The two great engines of the Earth’s interior

cartoon cross-section of Earth's core (yellow) and mantle (orange)The Earth is hot and space is cold and the first law of thermodynamics tells us that heat must flow from the former to the latter until they are the same temperature. The Earth is far from being just a boring hot rock however and has spontaneously developed some pretty impressive ways of transferring this heat up from its depths. Earthquakes, volcanoes, tsunamis, hurricanes and the Earth’s magnetic field are all consequences of our planet trying to lose heat as fast at it can and converting (temporarily) some of this thermal energy into mechanical energy as it does so.

At the heart of thermodynamics lies the concept of the heat engine: a system that converts thermal energy into useful work (mechanical energy). The idea was formulated from considerations of man-made steam engines increasingly brought into service during the industrial revolution. Nevertheless, the pioneers of thermodynamics were also doing a great service to the Earth Sciences because heat engines are actually at work all over and inside of our planet.

The two biggest engines in the Earth’s interior are in the mantle and outer core. Both regions are undergoing vigorous convection – that is they are transferring heat by mass movement (due to buoyancy) of hot and cold bits – though at very different rates. The creeping of the continents and oceans at rates of centimetres per year, and the resulting earthquakes that ensue, are down to convection in the mantle. Simply, the crust and top of the mantle loses heat to the adjacent oceans and atmosphere, becomes dense, and sinks at subduction zones. This pulls open a gap at the surface allowing the hotter mantle down below to rise at volcanic mid-ocean ridges. This process of mantle convection via plate tectonics is an extraordinarily effective means of transferring the Earth’s heat that is both much faster and far more interesting than simple conduction. It is not our planet’s only great and beautiful internal heat engine however.

The Earth’s liquid core is a lot hotter than the solid mantle above it and so, once more, heat must flow upwards and outwards. This core-mantle heat flow is the ultimate driver of mechanical flow in the liquid core and so we have another heat engine. In detail however, things in the core are a little more complicated and uncertain than in the mantle. The core fluid (an iron alloy) is a very good thermal conductor which gets in the way of thermal convection as is happening in the mantle.

Nevertheless, the core is vigorously convecting. As the core is losing heat, it is freezing from the centre (where the pressure is greatest) outwards. This means that the solid inner core is progressively growing at the expense of the liquid outer core above. As well as changing the size of the outer core, this freezing process is also affecting its composition. Only the densest, iron part of the fluid freezes onto the inner core so the outer part is left with the lighter, residual part of the alloy. Light means buoyant, buoyant means flow (upwards), flow means convection. But this time: bottom-up compositionally driven convection rather than the top-down thermally driven convection exhibited by the mantle.

In the core, the fact that the liquid is electrically highly conductive means that things get even more interesting. The mechanical energy generated by the heat engine can undergo a further conversion before it finally gets turned back into thermal energy: this time into electromagnetic energy. As you drive your car, part of your engine’s motion is converted into electricity and used to charge your car’s battery. This dynamo process – converting mechanical energy into electrical current (with an associated magnetic field) – is also happening, quite naturally, in the core. We call it the geodynamo and it’s what generates our planet’s magnetic field.

So, the Earth’s mantle and core both contain great heat engines producing work. Going back to the steam engines, the work they produced was put to good use – moving railway trains and the like. How does the Earth make use of all the work it produces? Well, if it is the aim of the Earth to lose its heat as fast as possible to space then this work is put to very good use indeed. I have already written of the massive efficiency gain in heat transfer that plate-style convection confers the mantle over simply waiting to cool down by conduction. It might surprise you to learn that the core goes to all the trouble of generating a huge magnetic field for much the same reason – to lose heat as fast as it can.

Relative motions between a conductor and a magnetic field generate electric currents in the conductor which, in turn, generate heat there. The core’s trick is to use its thermal energy to generate a moving magnetic field that, in turn, generates heat in distant conductors. Most of this magnetic-thermal energy conversion takes place within the core, but it also goes on in the mantle, the crust, the oceans and even out to space, rapidly allowing heat to migrate outwards. This is, again, a very good way of transferring heat long distances quickly.

I find it extraordinary that the creation of something as grand, complex and useful as the geomagnetic field can be neatly ascribed to the simple transfer of heat.

