Well it also depends on how fast the lava comes out too. Some lava comes out slow, and does damage slowly overtime, and other lava comes out fast and destroys everything in its path.
EX. In plymouth colony on Normal, when the lava comes out and is near your mine (the mine at your starter base), it doesn't automatically flow over it (most of the time anyway) and instead deals 10% damage to it till its destroyed. In other places the lava flows so fast that it destroys it completely right away.
Thats why sometimes a vehicle driving next to a lava flow will blow up even though lava hasn't entered into its space.