In spite of being posted on VC, the word ‘volcano’ does not appear in this post. Or at least, it wouldn’t have had I not written this preface. Which is why I wrote it, since we can not have a VC post which lacks the word ‘volcano’. (Of course now that I have mentioned ‘volcano’ twice (sorry, thrice), this apologetic preface is no longer needed!) This post is on popular request, as part III of the 2-part series on plumes and hot spots. I was asked to dig deeper. Here we go: to explore the core.
Imagine digging through a glacier of ice to a sea below. We are thinking about doing just this on Europa, the moon of Jupiter: it is covered by a 50-km thick layer of ice, with a liquid water ocean below. The only way to get into the ocean is by digging. In fact, we now know that many of the larger moons in the outer solar system contain such buried oceans, worlds of water we knew nothing about. There is a suggestion that even Pluto may have one, although that is rather speculative. And the Earth too has a hidden ocean underneath its coat of rock, at a depth of 2890±3 km below the surface, or 3485 km from the centre of the Earth.
But what a strange ocean it is. The dense rock of the mantle, denser then the densest basalt on the surface, floats on it. The ocean is hot. While the bottom of the mantle has a temperature of perhaps 2200 °C, the liquid below is more than 3500 °C – perhaps even as high as 4200 °C. There is a jump in temperature of at least 1300 °C! What on Earth is this liquid?
As befitting the surface of an ocean, the core-mantle boundary is flat, varying by less than 10 km in height over its surface. Most of that variation stems from a bulge at the equator, caused by the rotation of the Earth. We see the same effect on the surface of the Earth where the equator is further from the centre of the Earth than the poles are. (One of those funny sentences where the meaning changes if you write one word with a capital.) The local topography varies by no more than 500 meters from place to place. The top of the core coincides with the bottom of the Dʺ layer – the top of the Dʺ layer is much less flat. The Dʺ layer is one of the mysteries of the Earth. One suggestion is that is partly melted by the excessive heat from below, and perhaps even chemically altered by the interaction. Another suggestion (see below) is that it comes from a violent interaction. But interaction with what?
The outer core
The earliest speculation about the core of the Earth dates from long before its discovery. The story begins in 1798 when the first calculation of the density of the Earth were done. Remember that density equals mass divided by volume. We know the volume of the Earth very accurately (well – that excludes the flat-earthers who haven’t solved that basic problem yet). But how do we know the mass? Isaac Newton showed us how to do it: we can use the orbit of the Moon to calculate our mass. That is not at easy as it sounds. Newton’s law of gravity (derived from Kepler’s laws) only gives the product GM, where M is the mass and G is the ‘gravitational constant’ which you still need to measure. You do that by measuring the force of gravity between two large metal balls. It is a very difficult experiment because gravity is such a weak force. You may feel all kinds of forces between you and the person sitting next to you, attraction, repulsion, electric shock from their jumper, magnetic force from their iron piercings, but gravity is not one of them. Even if that person is of above average weight, gravitational attraction will still not move you. You would need to have the total human population of the world at 2 meter from you before the attraction you felt would reach even 1% of the Earth’s gravitational force. No, neither emotional attraction (or repulsion) nor peer pressure need to give any consideration to the gravity of the situation.
Still, people did manage to measure at least an approximate value for G. As early as 1798, this gave a mass of the Earth accurate to a few per cent (in hindsight). And as the volume of a spherical Earth is easy to calculate, this gave them the density of the Earth. This turned out to be 5.5 times that of water, or 5500 kg/m3 if you prefer. And that was a problem.
The densest rocks that we see at the surface are mid-oceanic basalts. They have a density approaching 3000 kg/m3. The measurement showed that the Earth overall was almost twice as dense. This could only be if a large part of the Earth consisted of something much denser than rock. Something that was very different from what we found near the surface.
