In the center of The Earth.
Scientists can only see it when they study seismic waves (earthquakes generated by earthquakes) passing through the planet’s solid iron. inner core. For some reason, waves travel through the core significantly faster when they travel between the North and South Poles than when they travel across the equator.
Researchers have known about this discrepancy – known as seismic anisotropy – for decades, but have been unable to come up with an explanation that is consistent with the available data. Now, using computer simulations of core growth over the last billions of years, a new study in the June 3 issue of Nature Geoscience offers a solution that finally seems appropriate: Every year, little by little, the Earth̵
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“The movement of liquid iron in the outer core removes heat from the inner core, causing it to freeze,” study lead author Daniel Frost, a seismologist at the University of California, Berkeley, told Live Science. “So that means the outer core takes more heat from the east side [under Indonesia] than in the west [under Brazil]. “
To visualize this bilateral growth in the core, imagine a tree trunk with growth rings radiating from a central point, Frost said – but “the center of the rings is offset from the center of the tree” so that the rings are farther away. on the east side of the tree and closer together on the west side.
The cross section of the Earth’s inner core may look similar. However, this asymmetric growth does not mean that the inner core itself is deformed or at risk of becoming unbalanced, the researchers said.
On average, the radius of the inner core grows steadily by about 0.04 inches (1 millimeter) each year. Gravity corrects single-leaf growth in the east by pushing new crystals to the west. There, the crystals accumulate in lattice structures that extend along the north-south axis of the nucleus. These crystal structures, arranged parallel to the Earth’s poles, are seismic superhighways that allow earthquake waves to move faster in that direction, according to team models.
Unpacking the snowball
What causes this imbalance in the inner core anyway? That’s hard to say without looking at all the other layers of our planet, Frost said.
“Every layer of the Earth is controlled by what is above it and affects what is below it,” he said. “The inner core slowly freezes from the outer liquid core, like a snowball, adding more layers. The outer core is then cooled by the mantle above it – so let’s ask why the inner core grows faster on one side than the other may ask why one side of the mantle is cooler than the other? “
Tectonic plates may be partly to blame, Frost said. Because cold tectonic plates dive deep below the Earth’s surface at subduction zones (places where one plate sinks under another), they cool the mantle below. However, whether cooling the mantle can affect the inner core is still a matter of debate, Frost said.
Equally puzzling is whether unilateral cooling in the core can affect the ground magnetic field. The modern magnetic field is driven by the movement of liquid iron in the outer core; the movement of this fluid is in turn driven by heat loss from the inner core. If the inner core loses more heat to the east than to the west, then the outer core will move more to the east, Frost said.
“The question is, does this change the strength of the magnetic field?” He added.
Such big questions are beyond the scope of the team’s new document, but Frost said he has begun work on a new study with a team of geomagnetists to explore some possibilities.
Originally published in Live Science.