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Why the Magnetic North Pole moves

 Over the past 30 years the magnetic North Pole (MNP) has moved almost the same distance as it had during the previous 180 years.

With this, very curious behaviour, in mind I thought it would be interesting to look at:

  • What creates the Earth’s Magnetic Field
  • Why has the Magnetic moved

The fact that the magnetic field has moved is nothing new; it’s perfectly normal.

Throughout geological history the MNP has at times been about where it is now; roughly following the axis of rotation, where the South Pole is now and at all points between the two.

These “flips” in field orientation can be confirmed anywhere new land has formed.  For example, either side of mid-ocean ridges small magnetic minerals, in the cooling erupted lava, will align in the direction of the Earth’s magnetic field. These magnetic anomalies can be read and the direction of the magnetic field lines, at the time the land was created, can be observed.

Rather than discussing large time-scales, it may be more useful to start on June 1st 1831. This is when James Clark Ross led the first group of Europeans to the MNP – I assume the local Inuit people had been there many times before this date. Seventy three years later the Norwegian explorer Roald Amundsen returned to the same spot and was surprised to find that he wasn’t at the MNP. Using a Dip Compass to navigate, he had to travel North West for around 40 miles until he found the MNP.

Note: a dip compass has a needle which rotates vertically, rather than horizontally like in a hand held compass.  This is useful at the Poles as the magnetic fields (to which the compass needle aligns) go straight up, at the South Pole or down at the North Pole.

What creates the Earth’s Magnetic Field

The reason the Earth has a Magnetic field is that the center of the Earth is hot. Or, to be more precise, it is the difference in temperature, between the various parts of the inner Earth which generates the field.

There are a number of reasons for the heat such as the remaining heat from the planet’s formation, friction between core materials, etc. However the main heating process is the radioactive decay of natural isotopes such as potassium, uranium and thorium.

It is often thought (and, sadly, still taught in some schools) that the Earth’s magnetic field is generated in it’s solid inner core. This is not correct.

The solid inner core of the Earth consists of mainly Iron which has a temperature of around 6,000K – this is well above 1043K which is Currie Point of Iron. The Currie Point is the temperature above which it is not capable of being magnetised – so the Earth’s magnetic field is not created in the Inner Core.

Surrounding the Inner Core is the liquid Outer Core which is for the most part Iron Nickel alloy, along with other materials such as Sulphur and other trace materials: it is here that the magnetic field is generated.

The process is quite complex, however it can be loosely stated as follows:

  • The Earth radiates heat into space, this cools the whole planet including the the solid Inner Core, which enlarges as it cools – as some liquid Iron, from the liquid Outer Core, solidifies at the boundary.
  • This leaves other lighter elements near the boundary which rise up in erratic convection currents of molten material.
  • When the plumes rise they stretch, becoming thinner and (like an ice skater pulling in their arms) start to rotate.
  • As the trillions of magnetic fields in the liquid Outer Core come into contact with the spiralling molten currents they are stretched (around the spiral) and increased in strength.
  • As the fields are generated faster than they are diffused (by resistance in the core and mantle) the Earth acts as a dynamo, producing many fields which combine to form a self generating magnetic field on a planetary scale.

Why has the Magnetic moved?

For most of the past few hundred years the MNP has been meandering around Northern Canada at just under a leisurely 10mph per year then, around 1990, it suddenly sped up to around 35mph – this may not sound much but remember that here we’re describing continental size features. 

Thanks to a number of satellites equipped with geomagnetic data collection devices (Oersted, CHAMP, Swarm, etc.) we now have a real idea of what is happening both above and below ground.

Satellite scans have shown that two extremely large up-swellings of rotating magnetic lignified rock in the Mantle, in Canada and Siberia, are the main cause of the wandering MNP.

There is a (very) large magnetic patch which stretches under Canada and Siberia.  There are two major Lobes (up-swellings of magnetic material) at either end of the patch, with a massive conduit (subterranean flow) between the Canadian and Siberian Lobes.  This allowed the molten (magnetic) material to flow out of the Canadian lobe into the Siberian lobe.

The result of this is that the MNP has been “pulled” towards Siberia

Patch Lobes

The red dot indicates the position of the MNP

Year 1990

Year 2020

The MNP has moved 686 miles and it’s still going.

 It is not so much the Siberian Lobe has increased: it has only risen in strength from −60.5 μT to −60.6 μT, but rather that, during the 20 year period, the Canadian Lobe has reduced significantly from −59.6 μT to −58.0 μT (a 2.6% drop).

Note: Magnetic field strength is measured by a gaussmeter.  1 μT (microtesla) = 10 mG (milligauss)

Surface magnetic field

The deeper yellow indicates field strength

Year 1990

Year 2020

Note that the Siberian field has remained, roughly the same, whilst Canadian has decreased so the Siberian field “pulls” the MNP towards it.

 How does this affect us when navigating around the UK?

As you may know a compass needle follows local magnetic fields which generally align towards with the MNP, so if it moves, so will the direction the needle points.  This deviation needs to be accounted for when navigating or, when following a compass bearing, you’ll walk in the wrong direction. 

On all Map Reading Co courses we teach how to compensate for the movement of the MNP.

Line A is the direction your compass would have pointed at 20 years ago.

Line B is where a compass points in 2020

So, for any old style navigators out there, the old “mag to grid”…….. has been well and truly “got rid”.  I honestly don’t know what the new rhyme (mnemonic) will be – any ideas?