The idea that the Earth had an iron centre developed at the end of the 19th century. It was based on the fact that some meteorites consisted of iron rather than rock – so perhaps the Earth was like an iron-nickel meteorite with a thick coating of rock. However, our actual knowledge really started with the work of Richard Oldham in 1906. In his paper, he writes “Many theories of the earth have been propounded at different times: the central substance of the earth has been supposed to be fiery, fluid, solid, and gaseous in turn, till geologists have turned in despair from the subject, and become inclined to confine their attention to the outermost crust of the earth, leaving its centre as a playground for mathematicians” before stating that “the subject is, at least partly, removed from the realm of speculation into that of knowledge by the instrument of research which the modern seismograph has placed in our hands”
He was in fact a seismologist, and had previously shown that earthquakes produce three different types of waves, now known as P, S and surface waves. In the 1906 paper, he begins with stating that P and S waves only becomes separated at around 1000 km from an earthquake, and that this ‘sorting out’ (as he calls it) can only happen in a more uniform material than found near the surface. In a spherical Earth, the line of travel to more distant locations goes deeper into the Earth. (I have no idea how flat-earthers accommodate this). At a distance of 1000 km, the waves arriving there had traveled through a deep region which is much better behaved than the crust. From his data, he found that the crust could be ‘no more than a few score of miles thick’, and that below it lies a different structure (now known as the mantle). (Oldham even shows that the crust is less thick around Japan than around Europe, so in a way he discovered the difference between oceanic crust and continental crust.) In the 1906 paper, he analyzes arrival times at seismic stations across the world, for a number of large earthquakes. They include the 1899 Alaska earthquake, and a large quake near the Moluccas islands in Indonesia in 1899 for which he used data from the mining engineer Rogier Verbeek – well known as the Krakatoa eruption expert, the only person who had had an inkling of the danger of that mountain.
Oldham’s result is shown above. Two curves are shown for arrival time versus distance, where the distance is measured in degrees (where 180 degrees is the opposite point on Earth). The bottom curve is for the faster waves (now known as P) and the upper one for the slower waves (or S). Up to a distance of 120 degrees, the change in arrival time is smooth. The more distant waves have traveled deeper into the Earth, and from the arrival times Oldham deduced that the waves traveled faster at greater depth but did so smoothly – there was no indication for a change in structure or composition, just changing pressure and temperature. But at 120 degrees, there is a sudden change. The waves slow down dramatically and there are distances where the waves are not seen at all, as in a shadow. From this, Oldham derived that the Earth must have a core with a very different structure to the outer region. His value for the radius of this core was approximately correct. What the core consisted of, he did not speculate on. Some claim that he discovered that the core was liquid, but that is not in the paper. In fact, that fact was not established for another 20 years. The propagation of the waves is illustrated in the first panel below, where the lines show the wave trajectories (note that in reality they are curved, something ignored here), the inner red circle is the core and the brown arc is the shadow zone.
The next major step in the unraveling of the core came from Inge Lehman, in 1936. She was head of the department of seismology at a Danish institute. Lehman used seismic waves from a major earthquake in New Zealand in 1929, detected in her home country of Denmark. They shouldn’t have been there: from New Zealand, Denmark is in the shadow zone which the waves cannot reach. She showed that this could be explained by waves reflecting inside the core, but that required a surface to reflect from. In other words, there was another transition zone, and there was something else inside the core. It is depicted in the second pane, where the black lines show the reflections. Her model was so convincing that it was adopted within a few years. Now the Earth had both an inner and an outer core.
Lehmann was a remarkable scientist, both famous and under-appreciated, in a world where science was largely a male profession and many of them were strongly protective of their own gender. She never got the professorship she fully deserved, although later on she did receive several medals. Lehmann reportedly once said, “You should know how many incompetent men I had to compete with — in vain.”
It was a small step to combine the lines of evidence for a high density component of the Earth and those for the existence of a core. This became the model of an iron core embedded in a mantle of rock.
