Pumice is a foam — volcanic glass full of holes. It floats on water for months, sometimes years, traveling thousands of kilometers across oceans. This floating is remarkable because everything about pumice says it shouldn't.
The pores in pumice are highly interconnected. The rock is not a closed-cell foam like styrofoam; it's an open-cell sponge where pathways connect the interior to the surface. Water wets volcanic glass readily. Put these facts together and the prediction is straightforward: water should wick into pumice through capillary action, fill the pore network, and the rock should sink within hours.
It doesn't. X-ray imaging of pumice soaking in water revealed why. As water enters through the connected pore network, it traps gas in the interior. The advancing water front isolates bubbles of air within pore throats, cutting them off from the atmosphere. Surface tension then locks these bubbles in place. The rock becomes buoyant not despite the water entry but partly because of it — the water does the trapping.
The mechanism is counterintuitive because the substance that should cause sinking is the substance that prevents it. Water entering through narrow pore throats acts as a valve, sealing gas behind it. The geometry of the pore network — the same interconnectedness that should enable flooding — creates bottlenecks where gas gets stuck.
Submarine pumice, rapidly cooled by seawater, retains even more buoyancy because the quenching creates truly isolated pores with no external connection. But the interesting case is subaerial pumice — erupted into air, with fully connected pore networks — which still floats for months because the water itself becomes the sealant.
The invasion is the defense. The water that would make the rock dense becomes the mechanism that keeps it light.