Freddie Heina, age 10, of Winston Salem, North Carolina for his question:

What causes bubbles in rocks?

The earth has several recipes for making rocks with pores, pockets and holes that look like Swiss cheese. Most stones are mixtures of minerals    hard ones as well as softer ones that dissolve and wash away with seeping ground water. When pieces of softish minerals are embedded in hard bedrock, they may dissolve away and leave empty holes. Sometimes the groundwater deposits its dissolved chemicals. It may line the walls of a rocky pocket and add a few crystals. Then we get a colorful bubble called a geode. However, the earth uses a very different recipe to put the bubbles into lava rocks.

For example, a pumice stone has so many airy bubbles that it is light enough to float. It looks like a piece of frozen foam    and that is just about what it is. It was made when a volcano erupted a molten mixture of minerals, hot gases and frothy steam. This foamy lava cooled fast because the air was very cold. The steamy gases had no time to escape. They were trapped and when the liquid minerals cooled and became solid, the gases left bubbles inside the lava rock.