But consider an ice cube sitting upon a wooden board which is floating in the water. The ice cube causes the board to displace a volume of water which has the same weight as the ice cube. Thus when the ice cube melts and the water runs off the board the water has a volume exactly equal to the volume which the board moves upward. Thus the water level would not rise.

Now consider a container with a layer of thick sludge at the bottom and a layer of water floating on the thick sludge. Suppose there is a board floating on thick sludge and there is an ice cube on the board. When the ice cube is placed upon the board the board displaces an amount of sludge equal to the weight of the ice cube. Since the thick sludge is presumed to more dense than water, the floating board displaces sludge of a smaller volume than the volume of the water coming from the melting of the ice cube. The water level would rise but by an amount corresponding to the difference in density of the water and the density of the thick sludge.

The continents are less dense rock floating in the denser rock of the Earth's mantel. Thus when continental ice melts the continent moves upward freeing space at the bottom of the ocean thus reducing the rise in the sea level due to the melting of continen ice. When the continental ice melts a continent rises. For example, the Scandinavian peninsula is still rising from a rebound resulting from the melting of the glaciers there at the end of the last ice age. The rise of the continents lowers the mantle and the sea above it. The sea level rises but not by the full volume of the water from the melting ice.

There is the additional consideration of much of the ice of Greenland and Antarctica being below sea level. Greenland and Antarctica are in actuality archipelagoes rather than islands. The melting of such ice, of course, does not raise the sea level. This is depicted below.