I don't think it works like that. If you are in the middle of an ice burg and drill down to the ocean below it, does water start shooting up through the hole?
Yes, if the bottom of the iceberg-hole is below sea level.
If the mantle at the bottom of this new bore-hole is under pressure and liquid, then it will go up the bore hole until the pressure above it equals the pressure below. If the bottom of the bore-hole hits a part of a liquid mantle with a lot of pressure, then we might get a volcano. Hence, Moria.
