The wording on this is awkward. Can you please revise it to something along the lines of:
"Lava Trail: Lava Snail creates
a lava field pool on your opponent's
field after each attack." (?)
Anyway, the concept is also pretty cool - creating 'dead zones' where you don't want your creatures to be, in case you Fractal a whole bunch of these guys and they all happen to target the immortalized creature you have on the field. Of course, you have to remember than Flooding doesn't kill immortalized/burrowed creatures, so technically this shouldn't affect them either.
Also, don't you think its attack is high for its non-upgraded cost? I imagine Snails to be quite pacified, if not weak in nature (the upgraded card can have a boost in attack, but as it is now, it seems like it's a bit too strong when you set it out). Of course, it's fodder for Paradox and nearly every other creature control killer out there, so I guess having the attack higher than the HP is a good thing.