Geomagnetic Reversal and Geomagnetic Poles
Geography › Interior of the Earth and Magnetic Field · Topic 2.7
Geomagnetic poles are the ends of the axis of the best-fitting dipole at the centre of the Earth, so they are exactly opposite each other, unlike the wandering magnetic poles. A geomagnetic reversal is the flip of the field in which north and south magnetic polarity swap, and it has happened many times, irregularly, with the last full reversal about 780,000 years ago.
5 min read · UPSC Prelims · Geography · Interior of the Earth and Magnetic Field
At a Glance
- Geomagnetic poles Ends of the best-fit dipole axis; antipodal
- Reversal Swap of north and south polarity
- Frequency Irregular; a few times per million years on average
- Last full reversal About 780,000 years ago
- Evidence Magnetic minerals in rocks and ocean-floor stripes
- Exam link UPSC Prelims, Geography: Interior of the Earth and Magnetic Field
Where Does It Fit?
This is the seventh topic of the category on the interior of the Earth and its magnetic field. The previous topic described magnetic poles, declination and dip, and this one separates the geomagnetic poles and explains reversals. The next topic covers the magnetosphere, auroras and the Van Allen belts. Previous topic: Magnetic Poles, Declination and Inclination. Next topic: Magnetosphere, Auroras and Van Allen Radiation Belts.
What Are the Geomagnetic Poles?
In short: the geomagnetic poles belong to an idealised bar magnet, whereas the magnetic poles belong to the real, irregular field.
The real magnetic field of the Earth is not a perfect dipole, because of local irregularities from the flows in the core and from rocks in the crust. Scientists therefore fit an imaginary bar magnet through the centre of the Earth that best matches the field, and the points where its axis meets the surface are the geomagnetic poles. These poles are exactly opposite each other, since they are the ends of a straight line through the centre, and they lie at about 80 degrees N and 80 degrees S, with the north geomagnetic pole in northwest Greenland. The magnetic poles, in contrast, are where the real field lines are vertical, as described in Magnetic Poles, Declination and Inclination. The geomagnetic poles are used in studies of the upper atmosphere, because the auroras and the radiation belts follow geomagnetic and not geographic coordinates.
What Is a Geomagnetic Reversal?
The magnetic field of the Earth has not always had its present direction. A geomagnetic reversal is a change in which the north and south magnetic poles swap places, so a compass needle would point south. During a reversal the field weakens, becomes complex with several local poles, and then re-establishes itself with the opposite polarity. Reversals are irregular: some periods of stable polarity, called chrons, last for a few hundred thousand years, while others last for tens of millions of years. The present normal polarity is the Brunhes chron, which began with the Brunhes-Matuyama reversal about 780,000 years ago. A partial change that does not complete, called a magnetic excursion, occurred about 41,000 years ago in the Laschamp event.
| Term | Meaning |
|---|---|
| Normal polarity | Field as at present |
| Reversed polarity | Compass would point to the south |
| Chron | Interval of stable polarity |
| Excursion | Short, incomplete swing of the field |
How Do We Know About Past Reversals?
Igneous rocks such as basalt contain tiny crystals of magnetic minerals, such as magnetite, that line up with the Earth’s field when the rock cools and then hold that direction. This is fossil magnetism, studied in paleomagnetism. Layers of rocks of different ages therefore show the polarity of the field at the time they formed. Symmetric stripes of alternating polarity on either side of mid-ocean ridges provided strong evidence for seafloor spreading, as seen in Seafloor Spreading and Paleomagnetism. The Middle Pleistocene stage of the geological time scale, the Chibanian, was named in 2020 after a site in Chiba, Japan, which preserves a clear record of the last reversal.
Is the Field Weakening and Is a Reversal Due?
The strength of the field has declined by about 9 per cent over the last 170 years, and a region of very weak field, the South Atlantic Anomaly, has grown. Some people have taken this as a sign of an approaching reversal, but most geophysicists note that the field has weakened in the past without reversing, that the present strength is still within the normal range, and that a reversal takes thousands of years, not a human lifetime. Reversals are also not linked to mass extinctions according to the geological record. As an analytical point, the main risks of a weaker field are practical: more radiation for satellites and a greater effect of solar storms on power grids and communication, rather than any direct danger to life on the surface.
