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“The Earth Itself Is a Great Magnet”

This post was written by Science Reference Specialist Nate Smith.

Lately there has been a lot of discussion about the migration of the magnetic north pole.  The magnetic north pole is different from the geographic North Pole and is part of the much larger magnetic field of the Earth.  Not only do the magnetic poles shift, they reverse so that north becomes south and south becomes north!  Some information we read seems dire, especially when taken at face value.  But in order to understand the significance of that movement, we need to understand how the magnetic field of the Earth operates and has changed over time.

Title page of De Magnete by William Gilbert. This tome helped to reshape how Europeans viewed the Earth and magnetism.

William Gilbert famously wrote “Magnus magnes ipse est globus terrestris,” or “The Earth itself is a great magnet.”  This is due to the inner workings of the place we call home.  The structure of the Earth, moving up from the center, is as follows: 1) the solid inner core, believed to be composed primarily of iron; 2) the liquid outer core, also believed to be composed primarily of iron; 3) the mantle, composed of silicate rocks; and 4) an outer silicate crust.  The part that comes into play for the Earth’s magnetic field is the liquid outer core, that sloshing molten iron that is still cooling from the formation of the planet.  Today it is generally accepted that the Earth is what is known as a geodynamo, where an electrically conductive fluid, through thermal convection, circulates around the solid inner core.  This fluid circulation becomes a corkscrew due to the rotation of the Earth by what is known as the Coriolis Effect, which also causes the helical spin of hurricanes on the surface.  The best way to picture what happens in the outer core is to watch a lava lamp, where the mineralized oil in the center is heated by the light source and travels to the top of the lamp, cooling on its journey.  Once it cools past a particular temperature, it sinks back to the bottom and is reheated by the light source only to circulate once again.  In the outer core, it is this very movement that creates the magnetic field.

This magnetic field has been one of the most misunderstood phenomena throughout recorded history.  Many theories on magnetism have been around since the ancient Greeks, and the ancient Chinese even used magnetized rocks, called lodestones, shaped into spoons to harmonize their environments and lives and began using the compass for navigational purposes as early as the 11th century CE.  Europeans followed suit shortly and by Columbus’ voyage of 1492 had figured out that when the compass pointed north, it wasn’t necessarily pointing to the geographic North Pole.  In fact, according to Campbell, Columbus’ voyage was “probably the first documented observation of the change in declination with changing longitude,” declination being the angle between magnetic north and geographic north that changes as you move either east or west.

Since the 15th century, navigators and scientists alike have taken magnetic measurements in hopes of creating a true longitudinal chart of the earth.  According to Turner, by the end of the 18th century, mariners were routinely measuring and recording the angles of magnetic declination and inclination at locations throughout their voyages.  This lead to numerous nautical charts that were obsolete within years of being distributed.

There were several theories as to why declination happened.  One prevalent theory early on was that there were lodestone mountains at the poles causing the declination from true north.  Even though Peregrinus argued against this in his 1269 Epistola de Magnete, the theory lasted several centuries.  There was a theory that the magnetic poles were moving in a circular motion around the geographic poles at 10° latitude.  Edmond Halley, most notably of comet fame, came to the conclusion that something within the earth itself was causing declination and theorized that four magnetic poles existed.  Two of these poles were located in the southern hemisphere close to the south pole and two in the northern hemisphere close to the north pole.

This era saw the birth of what we now call the field of geomagnetism.  The work of many scientists aided in the discovery of the shifting magnetic field of the Earth, but it wasn’t until the discovery by Bernard Brunhes in 1906 in a small town in France that the world first learned of an entire magnetic field reversal.  Several years later, Motonori Matuyama supported this discovery with his own evidence of a magnetic field reversal and because of their monumental contributions to the field, the last reversal event, which happened some 780,000 years ago, is called the Brunhes-Matuyama reversal.

These reversals have been happening roughly every 200,000 years going back eons into Earth’s history and our magnetic poles continuously shift.  The difference between then and now is that our society is heavily reliant on electronic technology.  But reversals take hundreds to thousands of years and by that time, we will have figured out how to protect our electronic infrastructure.  In short, in terms of geology the path that the magnetic north pole is currently on is the rule, rather than the exception.

For further reading, I recommend beginning with the following resources:

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