Crossword Genius

It can harden into igneous rock (4)

Ross

I believe the answer is:

lava

(Other definitions for lava that I've seen before include "Volcanic outflow" , "What volcano ejects" , "Molten volcanic rock" , "Hot molten rock" , "Flowing volcanic matter" .)

I've seen this clue in The New York Times.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.