Re: Why is the center of the Earth Not hollow?

That's a pretty good question! 

It is true that the Earth is not hollow, and that IF there were a cavity in 
the center an object would feel no gravity if placed there. It is however 
not correct that the centrifugal force would be particularly strong near 
the center - because centrifugal force goes as the distance from the axis 
of rotation and as we are near that axis here the centrifugal force is 
lowest near the center of the Earth. 

But what about that cavity? Well, first of all the Earth was formed by 
accumulation of rocks, gravel, gas and boulders from the inside and out, so 
there never was a cavity near the center. 

I think your thought experiment goes along the lines 'what IF there was a 
cavity near the center of the Earth - would it be crushed shut or would it 
remain?' 

I think the answer is that while there may be no gravity in the cavity 
there would be plenty of pressure in the rocks from the rocks above - this 
pressure would be felt all along the walls of the cavity and the pressure 
would be high enough (not to mention the heat from radioactivity that 
actually melts the rock) that the rock would flow and one place it sure 
would flow would be into that nice empty cavity. So the cavity would be 
gone very quickly!

Please feel free to resubmit this question as I am not too sure I answered 
it correctly!