How Do Geomagnetic Poles Differ from Magnetic Poles?
| Feature | Magnetic poles | Geomagnetic poles |
|---|---|---|
| Basis | Actual field, where lines are vertical | Idealised dipole through the centre |
| Position relation | Not exactly opposite each other | Exactly opposite each other |
| Movement | Faster and irregular | Slower and smoother |
| Used for | Compass navigation | Space physics, auroras |
The source of the field in the core is described in Dynamo Theory and the Magnetic Field of the Earth.
Why Is This Topic Important?
- Paleomagnetism Evidence for plate movement
- Pole types Frequent confusion
- Current affairs Field weakening in news
What Came Next?
The next topic describes the magnetosphere, auroras and the Van Allen radiation belts. Next topic: Magnetosphere, Auroras and Van Allen Radiation Belts.
Key People and Terms
- Geomagnetic pole End of best-fit dipole axis
- Reversal Flip of polarity
- Chron Stable polarity interval
- Excursion Incomplete reversal
- Paleomagnetism Study of ancient magnetism in rocks
- Magnetite Magnetic iron oxide mineral
Exam Corner
Points to Remember
- Geomagnetic poles are exactly opposite each other.
- Last full reversal was about 780,000 years ago.
- Rocks record the field as fossil magnetism.
- Ocean floor shows alternating stripes.
- The field has weakened about 9 per cent in 170 years.
Do Not Confuse With
- Magnetic and geomagnetic poles Real field versus idealised dipole
- Reversal and excursion Complete flip versus short swing
- Reversal and pole shift Polarity swap versus drift or rotation of the axis
Memory Hook
Rocks remember the field; poles flip rarely; geomagnetic is the ideal, magnetic is the real.
Mains Angle
Questions link reversals to plate tectonics. These points add depth.
- Record Rocks preserve polarity
- Stripes Support seafloor spreading
- Risk Weaker field affects technology
A Question You May Face
An original practice question, not a past paper question.
“What is a geomagnetic reversal? How does paleomagnetism provide evidence for seafloor spreading?”
How to Answer
- Define Reversal and polarity.
- Evidence Magnetic minerals.
- Link Ocean-floor stripes.
- Present Field weakening.
GS Relevance
Frequently Asked Questions
What are the geomagnetic poles?
The geomagnetic poles are the points where the axis of the best-fitting dipole, an imaginary bar magnet at the centre of the Earth, meets the surface. They are exactly opposite each other, unlike the magnetic poles, which are where the real field lines are vertical.
What is a geomagnetic reversal?
A geomagnetic reversal is a change in which the north and south magnetic poles swap, so a compass would point to the south. The field weakens, becomes complex and then re-establishes with opposite polarity. Reversals are irregular, and the last full one was about 780,000 years ago.
How do scientists know that reversals happened?
Magnetic minerals such as magnetite in cooling igneous rocks line up with the field of the time and keep that direction. Layers of rock of different ages, and the stripes of rock on the seafloor on either side of mid-ocean ridges, record the alternating polarity.
Is the magnetic field of the Earth weakening?
The field has weakened by about 9 per cent in the last 170 years or so, and the South Atlantic Anomaly has grown. However, it has weakened without reversing before, and a reversal would take thousands of years, so there is no immediate danger.
What is the difference between a reversal and a pole shift?
A reversal is a swap of magnetic polarity over thousands of years, while the drift of the magnetic poles is a slow movement of their position within the same polarity. A change in the rotation axis of the Earth is a separate matter altogether.
PYQ Practice — Statement Analysis
1 The geomagnetic poles are exactly opposite each other on the globe.
They are ends of the dipole axis.
2 Geomagnetic reversals occur at regular intervals.
They are irregular.
3 Magnetic minerals in basalt preserve the direction of the field when it formed.
This is paleomagnetism.
4 The last full geomagnetic reversal took place about 78,000 years ago.
It was about 780,000 years ago.
Related Topics
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