Experimentally, we can mimic this process by plucking melt from a flow and submerging it in a bucket of water. In this instance, the blob of melt literally puffs up. It expands abruptly as water enters the melt, perhaps along cracks and microfractures, while simultaneously expanding to steam. The liquid water expands to roughly 22 times its initial volume during conversion to steam at the Earth's surface. By creating gas pockets, the expansion leads to the lower density that allows the cooled melt, now rock, to float on the ocean surface.
Nothing floats forever.
Sinking results as the steam cools and liquid water penetrates the gas pockets. At first, the floating fragments of quenched lava remain so hot that the voids are pressurized against sea water. As cooling progresses, however, the steam condenses, the pressure is reduced, and liquid is able to infiltrate relentlessly. In the time it takes to unpack a camera and photograph, the fragment, a gently bobbing floater, slips beneath the sea and sinks like the rock it has become. There was a short pause in the eruptive activity of Kilauea Volcano.