More than 5,000 kilometers beneath us, Earth’s solid metal inner core wasn’t discovered until 1936. Almost a century later, we’re still struggling to answer basic questions about when and how it first formed.
These aren’t easy puzzles to solve. We can’t directly sample the inner core, so the key to unraveling its mysteries lies in collaboration between seismologists, who indirectly sample it with seismic waves, geodynamicists, who create models of its dynamics, and mineral physicists, who study the behavior of iron alloys at high pressures and temperatures.
Combining these disciplines, scientists have delivered an important clue about what’s happening miles beneath our feet. In a new study, they reveal how Earth’s inner core is growing faster on one side than the other, which could help explain how old the inner core is, and the intriguing history of Earth’s magnetic field.
Earth’s core was formed very early in our planet’s 4.5 billion-year history, within the first 200 million years. Gravity pulled the heavier iron to the center of the young planet, leaving the rocky, silicate minerals to make up the mantle and crust.
Earth’s formation captured a lot of heat within the planet. The loss of this heat, and heating by ongoing radioactive decay, have since driven our planet’s evolution. Heat loss in Earth’s interior drives the vigorous flow in the liquid iron outer core, which creates Earth’s magnetic field. Meanwhile, cooling within Earth’s deep interior helps power plate tectonics, which shape the surface of our planet.
As Earth cooled over time, the temperature at the center of the planet eventually dropped below the melting point of iron at extreme pressures, and the inner core started to crystallize. Today, the inner core continues to grow at roughly 1mm in radius each year, which equates to the solidification of 8,000 tonnes of molten iron every second. In billions of years, this cooling will eventually lead to the whole core becoming solid, leaving Earth without its protective magnetic field.
One might assume that this solidification creates a homogeneous solid sphere, but this isn’t the case. In the 1990s, scientists realized that the speed of seismic waves traveling through the inner core varied unexpectedly. This suggested that something asymmetrical was happening in the inner core.
Specifically, the eastern and western halves of the inner core showed different seismic wavespeed variations. The eastern part of the inner core is beneath Asia, the Indian Ocean and the western Pacific Ocean, and the west lies under the Americas, the Atlantic Ocean, and the eastern Pacific.
The new study probed this mystery, using new seismic observations combined with geodynamic modeling and estimates of how iron alloys behave at high pressure. They found that the eastern inner core located beneath Indonesia’s Banda Sea is growing faster than the western side beneath Brazil.
You can think of this uneven growth as like trying to make ice cream in a freezer that’s only working on one side: ice crystals form only on the side of the ice cream where the cooling is effective. In the Earth, the uneven growth is caused by the rest of the planet sucking heat more quickly from some parts of the inner core than others.
But unlike the ice cream, the solid inner core is subject to gravitational forces which distribute the new growth evenly through a process of creeping interior flow, which maintains the inner core’s spherical shape. This means that Earth is in no danger of tipping, though this uneven growth does get recorded in the seismic wavespeeds in our planet’s inner core.
Dating the core
So does this approach help us understand how old the inner core might be? When the researchers matched their seismic observations with their flow models, they found that it’s likely that the inner core – at the center of the entire core which formed much earlier – is between 500 million and 1,500 million years old.
The study reports that the younger end of this age range is the better match, although the older end matches an estimate made by measuring changes in the strength of Earth’s magnetic field. Whichever number turns out to be correct, it’s clear that the inner core is a relative youngster, somewhere between a ninth and a third as old as Earth itself.
This new work presents a powerful new model of the inner core. However, a number of physical assumptions the authors made would have to be true for this to be correct. For example, the model only works if the inner core consists of one specific crystalline phase of iron, about which there is some uncertainty.
And does our uneven inner core make the Earth unusual? It turns out that many planetary bodies have two halves which are somehow different to each other. On Mars, the surface of the northern half is lower-lying while the southern half is more mountainous. The Moon’s near-side crust is chemically different to the far-side one. On Mercury and Jupiter it’s not the surface which is uneven but the magnetic field, which doesn’t form a mirror image between north and south.
