Mega Golem is a rock and fire dual type, and certainly evokes images of magma with its coloring. The Pokémon has a new ability in its mega form, called Big Bang, which causes Mega Golem to detonate upon fainting. Dragonith, the creator, does not explain this ability further, but perhaps it is a bit like the ability aftermath, or like the move Explosion?
(Update: Dragonith has told me Big Bang functions like the move Self-Destruct)
The only Pokémon that currently has this type combination is Magcargo, but this Pokémon is relatively low tier competitively and another option, such as Mega Golem, would be a boon. Plus, exploding Pokémon (especially Wailords) are fun, right? (Except when it is that shiny Geodude self-destructing before you can catch it, then it is no fun.)