Pumice is a highly vesicular (porous) volcanic glass produced during explosive eruptions. It forms when gas-rich, frothy magma cools so rapidly that the gas bubbles remain trapped as holes within the rock. Because it is filled with air pockets, its density is extremely low—often lower than that of water—allowing it to float until it becomes waterlogged. ANSWER: (B) A light, porous volcanic glass that floats on water
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Geographically, pumice is a unique volcanic product associated with silica-rich, explosive eruptions. When magma containing high concentrations of dissolved gases is suddenly depressurized, it “foams” up like a carbonated drink. If this foam solidifies instantly, it creates pumice. The rock is so full of tiny vesicles (air bubbles) that it acts like a sponge. Large “pumice rafts” can sometimes be seen floating on the ocean after submarine eruptions, traveling thousands of miles. It is economically significant, used as an abrasive in industry and in horticulture to improve soil aeration and drainage.