“Descend, bold traveller, into the crater. . .” Jules Verne, A Journey to the Centre of the Earth
Forget the shovel, the drill or the dynamite. You aren’t going to tunnel to the centre of the Earth.
If you want to know what’s way down there, halfway to China, then you’ll have to trust the work of the likes of Illinois geology professor Xiaodong Song and Simon Redfern, a professor of mineral physics at the University of Cambridge.
Song and Tao Wang, a visiting postdoctoral researcher from China, have just published a paper in the academic journal Nature Geoscience on the so-called inner-inner core of the Earth.
They described how they used earthquake-reading technology to conclude that the centre of the Earth is made up of solid iron crystals, aligned roughly east-west.
Surrounding this are more crystals, aligned roughly north-south.
That matters because it can help us understand how the Earth was formed, they concluded.
Song and Wang are publicity shy and declined to be interviewed but Redfern was comfortable talking with the Toronto Star.
“Earth’s interior structure can only be determined indirectly,” Redfern said in an email interview. “You can’t drill down into the Earth more than about 10 km, so you can’t directly observe it.”
The indirect way of learning what’s way down beneath your feet involves studying the vibrations set off by earthquakes. That’s much like how a radiologist uses a CT scan to study the interior of your brain, Redfern said.
“The earthquake waves reveal the properties of the rock that they pass through, and can be detected by seismometers set up all around the globe,” Redfern said.
The Earth is about 12,700 km in diameter. If you could somehow tunnel way down, you would encounter the outer core about halfway into your journey to the centre.
You would be travelling through the solid inner core on the last 1,300 km of the trip.
Surrounding this solid core is molten material, Redfern said, adding it’s the molten outer core that gives Earth its magnetic field.
“No one knows when the solid inner core first appeared, but some estimate that it started growing around one billion years ago,” Redfern said. “It formed because the molten core is slowly solidifying. The solid inner core is composed of iron-rich crystals, crystallizing from the melt that surrounds it.
“It is an almost perfect environment for crystallization — zero gravity at the centre of the Earth, very, very hot, and with very stable temperatures. The crystals are probably very big, but they form together, under the immense pressure at the core (the weight of the huge column of rock above them) to make a polycrystalline metallic inner core.”
“The results don’t tell us much about how Earth was formed,” Redfern said. “The inner core formed a long time after the whole Earth had come into existence. But as you go deeper into the inner core you are, effectively, going back in time. The middle part most likely represents the oldest part, and we know that the outermost part of the inner core is the bit that is solidifying today, so is the newest.”
In Jules Verne’s 1864 pioneering science fiction novel, A Journey to the Centre of the Earth, the core of the Earth is hollow.
Redfern’s scientific knowledge doesn’t diminish his appreciation of the novel.
“I read it as a boy, and again more recently,” Redfern said. “Jules Verne tells a great story, and a journey into the interior landscape of the planet provides a fantastic setting for his narrative, just as space exploration has provided the dominant context for science fiction since he wrote his book.”