N7 - Earth\'s Magnetic Field and Plate Tectonics PDF

Preview

Summary

semester 1...

Description

Earth Systems L7: Earth’s Magnetic Field and Plate Tectonics Cause of the Earth’s magnetic field Early in Earth’s history, a separation of silicate and iron-rich material occurred. Iron, being denser, was drawn towards the centre of the earth and formed the core. Inner core: 6000°C, but remains solid due to pressure. Outer core: cooler, but lower pressures mean this is liquid. Convection in the outer core:  Liquid material in the outer core leads to convection taking place  Temperature differences and the Earth’s spin (Coriolis Effect) Generating the Earth’s magnetic field  Friction from flow of liquid iron generates electric potential  Rotation of the Earth causes helical flow  Produces electrical coil  magnetic field  But this wouldn’t be enough to generate the field we observe  Further convection (due to temp?) gives the effect of an iron ‘core’ moving through the loops  Induces a stronger electric field  stronger magnet  The outer core is therefore referred to as a Geodynamo Magnetosphere: the area of space near to the Earth in which charged particles are influenced by its magnetic field There is evidence from the aurorae (Borealis/Australis) of its existence. Aurorae caused from the interaction of the solar wind with the Earth’s magnetic field. Creation of aurorae:  Electrons travel down field lines towards poles  Collide with and excite air molecules in Ionosphere  Energy released from these as they calm down  light Variations in the magnetic field  There is evidence for reversals in the Earth’s magnetic field

    

Magnetic minerals (magnetite) in basalt become aligned with the Earth’s magnetic field The position of the pole shown by these can be seen to flip in the opposite direction This also occurs in sedimentary rocks Pattern remains remnant; this is remnant magnetisation Unless the curie temperature is passed, any remnant magnetisation is lost

Earth’s magnetic field is not perfectly aligned with the north/south poles. Exact position of magnetic pole varies. The north pole is currently at a latitude of 80°. However, there is evidence of much greater variation in the position of poles in the past. This is called pole wandering. You can determine the position of poles by magnetic declination and inclination: Declination: variation of compass needle from geographic north Inclination: direction through the Earth to the pole

The theory of continental drif Ortelius (1595):  Thesaurus Geographicus  First suggested that the Americas and Europe/Africa were once joined Edward Suess (1890’s)  Used fossil leaves (Glossopteris) to suggest that South America, Africa and India were once joined – Gondwanaland Alfred Wegener (1912)  ‘Continental drift’  From fossil evidence and Mid Ocean Ridges Keith Runcorn (1940’s/50’s):  Helped to establish the study of palaeomagnetism  Initially explained ‘polar wander’ by movement of the poles  Contradicting evidence from America/Europe became a proponent of continental drift but no mechanism Holmes (1928):  Convection of mantle (no evidence) Evidence accumulated:

   

Mountain belts can be linked across continents (e.g. Appalachians – Caledonides) Phylogenetic evidence: opossum found in South America and Australia Glaciation in South Africa: Evidence for glaciers in Vaal River at ~ 320 Ma Fossil evidence

 1962: Hess and Diaz suggested Seafloor Spreading  1963: Matthews and Vine used magnetic stripes o Stripes are symmetrical about a central ridge o Correlate with known sequence of magnetic reversals from rocks studied on land o Seafloor becomes older away from the central ridge  Led to suggestions that the seafloor was produced at this ridge = Mid Ocean Ridge Continental drif There was a supercontinent called Pangaea, formed of Laurasia and Gondwanaland. Modern continents that exist today from them. J. Tuzo Wilson (1965) said that plates are portions of rigid lithosphere (crust + uppermost mantle), resting on top of plastic asthenosphere. Much of the flow occurs in plastic asthenosphere, and the lithosphere ‘rides’ on this.

Drivers of plate tectonics Holmes (1928):  Mantle convection: large scale convection of the mantle  Friction between this and lithosphere causes plates to move Whole mantle or stratified convection? Density differences suggest stratified

Slab Pull:

 Subduction of cold, dense lithosphere causes movement of plates  ‘Sucked’ apart at Mid Ocean Ridges  However, North American plate is moving but is not being subducted  must be other drivers Ridge Push:  Buoyancy causes plates to sit higher at Mid Ocean Ridges  The weight of these acts on adjacent crust  Precise drivers unknown – probably a combination  Slab pull thought to be more important than ridge push...