Not all of the discoveries were made with seismographs. One of the most important findings came from the tides. We all know how tides affect the oceans – a source of endless play for children. But the Earth itself also has tides. The rocks underneath our feet move up and down by centimeters due to the tidal forces. How much the Earth responds to the tidal force depends on how strong the planet is. In particular, it depends on the shear modulus (see part I), its resistance to deformation. Already in the 19th century, Lord Kelvin used the earth tides to show that the strength of the planet (or its rigidity) is greater than that of glass but less than that of steel. By the 1920’s seismology had shown (from the velocity of the S waves) that the mantle had a much higher rigidity than this. The only way that the two results could be made to agree was if the core was very much less rigid. In fact, a fluid core was needed. This became the basis for the discovery that the core of the Earth was liquid. Once the inner core was discovered, it quickly became clear that this part was solid. And that is the story of how we came to know about the ocean under our feet, caught between the solid surfaces of the inner core and the mantle.
So the Earth in fact has two cores, just as the mantle has two hearts (part II). The outer core has a radius of 3485±3 km (compared to 6371 km for the planet) and the inner core has a radius of 1220 km. (The shapes are in fact ellipsoids, so the radii vary from pole to equator. The quoted radii are for spheres with the same volume as the real thing.) The temperature ranges from 3700±500°C at the core-mantle boundary, to 5000±400°C at the top of the inner core and 5500±400°C at the centre of the inner core. The outer core is liquid with a fairly low viscosity, perhaps similar to a low viscosity lava. The inner core, as already mentioned, is solid, sitting at the centre like an ancient Stone of Scone. But also, and this is much more like a black hole, it is growing and will eventually devour the outer core.
The original idea of a pure iron core doesn’t quite work. If you make the core a ball of pure iron, the Earth becomes a little too heavy. The core is about 10% less weighty than iron, so there needs to be a fraction of a lighter element included.
By the way, the density is of course affected by the sheer weight of the world above, including the weight of humanity. Material deep down is under considerable pressure and this compresses it a bit. The compression makes the density higher. But that compression is easily corrected for. The ‘uncompressed’ density of the Earth is very close to 4000 kg/m3, compared to the 5500 kg/m3 in reality. In case you are interested, the uncompressed density of Venus is very similar (perhaps a fraction lower), that of Mercury is higher at 5000 kg/m3 and that of Mars is lower 3700 kg/m3. Mercury appears to have a large core compared to the size of the planet, and Mars a smaller (or lighter) one. (Mars’ core was recently reported to be 1850 km in radius, from InSight data, making it around 55% of the radius of the planet. This is larger than had been expected. The ratio is now the same as that for Earth (also 55%). So Mars actually has the same size core (relative to the size of the planet) as Earth. However, Mars’ core may have a lower density than ours. Not all cores are the same – but they have a lot of similarity.
Based on what we find in iron meteorites (I have a small fragment of one at home but wouldn’t dare to use it for a destructive analysis!), the Earth core would be expected to contain iron mixed with between 5% and 20% of nickel. This is plausible but it is impossible to prove, as iron and nickel cannot be distinguished with seismology.
However, this iron/nickel combination is too dense by about 5%-10%, at least for the outer core. The density of the outer core ranges from 10,000 kg/m3 at the outer edge to 12,000 kg/m3 at the bottom (the increase with depth is caused by the higher pressure). Molten iron at these pressures and temperatures should have 11,000 to 13,100 kg/m3. This leaves a 10% deficit. The precise values do depend on the assumed temperature and some recent studies find a lower deficit of around 5%. But in either case, we need some lighter element mixed in – making it more like steel than iron.
Which element(s) is not known with certainty. Sulfur is plausible. It is depleted in the mantle, and the missing fraction could easily have gone into the core. Meteorites often contain iron-sulfide. But silicon has also been proposed, and even oxygen is possible if you can imagine a rusted core. Models can reproduce the properties with a range of compositions, with silicon from a few per cent to as high as 17%, oxygen at a few percent, and sulfur at 5 to 15%. These elements are to some degree mutually exclusive. The precise content remains a matter of speculation, as Richard Oldham would have pointed out.