So while the causes for all of these asymmetries vary, Earth appears to be in good company as a slightly asymmetrical planet in a solar system of lopsided celestial bodies.
- Jessica Irving – Senior Lecturer in Geophysics, University of Bristol
- Sanne Cottaar – Lecturer in Global Seismology, University of Cambridge
This article was first published in The Conversation.
For more on this research, read Is Earth’s Core Lopsided? Something Strange Is Going On in Our Planet’s Interior.
Reference: “Dynamic history of the inner core constrained by seismic anisotropy” by Daniel A. Frost, Marine Lasbleis, Brian Chandler and Barbara Romanowicz, 3 June 2021, Nature Geoscience.
“… leaving the rocky, silicate minerals to make up the mantle and crust.”
What is a “rocky” mineral? I tend to think of “rocky” as a textural term, such as in a “rocky terrain.” Do the authors mean “lighter,” or “rock-forming,” which is an often-used petrology term?
Not all of the primordial iron went to the core. At least five percent of the crust is still in the crust and has been oxidized. I suspect use of the term “rocky” for silicates was considered better understood than “lighter minerals.”
What annoys me are these people like Voldall who try to place ads in a scientific discussion.
Good Lord, I think this must have been written by the Congressman Hank Johnson who thought Guam would capsize if we built more housing on the military base.
I’ve always wondered what mechanism allowed the molten core to remain liquid after billions of years. It seems from this article that the primary factor is heat generated via radioactive decay?
“Theories about the age of the core are necessarily part of theories of the history of Earth as a whole. This has been a long-debated topic and is still under discussion at the present time. It is widely believed that the Earth’s solid inner core formed out of an initially completely liquid core as the Earth cooled down. However, there is still no firm evidence about the time when this process started.”
So, in general, we could say that the Earth’s axis changing would be the main factor behind climate change? After all, the seasonal path has changed due to the axis shift. If we consider the amount of snow on the land versus the water it would create if it melted, the sea level would only rise about 9 inches, due to a good portion being icebergs that the displacement is already compensated for. In short, those icebergs would never change the sea level. We could though by reducing waste and boats.
Tim, Tim, Tim !!!
Contradiction, contradiction, contradiction !!!
Quoting your one relevant paragraph:
“… snow on the land versus the water it would create if it melted, the sea level would only rise about 9 inches, due to a good portion being icebergs…”
You (correctly) acknowledge that
“icebergs” make zero difference to sea level — so how can “sea level… only rise about 9 inches, due to a good portion being icebergs…”?
Glaciers are conspicuously missing from these comments — glacier meltwater very much affects sea level rise — so Tim’s “9 inches” is a miniscule estimate, far too low, for Tim’s “snow on the land”
Climate is changing and crucially our atmoshere is becoming unbreathable or at least insufficient to sustain human health.
If suffocation doesn’t move the petrolhead Denialists then nothing peaceful will.
Not recent man made global warming, no, that is now a 3 sigma observation so beyond reasonable doubt.
You may be thinking of some earlier climate changes, where axis orientation has been a larger factor. Or you may be trying to convince yourself of some personal unreasonable doubt. But that should be done in private, don’t drag us into it.
The Eskimos elders state that the sun is not coming up in the same place. Weather is so complex who really knows. But it would not hurt to clean up the air in LA.
Okay, Tim, then how do you explain rising water levels if the ice is just replacing the water? The ice caps are melting indicating global warming.
See paper by Frank T Lee, Gondwana 8 Conference volume published by AA Balkema in 1993.An interesting approach to an argument for an off-centre core. Correct or not it is an interesting paper.
Found it – Lee rejects the existence of the viscous mantle and makes up “stress planes” from whole cloth. It is self announced as fringe, but looks like pseudoscience (no quantitative theory, cherry picked data).
Oh, and how would we know that this isn’t anonymous self promotion?
