In recent blogs I’ve looked at the mass of the Earth and the circumference of the Earth. But what about its depth, or to put it more correctly, it’s radius? Well, to cut to the chase, the average distance from the centre of the planet to the surface is 6371km. Scientists have worked this out over the years using the same basic techniques used in measuring the circumference of the globe. It’s a big number, but what does it mean?
The Big Picture
In this post you should see the big picture – literally. Although I’d seen loads of images of the Earth’s layers before, hardly any actually gave me a true feeling for the scale of each layer compared to the bit we live on. As the awesome Australian science-guru Dr Karl has said many times – the Earth’s crust is like a postage stamp stuck to a soccer ball. However, that’s not the impression I’d get from pictures like this (from the BBC).
So I made this image to scale. Sorry – you could be scrolling down for a while… Compared to the total depth of the Earth, you can see that Mount Everest is a mere pimple, and the deepest trench in the ocean isn’t much more than a scratch!
This image shows what are called the ‘compositional’ layers of the Earth, breaking up the planet into layers based on the stuff that they are made of. This gives us the crust, the mantle and the core, with various subdivisions. You can also divide the planet up into what are usually known as ‘mechanical’ layers, based on the how much they move around, or whether they are solid or more flexible. More on that later.
But how do we know all this? Certainly no-one has dug a hole right through the Earth to find out.
Back at the start of the 20th century, scientists discovered that measuring earthquakes could provide a window into the inner Earth. The pioneer of this method was Croatian physicist and mathematician Andrija Mohorovičić. After taking measurements of a large earthquake that hit Croatia in 1909, Mohorovičić noticed that some of the underground shock waves had arrived at measuring stations sooner than expected based on their speed at surface-level. Based on these measurements, he figured out that these underground shock waves – or seismic waves – were being caused to change speed and direction, or bounce back, when they hit different layers within the Earth.
This was happening because layers of material within the Earth are more or less dense than the other layers around them. In the same way that light is bent or reflected when it hits something denser than air – like the lens in your glasses, or the water in a swimming pool – seismic waves were being bent or reflected by different densities of materials within the planet. You can then work out the rough thickness and density of layers from the amount of bending or reflection.
Over the next century, scientists have continued to use basically this same technique, of measuring earthquake waves passing through the Earth, to build up a pretty good model of the inside structure of the planet.
The onion Earth
So the upshot of all this is that we are now pretty confident that under our feet the Earth goes from solid rock down to an inner core of super-hot solid iron 70% the width of the moon! By breaking up the Earth by the stuff it’s made of, five fairly clear layers emerge: crust, upper mantle, lower mantle, outer core and inner core.
The crust is the rocky shell of the planet, and its thickness varies pretty widely. Under the oceans, the crust is thinner – usually between 5-10km, and made of denser rock like basalt. The crust of land masses tends to be a lot thicker, ranging from 10km thick to 70km or more in mountainous regions, but are made of a wider variety of lighter rock types.
It feels pretty sturdy, but proportional to the size of the Earth it’s similar thickness (perhaps even thinner) to the shell on a hen’s egg! In fact, the crust represents less than 1% of the Earth’s volume.
The mantle begins where the crust stops, at a point known as the Moho disconuity, named after our old pal Andrija Mohorovičić. This layer starts under the crust and extends down to a depth of about 670km.
The mantle is thought to be made up of a range of rock types and minerals that we don’t see that much at the surface, such as garnet, olivine and peridotite, varying with depth. Overall, the mantle is mostly a mix of silica (which you’ll have seen up here on the surface in the form of quartz crystals) and magnesia, which together make up about 84% of the mantle’s mass.
The lower mantle is made of similar stuff to the upper part, but begins at the point where the weird rock types mentioned above are no longer stable. As we travel down through the mantle, it really begins to heat up too. The top of the lower mantle sits at around 1000C, with the lower levels hitting as much as 4000C – close to the burning point of diamond!
Like the upper mantle, the lower mantle is still essentially solid, and actually moves around less than the upper layer, mostly because of the extreme pressure it is under.
Extending down to about 5155km, the outer core is quite a different beast from the mantle. Unlike the fairly solid upper layers, it is a dense soup of liquid iron and nickel, ranging from 4000 to 5700 degrees C in temperature.
The movement of this thick layer of molten metal is thought to be a major factor in creating the Earth’s magnetic field. That’s good news for us! Not only does the magnetic field make our compasses work, it also keeps us nicely shielded from the radiation blasted out by the sun and other sources in deep space, which would otherwise have stripped away our atmosphere by now.
Although also made up of nickel and iron, the inner core is under such incredible pressure that it remains a solid lump, roughly 70% the diameter of the moon. The pressure of all the mass of the Earth pushing in raises the melting points of the metals, and forces them to stay as solids despite a temperature of more than 5400C.
Along with nickel and iron, the core is thought to contain large amounts of precious metals (such as gold and platinum). It has been estimated that if poured onto the surface of the Earth, these precious metals would form a layer 45cm deep around the whole globe!
Floating islands of rock
Earlier on I mentioned that you can also divide the Earth into slightly different layers than shown to the right, based on how rigid or pliable different parts are, and how much they move about (as opposed to what they are made out of). This different way of looking at things only really applies to the top layers, but it gives us a pretty important look at how the world we see at surface-level is created.
Although made up of pretty different stuff, the top layers of the upper mantle and the crust tend to stick together. The crust and top of the upper mantle make a relatively rigid layer, known as the ‘lithosphere’ (basically Greek for ‘rocky bit’), which has a maximum thickness of about 200km.
The parts of the mantle below 200km on the other hand are less rigid, and are at such high temperatures and pressures that they end up acting kind of like a very slow moving liquid (although they are mostly not made of molten magma, as is often assumed!). Because of this we have what are known as tectonic plates – basically big chunks of lithosphere – ‘floating’ on the rest of the upper mantle. These move about as the heat from lower layers shifts the mantle underneath in convection currents, in the same way that hot air from a heater might rise in a room, then spread across the ceiling.
The upshot of this all is the process called plate tectonics – where the surface of the planet is split into massive rocky ‘plates’ the size of continents. These plates then move around – at about the same speed as your fingernails grow – as new rock from deep within the Earth is pushed up by convection currents and pushes these chunks of lithosphere over the mantle below. These processes are also the main cause of earthquakes and volcanic activity, and have led to the development of many of Earth’s largest mountain ranges.
It’s because of plate tectonics that we have the continents that exist today, but it’s also because of plate tectonics that the continents were different in the past, and are constantly changing into something else at a rate too slow for most of us to notice. Until about 175 million years ago, for example, all of the continents we know today were joined together in a single continent called Pangaea, surrounded by a massive world-spanning ocean. This is one of the reasons that we see some similar animal and plant species on different continents today – because they used to be connected.
So, much like the layers of the Earth, the implications of its inner workings go pretty deep!