Question

Why does lava containing large amounts of dissolved gases generally produce a more violent explosion than lava that contains small amounts of dissolved gases?

Answer 1

Because the gases are the driving energy source for an eruption. At depth they are dissolved under high pressure; as magma rises the pressure drops and the gases exsolve into bubbles. If there is a large amount of dissolved volatile (especially H2O/CO2) those bubbles grow rapidly and expand violently. If the melt is viscous the bubbles cannot escape gradually, pressure builds until the magma fragments and is blown apart. More dissolved gas therefore means more rapid expansion and higher overpressure — and so a more violent, explosive eruption (whereas low-gas magma generally degasses more quietly and produces effusive lava flows).