Tiny iron particles fall from Earth’s molten outer core and pile on top of the planet’s solid inner core, according to new research published in the Journal of Geophysical Research: Solid Earth.
The inner Earth: white and black layers represent a slurry layer containing iron crystals; the iron crystals form in the slurry layer of the outer core (white); these crystals ‘snow’ down to the inner core, where they accumulate and compact into a layer on top of it (black); the compacted layer is thicker on the western hemisphere of the inner core (W) than on the eastern hemisphere (E). Image credit: University of Texas at Austin.
Our knowledge of the Earth’s core is mainly derived from seismic observations and mineral physics studies, which suggest the core comprises of a solid inner core surrounded by a liquid outer core with iron-nickel and some light element alloy.
The inner core grows as the molten outer core cools and solidifies, releasing heat and gravitational energy that drives the geodynamo and generates Earth’s magnetic field.
Geoscientists study Earth’s core by analyzing signals from seismic waves as they pass through the planet.
However, aberrations between recent seismic wave data and the values that would be expected based on the current model of the Earth’s core have raised questions.
The waves move more slowly than expected as they passed through the base of Earth’s outer core, and they move faster than expected when moving through the eastern hemisphere of the top inner core.
In the early 1960s, Soviet geophysicist S.I. Braginkskii proposed that a slurry layer exists between the inner and outer core, but prevailing knowledge about heat and pressure conditions in the core environment quashed that theory.
However, the new data from experiments conducted by Sichuan University’s Dr. Youjun Zhang and colleagues show that crystallization is possible and that about 15% of the lowermost outer core could be made of iron-based crystals that eventually fall down the liquid outer core and settle on top of the solid inner core.
“It’s sort of a bizarre thing to think about. You have crystals within the outer core snowing down onto the inner core over a distance of several hundred kilometers,” said Dr. Nick Dygert, a researcher at the University of Tennessee.
“The accumulated snow pack is the cause of the seismic aberrations. The slurry-like composition slows the seismic waves,” the scientists said.
“The variation in snow pile size — thinner in the eastern hemisphere and thicker in the western — explains the change in speed.”
“The inner-core boundary is not a simple and smooth surface, which may affect the thermal conduction and the convections of the core,” Dr. Zhang said.
The team compares the snowing of iron particles with a process that happens inside magma chambers closer to the Earth’s surface, which involves minerals crystalizing out of the melt and glomming together.
“The Earth’s metallic core works like a magma chamber that we know better of in the crust,” said University of Texas at Austin’s Professor Jung-Fu Lin.
“In magma chambers, the compaction of the minerals creates what’s known as cumulate rock,” the authors explained.
“In the Earth’s core, the compaction of the iron contributes to the growth of the inner core and shrinking of the outer core.”
“And given the core’s influence over phenomena that affects the entire planet, from generating its magnetic field to radiating the heat that drives the movement of tectonic plates, understanding more about its composition and behavior could help in understanding how these larger processes work.”
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Youjun Zhang et al. Fe Alloy Slurry and a Compacting Cumulate Pile across Earth’s Inner-Core Boundary. Journal of Geophysical Research: Solid Earth, published online October 22, 2019; doi: 10.1029/2019JB017792
