Like said before, this should be an card with ability cost. Volcanoes are mountains with lava coming out of them, and if we have to pick an element for a mountain, I cannot see any other choices than . To me would seem like a perfect fit, probably one of the easiest element choices ever.
But my biggest issue with this card is the fact that this card is a creature. A volcano.. not a permanent but a creature. Um.. what? This would lead to all kinds of silly things like being able to lobotomize a Volcano or even eat it. If a mountain is not a "permanent" then what is?
Thematically, I would make Volcano so that it is a permanent and you can make it erupt, say every 3 rounds or so, and it recharges in between eruptions. This would better fit to what a Volcano actually is and does in real life. But if it has to be a creature, then a new theme would be better.
Also, have you considered changing "Supermassive" into something else because it sounds like a lame techno band. Massive, Colossal or Enormous would fit better in the fantasy theme imo.
The problem then becomes that since this is a series, then what would fire's supermassive creature be? It's hard to come up with something in the natural world made out of fire big or imposing enough to warrant upwards of 70 health. A volcano is indeed a mountain that spews lava. However, if it didn't spew lava, it wouldn't be a volcano. It would be a mountain. The fire is what
makes it a volcano. Since it has a standing attack, it's meant to imply that it is continuously erupting, maybe from a side channel or something, so it is always creating a lava flow.
If it were just a mountain, then yes, maybe it could be earth. But it's not. It's a mountain spewing fire. Just because firefly queen is a bug doesn't mean it should belong to life. It's a bug that, above all else, lives in the air. Same for the ghost of the past. Saying "it should be death, because it's really just a ghost, from times since past" is akin to saying "it should be earth, because it's really just a mountain that spews fire" as both elemental choices ignore a key feature of the card's identity.
Also, yes, many of these creatures are more like landforms in their current form. That's more a result of me not being artistic and not taking the time to find royalty free art that truly fits. Same for the actual name of the card. The name can change; it could be "Volcanoid Beast" for all I care. In fact, that'd probably be a much better name for it. The problem is that then I'd have to find art that fits -- and creating art like that (making a volcano into a more human or animal like figure) is way above my ability.
I mean, honestly, when I make a card idea, it's more for the concept, not the aesthetics. The name "supermassive" is included in these aesthetics as something I don't really care about. Massive, Colossal, or Enormous are all also fine names in my opinion. The concept behind the name is what matters, such as the cost, ability, attack, and defense.