The inner core also has a density deficit compared to iron-nickel, but it is less than that of the outer core. It is solid, and the solidification will have removed elements with lower melting temperature, which in general are the lighter elements. The evicted elements have to go somewhere, and over time this may have increased the fraction of light elements in the outer core. For the inner core, the best guess at the moment is 5% nickel and 3-7% of light elements which may again be sulfur or silicon. It is even possible that there is a substantial amount of carbon in the inner core: this has recently been suggested, but seems speculative as during the formation of a planet, carbon tends to end up near the surface. Hydrogen has also been suggested. The only certainty is that when we finally get our hands on a bit of inner core, there will be surprises.
The outer core is liquid and is expected to be well mixed. But the inner core may have different compositions at different depths. Perhaps we have a plurality of inner cores.
The core of the Earth affects more than just seismology. It has given us the Earth’s magnetic field, providing a sense of direction, helping birds to migrate, protecting the atmosphere against the solar wind and protecting our electricity cables against coronal mass ejections.
The Earth’s magnetic field is generated in the outer core. Even though the core is iron, it cannot have a permanent magnetic field because it is too hot: an iron magnet looses its magnetism above a temperature of around 700 C, far below that of the core. Instead we have a dynamo: a circular current which generates a magnetic field inside the current loop. This magnetic field captures the intrinsic magnetism of the iron, and aligns it with itself. It also forces any charged particles (electrons) to move in a loop around the field lines, and these particles cause a current which strengthens the magnetic field, etc. Once the field is there, it continuously re-generates itself in this way.
The dynamo is started off by a combination of the rotation of the Earth and by convection inside the core. Imagine a blob of liquid iron moving around in the core. It is taking part in the daily rotation, and therefore moving at some speed, going around the world in one day. But as it changes location, the speed it needs for this changes. If it moves closer to the equator, it finds itself going too slow and therefore lagging behind. If it goes closer to the poles, it goes too fast. The same effect happens on the surface of the Earth. In fact this is what causes the air to move around in circles around low pressure or high pressure systems. It is called the Coriolis force, but it is all about keeping up with the locals. The same effect also happens when a blob rises or descends, and again finds itself with the wrong speed for the rotation of the Earth at its new location. In a highly viscous material, friction would immediately fix the problem but the liquid core has low viscosity.
The upshot is that the moving blob will be going in a large circle. And if it carries a charge, then there will be a circular current and therefore a magnetic field. That magnetic field will strengthen itself by grabbing the free electrons and forcing them into action, and the dynamo is in action. Details of the precise mechanisms are still being discussed: this is the general picture, but the interaction between the three processes (convection, currents and induced ferro-magnetism) remains an area of study.
The direction of this field is not pre-determined. The circles are shaped by the rotation of the Earth, and on average this will cause a polar field. But individual convective circles can deviate far from this direction. And the circles north and south of the equator will go around in opposite ways (just like low pressure systems on Earth) and produce opposite fields. So there is quite a chaotic situation in the core. As the dynamo takes hold, it begins to straighten out the field – but it can be many degrees off from the polar axis. And still has to decide whether to point north or south depending on which hemisphere wins out! Let battle commence – may the strongest field win.
The winner is not forever. The convective bubbles are still moving, sometimes strengthening and sometimes weakening but always distorting the field. The magnetic field may weaken enough that the battle between north and south is fought again. Most often, the existing field eventually wins out, but sometimes there is a different winner and there is a reversal of the Earth magnetic field. And suddenly all the birds which rely on magnetic sensors, find themselves at risk of migrating the wrong way. The book of Moby Dick tells how lighting reversed a compass so it pointed in the opposite direction, sending a whaler off course. Reversals are rare and may not happen for a million years or more. But they are a random inevitability. As far as we know, the Earth’s magnetic field has been present for at least 3 to 4 billion years, at more or less its modern strength. All this time, there will have been reversals.
