Final answer:
Pumice is the lava blown out of a volcano in an explosive eruption which then hardens around gas bubbles. It is a vesicular rock with low density that can float in water.
Step-by-step explanation:
Lava blown out of a volcano in an explosive eruption which then hardens around gas bubbles is called pumice. Pumice is a vesicular felsic igneous extrusive rock with very low density that it can even float in water. It is formed when magma cools quickly on the Earth's surface, trapping gas bubbles within the rock.