There are only 2 rocks that can float on water, and one of them has a fiery origin. Pumice. A light rock full of vesicles (bubbles) that has the ability to spread joy to anyone who has ever encountered large blocks of it in the wild.
Pumice contains at least 50% vesicles, or voids that formed from a large amount of bubbles within the original magma. When the erupted magma cools these bubbles remain in the rock as vesicles.
Many of the vesicles will be joined to other vesicles and form an interlocking maze of tubes throughout the rock; however, others will remain completely isolated and it is these that give the pumice it’s prolonged bouyancy in water.
Pumice from unobserved eruptions in our oceans can be found many hundreds to thousands of kilometres away from their source, and may be the only evidence of some eruptions in the ocean.
Eventually a floating pumice will become water-logged and sink to the depths of the ocean, becoming preserved within the rock record for millions of years.
The only other rock to float on water is called diatomite – a rock formed from the skeletal remains of billions of microscopic organisms called diatoms.