Why are some eruptions gentle and others violent?
Volcanic eruptions could be thought as a continuum between two end members. At one extreme is the gentle effusion of lava. Most Hawaiian eruptions would be a examples of this type of eruption. At the other extreme is the explosive ejection of ash from a vent. The May 18, 1980 Mount St. Helens eruption would be an example of this type of eruption.
The 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.