I'd say don't make it an energy control power but an Element Control like Air, or Water. Its like a plasma really, which is a semi-solid attack really with a Radiation (Heat) base. I say radiation as that seems to be heat in this game. If not you could say fire but I think we'd have problems with 'fire' and we could add the heat category to the types of energy. Heat actually is like a Fatigue effect of fire. It drains your energy so to speak, like when you get dehydrated and feel worn out, heat can drain you.
Lava generates heat no doubt, and can light stuff on fire too due to the sheer heat. I would say that it should be a higher pp cost power or come with a built in 'Area' of effect like 5' square per rank. Thus it is an area based attack and not a direct attack. Even a 'stream' of it could be directed but still effects an 'area' it is shot at. Thus it would be a Reflex save power perhaps. I would say it does Radiation damage, or heat, and that things that are flammable will ignite and possibly cause further damage in rounds thereafter to anything touching the Fire so it could have a dual damage sorta of function. I would also give it a built in Fatigue effect as it drains your energy to fight, and Distengration which 'destroys' stuff as that is all the lava is doing, destroying whatever it comes into contact with. I would say that anyone immune to Radiation or Fire would take stun damage from the lava, not lethal but that by its very nature, Lava is ONLY LETHAL damage, you can't stun someone with it. Sure if it were colder lava, the colder rock would stun them with kinetic damage perhaps and the heat of the lava is stun like in that it fatigues you but I would put as a restriction on the player who takes that power they can't use dual damage' stunt and they must select lethal. This means you won't find that many hero types running around with this power as it can easily destroy lots of stuff and kill innocents, but that is the point right. So its a great mutant 'I can't control my power' type of power or one for a villain like a Pyro type who loves to burn stuff.
My friend has a character like that. He is an 72 year old man, the living embodiment of the Hawain volcano god.
If you simply give yourself Power Immunity and just to be safe, immunity to radiation and fire you could do stuff like create Armor or Protection with Impervious extra for yourself as you encase yourself in lava like a shell. If you want just keep it flowing around you in molten form and use it as an Energy Field. All good extras for the power.
People would think your a lava monster or something, not knowing there is a cremey center in there, your little hero/heroine. Or you could simply do 'Alternate Form - Semi-Solid which could already include molten lava. Since I already want to interpret that power as granting you only one type of each form you take ie you can only turn into molten lava and then if you pick solid you could turn into solid lava rock'. Ofcourse that power is open to interpretation.
If you want this to be a very comic book like hero then give him the flaw like Pryo where he cannot generate nor summon lava, only control the flow of it, thus he'd need to hang out by a volcano to do anything really. Or he could technically carry around a little rock of lava, not molten and quite cold but since he can control the flow he could turn it into molten form and with power immunity thus hold it in his hand and shape it, grow it, and collect more rock and start his own large pool of lava to work with.
That would be a great villain. Encase a School in lava, and then ask for money or you won't let anyone out. I don't think the police can easily get through a bunch of lava.