Ryan McAvoy said:

A question:

I always remember growing up with the image of Anakin and Obi-Wan dueling in a Volcano (Or at a push a lava planet) firmly fixed in my mind. Me, my brothers and my friends. So where did I get this notion from? An article? A EU book? An interview?

Rolling Stone interview with George Lucas in 1977:

"Vader kills Luke's father, then Ben and Vader have a confrontation, just like they have in Star Wars, and Ben almost kills Vader. As a matter of fact, he falls into a volcanic pit and gets fried and is one destroyed being."

full article here:

http://www.rollingstone.com/movies/news/the-wizard-of-star-wars-20120504