magnetic earth

• The earth has a magnetic field that is very similar to having a bar magnet inside the planet. 
• The earths core is made of iron and nickel, but the inner core is at temperatures of 5700-6000 dc and the outer core is 3500-5700 dc. These temperatures are above the curie point. 
• The curie point is the temperature at which materials lose their permanent magnetism. 
• Earths magnetism is not permanent and must be generated constantly. 
• The temp difference between the cores is what induces convection currents. Convecting iron moving through an existing magnetic field generates electricity, which induces magnetism. This is called a self exciting dynamo. 
• The rotation of the earth causes motion of the outer core, aiding the magnetic field generation. 
• Convection in the core changes, the magnetic field gradually fades over thousands of years, when it increases again the poles are the opposite way round. 
• Magnetic magnetite is iron rich, in lavas that cool through the curie point, they align with the earths magnetic field. The permanently retain palaeomagnetism like a frozen compass. 
• The lava is dated by radiometric dating. 

  Ocean floor basalts show many reversals in their remanent magnetism. 

   
• If the sea floor is spreading the rock stripes should show symmetrical magnetic stripes moving away from the ridge.
• Perfect stripes are uncommon because:

  • Magma does not erupt at a constant rate 
  • Lines can be offset by transform faults 
  • One plate may move faster than the other 
• Inclination is the angle of dip the magnetic field makes with the ground. Which can determine the latitude of eruption. This can be used as evidence of continental drift. 
• Magnetic surveys measure the variation of magnetic fields, variations are called anomalies. 
• Mapping of anomalies is used to locate metalliferous ore deposits which includes metals that respond to magnetism. Deposits give a higher reading on a magnetometer than the surroundings.