-> Remember, the *entire* crust of the earth is only about 50 miles thick,
-> and the radius of the earth is about 4000 miles, making the crust only
-> about 1.25% of the radius. The core of the earth is believed to consist
-> mainly of nickel-iron and spinning; thought to be the mechanism that
-> generates the Earth's considerable magnetic field - very much stronger
-> than any of the other rocky planets. Unless the core is absolutely
-> uniform in consistency, there will undoubtedly be shifting of mass as
-> it spins. Even if it is uniform, precession of the spinning mass would
-> cause some shifting, just as the earth itself wobbles. The amount of
-> material in the core is hundreds, perhaps thousands, of times more
-> massive than the entire crust of the earth, let alone just the water,
which
-> is only a tiny fraction of the mass of the crust itself. And the core
is
-> moving fairly rapidly (hence the magnetic field), which the crust
isn't.
:-)
But is the core material moving *radially* at any significant speed? It
would have to do so in order to change the earth's radius of gyration
(or moment of inertia).
Remember that the moment of inertia is <sigma>M.R^2. If R changes, i.e.
material moves toward or away from the axis of rotation, the moment
will change, but the change will be bigger if R is already large. So
moving material in the crust will have a larger effect than a similar
motion in the core.
I think I read that the earth's rotation speed changed measurably when
the earthquake occurred that caused the big tsunami in Asia a couple
of years ago.
dow
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