 |
 |
Composition of the Earth's core
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
Click here to return to the General Topics Archives
| |
Update: June 2012
|
|
