Quiet eruptions are volcanic eruptions that explode gently, with broad sheets of slowly flowing lava. Shield volcanoes, such as those in Hawaii, are commonly associated with quiet eruptions.
In contrast to quiet eruptions, other volcanoes erupt explosively. Mount St. Helens, for instance, spewed lava high in the air when it erupted. Two things control the type of eruption: how much water vapor and other gasses are in the magma and whether the magma is basaltic or granitic. Basaltic magma tends to ooze out gently in a thin, quiet eruption, while granitic magma is thicker and becomes trapped inside the volcano's vents. Once the pressure grows enough to force out the magma, it explodes.