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Name: susan b hieber
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999


Question:
My fourth grade class wants to know what the Earth's inner and outer core is made from, and how do scientists know?


Replies:
It is curious that earthquakes occurring near the Earth's surface have given us most of our information about the Earth's core. When an earthquake occurs, it sends out seismic waves that travel throughout the earth. Instruments far away from the earthquake pick up the motions caused by these waves. By examining the motions of these waves, we can learn about the structure of the Earth, including the core.

For example, we can say with much confidence that the outer core of the Earth is a fluid. How is that known? There are two kinds of waves that travel through the Earth: p-waves and s-waves. The p in p-wave stands for "primary"; p-waves were given this name since they are observed to arrive first after the earthquake. The s in s-wave stands for "secondary," since these waves arrive after the p-waves. It is an interesting property of s-waves that they do not pass through fluids, while p-waves do. Following a large earthquake, say in California, seismographs all over the world pick up the seismic waves generated by it. But some of these stations show only p-waves, not s-waves. This tells scientists that part of the earth is fluid and not solid.

By more complicated analyses of these earthquake recordings, scientists can estimate the density of the material in the Earth's core. Knowing this, scientists can make a guess at what makes up the core. Because the core is determined to be very dense, it is thought that iron and nickel may be major constituents. But this is very speculative: nobody is certain what is down there.

-Grant


Grant has told us truth as far as it is known. But I would like to point out that a recent article in the Science section of the New York Times discusses the growing fraction of scientists that believe that the core of the Earth is a giant mega-crystal, and not molten at all! There is a variety of data, but mostly we have the fact that there have been a number of mega- earthquakes in the last few years, and measuring the time it took the reverberations to pass through the Earth in various directions shows that there are significant anomalies. Believing that the interior of the Earth may be a megacrystal would explain (1) why sound waves which pass from china to bermuda take a different amount of time than those which travel freom the north pole to the south pole (different crystal axes transmit vibrations at different rates) and (2) the origin of the strange magnetic properties of the Earth, including the wandering of the magnetic pole and its periodic reversal (which is reminiscent of a ferromagnet near its critical point). Food for thought, eh?

prof topper


Very interesting. I have only heard of this idea in passing, and this is not my field of expertise; however, it seems to me that the megacrystal theory must apply only to the solid inner core, and not the outer core. It is known that S-waves (shear waves) are strongly attenuated in the outer core. This means that the outer core does not have much resistance to a shearing force, which is basically the definition of a fluid. It seems to me that if the outer core were a crystal, it would be able to transmit shear waves.

Rereading my message, I see I was not clear in stating that the inner core is believed to be solid, while the outer core is believed to be fluid. Perhaps this is where the confusion came from.

-Grant



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