Question: how long does lava take to turn into obsidian

Chris Werner answered on 15 Nov 2017:

Yup, exactly as Sarah said, only a few minutes. You would usually get obsidian forming when lava cools in water. The larger crystals in a volcanic rock, the longer it takes to cool. Look at granite, blacks, whites, creams and tiny, but large crystals. Basalt on the other hand, like in Hawaii you’d need a microscope to see the crystals, usually less than 0.05 mm across, and that cools quickly too!