Lava contains water in vapour form. The amount of vapour that's dissolved in a volcano's lava controls how that volcano erupts. Lots of water equals explosive eruptions. Low levels of water vapour means the lava oozes out without a big explosion.
Solidified lava that had lots of water vapour when it was flowing tends to have lots of bubbles and weighs very little. Solidified lava that had hardly any water vapour in it is heavier and has fewer bubbles trapped inside.