Pumice or Scoria
Pumice
is a highly vesicular rock that can float on water. It was formed when bubbly lava erupted by a
volcano cooled down rapidly but did not crystallize.
The density of pumice is as low as 0.3-0.4 g/cm³ because
of amount of vesicles. It usually refers to light-colored vesicular acidic glassy volcanic rocks, formed by bubbly
andesitic, dacitic and rhyolitic lavas. But here the
dark-colored basaltic vesicular rock fragments belong to a sort of scoria which
is different from the white silicic pumice.