Convection at the Center of the Earth
The inner core of the Earth, its innermost part, is a primarily solid ball with a radius of about 760 miles, according to seismological studies. It is believed to consist primarily of an iron—nickel alloy, and to be about the same temperature as the surface of the Sun. A new study, by an international team of researchers from Leeds, London and California, states that rocks could be circulating in the inner core which may explain the unusual behavior of seismic waves passing through it.
All models that invoke convection to explain the observed seismic variations in Earth's inner core require unstable inner core stratification. Previous work has assumed that chemical effects are stabilizing and focused on thermal convection, but recent calculations indicate that the thermal conductivity at core temperatures and pressures is so large that the inner core must cool entirely by conduction.
Natural convection is a mechanism, or type of heat transport, in which the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature gradients. In natural convection, fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current; this process transfers heat energy from the bottom of the convection cell to top.
"Slow cooling of the whole Earth is causing the liquid outer core to solidify from the bottom up, adding to the edges of the solid inner core. That material, at the top of the inner core, is denser than the material below," says Dr Chris Davies, one of the authors of the study, from the University of Leeds, "When you have dense material overlying light material the light material wants to rise and the dense wants to sink - making it unstable. It is this instability which causes convection to occur."
Some researchers assumed the inner core was hotter in the center and that this change in temperature — from the center to the edge — could also cause convection as the cool material at the edge wants to sink.
But Davies says the convection is due to heavier, not cooler, material. "We show the driving force of the convection is different to previous assumptions. Instead of the driving force being a difference in temperature, it's now a difference in composition."
"Whilst previous work put a dampener on the convection argument, and it seemed convection wasn't enough to prove these seismic anomalies, we show that convection could be behind the striking inner core complexity we observe." says Davies. "We show that this mechanism is possible, in principle."
For further information see Convection.
Core image via Wikipedia.