Obsidian pools in deep ocean floors

Deep ocean floors should sometimes generate ‘pools’ of obsidian, as if an overworld lava pool had generated underground. Or maybe like Hawaiian oceanic lava flows? Why would lava only bubble up to land surface biomes, shouldn’t also bubble up to the surface of the ocean floors too. I know lava flowing into water should create cobblestone and stone, but I think obsidian would make it more obvious that it was lava. Maybe it could be a mix of both.