Problem with emmisive materials is that they have a very hard decay in illumination.
So those are very practical for very close distance lights; but the decay of illumination is very fast. Many times you would not be able to light your room using this kind of materials unless you use huge numbers that will burn the objects that are close to the emmiter.
You can also use spotlights that have a radius and a length (like 3d properties), that will make the emmiter to have a different shape than a point.
Finally you can also use an interesting trick: You add a white plane thar “refracts” the light, as a photographer would do. (just by pointing a spot on that plane that floats over your lava). After you bake the scene illumination, you just make that plane not to be renderable in it’s properties. For some extra control, you can set the plane to cast shadows or not cast shadows over the lava underneeth, it’s up to you how it works better.
That would build up a smooth scattered area light from the refraction of the plane. Very usefull trick to fake area lights, and make great architectural ullumination.