Sea-floor Spreading / Paleomagnetism
- Examination of the ocean crust either side of the Mid-Atlantic Ridge suggested that sea-floor spreading was occuring.
- The evidence for this is the alternating polariity of the rocks that form the ocean crust.
- Iron particles in lava erupted on the ocean floor are aligned with the Earths magnetic field.
- As lava solidifies, these particles provide a permenant record of the Earths polarity at the time of the eruption (paleomagnetism).
- However, the Earths polarity reverses at regular intervals (approx every 400,000 years).
- The result is a series of magnetic 'stripes' with rocks aligned alternately towards the north and south poles.
- The stripped pattern, which is mirrored exactly on the other side of the ridge, suggests that the ocean crust is slowly spreading away from this boundary. Moreover, the oceanic crust gets older with distance from the mid-ocean ridge.
- Sea-floor spreading implies that the Earth must be getting bigger.
- As this is not the case, then plates must be getting destroyed somewhere to accomodate the increase in their size at mid-oceanic ridges.
- Evidence of this was found with the discovery of huge oceanic trenches.
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- Newly formed oceanic lithosphere at a constructive/divergent plate boundary, eg. at the Eurasian and North American plates, is less dense than the asthenosphere.
- As it becomes denser with age, due to it cooling and thickening, it sinks into the mantle with a downward pull at subduction zones. This results in an ocean trench.
- E.g. MAR - Mid-Atlantic Ridge.
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- Radioactive decay of some elements in the mantle and core, e.g. uranium, generates a lot of heat.
- When lower parts of the asthenosphere heat up they become less dense and they slowly rise.
- As they move towards the top of the asthenosphere they cool down, become more dense, then slowly sink.
- These circular movements of semi-molten rock are called convection currents.
- convection currents in the asthenosphere create drag on the base of the tectonic plates (which are solid and rigid) - and causes them to move.
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