I'm saying that the lava generated by the base effect does not need Ignem, however it will need ignem to scale up damage.
eg.
CrTe10 = Creates lava inflicting +15 when it splashes somebody and up to +60 if they are full immersed in it (CrTe base 10)
CrTe/Ig 15 = If you increase that damage to +20 the spell must become (CrTe Base 10, +1 mag for +5 extra damage and Ig requisite)
CrTe/Ig 20 = If you increase that damage to +25 the spell must become (CrTe Base 10, +2 mags for +10 extra damage and Ig requisite)
To envelop a target you'll need a fair amount of lava, so +1 mag might be enough, but I think +2 mags is better for size. It depends on the base size of lava, which I cannot find at present (is it 1 inch, 1 foot, 1 pace, 10 paces?).
The target of the spell for Creo is always the thing being created. The moving the created material around is sometimes cosmetic, or a secondary effect, or another spell (varies with the form of the spell). Where is appears is limited by the range of the spell, and how precise is sometimes a contention. After reading some Ignem and Aquam spells, I'm thinking now that you may not need it anyway as some of those spells to not need targeting either (like Mighty Torrent of Water, Ars p.121); however Terram effects sometimes require Finesse to drop stones on people but other forms do not.
Should lava be guided by terram (which typically needs Rego or Finesse), or from Aquam or Ignem (which does not)? No idea really, perhaps as terram spells as it is terram, but thats a real line call.
...roughly spec'ed as spells ... (I have no idea what the base amount created by Lava is, so I've assumed it is 1 cubic foot)
Ball of Burning Lava
Creo Terram 20, R: Voice, D: Momentary, T: Individual
A ball of lava covers a small area within range. This inflicts +15 damage to items and people splashed by the lava. This spell creates 1 cubic foot ball of lava.
(Base 10, +2 Voice)
Spray of Molten Lava
Creo Terram 35, R: Voice, D: Concentration, T: Individual
for as long as you concentrate a jet of lava sprays from your outstretched hand and covers an area within range. This inflicts +15 damage to items and people splashed by the lava, +20 for those partially covered, and up to +60 damage for anyone fully enveloped by the stream. This spell creates a cube of lava 100 cubic feet in size each round, enough to cover several people or the side of a building.
(Base 10, +2 Voice, +1 Concentration, +2 size)
Erruption of Molten Lava
Creo Terram / Ignem 45, R: Voice, D: Momentary, T: Individual
A massive spraysof lava is created within the target area. This inflicts +25 damage to items and people splashed by the lava, +30 for those partially covered, and up to +70 damage for anyone fully enveloped by the stream. This spell creates a cube of lava up to 1000 cubic feet, enough to envelop a small building.
(Base 10, +2 Voice, +3 size, +2 inflicts +10 extra damage, Ignem Req)
(edited for horrible typos)