Study on Earth’s inner core

Context: Earth’s inner core, a hot iron ball the size of Pluto, has stopped spinning faster than the planet’s surface and might now be rotating slower than it, research suggested on Monday.

About the Study

  • In research published in the journal Nature Geoscience on Monday, Yi Yang, associate research scientist at Peking University, and Xiaodong Song, Peking University chair professor, studied seismic waves from earthquakes that have passed through the Earth’s inner core along similar paths since the 1960s to infer how fast the inner core is spinning.
  • A 1996 study in Nature also revealed that the travel times of seismic waves that traverse the Earth’s inner core show a small but systematic variation over the past three decades. This variation is best explained by a rotation of the inner core and the rotation rate is on the order of 1° per year faster than the daily rotation of the mantle and crust.

Key Findings of the Study

  • The study published in Nature Geoscience states that the globally consistent pattern suggests that inner-core rotation has recently paused. The rotation came to a grinding halt in 2009 and then it surprisingly turned in an opposite direction. Researchers have long believed that the inner core rotates, relative to the Earth’s surface, back and forth, like a swing.
  • “One cycle of the swing is about seven decades, meaning it changes direction roughly every 35 years. It previously changed direction in the early 1970s, and predicted the next about-face would be in the mid-2040s,” researcher said.
  • The team from Peaking University analysed earthquakes mostly from between 1995 and 2021 and the analysis revealed that sometime around 2009 the core stopped spinning and might be in the process of changing the spinning direction.
  • The spin of the inner core is driven by the magnetic field generated in the outer core and balanced by the gravitational effects of the mantle. Knowing how the inner core rotates could shed light on how these layers interact and other processes deep in the Earth.

What Is Earth’s Inner Core?

  • Earth’s layers are divided into three parts: the crust, mantle, and core
  • Earth’s inner core was first discovered in 1936 as researchers were studying seismic waves from earthquakes that travel throughout the planet.
  •  It was the change in the waves that revealed Earth’s core, which is around 7000 kilometers wide and made up of a solid center of iron wrapped inside the shell of liquid iron.

Structure Of The Earth

The Crust

  • It is the outermost solid part of the earth. It is brittle in nature. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region. It is made up of heavier rocks having density of 3 g/cm3. This type of rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm3.

The Mantle

  • The portion of the interior beyond the crust is called the mantle. The mantle extends from Moho’s discontinuity to a depth of 2,900 km. The upper portion of the mantle is called asthenosphere. The word astheno means weak. It is considered to be extending upto 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions. It has a density higher than the crust’s (3.4 g/cm3). The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lower mantle extends beyond the asthenosphere. It is in solid state.

The Core

  • As indicated earlier, the earthquake wave velocities helped in understanding the existence of the core of the earth. The core mantle boundary is located at the depth of 2,900 km. The outer core is in liquid state while the inner core is in solid state. The density of material at the mantle core boundary is around 5 g/cm3 and at the centre of the earth at 6,300 km, the density value is around 13g/cm3. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.


  • Researchers have said that the rotation of the core is related to the changes in the length of the day, and it could lead to small variations in the exact time it takes for Earth to rotate on its axis and that there are links between the different layers of the planet -crust, mantle, and core.
  • The team said that the observations provide evidence for dynamic interactions between the Earth’s layers, from the deepest interior to the surface, potentially due to gravitational coupling and the exchange of angular momentum from the core and mantle to the surface.

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