What determines the viscosity of magma? what else determines whether a volcano will erupt violently or calmly  answerthe two main factors that influence how a volcano will erupt are  viscosity  and gas content. both are related to the composition of the  magma. hawaiian volcanoes tend to erupt  basalt, which is low in viscosity and low in gas content (about 0.5 weight percent). the gas that is present can readily escape and little pressure builds up in the magma. at the other extreme,  rhyolite  magmas are very viscous and can contain a lot of gas (up to 7-8 weight present). as the magma moves into the vent and the pressure drops, the gas wants to escape. the magma is very sticky and resists the expansion of the gas bubbles. ultimately, enough bubbles grow and expand to blow the magma into ash size fragments and eject them violently into the atmosphere.