Will a dense area of rock under the surface increase gravity locally?

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Meghan asked the Naked Scientists:
Will a dense area of rock under the surface increase the local strength of gravity? If so, why?

What do you think?
« Last Edit: 05/12/2010 22:30:03 by _system »


Offline graham.d

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Yes. Such gravitational variations are used as a means of predicting what materials may be present underground as an aid to geological surveying for minerals and oil reserves.

Mathematically, the only reason we consider gravity as reasonably constant over the earth's surface is that we assume spherical symmetry and a regular distribution of density. This is not a bad assumption for everyday use but is inadequate at describing the fine detail. Gravity does vary both in magnitude and direction but only to a small degree. This is not noticeable by human senses but is detectable by instruments.


Offline granpa

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mountains on the other hand are like icebergs.
They are less dense than basalt and the displace their own weight in it.
as a result they have less of an effect on gravity than you might otherwise expect


Offline Chemistatwork

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Yes it would. That's why if you look at a "gravity map" of the moon's surface, you would see patches with microscopically higher gravity than the other places.



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Offline CliffordK

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The more dense "stuff" closer to the surface will affect the gravity locally. 

If you look at the Grace Project Geoid Map of the earth, you will see that the gravity (as viewed from space) of the surface of the Planet is not at all uniform).  And, in fact, it is not constant over time either (atmospheric density changes?).


There is a huge fallacy of the Grace Project which is not discussed thoroughly. 

This is a view of Earth's Gravity from space.  However it ignores the effect of elevation.  So, while the major mountain ranges (Andes, Rockys, Himalayans, etc) show up as higher gravity from space, there is an altitude effect on earth which would lead you to actually experience lower gravity on the surface of the planet at the mountain peaks.

Your question, however, could be answered by looking at the oceans.  If you consider the oceans as having a constant elevation (ignoring tides).  Then you could consider water as being low density, and rock as being high density.  One would expect that the more rock near the surface of the ocean, the higher the gravity. 

And, if you look at the Geoid above, it can be difficult to make out a lot of subsurface topography, but the Lapu-Lapu ridge Between Japan and New Guinea is quite obvious (I think that is the formation).  The relatively shallow ocean south of Iceland is also apparent, as well as east of Australia.

Compare the geoid above to the Ocean View on Google Earth.