But there have been some periods where there were no reversals for as long as 50 million years. How can that be? Perhaps these were times of reduced mantle convection, reducing the heat flow through the mantle which in turn could reduce the vigour of the core convection. But the precise opposite explanation has also been suggested! Another option is that the mantle convection at these times was stronger in the north or south, perhaps because of the location of the continents. It is speculation – we just don’t know. The core convection may be suppressed below the LLSVPs (remember them?) because they are hotter, or the LLSVPs may be hotter because of stronger upwelling in the core below them!
The magnetic field requires both a convecting liquid and a rotating planet. There is one more requirement: it needs an energy source. Nothing comes for free. Where does the energy come from? There are two energy sources which drive the Earth core’s convection. One is the heat from the inner core, and the other is buoyancy caused by the loss of heavy elements to the core below and light elements to the mantle above. At the top, magnesium from the liquid core may attach itself to the mantle, leaving the core liquid a little heavier and ready to sink. At the bottom, the heavy iron and nickel may freeze out on the inner core, leaving the liquid there a bit less dense and ready to rise. This process can drive convection, and models suggest this is the main source of energy.
And as a final point: because the convection itself carries the magnetic field around, we can actually trace the convective flows in the outer core from the changes in the magnetic field at the surface. In the plot below, green shows descending flows in the core, arrows show horizontal flows and yellow/brown shows upwelling. It is amazing how we can map movements so deep inside the Earth. And notice the velocity, reaching a staggering 15 km per year. Well, it is staggering compared to the mantle doing a mere 0.5 meter per year. That is the advantage of being a liquid. It is giving our snails a run for their money.
The inner core
Inside the liquid outer core sits the solid inner core. It hasn’t always been there: the inner core has formed by slow solidification of the outer core. At the moment, the inner core grows by about 1 mm per year. At this rate, in 2 billion years from now the entire core will be solid and seismology will become a lot easier, to those of us not as competent as Inge Lehmann, but navigation will become harder for lack of a magnetic field.
There are suggestions that the core growth is not the same everywhere, and that it is higher around the equator and lower at the poles, and also that it is higher in the western hemisphere. The cause is speculation! A possibility is that the rotation of the Earth causes stronger convection at the equator, meaning faster heat transfer and therefore faster core growth.
It is part of a general asymmetry of the inner core, which has seismic waves traveling along the poles being faster by between 2 and 9 seconds compared to ones traveling along the equator. (It depends on where on the equator the delay is measured, and that depends on where the particular earthquake originated!) (The lack of a subduction zone near the poles presents a problem, since it means we rarely have a strong earthquake with a seismic wave traveling very close to the polar axis.) The explanation for the asymmetry is hotly debated. As it solidifies, the iron forms crystals which can be rather large – they may be a kilometer or more. The crystal structure is a bit more rigid in one direction, by a few per cent. If all the crystals in the core have become aligned, this could explain the observed asymmetry. In fact, the faster equatorial growth might do this: it requires a slow creep along the core of the solidified material to the poles (to maintain the spherical shape of the core) and this creep could cause the alignment.
Some measurements suggest that the inner few hundred kilometer of the inner core have an opposite alignment, where the waves travel faster along the equator. This has been interpreted as an ‘innermost core’ inside the inner core. It seems to be re-discovered by different groups every few years, ever since the original finding 20 years ago, but whether it constitutes a distinct core region remains disputed. There is no clear and sharp transition radius in the data.
There is also a change in seismic properties some 200 kilometers below the outer edge of the inner core, which may relate to a phase change of the iron crystals.
The inner core appears to rotate slightly faster than the rest of the Earth, although only minimally so. The first measurements gave an excess of 0.2 degrees per year: like a sluggish version of Phileas Fogg, the inner core is gaining one extra rotation every 1800 years. Newer measurements has reduced it to 0.07 degrees per year. The faster rotation can come from the slight contraction during solidification. If a rotating object becomes smaller, it rotates faster.
