The Solid and Liquid Cores

The core of the Earth takes up about half the Earth's radius and 32.5 percent of its mass. Studies of the Earth's moment of inertia factor along with the movement of seismic waves through the Earth's interior show that the core is made of very dense material. The only real options, in terms of elements that are common enough in the inner solar system and also heavy enough to create the Earth's core, are iron (Fe) with the addition of a little nickel (Ni).These ideas are supported by finding iron meteorites made of mixtures of iron and nickel, thought to represent the broken-apart cores of planetesimals that did not survive the chaos of the early solar system.

To fit the constraints on the density of the core, knowing its radius, the core has to consist of 84.6 percent iron (about 27 percent of all the iron in the Earth), 7.7 percent nickel, and then an additional ~7.7 percent of some lighter element. Debate over what the lighter component might be is ongoing in the geological sciences. The best candidates are sulfur (S), potassium (K), oxygen (O), carbon (C), hydrogen (H), and silicon (Si). For each of these elements, there are reasons to support its existence in the core, and reasons to argue against it, and there are scientists who are protagonists for each and opponents of others. The light element in the core has to be soluble in the liquid iron in the outer core and abundant enough that some would be available to leave the mantle and enter the core during core formation. The fact that the outer core is liquid was first discovered by seismologists trying to understand the very complex waves caused by earth-quakes.When two parts of the crust are being forced past each other,

A major earthquake can cause the whole Earth to move according to patterns called free oscillations.

most often by plate tectonic forces, stress builds up in the rock to the point that the rock suddenly breaks and is able to slide past itself to relieve the pressure.This sudden breaking and slipping is the cause of the earthquake.

At the site of an earthquake, four kinds of waves are produced in the Earth as a result of the energy release and movement of the rocks. The first two are called body waves, because they move through the earth itself. The second two are called surface waves, because they propagate along the interface between the atmosphere and the earth. The two kinds of body waves are P-waves (P for primary, and also for pressure) and S-waves (S for secondary, and also for shear). P-waves move as fronts of compression and decompression, exactly like sound waves in the air. P-waves can move through liquids, gases, and solids (remember that you can hear while underwater, because this kind of wave can move through liquid). S-waves move as up-and-down shearing motions, like an oscillating string or a moving snake. The movement of this kind of wave requires that the material it is moving through holds together, having some strength. Shear waves cannot move through liquid. The two kinds of surface waves are called Love waves and Rayleigh waves, and they cause much of the earthquake damage experienced at the surface. Earthquakes also cause the whole Earth to oscillate, like a floating bubble after it has gently bumped into an object.These movements are called free oscillations and over

Free Oscillations

Radial

Spheroidal ("Football")

Radial

Spheroidal ("Football")

Toroidal (twisting)

5- and P-wave shadow zones are regions on the Earth's surface that cannot be reached by 5- and P-waves from a given earthquake because of the inability of 5-waves to move through the liquid outer core and by the refraction of P-waves through the same liquid.

1,000 modes have been found; a few of the simplest are shown in the figure on page 44. Sometimes the press announces that a huge earthquake has caused the Earth to "ring like a bell."This analogy refers to free oscillations, though they occur for all sizes of earthquakes, just less strongly for small earthquakes. (After major earthquakes the press has sometimes reported that the Earth moved out of its orbit, but it is physically impossible for earthquakes to cause that.)

So what do earthquake waves have to do with identifying the outer core? Since S-waves cannot pass through liquids, the S-waves from an earthquake are prevented from reaching the other side of the Earth if they have to pass through the outer core. There are zones on the surface of the Earth about 110 degrees from earthquakes where S-waves are not received at the surface.These are called shadow zones (see figure above). They were discovered in about 1910, and from them it was deduced that the outer core at least had to be liquid.

Inge Lehmann, a Danish geophysicist, subsequently discovered in 1936 that the Earth has a solid inner core. Her work is a great story of early success by a woman in a field traditionally populated only by men. By studying seismic waves she was able to discern the strange waves that had passed through the liquid outer core, through the solid inner core, and back out through the other side of the liquid core before traveling back up to the surface to be detected on seismometers. These waves had properties that could only be explained if they had passed through a solid inner core:The travel time of P-waves that go through the center of the core are appreciably faster then those that skirt the outside, indicating the center contains some dense, solid material through which waves travel faster.The discontinuity between the liquid outer core and the solid inner core, 3,200 miles (5,150 km) down, is named the Lehmann discontinuity.The name was given only recently, but Lehmann was still alive, in her nineties, and could enjoy the honor.

The liquid outer core convects much as the mantle convects, though 106 times faster and possibly more turbulently. Fluid motions in the outer core reach 10 to 100 kilometers per year, meaning that it is moving several meters per hour, movement fast enough to see if you were able to watch it.

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