A cut-away of Earth's interior shows the solid iron inner core (red) slowly growing by freezing of the liquid iron outer core (orange). Seismic waves travel through the Earth's inner core faster between the north and south poles (blue arrows) than across the equator (green arrow). The researchers concluded that this difference in seismic wave speed with direction (anisotropy) results from a preferred alignment of the growing crystals--hexagonally close packed iron-nickel alloys, which are themselves anisotropic--parallel with Earth's rotation axis.
Credit: Daniel Frost
For reasons unknown, Earth's solid-iron inner core is growing faster on one side than the other, and it has been ever since it started to freeze out from molten iron more than half a billion years ago, according to a new study by seismologists at the University of California, Berkeley.
The faster growth under Indonesia's Banda Sea hasn't left the core lopsided. Gravity evenly distributes the new growth—iron crystals that form as the molten iron cools—to maintain a spherical inner core that grows in radius by an average of 1 millimeter per year.
But the enhanced growth on one side suggests that something in Earth's outer core or mantle under Indonesia is removing heat from the inner core at a faster rate than on the opposite side, under Brazil. Quicker cooling on one side would accelerate iron crystallization and inner core growth on that side.
This has implications for Earth's magnetic field and its history, because convection in the outer core driven by release of heat from the inner core is what today drives the dynamo that generates the magnetic field that protects us from dangerous particles from the sun.
"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 3 billion years ago, so other processes must have driven convection in the outer core at that time."
The youngish age of the inner core may mean that, early in Earth's history, the heat boiling the fluid core came from light elements separating from iron, not from crystallization of iron, which we see today.
"Debate about the age of the inner core has been going on for a long time," said Daniel Frost, assistant project scientist at the BSL. "The complication is: If the inner core has been able to exist only for 1.5 billion years, based on what we know about how it loses heat and how hot it is, then where did the older magnetic field come from? That is where this idea of dissolved light elements that then freeze out came from."
Freezing iron
Asymmetric growth of the inner core explains a three-decade-old mystery—that the crystallized iron in the core seems to be preferentially aligned along the rotation axis of the earth, more so in the west than in the east, whereas one would expect the crystals to be randomly oriented.
Evidence for this alignment comes from measurements of the travel time of seismic waves from earthquakes through the inner core. Seismic waves travel faster in the direction of the north-south rotation axis than along the equator, an asymmetry that geologists attribute to iron crystals—which are asymmetric—having their long axes preferentially aligned along Earth's axis.
If the core is solid crystalline iron, how do the iron crystals get oriented preferentially in one direction?
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