New insight into Earth’s crust, mantle and outer core interactions

A correlation between the movement of plate tectonics on the Earth’s surface and the rate of reversal of the Earth’s magnetic field which has long been hypothesized.

Earth's crust
Image credit: Kay Lancaster

A new study by the University of Liverpool, in collaboration with the Universities of Lancaster and Oslo enlightens a correlation between the movement of plate tectonics on the Earth’s surface, the flow of mantle above the Earth’s core and the rate of reversal of the Earth’s magnetic field which has long been hypothesized.

According to their findings, it takes around 120-130 million years for chunks of the old sea floor to sink (subduct) from the Earth’s surface to an adequate depth in the mantle where they can cool the core, which thus causes the liquid iron in the Earth’s outer core to flow more vigorously and produce more reversals of the Earth’s magnetic field.

Liverpool palaeomagnetist, Professor Andy Biggin, said: “Until recently we did not have good enough records of how many global rates of subduction had changed over the last few hundreds of millions of years and so we had nothing to compare with the magnetic records.

“When we were able to compare them, we found that the two records of subduction and magnetic reversal rate do appear to be correlated after allowing for a time delay of 120-130 million years for the slabs of ocean floor to go from the surface to a sufficient depth in the mantle where they can cool the core.

“We do not know for sure that the correlation is causal but it does seem to fit with our understanding of how the crust, mantle, and core should all be interacting and this value of 120-130 million could provide a really useful observational constraint on how quickly slabs of ancient seafloor can fall through the mantle and affect flow currents within it and in the underlying core.”

Scientists analyzed the data from using records and proxies of global rates of subduction from various sources including a continuous global plate reconstruction model developed at the University of Sydney. These records were contrasted with another compilation of magnetic field inversions whose event is bolted into volcanic and sedimentary rocks.

This magnetic field is formed deep inside the earth in a fluid outer core of iron and other elements that creates electric currents. The core is encompassed by an about 3,000 km thick mantle, made of strong shake, streams gradually (mm every year).

The mantle produces convection currents which are strongly linked to the movement of the tectonic plates but also affect the core by varying the amount of heat that is transferred across the core-mantle boundary.

The Earth’s magnetic field occasionally flips its polarity and the average length of time between such flips has changed dramatically through Earth’s history. For example, today such magnetic reversals occur on average four times per million years but one hundred million years ago, the field essentially stayed in the same polarity for nearly 40 million years.

This study is published in Tectonophysics.