It can melt a boulder? What is this stuff that's so hot? It's lava. It's also probably made from melted boulders too, come to think of it.
This word uses the on'yomi reading for the kanji, but you haven't learned the on'yomi reading for 岩, which is がん. When you think of lava, think about Gandhi, coming out of the lava. He's immune, because he's a robot.
We can’t go any closer because the volcano is spewing out lava and it’s dangerous.