Reversal of the Earth's Magnetic Poles


NASA Goddard Space Flight Center/CC BY 2.0/Flickr 

In the 1950s, ocean-going research vessels recorded puzzling data based on the magnetism of the ocean floor. It was determined that the rock of the ocean floor had bands of embedded iron oxides that alternately pointed toward geographic north and geographic south. This was not the first time such confusing evidence had been found. In the early 20th century, geologists had found some volcanic rock was magnetized in a manner opposite to what was expected. But it was the extensive 1950s data that prompted a widespread investigation, and by 1963 a theory of the reversal of the earth's magnetic field was proposed. It has been a fundamental of earth science ever since.

How the Earth's Magnetic Field Is Created

The earth's magnetism is thought to be created by slow movements in the liquid outer core of the planet, which consists largely of iron, caused by the rotation of the earth. Much the way the rotation of a generator coil creates a magnetic field, the rotation of the liquid outer core of the earth generates a weak electromagnetic field. This magnetic field extends out into space and serves to deflect solar wind from the sun. The generation of the earth's magnetic field is a continuous but variable process. There is a frequent change in the intensity of the magnetic field, and the precise location of the magnetic poles can drift. True magnetic north does not always correspond to the geographic North Pole. It also can cause the complete reversal of the earth's entire magnetic field polarity.

How We Can Measure Magnetic Field Changes

Liquid lava, which hardens into rock, contains grains of iron oxides that react to the earth's magnetic field by pointing toward the magnetic pole as the rock solidifies. Thus, these grains are permanent records of the location of the earth's magnetic field at the time the rock forms. As new crust is created on the ocean floor, the new crust solidifies with its iron oxide particles acting like miniature compass needles, pointing to wherever magnetic north is at the time. Scientists studying the lava samples from the bottom of the ocean could see that the iron oxide particles were pointing in unexpected directions, but to understand what this meant, they needed to know when the rocks formed, and where they were located at the time they solidified out of liquid lava. 

The method of dating rock through radiometric analysis has been available since the early 20th century, so it was an easy enough matter to find the age of the rock samples found on the ocean floor

However, it was also known that the ocean floor moves and spreads over time, and it was not until 1963 that rock aging information was combined with information about how the ocean floor spreads to produce a definitive understanding of where those iron oxide particles were pointing at the time the lava solidified into rock. 

Extensive analysis now shows that the earth's magnetic field has reversed about 170 times over the last 100 million years. Scientists continue to evaluate data, and there is much disagreement over how long these periods of magnetic polarity last and whether the reversals happen at predictable intervals or are irregular and unexpected.

What are the Causes and Effects?

Scientists do not really know what causes the reversals of the magnetic field, although they have duplicated the phenomenon in laboratory experiments with molten metals, which also will spontaneously change the direction of their magnetic fields. Some theorists believe that magnetic field reversals may be caused by tangible events, such as tectonic plate collisions or impacts from large meteors or asteroids, but this theory is discounted by others. It is known that leading up to a magnetic reversal, the strength of the field declines, and since the strength of our current magnetic field is now in steady decline, some scientists believe we will see another magnetic reversal in about 2,000 years. 

If, as some scientists suggest, there is a period during which there is no magnetic field at all before the reversal occurs, the effect on the planet is not well understood. Some theorists suggest that having no magnetic field will open the earth's surface to dangerous solar radiation that potentially might lead to global extinction of life. However, there is currently no statistical correlation that can be pointed to in the fossil record to verify this. The last reversal occurred about 780,000 years ago, and there is no evidence to show that there were mass species extinctions at that time. Other scientists argue that the magnetic field does not vanish during reversals, but merely grows weaker for a time.

Although we have at least 2,000 years to wonder about it, if a reversal were to occur today, one obvious effect would be mass disruption to communications systems. Much the way solar storms can affect satellite and radio signals, a magnetic field reversal would have the same effect, though to a much more pronounced degree. 

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Your Citation
Rosenberg, Matt. "Reversal of the Earth's Magnetic Poles." ThoughtCo, Aug. 28, 2020, Rosenberg, Matt. (2020, August 28). Reversal of the Earth's Magnetic Poles. Retrieved from Rosenberg, Matt. "Reversal of the Earth's Magnetic Poles." ThoughtCo. (accessed March 22, 2023).