this is technically an episode of the 1966 Thor cartoon, but it's adapted from the Avengers (issue #5). possibly the first animated Avengers story, though there are several others adapted into cartoon form in the other series as well. I love how the heroes do not look roided out- they are muscular, but still look human.
this episode actually presents both sides of an issue- the Lava Men aren't actually evil, which is a nice thing. I want to read the original story now. it's that good!