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.