However, Halemaumau seems to be the principle conduit where lava reaches the surface. The unifying agent among volcanoes is the active ingredient of gas. The primary gas given off in an eruption is water vapor or steam. When water changes from a liquid state to a solid state, the expansion is about one ninth of its volume. Before eruption, water and dissolved gases are confined by the pressure of the overlying rocks, but as the magma rises to the surface the pressure drops and the gases are released.
Gas is necessary to move the magma, and magma is the vehicle carrying the gas.
Some flows issue from the vent as pahoehoe and change to aa as they are stirred, cooled, lose gas, and increase in crystallization. Lava does not convert from aa to pahoehoe. Once a crust has formed on pahoehoe, lava tubes below continue to move the remaining liquid for long periods of time. The size of the tubes range from only a few inches to many feet in diameter. When the source of lava stops, the liquid continues to drain, leaving lava tubes or caves.