This is a very interesting article,and proves that the Earth’s core is the reason that our weather and planet is changing on how it rotates on it’s axis due to an uneven core which changes our gravitational pull ,not global warming.
How can not observed global warming be global warming?
And no, nothing of the sort you write is described in the discussed research.
Im very interested in earth humans and also space, I really thought this was very interesting and also important and can help me with some of my research.
How did they forget to consider the impact with Thea?
It is believed from modeling it that Earth gained the last few percent of mass and re-differentiated by the impact heat.
I think the earth is the sun’s baby for lack of a better term
It would be interesting to consider whether the pre-solid core formation produced a magnetic field that shielded earth from cosmic rays. It would be an externality affecting the ability of life to emerge.
In general magnetic fields – apart from Sun’s massive – do not protect from cosmic rays but solar wind and ejections. It is the atmosphere that shields us from the cosmic rays Sun did not manage to shield us from.
Besides, the current phylogeny (such as it is) imply life evolved in the early global ocean.
Because God made it fixed, immovable, and on foundations.
No, definitely not!
I would like to make a hypothesis about why the earth’s core is crystalizing. As the article suggests, the western half of the core is cooling down faster than the eastern half, under Asia and the western Pacific. The Marianas Trench has been described as a way to explain the cause of mid-ocean ridges, subduction is the ocean basin growing so dense over time as to collapse into the mantle and as it does it drags the crust with it. It creates a flow of which all that comes after is compelled to follow, until I figure a equilibrium would be met. This is a cooling factor that most likely explains the eastern half of the core to crystallize more rapidly. Getting back to the equilibrium side of things, one day the reverse could occur, the Atlantic Ocean and the Juan de Fuca ridge, the Mediterranean, and the ridge that composes the Red Sea, the Great Rift Valley will wane. Many of the these could be able to create great volumes of magma at some point. The Siberia flood, the Deccan Traps. Boiling oceans, biblical proportions. There’s a lot of ground to consume from the Pacific basin. Perhaps the Pacific Oscillations are a result of this cooling affect and then it changes the gravity of the region. As a fluid change in a lake can effect the gravitational readings of the two satellites in space, I think they are called Grace so that could be taken as a causation for the welling of the Pacific Ocean.
Plate tectonics could affect and be affected by the growth.- But we don’t know the particulars yet.
Nitpick: Please don’t use superstitious terminology when discussing science.
I think the explanation to the core is that the mass of the surrounding rock being pulled inwards by this dense molten core is what if keeping it in liquid form. The pressure is so great the particles are undergoing fusion constantly generating heat and sliding about each other. We need to send down a submarine made of diamond to see what’s going on in that big burning hot ball of molten flame.
The generic hydrostatic equilibrium – round surface at certain radius – imply that heat (whether residual or radioactively produced) is flowing from the core and mantle out.
I know very little about this subject but I do know is that I would not trust the information in an article that has the same advertisement in it like 4 or 5 times.
That goes more towards trusting sites – and SciTechDaily was founded for educational purposes.
I believe in UFOs…We are just ants here on this little pressurized hot ball to be observed.
Conspiracy theory … that’s so wrong and boring to boot.
I believe it happens in cycles like climate change, the earth is still actually growing and ever changing like all living creatures. It adapts and reacts to reactions either man made or planet made .this is how the planet adapts The earth sort of heals itself like a living thing . These affects from the earth core magna pushing on seismic plates and releasing gases may cause planet shifting currents warming and cooling in other oceans causing weather patterns to change dramictly it’s inevitable for ice ages and position shifts poles moving further north or south who knows one day .All it needs is an catastrophic event,such an asteroid or major seismic eruptions which can be brought on by man’s interference. But no doubt th e shifts and changes kilter from time to time
Humans can’t cause impacts or volcanoes.
Look it up.
What if black holes are actually creating a planet.if you look at the core and the spyrl spinning like spinning a web but similar
Assuming the inner core formed about 500 mya and we think the Cambrian Exploslon of life form types occurred about the same time, I wonder if there’s any connection?