That's an excellent question! The answer to this is the same as the answer to another question: Why does ice float?
answer both of these questions we must first realize that something
very interesting happens to water when it freezes: It takes up more
space! To see that this it true, fill a glass half full of water and
make a pencil mark on the glass exactly at the top of the water (better
ask your mom or dad to help with this experiment). Now put the glass in
the freezer overnight. When you look at the glass in the morning it
will have ice in it, and the top of the ice will be slightly above the
pencil mark. This is because the water expanded as it froze.
the ice still weighs exactly the same as the water did the night
before, but is now taking up more space, it will float. You know this
is true...just drop an ice cube in a glass of water and you will see it
stay at the surface.
OK, so what does this have to do with
pressure and melting. Well, the answer is exactly the same as the
above, only in reverse. If we take some ice and push on it hard by
putting lots of pressure on it, we are really just trying to make it
take up less space. One way for ice to take up less space is for it to
turn back into water, which is the answer to your question.
verify that water takes up less space than ice, just leave your glass
of ice (from the last experiment) on the counter until it melts and you
will see that the water is back below the line you drew on the glass.
Hope this helps. MS
(republished on 07/25/06)