The style of a volcanic eruption is controlled by the characteristics of the erupting magma (molten rock), and the volume of and rate at which the magma is emitted. Magma characteristics include the chemical composition, crystal content, and temperature of the magma; the composition and amount of dissolved gases within the magma; and by any interaction with water (e.g. groundwater) that occurs prior to or during the eruption (learn more about magma characteristics).
As a result of these factors, eruption styles range from (1) effusion of lava flows and eruption of lava fountains that produce very little ash, and is typical of eruptions of low-viscosity basaltic magma (click here to learn more about effusive volcanism); to (2) extremely explosive eruptions that inject large quantities of ash high into the stratosphere, typical of eruptions of rhyolite and dacite magma that are highly viscous and gas-rich. The volume, grain-size and chemical composition of volcanic ash and its spatial distribution are therefore dependent upon various factors associated with the characteristics of the erupting magma and the resulting eruption style.