Earth’s Structure
Shortly after Earth formed, the decay of radioactive elements and the heat released by colliding particles produced melting of the interior. The denser materials, mainly iron and nickel, sank to Earth’s centre and lighter rocky materials floated towards the surface. This sorting of material by density is believed to be continuing today but on a much smaller scale.
Earth’s interior is therefore not homogeneous. It is arranged in concentric shells or spheres of materials with different densities and properties. The partly molten core is surrounded by a shell of molten rock called the mantle, which in turn is surrounded by a thin crust at the surface. As Earth cooled, the crust may have covered the whole surface of the planet.
About 4300 mya infalling asteroids broke up the crust. Mantle convection currents pulled the thin crustal blocks together to form the early continents. The continents reached their present thicknesses and about half their present area by about 2500 mya.
Palaeomagnetic Timescale
Evidence of Earth’s magnetic field through geological time is preserved in rocks because when rocks are formed, iron-containing minerals are lined up like tiny magnets. Rocks of the same age from different continents often give different pole positions. This suggests that during geological time the continents moved over the surface of Earth. Thus the latitude of a rock when it was formed can be found but not its longitude. Field reversals seem to have taken place fairly regularly over the last 4.5 million years; they occurred before this time but the rocks cannot be dated with sufficient accuracy.
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