A very recent paper claims that the inner core has changed from slower to faster rotation over the past few decades. That is a surprising result which needs confirmation. (A series of large earthquakes over the next few years would help!) . It is not impossible, and could arise from interaction between the convection in the outer core and the rotation of the inner core. A sinking blob attaching itself to the inner core needs to shed some of its rotation (angular momentum) and in doing so speeds up the rotation of the inner core. A rising blob would do the opposite and over time they average out. If this is correct, it would show how dynamic the zone between the inner and outer core is. We know from seismology that the top of the inner core attenuates seismic waves a bit more than expected. It is not a perfect, sharp boundary: the top of the inner core is a bit mushy and perhaps sticky. Maybe the La Brea tar pits are a good comparison.
The world underneath the mantle is a very different one, almost like another planet hiding inside ours. Like the Tardis, the Earth seems bigger on the inside. How did this happen?
The formation of the core requires that the iron separated from the rock, as in an ancient smelter, That required a molten Earth, a magma ocean where the heavier iron could drop to the bottom. But the only time the entire Earth was molten was during its formation, when the continuing collisions kept the planet hot. The core as we know it must be from this very beginning, 4.57 billion years ago. It appears that the formation of the core was largely complete before the Earth was 30 million years old, perhaps even before the Earth had reached its full size.
Some elements are called ‘siderophile’: in chemistry, they side with iron. Others are ‘lithophile’ (barium, rubidium) and side with rock (uranium, lead). When the core formed, the siderophile elements would have gone into the core, and these elements would become depleted in the mantle. But in the oldest rocks we know on earth, these elements have the same proportion as modern material. The depletion of these elements did not increase over that time, and this shows that the core stopped forming before these oldest rocks formed, more than 4 billion years ago.
Very early in its life, the Earth was hit by a Mars-sized body which we have optimistically called Theia. (In mythology, Theia is the daughter of Earth (Gaia) who gave us gems, but in the real world she nearly killed us and the debris of the collision formed the Moon – not quite the same thing.) Apart from the Moon, Theia must also has left remnants in the Earth. One recent paper suggested that the LLSVPs are these remnants – that is very unlikely. But there may be a signature in the core. The Dʺ layer and the upper few kilometers of the outer core are the most likely: the suggestion is that shock waves from the collision mixed the top of the outer core (at that time, there was no inner core) and the bottom of the mantle. The Theia impact appears to have happened around 40 million yers after the Earth began to form, at a time when the core of the Earth had already fully formed. The idea that the core is itself Theia’s remnant can be ruled out. The fact that the core of Mars and the Earth are so similar in relative size suggest that core formation is a normal part of planet formation, and not related to particular events such as this. The huge impact had little or no effect on the size of the core. It also means that the Moon is younger than the Earth’s core.
How about the inner core? At the current growth rate of 1 mm/year, it would only be 1.2 billion years old. There are a variety of estimates, ranging from 2.5 billion years at the oldest and 0.5 billion years for the youngest. In any of these estimates, the inner core is the youngest component of the Earth. Our liquid core is disappearing. This should be a major topic for the next election: what is the government going to be do about this? For if the entire core solidifies, as may happen in another 2 billion years, we would lose out magnetic field and that would be a major environmental disaster.
I propose an action group, called IronicAction, with slogans ‘Save Our Core (SOC); Cease The Freeze; Keep The Iron Curtain (KIC); Stop Iron Change (SIC)’. A Hollywood movie will be made about remelting the inner core, called Extreme Ironing. Scientists demand to limit core growth to no more than 2 meters, to avoid catastrophic change. The Navy spends 10 billion dollar finding new ways to sink its ships. Politicians will talk, decide, and inact (politics being the only activity where inaction is a verb). And Elon Musk develops plans to use the core as a giant battery. But that is a story for another, distant day.
Albert, June 2022
This post is largely based on the book by Ken Condie, The Earth as an evolving planetary system (4th edition), 2022, Chapter 5