The inner core of the Earth, which is believed to be growing about one millimetre per year, may be melting, scientists have claimed.
According to researchers at the University of Leeds in England, this melting could actually be linked to activity on the Earth's surface and the findings could help explain how the core generates the planet's magnetic field.
The 2,400km wide inner core which is a ball of solid iron about the same size of the moon is surrounded by an outer core made up mostly of liquid iron-nickel alloy, a highly viscous mantle layer and, topping it off, a solid crust that forms the surface of the planet.
As the Earth cools from the inside out, it is believed that the molten outer core is slowly freezing, leading the inner core to grow at a rate of about one millimetre per year.
"The standard view has been that the inner core is freezing all over and growing, but it appears that there are regions where the core is actually melting," said Sebastian Rost, a seismologist who led the research.
"The net flow of heat from core to mantle ensures that there's still overall freezing of outer core material and it's still growing over time, but by no means is this a uniform process," he was quoted as saying by LiveScience.
As the Earth's interior cools, relatively hot and cold matter churns around inside the planet a process known as convection. The roiling of material in the core, coupled with the spinning of the Earth, is what generates the planet's magnetic field, the scientists said.
Using computer models of convection in the outer core in conjunction with seismology data, the researchers found the flow of heat at the boundary of the core and mantle depended on the overlying mantle.
At times, the nature of the mantle is enough to force heat from the mantle back onto the core, leading to melting in places, the researchers reported in the journal Nature.
"Only a small fraction of the inner core's surface may be melting at any given time," co-researcher Jon Mound said.
"However, given the size of the inner core, even if only one per cent of the surface is melting, which is certainly possible, that corresponds to just under 200,000 square km."
For instance, when it comes to large regions under Africa and the Pacific where the lowermost mantle is hotter than average, the outer core below those areas can become hot enough to start melting the inner core.
On the other hand, beneath seismically active regions around the so-called "Ring of Fire" the cold remnants of oceanic plates sucked to the bottom of the mantle are drawing a lot of heat from the core, helping it freeze.
These findings suggest "that the whole dynamics of the Earth's core are in some way linked to plate tectonics, which isn't at all obvious from surface observations," Mound said.
"The localised melting theory could also explain other seismic observations for example, why seismic waves from earthquakes travel faster through some parts of the core than others," Rost said.