About AndyB

Andy Biggin is a researcher interested in the behaviour of the ancient geomagnetic field and what this can tell us about the Earth's interior. He has been based at the the University of Liverpool's Geomagnetism Lab since 2009.
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5 Responses to The two great engines of the Earth’s interior

  1. Claudio says:

    Hi Guys. Congratulations for your site, you´ve got excellent posts spread all over. I was reading an old post by Neil about the dropping of the earth´s magnetic field when I found the interesting observation that if the geodynamo should stop the field would decrease at a rate of just 1% for century. I started wondering what he meant, what he was thinking about. I guess he was imagining the interruption only of the part supplied by the convective motion in the outer core because to prevent or reduce the portion of magnetic field supplied by the inner core you will have to slow down earth´s rotation…
    But now if you consider that this rate of decrease (caused by something that I still don´t understand, but let´s just imagine and play what if…) would extinguish or reduce to a small portion of its strength the magnetic field in something like 10.000 years, you will find that this time is longer than the time expected for the magnetic field to drop during a magnetic reversal (for most of the authors and at least for some reversal event). So, if I believe in your source I must start believing also that during a magnetic reversal the rotation of the inner core is affected somehow, which is something I have been denying in my investigation on magnetic reversal.
    I´d be very grateful if you could explain this to me.
    I´ve also seen that unfortunately your section for conquering tax payers funds is lacking a bit of enthusiasm, so I would like to suggest to add the following claims: Because life on earth without magnetic field is impossible, Because the occurrence of the Magnetic Reversals are the key of the speciation events and the real engine of evolution and Because earth´s magnetic field affect living beings far more than just helping birds to migrate, affecting embryonic development, cells signaling and storage of memories (among others biologic features).
    I´m sure you are not new to most of these theories but they are too much underestimated, so I had to write a book on it.
    Relax, I´m not here to promote my book which by the way has not been published and it probably never will. I wrote it as a reflex to the incongruities and weaknesses of the actual Darwinian paradigm when It occurred to me that the reversals could solve all of them. I wrote it because it would have been a pity to keep just in my mind the wonderful picture of evolution that arose after four years of intensive research, but I´m satisfied enough by the certainty that one day, when someone else will finally drop the curtains, I´ll taste the pleasure to say to my children: see, your dad had told it…
    The fact is that I have the feeling that I missed something in not considering your site during my research and writing, but, in my defense, please consider that I tried to keep away from sites or blogs and used just peer reviewed articles as I wanted to be credible.
    Anyway, despite my efforts in the last years I´m sure that you guys have a better and deeper knowledge of the earth´s magnetic field and as I´m always willing to understand and learn more I would really enjoy if we could discuss a little bit about the issues I´ve just mentioned.
    Thank you again for your valuable site and your attention.
    Best Regards
    Claudio Farina

    • AndyB says:

      Hi Claudio,

      Many thanks for your questions and comments. To try and answer your first question: we do not expect the rotation of the inner core or any other part of the Earth to be strongly changed during magnetic reversals. The outer core convection generates most of the magnetic field and this is largely fuelled by the inner core’s solidification, not by its rotation. You are dead right that the field may reverse in less than the time it would take the field to passively “diffuse” away. This is possible without any change to the amount of energy driving the convection process however. It simply means that the dynamo is using the available energy in a different way – in this case, actively generating field in parts of the core that is in the opposite direction to the overall, causing rapid decrease in net field strength.

      I am sorry you felt my taxpayer’s piece lacked enthusiasm – I gave it my best! I am afraid that it would not have been honest of me to include the extraordinary claims you are making because the evidence to back them up is simply not there. Believe me, I would like it to be – it would certainly make writing the grant applications much easier! More details to follow – I am trying to encourage my colleage Richard Holme to write a piece on the likely impact of a future geomagnetic reversal for humans and life as he has given public lectures on this in the past. Without stealing his thunder, the gist is no massive catastrophe.

      Thanks again for your post, Claudio.

