction and strength of the earths magnetic field at the time of the rocks formation
- when magma cools below curie point magnetic iron-bearing minerals align with the earths magnetic field

Question 7

Polar Wandering

Answer 7

• 1950s: discovered that magnetism of ancient rocks showed different orientations from the present magnetic field
• magnetic poles apparently moved. the apparent movement was called polar wandering. different continents had different paths.

Question 8

Plate tectonic reconstructions

Answer 8

• by looking at the remanent magnetisation of ancient rock we can model where these rocks formed relative to the earths magnetic field.
• this can tell us the latitude of where the rock formed but not the longitude

Question 9

Magnetic field reversals

Answer 9

• these occur infrequently from 10,000 to millions of years apart
• field strength decays to 10-30% of normal field
• reversal may take 10s to 100s years
• this can be used to set up magnetic timescale with periods of normal and reverse polarity

Question 10

Seafloor spreading

Answer 10

• geomagnetism and the ocean floor
• symmetrical magnetic anomalies in the ocean (give dates fir the age if the crust)
• ocean lithosphere consists of upper mantle and the ocean crust
• oceans form by adding material symmetrically on either side of the mid ocean ridge

Question 11

Summary

Answer 11

• magnetic minerals within igneous rocks align with the earths magnetic field as the rocks cools and solidifies
• ancient rocks on continents often have remanent magnetism that is different to the magnetic field of their current location
• this means that either the magnetic pole has moved or the continents have moved