Terraforming volcanic ?
Why not.. for the game.
But have in mind that terraforming planet is a biological operation.
Upgrade a barren world => You add some basics species, like lichens or some insects.
Upgrade a desert => You add some grass, some tree...
A biome downgrade due to pollution is also a modification of the biology of the planet.
Terraforming up or down is a biological modification.
On the other side, terraforming a volcanic planet is a "geological" operation.
This is not a terraforming like the other in the game.
So, in the tech tree this should not be the same tech.
Technology for this volcanic modification is close to technologies to build planet from asteroid.
If you want to add this, i suggest to put this "volcanic terraforming" here in the tech tree.
Also and IMO, I thinks that "volcanic terraforming" for the Silicoid should be moved in the tech tree.
Their terraforming to infernal planet is described as "explosions well placed"... This is not a "terraforming" operation like the others are.
And IMO, because it is a "geological" operation, you don't change the biome, you simply destroy it.
So => Player should be able to terraform any biome to a volcanic one, at the same cost whatever the biome is.
... Silicoid will be happy ... balance problems incoming IMO... 
Terraforming a volcanic to... barren ? (this is what the owner of this post want i suppose ?) should also be a geological operation => Put this in the tech tree with planet construction technologies.
Have in mind that "cooling" a volcanic planet require powerfull technologies... but in a game where you can build planets... Why not...
IMO it is not be only a problem of technology: It is physically impossible to stop a volcanic activity (how and where do you move the energy of the planet core ?)...
But in a game where you can prevent supernova to explode... why not 
Edited by Laegad, 18 July 2016 - 08:42 PM.