Why do some rocks float on water?
Quick Answer
Some rocks, especially pumice, float because they are full of trapped gas bubbles that make them less dense than water, at least until the pores fill and they eventually sink.
The Full Story
Most rocks are denser than water and sink immediately, but pumice is a striking exception. Pumice forms when frothy, gas-rich lava erupts violently and cools so quickly that the gas bubbles are frozen in place, leaving the rock riddled with tiny sealed pores. This trapped air lowers the overall density of the rock below that of water, so it floats. Over time, water gradually seeps into the pores and the pumice becomes waterlogged and sinks. Large volcanic eruptions can create floating rafts of pumice that drift across the ocean for months, carrying marine organisms to new locations.
Key Facts
YouTube Angle
Suggested video title for this topic:
"The Rock That Floats — How Pumice Cheats Gravity on Water"