why does the magnetic polarity of the earth invert , and how often does it occur?

Answer 1

We don't know why it inverts, although you are probably aware that the magnetic poles do tend to drift around a bit. James Ross eventually found it in 1831 but, by the time Roald Amundsen rediscovered it in 1904, it had moved over 50kms.

Polar inversion happens on average every 300,000 years but at irregular intervals, so it is impossible to predict when the next one will happen. The last inversion occurred 780,000 years ago.

Answer 2

Magnetic reversal is where what we know as magnetic north switches to what we now call the south pole. It was discovered because of sea floor spreading when the molten iron particles would align themselves to the magnetic field as the rock cooled - along the ridge where the sea floor is spreading, there are symmetrical stripes where there are periods where the iron particles are aligned this way, and other periods where they are aligned that way. Knowing how fast the sea floor is spreading, they can tell how long the magnetic north was our north and how long it was in a reverse period.

P.S. I don't earth scientists really know why this happens or what becomes of the Earths magnetic field as it is switching (becomes erratic because, as you may know, the magnetic north can move around - then weakens - then switches, I'd guess) We may be moving into a period where the poles are erratic an weakening - perhaps the Earth is due for a reversal.

Answer 3

Magnetic polarity reverses, it does not invert. The reasons why are not understood, but it probably has something to do with the rotation of the solid inner core within the liquid outer core. The reversal interval varies, but it is on the order of hundreds of thousands of years. The periods of reversals last on the order of tens of thousands of years.