The growth on one side of the molten metal is the product of iron crystals that form as the molten iron cools, but something in the Earth’s outer core or mantle under the south Asian country is removing heat at a faster rate than on the opposite side, under Brazil. The faster the cooling, the faster that iron crystallisation occurs – and the faster the growth increases.
Such a disparity has significant implications for the Earth’s magnetic field, and the convection currents in the core that generate the field are what protects us from dangerous solar particles.
While the core is solid iron, it is surrounded by a fluid outer core and then a mantle of hot rock. In the mantle and the outer core, heat from the crystallising iron and hotter rock in the mantle moves upwards towards the surface, pushing colder material down. This movement is what generates the magnetic field.
“We provide rather loose bounds on the age of the inner core — between half a billion and 1.5 billion years — that can be of help in the debate about how the magnetic field was generated prior to the existence of the solid inner core,” said Barbara Romanowicz, UC Berkeley Professor of the Graduate School in the Department of Earth and Planetary Science and emeritus director of the Berkeley Seismological Laboratory (BSL).
“We know the magnetic field already existed three billion years ago, so other processes must have driven convection in the outer core at that time.”
The comparatively young age of the inner core suggests that, in our planet’s history, the heat that keeps the iron liquid came from light elements separating from iron, not from the crystallisation of iron. But a complicated question remains: “If the inner core has only been able to exist for 1.5 billion years … then where did the older magnetic field come from?” assistant BSL scientist Daniel Frost posed. “That is where this idea of dissolved light elements that then freeze out came from.”
One possible explanation could be tectonic plates, with cold plates cooling the mantle when they sink into subduction zones, but it is not known for certain whether mantle cooling could impact the inner core.
However, the asymmetric growth of the core does answer a mystery that scientists have been looking to solve for 30 years – why does the crystallised iron core seem aligned along the rotational axis of the Earth more in the west than in the east? Seismic waves travel faster in a north-south direction than along the equator, due to the asymmetry of the iron crystals, and this difference in growth is a possible explanation.
“The simplest model seemed a bit unusual — that the inner core is asymmetric,” Frost said. “The west side looks different from the east side all the way to the center, not just at the top of the inner core, as some have suggested. The only way we can explain that is by one side growing faster than the other.”
As iron crystals grow, gravity redistributes the excess growth in the east towards the west within the inner core. That movement within the soft solid of the inner core aligns along the crystal lattice, the scientists’ computer model suggests. In the east, the core grows 60 per cent greater than in the west, explaining differences in seismic wave velocity that can result in earthquakes, volcanic eruptions, and other phenomena.
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