Understanding polar reversals helps us to understand an important piece of evidence for Plate Tectonics.

When molten rock cools, some of the minerals that form contain enough iron to be magnetic such as magnetite and hematite.  This means that once they form, they tend to line up within the molten rock with the Earth’s magnetic field, just like tiny compass needles.  Not only do they point towards magnetic north, but they follow the Earth’s magnetic flux lines and ‘dip’ towards the poles.  That dip depends on the latitude ie. how far away they are from the poles.  Rocks forming at the Earth’s equator will have no dip – they will be horizontal.  The dip increases as you move towards the poles, with rocks forming right at the poles dipping vertically into the ground.  These needles are then locked into place when the rock completely cools, forming a permanent magnetic record of where the rock was formed relative to the north pole (direction and latitude).

understanding polar reversalsScientists use instruments, called magnetometers, to measure the direction and dip of these magnetic minerals frozen in rocks.  Using the information, they can work out the latitude and what direction magnetic North was from the rock when it formed.  This a great information when trying to reconstruct the movement of the Earth’s tectonic plates.

Back in the early 1900’s scientists realized that some rocks seemed to show that at some periods of time the North and South poles had reversed, as the rocks of certain ages magnetic minerals point in the opposite direction to the minerals in rocks in neighboring but slightly different aged rocks.   At the time, however, scientists did not have an accurate way to work out the age of these rocks.

As radiometric dating became available int he 1950’s, and with the mapping of the rocks of the sea floor which are made up of predominantly the igneous rock basalt which has loads of magnetic minerals, scientists discovered that the Earth has been going through these magnetic polar reversals a lot – around 183 reversals over the last 83 million years.  The last reversal took place around 780,000 years ago.

understanding polar reversals
Black are periods of ‘normal’ magnetic fields and white of ‘reversed’ magnetic fields.

Scientists think that the reversal takes places after a slow decline int he strength of the magnetic field, and the reversal may take place over a few decades.  Recently scientist announced that they found one reversal that lasted only 200 years.  It is not known what effect a reversal would have on life on the planet, as the magnetic field plays a major role in keeping out solar radiation.  However, the known polar reversals do not seem to be related to any major extinction event on Earth.

Understanding polar reversals as evidence for Plate Tectonics.

These polar reversals are important evidence in Plate Tectonics, as the rocks forming at mid-ocean ridges (divergent plate boundaries) display distinctive magnetic reversal striping that is mirrored across the mid-ocean ridge.  This proves that rocks of similar ages are being created and added to the plates on either side of the ridge.  Those rocks exhibit the same magnetic properties.

And so, the polar reversal strips provide more evidence that the sea floor is truly spreading at the mid-ocean ridges.

Understanding Polar Reversals

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