Re: Would ocean levels rise if the force of gravity decreased?

One of the great unsolved problems of physics is how the force of gravity
ties in with the other forces: the electromagnetic and nuclear forces that shape
the structure of all matter. We simply do not know whether the world could work
with a different size force of gravity without it affecting a whole lot of other
things.

But lets just suppose that it could work like that. We all wake up tomorrow
morning, and gravity is just half the size of what it was, but none of the other
laws of physics have been affected in any way. What effect would that have on
the Earth?

A good way to think of the result is that the effect would be felt wherever
we might use the word "pressure". Starting with the oceans, the hydrostatic
pressure at any depth would halve. The water would therefore become less
compressed. But water is very incompressible anyway. The compressibility is
roughly 4 * 10^-10 /Pa (CRC Handbook of Chemistry & Physics, Edition 56, page
F-18). The very deepest water in the ocean is now at a pressure of about 1000
atm = 10^8 Pa. Even this water is only compressed by 4%, which would become 2%
when the pressure is halved. On average, the loss of compression in ocean water
would amount to much less than 1%.

The mean depth of the world's oceans is 3800 m (CRC p. F-192), which means
the average piece of ocean water is at a depth of just half that, or 1900 m.
This corresponds to a pressure of 190 atm, and a compression of 0.76%. Loss of
half of that compression means an expansion of 0.38%, or about 11 metres of
sea-level rise.

However, there is also the small matter of expansion of the solid earth. This
is much more problematic and unpredictable. The big question is how plastic the
crust would be: could it simply flow in a uniform expansion, or would there be
major cracking and movement, with a lot of associated earthquakes and volcanic
activity? There are different compressibility factors associated with each
different type of rock or metal in the earth's interior. They are roughly one
tenth the size of that of water at ordinary pressures, much smaller at the
extreme pressures of the Earth's interior. 

But when you allow for the depth of rock being around 1000 times larger than
the average depth of the oceans, you can see that the expansion of the solid
earth, whether it occured smoothly or chaotically, would be a much larger factor
than expansion of the ocean water. If everything was smooth, it would actually
look as if the sea level had fallen rather than risen, because the expansion of
the solid earth would make a much larger basin for the oceans to fill. 

All of this is quite speculative, and it is not really science, because it
can only be about an imaginary world, and science is about the real world!