MadSci Network: Earth Sciences

Re: Why has the Earths Magnetic Polarity changed so many times?

Area: Earth Sciences
Posted By: Steven Schimmrich, Grad student Geology
Date: Fri Jan 10 14:14:13 1997
Message ID: 852345716.Es

Why has the Earths Magnetic Polarity changed so many times?

I have read that the earth's poles have reversed themselves or changed many time throughout history. But I can not find out why this has happened. Can you tell me?

That's a good question Kimberly!

Why does the earth have a magnetic field in the first place? To understand that, we have to understand what the interior of the earth looks like. About 2000 miles below the surface of the earth lies its core. The core of the Earth is about 4,200 miles across and is divided into an outer core of liquid iron and a nearly solid iron inner core which is some 1,500 miles in diameter. Because the earth rotates, so do the inner and outer cores (in fact, scientists recently discovered the surprising fact that the inner core spins slightly faster than the earth does!).

When the liquid iron in the core of the earth rotates, electrical currents are generated and these electrical currents form a magnetic field since electricity and magnetism are closely related. This is how scientists believe the earth's magnetic field and north and south magnetic poles are formed.

Geologists, looking at small iron minerals in rocks, have found that north has not always been north. At various times in the earth's history, the earth's north magnetic pole has been near the earth's north geographic pole (like today) and at other times, the earth's north magnetic pole has been near the earth's south geographic pole (like it was, for example, during much of the Cretaceous Period when dinosaurs roamed the earth).

Now back to your question. Why does the earth's magnetic field periodically flip-flop in polarity? No one really knows for sure. The science of magnetohydrodynamics which attempts to describe the generation of the earth's magnetic field from movements in the earth's core is very mathematically complicated and scientists have had to use supercomputers to study this problem. Most scientists think that the changes in polarity result from random, chaotic instabilities in the way the liquid iron moves around in the core. In other words, every once in a while the earth's magnetic field changes because the movement of liquid iron in the core of the earth changes. Exactly why it changes, or exactly when it will change, we simply don't know yet.

Hope this helps!

Steven Schimmrich

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