Subject: The Earth - The Floating Crust

The following has to be explained in terms of isostasy.  An area 
covered by a large ice cap sinks.

I believe it has to density.  When the density of the earth's 
crust decreases its thickness increases causing the area covered 
by the ice cap to sink because the ice is heavy.  I can't find 
anything specific in my book.  Is this correct?

Re: The Earth - The Floating Crust