At the centre of the Earth, a giant sphere of solid iron is slowly swelling. This is the inner core and scientists have recently uncovered intriguing evidence that suggests its birth half a billion years ago may have played a key role in the evolution of life on Earth.
At that time, our planet’s magnetic field was faltering – and that would have had critical consequences, they argue. Normally this field protects life on the surface by repelling cosmic radiation and charged particles emitted by our sun.
But 550m years ago, it had dropped to a fraction of its current strength – before it abruptly regained its power. And in the wake of this planetary reboot, Earth witnessed the sudden proliferation of complex multicellular life on its surface. This was the Cambrian explosion, when most major animal groups first appeared in the fossil record. Now scientists have linked it to events at the very centre of the Earth.
Our planet consists of spheres. There is a 5-70km-thick layer of rock that covers Earth like an eggshell. This is called the crust and below it lies the mantle, made up of a 3,000km layer of silicates. Further down, there is the outer core, made of molten iron, and inside it there is another sphere – of solid iron. It is more than 2,000km in diameter and is growing by about a millimetre a year.
“Earth’s magnetic field is generated by swirling iron in the outer core,” said John Tarduno, professor of geophysics at the University of Rochester, New York. “Before the Cambrian explosion the core was entirely molten and its ability to generate a magnetic field was collapsing.” [Continue reading…]