Let me take you back in time. No, not 4.54 billion years ago to when the Earth was forming – I want to take you back to high school geography lessons (sorry). As you may recall, the Earth is formed of a number of layers.
At its centre is the inner core, a solid lump of iron about the same temperature as the surface of the sun. Surrounding this is (surprise, surprise) the outer core, which is liquid. Further out still we have the mantle, a large layer of melted rock that is neither quite liquid nor quite solid. We sometimes get to see it break through the crust (the rock we stand on) in the form of a volcanic eruption. Additionally, the movements of this mantle layer are responsible in part for plate tectonics and the earthquakes we suffer from time to time.
I can see heads nodding, so I’ll leave the geography lesson there.
For a long time, the process by which the Earth got these characteristic layers has been a mystery to scientists. The early Earth was a chaotic place, it’s ever-shifting surface under constant bombardment by rocks from outer space, so scientists have struggled to understand how such neat and ordered layers could be formed. Recently though, scientists at Stanford University have conducted experiments that may provide the beginnings of an answer.
Researchers took tiny amounts of iron and silicate rock and squeezed them together between the tips of two diamonds to recreate the intense pressure found at the centre of the planet. They then fired laser beams into the mix to simulate the extreme heat and watched what happened. Using x-ray imaging, they came to the conclusion that – under these extreme conditions – iron can flow between the silicate rocks in a process known as percolation.
You will be familiar with percolation if you have ever prepared a pot of coffee. The process by which water passes through the coffee grounds and into the pot below is percolation, and the theory is no different here. The iron on the surface of the Earth, made liquid by heat, was filtered through the rock, joining up with other streams from the surface until it formed a central pool that now makes up the core.
There are competing theories to explain the layers, of which this is only one (search “magma ocean theory” if you want to find out more). However, I rather like the idea that the Earth is one big pot of coffee, so I’ll be sticking with this one.