The Earth’s deepest layer – the inner core – was formed between a billion and 1.5 billion years ago as it ‘froze’ from the surrounding molten iron outer core, according to a new study published today in the journal Nature.
The inner core of Earth has an inner core of its own, with crystals aligned in a different direction. Image credit: Lachina Publishing Services.
The Earth’s inner core is a ball of solid iron smaller than the Moon which is surrounded by a liquid outer core.
The inner core is a relatively recent addition and establishing when it was formed is a topic of scientific debate with estimates ranging from 0.5 billion to 2 billion years ago.
According to the new study, led by Dr Andy Biggin from the University of Liverpool, UK, there was a sharp increase in the strength of the Earth’s magnetic field between 1 and 1.5 billion years ago. This increased magnetic field is a likely indication of the first occurrence of solid iron at the center of our planet and the point in its history at which the solid inner core first started to ‘freeze’ out from the cooling molten outer core.
“This finding could change our understanding of the Earth’s interior and its history,” Dr Biggin said.
“The results suggest that the Earth’s core is cooling down less quickly than previously thought which has implications for the whole of Earth Sciences.”
“It also suggests an average growth rate of the solid inner core of approximately 1 mm per year which affects our understanding of the Earth’s magnetic field.”
The Earth’s magnetic field is generated by the motion of the liquid iron alloy in the outer core.
These motions occur because the core is losing heat to the overlying solid mantle that extends up to the crust on which we live producing the phenomenon of convection.
Once the inner core started to freeze, this convection received a strong boost in power because light, non-metallic elements remained molten in the outer core and were buoyant relative to the overlying liquid.
The process continues today and is thought to be the main source of ‘fuel’ for generating the Earth’s magnetic field.
“The theoretical model which best fits our data indicates that the core is losing heat more slowly than at any point in the last 4.5 billion years and that this flow of energy should keep the Earth’s magnetic field going for another billion years or more,” Dr Biggin said.
“This contrasts sharply with Mars which had a strong magnetic field early in its history which then appears to have died after half a billion years.”
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A.J. Biggin et al. 2015. Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation. Nature 526, 245-248; doi: 10.1038/nature15523
