Standing on the Earth, you’d probably think it’s made of rock. And you’d be right – mostly. In fact, what you’re standing on is the Earth’s crust.
The crust is broken into plates which fit together like a jigsaw, moving around the surface very slowly (a few centimetres a year – like fingernail growth). The thickness of the crust varies, but is generally between 5 and 50 km, so when compared to the size of the Earth, it is very, very thin, like the skin of an apple.
Beneath the crust is the mantle – split into three parts – the upper mantle, the transition zone and the lower mantle. This is all solid (apart from very close to the surface where some rock melts) and is mostly silicate rocks, down to about 2900 km.
Here, as we reach nearly halfway through the Earth, we hit a big change – to liquid iron (well, actually, an alloy of mostly iron with some nickel and other lighter elements). This is the outer core. At about 5200 km, we reach the inner core – solid iron (frozen by increased pressure, also alloyed with nickel and light elements). The centre is finally reached at around 6400 km.
So why am I telling you all this? Why do we care about the Earth’s core? Well, for a start, the outer core is the source of our magnetic field which protects us from the harmful rays of the sun. Without the magnetic field, our atmosphere would eventually be stripped away by solar winds, and all the water would evaporate – the Earth would become a completely inhospitable planet. Secondly, the Earth has been cooling from a molten soup, formed about 4.5 billion years ago, to the layered mostly solid structure it is today. Some of the original heat from when the Earth was first formed is still being transported out of the core and mantle. This is the energy source responsible for the dynamics of our planet, leading to surface events, like the mountain building, earthquakes and volcanoes that we observe today.
Now, you must be wondering how on earth we know anything about what is going on more than 6000 km beneath our feet. Did we dig a hole? Well, yes. In fact, the deepest hole that’s ever been dug is on the Kola Peninsula in a remote region of Russia, and it’s just over 12 km deep. Unfortunately, as you dig down, the pressures and temperatures increase so much that by the time you reach around 12 km, all the drill mechanisms start to get mangled. It’s an engineering problem that is currently too expensive to resolve.
Another way of getting information about what’s beneath our feet is to look at material ejected from volcanoes. While most volcanoes spew out stuff from just below the surface, there are some particular volcanoes that eject material from more than 200 km deep. That’s deep - but to reach the Earth’s core we still have over 6000 km to go!
It turns out that earthquakes, while disastrous for some, are the perfect tool for investigating the deep Earth. Whenever there is an earthquake (and there are thousands every year), energy passes through the Earth in the form of waves. There are two types of wave that travel right through the Earth – a compressional P-wave and a shear S-wave, and these so-called body waves are picked up by seismometers located all over the surface of the Earth. The squiggly pattern drawn out by these seismometers (that you sometimes see in disaster movies) give us lots of information, including the speed at which the waves pass through the Earth. The speed of waves travelling through a material depends critically on the physical properties of that material: specifically, the density, the bulk modulus (how squishy something is) and the shear modulus (how its shape distorts). Scientists can measure or calculate these seismic velocities, and try to match them up with those of materials likely to exist in the Earth. In this way, we can determine what the Earth is made of, when it changes from one material to another, whether it is solid or liquid, and much more. Unfortunately, this is not as straightforward as it may seem, since by the time you reach the centre of the Earth, the pressures and temperatures have increased to over 6000 degrees and 3.5 million times atmospheric pressure. This is equal to the temperature of the surface of the sun and the pressure of 750,000 elephants standing on top of each other (or 700 if they are wearing stilettos!). The technical challenges are immense.
But lots of people have been working on this subject for over a hundred years, and we have made great leaps in our understanding. For example, we know that the Earth’s liquid outer core has been crystallising for around a billion years to form the much younger inner core, and that both are made of iron alloyed with nickel and light elements such as silicon, sulphur and carbon. We know that the outer core is a convecting liquid that is runny like water. We know that in the inner core, waves travel faster pole-to-pole than they do along the equator – suggesting some sort of texture or crystal alignment channelling the waves. We know how iron alloys melt or solidify under pressure, allowing us to determine the temperature at the boundary between the inner and outer core (that’s where we get the 6000 degrees).
Looking forward, the biggest challenge will be getting an exact composition that matches both the seismological observations of the body wave velocities and the density. This is one of the “holy grails” for understanding the centre of the Earth. If we know exactly what the core is made of, we can then work out all sorts of relevant things that have consequences for the entire planet. Research in this area continues and exciting new discoveries about the Earth’s core are on the way. Understanding the material at the centre of our Earth is key to understanding geological events which have shaped Earth’s history.