Why do some volcanic eruptions produce pumice that can float on water?
Quick Answer
Pumice forms when gas-rich magma cools so quickly that bubbles get trapped inside, making the rock extremely porous and light. Some pumice is less dense than water because it can be mostly empty space.
The Full Story
Pumice is created during explosive volcanic eruptions when magma is rich in dissolved gases such as water vapor, carbon dioxide, and sulfur gases. As pressure drops near the surface, those gases expand rapidly and leave behind countless tiny holes when the lava solidifies. This can make pumice so porous that its density falls below 1 gram per cubic centimetre, which is why some pieces can float at first. Over time, the pores fill with water and the rock may sink. Pumice is closely related to volcanic ash and is common in eruptions that are highly explosive, such as those at Mount St. Helens in 1980 and Mount Pinatubo in 1991.
Key Facts
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Suggested video title for this topic:
"How can a rock float? The volcano-made stone with thousands of holes"
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