Earth’s inner core is growing lopsided

As the Earth’s nether regions cool and solidify, the solid-iron inner core at its heart expands by about a millimeter each year. According to a study, one side appears to be growing faster than the other — but scientists have no idea why.

This phenomenon is most likely related to the creation of the inner core, which occurred between 1.5 billion and half a billion years ago. After billions of years of cooling, the Earth’s fiery interior had finally lost enough heat to initiate a continuous process of crystallization. As the molten iron in the outer core loses heat, it crystallizes to form the newest layer of the inner core.

The center of this hyperactive hemisphere is located 1,800 miles (2,896 kilometers) beneath Indonesia’s Banda Sea, where 60 percent more iron crystals form than on the other side of the world.

The inner core now has a radius of about 750 miles (1,207 km) and a temperature of more than 9,000 degrees Fahrenheit (4,982 degrees Celsius). Despite this, even after an eon of lopsided growth, it has not deformed. Gravity constantly shapes it, redistributing excess in the east and keeping it spherical. This asymmetrical growth may help power the Earth’s magnetic field, in addition to providing an intriguing puzzle (and enable our survival).

Planetary air conditioning

The inner core has enormous influence on us surface dwellers despite being such a small and distant layer in the colossal onion we call home. Daniel Frost, lead author of the new study and a geophysicist at the University of California, Berkeley, jokes that he is “always having to justify the importance of the inner core.” In fact, we exist because it cools, releasing heat and causing convection in the outer core. Ultimately, that churning liquid iron (known as the geodynamo) generates the magnetic field that protects life on this planet from dangerous solar winds.

Similarly, the upper layers have an impact on the core. “Everything is affected by what’s above it,” Frost says. The inner core is surrounded by the outer core, which is surrounded by the mantle, which is surrounded by the crust. So, in order for the inner core to grow, it must transfer heat to each successive layer — some residual from Earth’s formation, some radioactive from decaying elements. Each layer, in turn, must be capable of receiving heat.

While these “ice cubes” are small, he believes they are sufficient to tip the scale. “This is all a delicate balance,” he says. “I don’t think you need much to precipitate a difference like this.” This explanation, however, may be overly simplistic: It’s unclear whether heat from the bowels of the Earth dissipates in a vertical line. The diving Indonesian crust could just as easily cool the core underneath the China or Saudi Arabia as it could below Indonesia.

Earthquake superhighway

For now, the asymmetry itself remains unexplained, but it does offer one solution to another longstanding enigma: why the iron crystals in the inner core align parallel to Earth’s North-South rotation axis. (Whilst also no one has directly observed the structure of the core, seismologists have discovered that earthquakes travel faster through the core between the North and South Poles than they do across the equator.) All things being equal, the crystals should be aligned randomly.

The answer, according to the Berkeley researchers, is in the lopsided formation of the core. According to their computer model, gravity redistributes the crystals, causing them to “flow.” Frost compares it to “throwing sticks into a river.” “If the river is flowing, the sticks will line up with it.” Similarly, because the inner core is flowing, the crystals align with it and form an orderly lattice that serves as a high-speed interstate for subterranean tremors.

More research is needed to understand the relationship between this asymmetry and the magnetic field. But given the vital role this planetary armor plays in our existence, it’s worth investigating the processes that underpin it. Scientists have known for a long time that the magnetic field reverses every so often (we’re due for a switch), and that it weakens temporarily during the transition. But it’s unclear why. Frost says that when new discoveries about the Earth’s core are made, “the question is always, ‘Does this relate to the reversal of the magnetic field?'”

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