Does Australia float as an island?

Answer 1

Australia is known as 'the island continent' for it is large enough to qualify as a continent, but is connected to no other land mass. [Similarly with Antarctica.]

Australia is a very ancient craton - part of the earth's original surface land mass. The surface lands of the Earth, well, the larger ones anyway, are mainly of a generalized rock known as sial, a light weight silica rich rock type.

Lying under the sial skin, lie the heavier rock type called sima - emphasizing the manganese and iron content of the rock, and its higher density. This rock is erupted as the heavier (and darker) volcanic rocks.

The lighter crust rocks may be considered to 'float' on the denser rocks, and indeed the continental rocks have a deeper root below them, and the crust is much thicker there.

Incidentally - islands do not float. Islands are merely mountains rising above the surface of the ocean.