PSN-L Email List Message

Subject: Re: Articles on gravimeters
From: John Hernlund hernlund@.......
Date: Mon, 01 May 2000 07:47:13 -0700 (MST)

On Mon, 1 May 2000, Tom Schmitt wrote:
> The stories about plumb bobs not hanging vertical are wonderful.  The
> British Royal Surveyers in India
> found the plumb bobs leaning away from the mountains.  They figured this out
> because the distance between
> two points was different when measured by astronomical and standard survey
> techniques.  There is a little more
> to the story and this effect was  known before that.  However  that is the
> example used in text books because the
> story is so good.  (So were the surveyers, thousands of miles and they were
> worried about less than few meters errror
> if someone has a text book fill us in on the ammount of error they found).
> By the way, plumb bobs (lacking wind electostatic and or magnetic
> influences) always point in such a way
> to minimize their potential energy.  Because of density variations in the
> earth, that may not be "down".
> The ammount of variation is slight and insignificant for most purposes.
> It is important for scientific purposes, it is important for  very critical
> surveys is a few areas in the world,  and it is very important for launching
> nuclear weapons  at people.  The later is why the Defense Maping Agency is
> concerned with it.  The missles follow potential energy paths, which may be
> curved relative to other coordinates.  If you want to hit dead on you need
> the shape of gravity field between where you are and where you want it to
> hit.
> Tom Schmitt
> tschmitt@..............

Yes, these are some of the origins of the ideas of isostatic equilibrium.
Many mountain ranges did not deflect a plumb as expected (i.e. toward the
mountain), and the explanation had to include something about the density
under the mountain range.  Two theories came out, one that said that the
mountain ranges had to be less dense than the lower country-side, and the
other that said the root of the mountains containing less dense crustal rock
had to be deeper than the low lowland crustal roots.  The second explanation
has proven to be the better one, because we find that the crustal root in
mountainous zones is far deeper.  This is like a boat sinking down further
into the water under an increased load.  Because the mantle is higher in
viscosity than water of course the mountain range takes some time to sink to
an equilibrium value.  For mountain ranges which are growing very fast (like
the Himalayas) an equilibrium is never achieved because the time for growth is
much shorter than the time for sinking to equilibrium.  Therefore you can use
the deflection of the plumb to tell how fast the range is growing.  The same
effect is observed when removing material from the surface such as erosion and
melting of glaciers, and people studying the latter have used the rate of
uprising to calculate the viscosity of the mantle.

All this came from the initial gravimeter studies...

John Hernlund
E-mail: hernlund@.......



Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]

Larry Cochrane <cochrane@..............>