small volcanic rocks nearly the size of Belgium
is Belgium that small
... never knew pumice floated ...
It's formed from lava that has a lot of dissolved gases in it. When the lava hardens before the gases can escape, a rather sponge-like rock that typically can float is created.
But it's interesting that they'd find a mass of it floating free. It's usually only found in areas with current or past geologic activity. And those lumps have rounded edges, they may have been in the water a long time.