Anyone with a lick of knowledge of geology is cringing over that chapter. What happens when a deep magma conduit is unable to break through to the surface? Pressure builds up until it finds a path of least resistance in the crust and creates a magma dyke. If fact, the Reykjanes volcano that's currently erupting in Iceland is being fueled by one of these dykes. It doesn't matter what direction the flow of magma needs to take. As long as there's enough pressure in the magma conduit, it will try its hardest to erupt somewhere on the surface even if the direct path straight up is blocked.