What is polarity? What happens if it reverses or changes direction? Did this effect the dinosaurs? What will happen to the people?
In the general sense, polarity is the quality of having paired but opposite
tendencies. In terms of your question, you are refering to the polarity of
the earth's magnetic field, which we know has reversed many times during
Earth history. The Earth's magnetic field is believed to be created by
dynamic movements within the metallic liquid core of the Earth, which
generates electrical currents and thus a magnetic field. The mechanism
by which this field "flips" is poorly understood but is believed to be due
to changes in relative motions within the liquid core.
We see polarity reversals in the rock record because magnetic minerals (iron oxides, for instance) align themselves with the field as the rock is formed. If we determine the age of the rock, we know the field at that point in time. By dating lots of rocks, we know that the field reverses every 500,000 years or so, although there have been periods of many millions of years when it hasn't reversed (such as the Cretaceous Period).
Revesals are very quick (a few tens of thousands of years) but our resolution on rock ages is not great so this really limits our precision in determining this number.
Many things could happen during a reversal but it is not easy to predict their effects on life on Earth. We can predict a few things, however. The Earth's magnetic field is important because it shields the Earth from a large percentage of cosmic rays (the field focuses these rays towards the poles and away from other latitudes). The field is also used by many organisms to navigate. During reversal times, things would probably go haywire with electronic communications and other electronic devices due to bombardment by more cosmic rays in low latitudes. Animals may also have trouble navigating.
How would we deal with this? I don't know. The effect of a reversal would come on gradually and could stay with us for a time period that is longer than human civilization has existed. We would probably adapt. How did reversals affect other organisms in the past (such as the dinosaurs)? This is very difficult to answer and I'm really not qualified to venture a guess. If any part of their daily routine involved using the magnetic field, or if their food supply depended on organisms that needed the field to survive, the effects could have been dramatic.
Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822