How does lava get enough pressure to break through the crust?
The pressure to break the crust comes from two sources: gas and buoyancy.
The gas pressure builds as the molten rock approaches and eventually may crack the crust.
The buoyancy of molten rock is related to the temperature and composition of the rock. It is hot and less dense then the surrounding rock so it can, with a little help from gas, fracture rock.
Dr. Mike Garcia, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822