Earth's inner core is melting... and freezing

May 18, 2011
Earth

The inner core of the Earth is simultaneously melting and freezing due to circulation of heat in the overlying rocky mantle, according to new research from the University of Leeds, UC San Diego and the Indian Institute of Technology.

The findings, published tomorrow in Nature, could help us understand how the inner formed and how the outer core acts as a 'geodynamo', which generates the planet's magnetic field.

"The origins of Earth's magnetic field remain a mystery to scientists," said study co-author Dr Jon Mound from the University of Leeds. "We can't go and collect samples from the centre of the Earth, so we have to rely on surface measurements and computer models to tell us what's happening in the core."

"Our new model provides a fairly simple explanation to some of the measurements that have puzzled scientists for years. It suggests that the whole dynamics of the Earth's core are in some way linked to , which isn't at all obvious from surface observations.

"If our model is verified it's a big step towards understanding how the inner core formed, which in turn helps us understand how the core generates the Earth's magnetic field."

The Earth's inner core is a ball of solid iron about the size of our moon. This ball is surrounded by a highly dynamic outer core of a liquid iron-nickel alloy (and some other, lighter elements), a highly viscous mantle and a solid crust that forms the surface where we live.

Over billions of years, the Earth has cooled from the inside out causing the molten iron core to partly freeze and solidify. The inner core has subsequently been growing at the rate of around 1mm a year as iron crystals freeze and form a solid mass.

The heat given off as the core cools flows from the core to the mantle to the Earth's crust through a process known as convection. Like a pan of water boiling on a stove, convection currents move warm mantle to the surface and send cool mantle back to the core. This escaping heat powers the geodynamo and coupled with the spinning of the Earth generates the magnetic field.

Scientists have recently begun to realise that the inner core may be melting as well as freezing, but there has been much debate about how this is possible when overall the deep is cooling. Now the research team believes they have solved the mystery.

Using a of convection in the outer core, together with seismology data, they show that heat flow at the core-mantle boundary varies depending on the structure of the overlying mantle. In some regions, this variation is large enough to force heat from the mantle back into the core, causing localised melting.

The model shows that beneath the seismically active regions around the Pacific 'Ring of Fire', where tectonic plates are undergoing subduction, the cold remnants of oceanic plates at the bottom of the mantle draw a lot of heat from the core. This extra mantle cooling generates down-streams of cold material that cross the outer core and freeze onto the inner core.

Conversely, in two large regions under Africa and the Pacific where the lowermost mantle is hotter than average, less heat flows out from the core. The outer core below these regions can become warm enough that it will start melting back the solid inner core.

Co-author Dr Binod Sreenivasan from the Indian Institute of Technology said: "If Earth's inner core is melting in places, it can make the dynamics near the inner core-outer core boundary more complex than previously thought.

"On the one hand, we have blobs of light material being constantly released from the boundary where pure crystallizes. On the other hand, melting would produce a layer of dense liquid above the boundary. Therefore, the blobs of light elements will rise through this layer before they stir the overlying outer core.

"Interestingly, not all dynamo models produce heat going into the inner core. So the possibility of inner core melting can also place a powerful constraint on the regime in which the Earth's dynamo operates."

Co-author Dr Sebastian Rost from the University of Leeds added: "The standard view has been that the inner core is freezing all over and growing out progressively, but it appears that there are regions where the core is actually melting. The net flow of heat from core to ensures that there's still overall freezing of material and it's still growing over time, but by no means is this a uniform process.

"Our model allows us to explain some seismic measurements which have shown that there is a dense layer of liquid surrounding the inner core. The localised melting theory could also explain other seismic observations, for example why seismic waves from earthquakes travel faster through some parts of the core than others."

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User comments : 10

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StillWind
1.2 / 5 (18) May 18, 2011
I read this malarkey, and I just have to wonder if anyone even took physics. Certainly, no one who understands, even basic physics would believe that crustal material could penetrate a more dense mantle, let alone "sink".
Please stop insulting my intelligence with this stuff.
Soylent_Grin
4.8 / 5 (17) May 18, 2011
Memo from the University of Leeds: "I know you geophysicists have your PhD's and all, but you should really clear your peer-reviewed submissions to Nature with this StillWind guy in the future. Apparently, you're insulting his intelligence."
omatumr
2.3 / 5 (9) May 18, 2011
I read this [stuff] and I just have to wonder if anyone even took physics . . .


Earth's inner core in fact accreted first out almost pure Fe-Ni produced near the the core of the supernova that gave birth to the solar system ~5 Gyr ago

It was probably layered with iron meteorites containing troilite inclusions (FeS). That may explain why the outer core is molten but the inner core is not.

It was then layered with the silicates that formed the mantle.

Only the upper mantle melted to produce the crust and atmosphere.

1. TUREKIAN, K. K. and CLARK, S. P., JR. (1969) Inhomogeneous accumulation of the
earth from the primitive solar nebula. Earth Planet. Sci. Lett. 6, 346-348.

2, "The noble gas record of the terrestrial planets", Geochemical Journal 15, 247-267 (1981).

http://www.omatum...eGas.pdf

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
mrlewish
5 / 5 (3) May 18, 2011
Why is the inner core of the Earth solid? Because melting temperature is dependent on pressure. The higher the pressure the higher the melting temperature. There is enough pressure that far down that what is liquid at a lesser pressure above turns to a solid. Simple.
TheFlynn
5 / 5 (1) May 19, 2011
I think it's comical when people inaccurately asses their intelligence in relation to others...

The article doesn't claim the subducted crustal material sinks to the core. It suggests that this cooler material cools the underlying molten core, inducing downward flow or convection of cooler material. It would also seem plausible that heavier material may precipitate from the subducted crust into the molten core, as well, though this is not mentioned in the article.
astro_optics
1 / 5 (3) May 19, 2011
in other words...they have no clue :(
mmead
1 / 5 (3) May 19, 2011
Wow really, the Earths inner core structure is more complicated than we originally believed...no way....
Please. I for one do not believe that our inner core is a solid iron chunk. No one really knows anyway. They can guess, theorize but really they have no clue.
In my opinion there is a geo-nuclear reaction at our core that is providing all the inner heat and magnetism and gravity that our planet needs.
woofwoofwoofoioioi
1 / 5 (3) May 19, 2011
Based on this article, I reckon the inner core must be shaped like a Sherrin.
El_Nose
5 / 5 (1) May 19, 2011
@Still WInd

Because of you most of us were forced to give Omatumr a high positive rating -- you obviously don't know what that means on this forum...

Nothing personally against you omatumr but hey , you know we love to fight you.

;-)

kind regards
El_Nose
not rated yet May 24, 2011
in retrospect omatumr thinks the core of almost everything in the solar sytem is pure iron -- rethinking my vote now