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Mid-oceanic ridges and the geological record

Mid-oceanic ridges emit a flow of lava that is consistent over the ages.

The constancy of this rate of discharge is important for the dating of geological events, and for the mapping of tectonic movements.

When magma cools, the iron particles within it record the polarity of the Earth magnetic field at that moment.

And Earth's magnetic field reverses polarity aperiodically.

The polarity of an iron-bearing igneous rock deposit helps to determine where and when it formed.