SCIENCE FOCUS
Take even a quick peak beneath Earth’s surface and you soon discover just how much we don’t know about what’s happening right under our feet.
We spend so much of our time focused on the world around us that we rarely give much thought to what’s going on at the core of our planet. If Earth were an apple, the crust that we live on would only be as thick as the apple’s skin.
The structure of the Earth
How hot is the Earth’s core?
Like an apple, Earth also has a core tucked away within, buried beneath a layer called the mantle. The core formed early, just 200 million years after Earth itself coalesced, some 4.5 billion years ago. Earth’s core is large – almost equivalent to half the size of Mars – and there’s such extreme pressure crushing down on it that its temperature is as hot as the surface of the Sun.
To put that into numbers, that’s around 6,000°C! Keep in mind, too, that Earth’s core is only around 3,000km from the surface – if the Sun were as close as that, it would melt us entirely.
What is the Earth’s core made of?
There are two main parts to the Earth’s core; an inner core and an outer core, followed by the mantle and the crust:
1. Inner Core
A solid, crystallised iron structure that’s under immense heat and pressure. Each layer of the crystal structure is thought to be hexagonal in shape, although there may actually be two separate crystalline structures present. The crystals are believed to align roughly north-south to match the orientation of the Earth’s rotation axis and its magnetic field.
2. Outer Core
This is the only truly liquid layer of Earth’s internal structure. Around 2,000km thick, the outer core is mostly iron and nickel, with between five and ten per cent made up of lighter elements. The transition between the inner core and outer core is located approximately 5,150km beneath Earth’s surface.
3. Mantle
Together, the crust and the top half of the mantle make up the lithosphere, which is broken into tectonic plates that shift. These shifts cause earthquakes and the continents to drift. The mantle is by far the largest part of the Earth, making up 84 per cent of its total volume.
4. Crust
The crust is split into the oceanic crust, which is a maximum of 10km (6.2 miles) thick, and the continental crust, which can be as much as 80km (49.7 miles) thick in places. The crust rises and falls by up to 25cm each day as the Moon pulls on it.
How do we know the composition of the core?
Earthquakes have played an indispensable role in our understanding of this internal structure. The modern seismometer, invented in 1880, measures the vibrations from earthquakes as they ripple through the planet. In the early 20th century, scientists assumed that Earth’s core was completely molten and the material’s movement was responsible for generating the planet’s magnetic field.
Then, in 1936, the Danish seismologist Inge Lehmann was able to determine, through the use of seismometers, that seismic waves were bouncing off something deep inside Earth. She correctly concluded that the planet’s core was composed of two parts: a solid inner core, nested Russian-doll-style, inside a molten outer core.
But more recent work is revealing that the reality could be a touch more complicated. Dr Thanh-Son Phạm and Prof Hrvoje Tkalčić from The Australian National University tried something different. “We claim the detection for the first time of ricocheting seismic waves, which propagate from the earthquake source to the other side of Earth, and back, up to five times,” Phạm says.
“The detection is significant because it allows a new way to probe the very centre of Earth, which was very unlikely in the past.” It’s a technique that’s often been used in the search for new minerals but not for probing Earth’s inner structure.
Publishing their findings in February 2023, Phạm and Tkalčić analysed data from the growing network of seismometers set up across the planet. The important part was getting data from close to the epicentre of the earthquakes and then from the exact opposite spots on the other side of the planet, known as the antipode.
The reason this has been tricky in the past is because earthquakes tend to cluster around an equatorial belt dominated by oceans and other remote areas.
When an earthquake strikes, the ensuing vibrations reverberate inside the planet for days. They take about 20 minutes to cross from one side of the Earth to the antipode. Phạm and Tkalčić saw up to five back-and-forth bounces from several magnitude-six earthquakes.
