Journey to the REAL Center of the Earth

Illustration for article titled Journey to the REAL Center of the Earth

This Friday sees the release of Journey to the Center of the Earth, the second major film adaptation of Jules Verne's groundbreaking 1864 novel. While director Eric Brevig's sweet 3D imagery depicts the Earth's core as a biosphere in a bubble of lava, the real center of the Earth might be even more fascinating. We've hit the geophysics books and brought back for you . . . a realistic journey to the center of the Earth, and the burning gravitational well you'd really find there.


Anyone who's spent time with Newton's law of universal gravitation can tell you that we're all being pulled toward the center of the Earth. In fact, if you were to drill a hole through the Earth and jump through, you wouldn't fall to the other side and stay there — you would fall through the center, heading for the surface on the other side, and then be pulled back to the center by the gravitational force of the Earth. You would oscillate back in forth this way (with a period of about an hour and twenty minutes, by the way) until you ended up suspended at the exact center of the Earth. And you'd be stuck, unless you had some serious propulsion.


The virtual journey we're about to go on should be a little more straightforward — and a lot less dangerous.


This is the outer shell of the Earth, and it comes in two categories: the continental crust, or the part right under our feet; and the oceanic crust, which is the layer of rock at the bottom of the seas. While the continental crust is composed mainly of silicon and aluminum oxides, the oceanic crust is made up of mafic rocks — that is, mostly magnesium and iron. The continental crust tends to be less dense than the oceanic crust, but both are formed by lower-density materials in the mantle that rise to the surface over time. Temperatures of the crust get higher as you venture further down, and they can reach up to 1000 degrees Fahrenheit on the boundary with the upper mantle.


Here's where it gets interesting. Though the crust and the upper mantle (up to about 660 km down) are effectively solid, after that temperatures become so high that the material of the mantle becomes plastic — which means that under a certain level of stress, the body of the lower mantle actually begins to flow like a liquid. In the movie Journey to the Center of the Earth, what sent our heroes running was lava at 200 degrees Fahrenheit; I wonder what they'd do if someone told them that temperatures in the mantle range between about 1000 to 7200 degrees Fahrenheit.


Because of the mantle's plasticity, deformation of the metals can occur in areas called "subduction zones," and over time these metals change location within the mantle layer. Extreme pressures near the bottom — try about 1.4 million atmospheres — cause the material to deform like a fluid, and it rises up to relieve the pressure. Below about 650 kilometers, all of the minerals in the mantle start to destabilize and convect like this. Once they reach a higher point in the mantle, however, the release of pressure and the cooler surrounding area lead to a drop in temperature of the stressed metals. The lower-density metals can also rise to the upper boundary, eventually becoming part of the Earth's crust. This entire "downwelling" process is extremely chaotic — that is to say, freakishly nonlinear. But it works.


As less dense materials and their chemically-bound buddies migrate to the Earth's crust, the denser metals remain in liquid form as a layer surrounding the Earth's core. In the outer core, these metals are mostly iron and nickel, heated at temperatures approximately between 7200 and 9000 degrees Fahrenheit and pressurized to over 1 million atmospheres. The film features a biosphere at the core heated by explosive gases, but that's, um, just plain wrong. It also shows our intrepid explorers heading to the core through "volcanic tubes," but as classic as the wooden-railed mine cart ride is, that's a fantasy that would disappear instantly into molten rock under the extreme heat and pressure of the outer core — not to mention the mantle first.


Since obviously no one can get down this far (the deepest humans have gotten is a smidge over 12 kilometers), seismologists have determined details about the mantle and the core by analyzing reflections and refractions of earthquake waves at the surface. They also estimate that convection in the outer core is responsible for sustaining the Earth's magnetic field, or magnetosphere — that's a concept that's very confused in Journey when Fraser and his cohorts leap across "magnetic rocks" at the center of the planet. In case you're also confused by the giant overhanging mushrooms and the ethereal beauty of the caverns and waterfalls, let me clear that up for you: They don't exist, either.



The inner core of the Earth is a solid ball of metal — specifically, nickel and iron (just like in the outer core). So you can say goodbye to the fantastic creatures of Verne and Brevig, because there's nothing alive down here. The pressure at the center of the Earth is over 3 million atmospheres, and geologists estimate that the temperature can rise to over 10000 degrees Fahrenheit; even Brendan Fraser can't withstand that. It's the extreme pressure that "freezes" the inner core into a solid, and in fact, scientists believe that the inner core was liquid until 2-4 billion years ago.


A 2005 report in Science claims that the inner core rotates about 0.3 to 0.5 degrees more per year than the Earth's surface. This "super-rotation" probably acts to stabilize the magnetic field created by the outer core, so you have the inner core to thank for your working compass. And in case mine-cart rollercoasters, jagged stalactites, and huge 'shrooms don't do it for you, remember that the real center of the Earth is a spinning sphere of superheated metal. Not too shabby, eh?

"Structure of the Earth" [Wikipedia]

World Book at NASA: Earth [NASA] Earth images by Jeremy Kemp and SEWilco from Wikipedia.


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Stephan Zielinski

The short answer is within a sphere, only the mass "lower" than you are counts for the purposes of the downward acceleration you feel. Alternately, within a hollow shell of uniform thickness, there's no net acceleration due to gravity.

The math: