The Earth is not a single uniform mass. It comprises a number of different layers, starting with the outer layer or crust. Beneath this is the mantle and then a liquid outer core. The inner core lies at the centre of the planet. Danish seismologist Inge Lehmann first deduced there was a solid inner core distinct from the liquid outer core in 1936 after studying seismic waves bouncing back from the core.
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The Earth's inner core is thought to be about 1,516 miles in diameter. This makes it about 70 percent the size of the Moon, according to Universe Today. It lies at a depth of between 3,219 to 3,981 miles beneath the Earth's surface and accounts for approximately 1.7 percent of the Earth's mass.
The inner core is mostly iron but is also thought to contain significant amounts of sulfur and nickel. Estimates of its temperature vary, but it is probably somewhere between 5,000C and 7,000C (9,000F and 13,000F), according to National Geographic. This is around the same temperature as the surface of the sun and more than enough to melt iron, but the vast pressures exerted produce the solid inner core.
The inner, inner core
Research published in 2008 by geologists from the University of Illinois suggested the Earth may have an "inner, inner core." This is still largely made up of solid iron but has a different crystalline structure, according to Universe Today. It is believed to have a diameter of 733 miles, just less than half that of the inner core as a whole.
The Earth's magnetic field is generated mainly at the core. Electric currents and an accompanying magnetic field are generated by the rotation of the fluid outer core around the solid inner core. The Earth's magnetic field is essential to life on the planet as it helps shield the planet from solar radiation.
Rate of spin
The inner core is believed to spin independently of the rest of the Earth, according to the Hawaiian Volcano Observatory. It rotates in the same direction but at a different speed. Estimates of the rate of spin vary. Some scientists believe the inner core has gained a quarter-turn on the planet as a whole over the past 100 years. Others believe it would take 1,000 years or more for the inner core to completely "lap" the planet.
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