What controls volcanic eruptions What controls an eruption? Well I believe that the magma controls the whole thing. One example is in 2004 a big sticky mass of lava began flowing from a mountain. It was that way because of it's composition. That is what scientist have said . I think it was that way because of the magma . So the magma can make the lava really thick or really thin. That is just my opinion though. What do you think?