Re: What, if any are the effects of mtrl. being removed from inside the earth?

Moving mass can produce local changes in the earth's gravitational field and 
concentrations of ferrous materials (iron and steel) can produce local 
changes in the magnetic field, however, these effects are very local (they 
die out within a distance away from the mass that is roughly equivalent to 
its hight or depth).  If you have technical instruments you can use them to 
detect these anomalies (values of the fields that are either higher or lower 
than youwould expect), but the ordinary observer would never notice.  For 
example the mass of a large building neaxt door will alter a gravimeter's 
reading, but if you held a plumb bob up, you would not be able to detect the 
tiny little bit that the mass of the building would pull the bob to the 
side.  Likewise, you can use a magnetometer to find buried steel pipes, but 
unless you put your compass right next to the steel, you would not see the 
needle wavering at all.

Human efforts to reaarange the world, while profoundly affecting the 
surface, are small compared to the whole of the earth.  For example, a large 
city might contain on the order of 10 billion kilograms of steel and another 
10 to 100 billion kilograms of other materials.  That's a lot, but the earth 
as a whole contains six million billion billion killograms of rock, so the 
mass of that city is only about one million billionth of the mass of the 
earth (1 part in 1,000,000,000,000,000).  Even the very small changes in 
magnetic and gravitational force due to the different densities and iron 
contents of different rock types near the surface can be significantly 
bigger than the signals from buildings or other human artifacts.

David Smith
Geology and Environmental Science
La Salle University, Philadelphia, PA