This pumice has the same composition as the obsidian next to it. In fact, they formed in the same eruption. The pumice looks very different because it is filled with vesicle, or gas bubbles. These were produced when the magma was depressurized, allowing the gases originally dissolved in the magma to separate as bubbles. This pumice floats on water because its density is so low.
Provided by Echo Moutain Quarry, California
Topic: Earth Science
Keywords: Medicine Lake (Mont.), Obsidian, Pumice, Volcanic eruptions, Volcanism