In the first Austin Powers movie, Dr. Evil steals a 50 kiloton nuclear weapon, straps it to a drill and is going to fire it into the mantle to trigger volcanic eruptions world wide.
My instinct is that, compared to a lot of nuclear weapons in the megaton range, 50 kilotons is not really a lot, and using it to try to move rock (even molten rock) is not going to accomplish his goal. Would it even cause a measurable change in the pressure that far down?
No. I don’t have expertise, but common sense tells me that the weight of the earth on top of the magma, miles of earth, couldn’t be moved by even our largest nuclear weapons.
If you found a spot with magma chambers you could bust them open with nukes sure, not the same though. The earth weighs a lot. A lot. And they already have nuclear action down there I presume.