68. Telling the 6th grade about polarity reversals
I found your site on the web as I was searching for more information about the mid ocean ridge and magnetism. My 6th grade students want to know, and so do I, the reason why the magnetic poles of the Earth switch? what causes this, and at which time interval does this happen? Is it an on-going process, like the tilting of our planet? I have been unable to find a concise yet simple explanation about this.
That is a very difficult thing to explain at 6th grade level!
In "The Great Magnet, the Earth" your students will find many answers, and there is also the outline of a course in 3 parts, starting at
The 3rd part discusses reversals.
I would recommend for you to read the material of these web pages, to get for yourself a good understanding. After that you might try to explain it to the class, though the full story seems too detailed for 6th graders.
To a 6th grade asking about "the reason why the magnetic poles of the Earth switch" I would say, you are going too far too fast--as if you were trying to build a house starting with the 4th floor. First come the foundations!
- What is magnetism ?
- What causes the Earth to be magnetic at all?
- How do we know about past reversals of magnetic polarity?
And only then
- What is the reason why the magnetic poles of the Earth switch?
"What is magnetism?" Most of our adult population seems to misunderstand this. They are familiar with iron magnets, and therefore regard magnetism as a mysterious property of iron. They may perhaps know about electromagnets--turning a bar of iron into a magnet by running an electric current through a coil wrapped around it--but iron is always needed.
IT ISN'T SO. Iron has interesting magnetic properties, but basically, magnetism is a force created by electric currents--a force that can act on iron magnets, or on other electric currents.
I recommend three experiments here, described at
but in particular the last of them, also described in
(see the end, on how to perform the experiment in a classtoom)
You may run into a problem here--someone asking "but what is an electric current?" You can just say then "the flow through material of tiny particles known as electrons, carrying an electric charge", and you may compare this to a flow of water in pipes, as I did in the first part of
"What makes the Earth magnetic?" MUST be electric currents, because careful measurements show the field is constantly but slowly changing. Navigators using the magnetic compass (that was before GPS!) needed to update their magnetic charts every 10-20 years, for that reason.
We know the inside of the Earth is hot (volcanoes, geysers!). We also know from study of earthquake waves--vibrations like sound, spreading world-wide--that a denser core exists, and is liquid. We guess the core is iron (the density fits, and iron is a very common element), too hot to be permanently magnetic but a good conductor of electric current.
The heat (produced perhaps by radioactivity, perhaps by solidification of liquid iron) flows outwards, and can cause swirling motion.
It is like the heat deposited by the Sun on the Earth, which is carried by swirling motions of air to high level, from where it is lost to space (as infra-red light, radiated by warm air). The swirling in the atmosphere goes by the name "weather."
Swirling of a fluid conducting electricity CAN produce magnetism. Mathematically, this is a very difficult thing to prove, only achieved recently, but computers have now simulated it, too.
How do we know about past reversals? That is the interesting part of the story. See
"What is the reason why the magnetic poles of the Earth switch?" The patterns of swirls changes constantly. Besides the north-south field, irregularities also exist, and at the core these are actually larger than they seem on the surface (because their effect decreases faster with distance than the main north-south field).
There is no physical reason for a preferred North-South polarity, the math suggests either polarity has equal likelihood. As the field changes, sometimes the "main" north-south magnet weakens to become just an irregularity, and some other swirl may become dominant. Sometimes when the new swirl grows, it returns the Earth to the same north-south polarity, but sometimes the new polarity is a reverse of the old one. Computers have simulated reversals, too.
Tidbit: The evidence of magnetization of the seafloor suggests that in the era of dinosaurs, there was a stretch of tens of millions of years, a "superchron", when no reversals occurred at all (usual intervals are bout half a million years). Interesting, but probably unrelated to dinosaurs.