A pole shift is considered as the magnetic field of the Earth that reverses its polarity. If the Earth face magnetic reversal anytime for say today, your compass will then show you opposite direction or south rather than showing north.
Talking about science, scientists since past 15 million years have revealed that pole shifts basically occurs four times in every 1 million years. This means in every 250,000 years, you will find yourself lucky enough to see this great change in the planet Earth. Even it was a period in Cretaceous when polarity stayed like the same for around 30 million years, but it is believed as anomaly. It was also declared that the last pole shift was experience some 790,000 years ago that made several scientists to believe that we are due whereas; other scientists have speculated that a reversal is already underway.
Basically, dynamic processes are taking place deep inside the earth that helps generating Earth’s magnetic field. It is a core of molten iron that is surrounding the inner core of solid iron. Both the cores are rotating at their separate rates. However, scientists have never understood the interaction of both the core, not even other geophysical processes. Well, this process has been named by scientists as hydromagnetic dynamo. This action of self-perpetuate electric field somewhere seems like a gigantic magnet bar. The magnetic field of Earth extends into space for some thousands of miles from its poles. The Earth remains protected due to the solar radiation that plays a vital role in changing weather patterns, overall climate and migratory habits of animals.
However, these poles cannot be said as beneficial if reversed instantly, but they could bring global destruction including earthquakes, volcanic eruptions, vast flooding, and Arctic ice melt down. On the other hand, evidences suggests that pole shifts occur gradually and takes from 1000 to 28,000 years and the last four flip-flops took around 7000 years each.
The actual mechanisms behind each pole shift have still not discovered. Some of the theories imply that comet impacts perhaps play a significant role while others say that the magnetic field of the Earth is intrinsically prone to flip-flops. A conclusive answer in this matter would bring better understanding of the dynamics of this very interesting geophysical phenomenon.