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Gravitational forces stop Earth’s asymmetrical core from tipping us over

todayAugust 1, 2021 1

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More than 5,000 kilometers beneath us, Earth’s solid metal inner core wasn’t discovered until 1936. Almost a century later, we’re still struggling to answer basic questions about when and how it first formed.

These aren’t easy puzzles to solve. We can’t directly sample the inner core, so the key to unravelling its mysteries lies in collaboration between seismologists, who indirectly sample it with seismic waves, geodynamicists, who create models of its dynamics, and mineral physicists, who study the behavior of iron alloys at high pressures and temperatures.

Combining these disciplines, scientists have delivered an important clue about what’s happening miles beneath our feet. In a new study, they reveal how Earth’s inner core is growing faster on one side than the other, which could help explain how old the inner core is, and the intriguing history of Earth’s magnetic field.

Early Earth

Earth’s core was formed very early in our planet’s 4.5 billion-year history, within the first 200 million years. Gravity pulled the heavier iron to the center of the young planet, leaving the rocky, silicate minerals to make up the mantle and crust.

Earth’s formation captured a lot of heat within the planet. The loss of this heat, and heating by ongoing radioactive decay, have since driven our planet’s evolution. Heat loss in Earth’s interior drives the vigorous flow in the liquid iron outer core, which creates Earth’s magnetic field. Meanwhile, cooling within Earth’s deep interior helps power plate tectonics, which shape the surface of our planet.

As Earth cooled over time, the temperature at the center of the planet eventually dropped below the melting point of iron at extreme pressures, and the inner core started to crystallize. Today, the inner core continues to grow at roughly 1mm in radius each year, which equates to the solidification of 8,000 tons of molten iron every second. In billions of years, this cooling will eventually lead to the whole core becoming solid, leaving Earth without its protective magnetic field.

Core issue

One might assume that this solidification creates a homogeneous solid sphere, but this isn’t the case. In the 1990s, scientists realised that the speed of seismic waves travelling through the inner core varied unexpectedly. This suggested that something asymmetrical was happening in the inner core.

Specifically, the eastern and western halves of the inner core showed different seismic wavespeed variations. The eastern part of the inner core is beneath Asia, the Indian Ocean and the western Pacific Ocean, and the west lies under the Americas, the Atlantic Ocean and the eastern Pacific.