  2. Claudio says:

    Hi Andy,
    Thanks a lot for your simple and clear answer.
    I agree with you in not expecting any great change in the inner or outer core´s rotation during a reversal, but then you say something that it´s different from what I thought I had understood about the geodynamo process.
    I knew that the process of solidification (even if I would say the solidification of the liquid OUTER core) is the cause of the convective motion which in turn generates magnetic lines of force but I believed that these lines would increase/modulate the magnetic fields already produced by the rotation of the inner solid core.
    That´s why I asked you which part you imagined to be responsible of the passive dissipation and I believe now that your explanation about the opposite flow of energy has solved my doubt.
    With regards to the impact of a reversal on life on earth I beg to disagree with you and your colleague Richard.
    Once again: don´t worry, I´m not a doomsday jerk (I have a six months old daughter and I can´t avoid wondering how there can be so many folks so much looking forward for the end of the world to come…) and I´m not interested in Tesla´s secret research or stuff like that.
    Nowadays more than ever to be a man of wisdom means to be able to select the sources of your knowledge.
    As I told you the information I´ve got (and 90% of the hundreds of articles cited in my book) come from peer-reviewed articles (there are plenty from Nature, for example).
    It seems to me that generally, in the literature about possible effects of a magnetic fields, researchers who find evidences always say it in a soft, shy, almost embarrassed way, while who doesn´t scream it´s not possible; and this is what makes the real difference in the general opinion (and it´s definitely not your case, being specialist you must speak supported by facts and thoughts and that´s way It´s a pleasure to discuss and learn with you guys).
    See this example: NASA´s website repeatedly claim that magnetic reversals are not big deal, but than in a different section relates of a report which claims that in case of a Carrington´s event to occur in present time the disruption of the electric lines and the weak production capacity for huge transformers would mean 40 years of waiting before the U.S electric line would be finally fully restored. And the report is NOT considering the possible lack of protection from our magnetic field.
    Actually most of researchers who are not worried about magnetic reversals says that´s because our atmosphere would easily compensate the lack of the protection from the magnetic field.
    How can they be so sure? Did they consider the fragility of the equilibrium of our atmosphere itself?
    Isn´t it true that on the contrary the higher flux of radiation would probably threaten this equilibrium that keeps our climate relatively stable?
    And why then always from NASA´s site we learn that is just the lack of of a magnetic field (together with its weak gravity, of course) that caused the loss of the atmosphere on Mars, as it can be seen happening on Venus right now.
    And all these effect are not even my biggest concern, for I suspect (once again supported by tens of researches on the effect of different magnetic fields applied on living beings as a whole and specifically on tissues, embryos, eggs, cells, membranes and even DNA) that all living beings are suited to live on a determined magnetic environment and that any change in it would drastically affect their general health promoting celular mechanisms still to be investigated.
    As for the possibility that the magnetic field could participate in the process of memories storage I have to admit that this is a personal theory, even if is indirectly supported by researches about bio-magnetite grain, which are present in our brains and many other organs, are magnetic susceptibles and presents peculiar features of permanent and temporary magnetization.
    Thank you very much for your time.
    Claudio

  3. Claudio says:

    Sorry Andy,
    I almost forgot the main issue, the one which in my opinion should grant you and your fellows more funds!!!
    It´s the claim that Magnetic Reversals of the Earth´s Magnetic Field are the true engines of the evolution of species.
    Trying to convince you in a few lines it´s quite hard.
    (You should read my book, which is even harder for apart from the original articles cited, which are in the original English, the rest is written in Portuguese!!!).
    In the most simple and short way let´s say that if you take almost any of the books from the famous paleontologist Stephen Jay Gould, keep for good all of his reasoning and just add a magnetic reversal as the cause of the jumps he saw in the fossil record, you would see how this phenomena is able to put all the pieces of the evolution puzzle in the proper place.
    Best Regards
    Claudio

  4. AndyB says:

    Hi again Claudio and sorry for the slow reply,

    Just to respond to one of your points – I entirely agree that a big magnetic storm such as the one associated with the 1859 Carrington Event has the potential to be extremely disruptive to the industrial world through producing geomagnetically induced currents in power networks.

    One thing to appreciate about large magnetic storms is that they involve the Earth’s magnetic field essentially being “opened up” by anomalous patches of interplanetary magnetic field (IMF) through a process called magnetic reconnection. While a weaker geomagnetic field maybe opened up more easily, my understanding (and this is not my research specialisation) is that the strength and size of the anomalous patch of IMF is the governing factor that determines the severity of the magnetic storm. Consequently a major coronal mass ejection carrying just the right (or rather wrong) IMF with it and hitting Earth could be very bad news regardless of whether the core-generated field was a little stronger or weaker.

    That said, the geomagnetic field currently does a pretty good job of shielding near-Earth space from the solar wind most of the time. A weaker field would bring the magnetopause closer leading to more solar wind penetration and lower latitude aurora. For info, this is the domain of the relatively new discipline of “palaeomagnetospherics”.

    Andy

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