Magnetism discovered in the Earth's mantle

Magnetism discovered in the Earth’s mantle
his is what the Earth inside looks like: Deep down lies the core of the Earth, followed by the Earth's mantle. The Earth's crust begins 35 kilometres below the surface. Credit: Peter Eggermann / Adobe Stock

The huge magnetic field which surrounds the Earth, protecting it from radiation and charged particles from space—and which many animals even use for orientation purposes—is changing constantly, which is why geoscientists keep it constantly under surveillance. The old well-known sources of the Earth's magnetic field are the Earth's core—down to 6,000 kilometres deep down inside the Earth—and the Earth's crust: in other words, the ground we stand on. The Earth's mantle, on the other hand, stretching from 35 to 2,900 kilometres below the Earth's surface, has so far largely been regarded as "magnetically dead." An international team of researchers from Germany, France, Denmark and the U.S. has now demonstrated that a form of iron oxide, hematite, can retain its magnetic properties even deep down in the Earth's mantle. This occurs in relatively cold tectonic plates, called slabs, which are found especially beneath the western Pacific Ocean.

"This new knowledge about the Earth's mantle and the strongly magnetic region in the western Pacific could throw new light on any observations of the Earth's ," says mineral physicist and first author Dr. Ilya Kupenko from the University of Münster (Germany). The new findings could, for example, be relevant for any future observations of the magnetic anomalies on the Earth and on other planets such as Mars. This is because Mars has no longer a dynamo and thus no source enabling a strong magnetic field originating from the core to be built up such as that on Earth. It might, therefore, now be worth taking a more detailed look on its mantle. The study has been published in Nature.

Background and methods used

Deep in the metallic core of the Earth, it is liquid iron alloy that triggers electrical flows. In the outermost crust of the Earth, rocks cause magnetic signal. In the deeper regions of the Earth's interior, however, it was believed that the rocks lose their due to the very and pressures.

The researchers now took a closer look at the main potential sources for magnetism in the Earth's mantle: iron oxides, which have a high critical temperature—i.e. the temperature above which material is no longer magnetic. In the Earth's mantle, iron oxides occur in slabs that are buried from the Earth's crust further into the mantle, as a result of tectonic shifts, a process called subduction. They can reach a depth within the Earth's interior of between 410 and 660 kilometres—the so-called transition zone between the upper and the lower mantle of the Earth. Previously, however, no one had succeeded in measuring the magnetic properties of the iron oxides at the extreme conditions of pressure and temperature found in this region.

Magnetism discovered in the Earth’s mantle
The interior of the Earth and the experiment graphically illustrated. The blue dotted lines show the magnetic field surrounding the Earth. The researchers pressed and heated samples of the iron oxide hematite found in the Earth's mantle between two diamonds (right) to simulate the extreme conditions in the Earth's mantle. They observed that the iron oxide is magnetic under these conditions. Credit: Timofey Fedotenko

Now the scientists combined two methods. Using a so-called diamond anvil cell, they squeezed micrometric-sized samples of iron oxide hematite between two diamonds, and heated them with lasers to reach pressures of up to 90 gigapascal and temperatures of over 1,000 °C (1,300 K). The researchers combined this method with so-called Mössbauer spectroscopy to probe the magnetic state of the samples by means of synchrotron radiation. This part of the study was carried out at the ESRF synchrotron facility in Grenoble, France, and this made it possible to observe the changes of the magnetic order in iron oxide.

The surprising result was that the hematite remained magnetic up to a temperature of around 925 °C (1,200 K) – the temperature prevailing in the subducted slabs beneath the western part of Pacific Ocean at the Earth's transition zone depth. "As a result, we are able to demonstrate that the Earth's mantle is not nearly as magnetically 'dead' as has so far been assumed," says Prof. Carmen Sanchez-Valle from the Institute of Mineralogy at Münster University. "These findings might justify other conclusions relating to the Earth's entire magnetic field," she adds.

Relevance for investigations of the Earth's magnetic field and the movement of the poles

By using satellites and studying rocks, researchers observe the Earth's magnetic field, as well as the local and regional changes in magnetic strength. Background: The geomagnetic poles of the Earth—not to be confused with the geographic poles—are constantly moving. As a result of this movement they have actually changed positions with each other every 200,000 to 300,000 years in the recent history of the Earth. The last poles flip happened 780,000 years ago, and last decades scientists report acceleration in the movement of the Earth magnetic poles. Flip of magnetic poles would have profound effect on modern human civilisation. Factors which control movements and flip of the magnetic poles, as well as directions they follow during overturn are not understood yet.

One of the poles' routes observed during the flips runs over the western Pacific, corresponding very noticeably to the proposed electromagnetic sources in the Earth's mantle. The researchers are therefore considering the possibility that the magnetic fields observed in the Pacific with the aid of rock records do not represent the migration route of the poles measured on the Earth's surface, but originate from the hitherto unknown electromagnetic source of hematite-containing rocks in the Earth's mantle beneath the West Pacific.

"What we now know—that there are magnetically ordered materials down there in the Earth's —should be taken into account in any future analysis of the Earth's magnetic and of the movement of the poles," says co-author Prof. Leonid Dubrovinsky at the Bavarian Research Institute of Experimental Geochemistry and Geophysics at Bayreuth University.

Explore further

New insight into Earth's crust, mantle and outer core interactions

More information: I. Kupenko et al. Magnetism in cold subducting slabs at mantle transition zone depths, Nature (2019). DOI: 10.1038/s41586-019-1254-8
Journal information: Nature

Citation: Magnetism discovered in the Earth's mantle (2019, June 6) retrieved 17 September 2019 from
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User comments

Jun 06, 2019
So, in addition to the geo-dynamo multi-poles, there are 'fixed' contributors, too ?

As part of subducted plate of a certain age finally reaches transition temperature, its remnant magnetic contribution is lost, causing magnetic polar wander...

Jun 06, 2019
Not a bad conjecture, @Nik, and it may be something these scientists are thinking of too. The critical temperature is also called the Curie Point. That will help you look it up.

Jun 08, 2019
In the Earth's core exists an electric current

The Earth's core simmers at 6000° C
Where as in Earth's mantle
Iron oxides occur
Iron oxide hematite
Hematite remains magnetic up to a temperature 1000°C
Therein lies a temperature gradient
From liquid mantle to the ground underfoot
Liquid iron oxide hematite to solid iron oxide hematite
With convection currents
Stirring this cauldron
This cauldron is causing this flow of electrons
Electrons in liquid iron oxide hematite
In the Earth's mantle exists an electric current
For to produce an electric current
In a moving iron oxide hematite
Requires a magnetic field
For simply
Pumping liquid iron oxide hematite
In a circular crucible
Does not in the presence of the earth's field
Produce an electric current to flow
Not one that is measurable in this industrial forge
The question
How is this earth's magnetic field able to induce an electric current?

For in the Earth's core exists an electric current

Jun 08, 2019
This Mysterious Earthly Phenomenon - Magnetism

Whether solid
Whether liquid
Iron oxide hematite
Simply put into motion
As in Sir Isaac Newton's first law
Will not produce and electric current
Unless acted on by a force
As this force is magnetism
Unless moving liquid iron oxide hematite
Moves through a magnetic field
It will not produce an electric current
Conversely no electric current flowing
Means no magnetic field
As not having this magnetic field in the first instance
As it requires this magnetic field
Brings into focus
This Mysterious Earthly Phenomenon - Magnetism

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