After solidification, dissolved gases are suddenly released, leading to the characteristic frothy nature, but if cooled at greater pressures this pyroclastic igneous rock would have formed obsidian.  
It floats due to the vesicles produced during formation.  Essentially liquid when effused, it cools quickly enough to prevent crystallization.  After solidification, dissolved gases are suddenly released, leading to the characteristic frothy nature, but if cooled at greater pressures this pyroclastic igneous rock would have formed obsidian.  For ten points, name this porous volcanic rock.
It floats due to the vesicles produced during formation.  Essentially liquid when effused, it cools quickly enough to prevent crystallization.  
pumice