there is a mod called tropicraft(i don't know if it is still being updated though), that doesn't only add volcanoes with a large amount of lava, but they even spawn fireballs of death spawning lava where they hit(once they are full, before they are, they fill up).
Volcanoes with Surface-Accessible Central Lava Chambers™
Lava does not exist in "chambers", because it is by definition exposed to the surface (although lava lakes found in vanilla Minecraft would satisfy these conditions). Magma chambers are a real thing, but by definition they are not exposed to the surface; in fact, magma chambers are at least 1km underground.
drain below Bedrock (which is presumably supposed to be the mantle)
Since magma chambers are farther underground than 64m (the accessable world height in Minecraft) this sort of thing would be (quite) below the Bedrock layer. I think the traditional mindset is that the underside of the Bedrock layer is the Nether, however. Maybe that's why there aren't many volcano mods?
