The Earth's core melts and freezes all at the same time

The inner core of our planet is roughly as big as the Moon, and we can only guess what's going on deep inside our planet. We might now have an answer...and it's even more volatile and weird than we thought.

Here's what we know (or at least are pretty sure we know): the inner core itself is a ball of solid iron. It's surrounded by a liquid outer core that's primarily composed of a liquid iron-nickel alloy, then by the viscous mantle, and finally by the solid crust. The core was once completely molten, but as the Earth has slowly cooled over the last few billion years, it's become more and more frozen.

This process leads to a couple of things. One, because solid iron has a larger volume than the same amount of liquid iron, the core is slowly growing by about one millimeter a year. Two, this freezing process gives off excess heat, which travels all the way to the Earth's crust through the process of convection. This heat, among other things, is partially responsible (along with the Earth's spinning) for generating our planet's magnetic field.

While we understand a decent amount about what's going on far down below, there's still quite a bit that remains unexplained or poorly understood. University of Leeds researcher Dr. Jon Mound explains the basic problems, and why his team's new research might go a long way to solving some of these mysteries:

"The origins of Earth's magnetic field remain a mystery to scientists. We can't go and collect samples from the centre of the Earth, so we have to rely on surface measurements and computer models to tell us what's happening in the core.

"Our new model provides a fairly simple explanation to some of the measurements that have puzzled scientists for years. It suggests 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. If our model is verified it's a big step towards understanding how the inner core formed, which in turn helps us understand how the core generates the Earth's magnetic field."

Here's how it all fits together. The researchers examined heat flow at the boundary between the core and the mantle. In areas like the Pacific Ring of Fire, the tectonic plates are undergoing subduction, which involves one of the plates being pushed underneath into the other and into the mantle below. Because the crust is cooler than the mantle, this subduction process causes a net loss of heat in the mantle. As the subducted ocean plates sink to the bottom of the mantle, they begin to suck up heat from the core itself. All this exacerbates the freezing process down in the core.

But here's where things get interesting - it's also possible for plate tectonics to produce the opposite effect. There are areas beneath Africa and the Pacific Ocean where the plates are so large that very little heat can escape upwards, meaning the mantle is hotter than average. This in turn means that there is less heat flow from the core. The heat stays further down, and the core can actually become hot enough for the solid inner core to start melting again, in a reversal of the general trend.

This means that Earth's inner core is simultaneously melting and freezing, which is a pretty neat trick. And, as fellow researcher Dr. Binod Sreenivasan of the Indian Institute of Technology points out, it may mean understanding the core just got a lot more complex:

"If Earth's inner core is melting in places, it can make the dynamics near the inner core-outer core boundary more complex than previously thought. On the one hand, we have blobs of light material being constantly released from the boundary where pure iron crystallizes. On the other hand, melting would produce a layer of dense liquid above the boundary. Therefore, the blobs of light elements will rise through this layer before they stir the overlying outer core. Interestingly, not all dynamo models produce heat going into the inner core. So the possibility of inner core melting can also place a powerful constraint on the regime in which the Earth's dynamo operates."

Overall, this is a promising result, and it could indeed explain some basic questions about the inner core, as team member Dr. Sebastian Rost, also of the University of Leeds, explains:

"The standard view has been that the inner core is freezing all over and growing out progressively, but it appears that there are regions where the core is actually melting. 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. Our model allows us to explain some seismic measurements which have shown that there is a dense layer of liquid surrounding the inner core. 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."

Via Nature.