The sudden escape of high-pressure volcanic gas from magma is the driving force for eruptions. Gases come from the magma itself or from the hot magma coming into contact with water in the ground. Volcanic plumes can appear dark during an eruption because the gases are mixed with dark-colored materials such as tephra. In general, the more viscous, or stiffer, the lava, the more explosive the eruptive activity. During explosive eruptions, the lava erupted is torn into shreds, forming a variety of fragmental or pyroclastic materials depending on the physical state of the lava and on the force of the explosions.
Explosive eruptions can eject a large amount of material into the air.
Nonexplosive eruptions produce lava flows and eject very little pyroclastic material into the air. A Explosive Eruptions Explosive eruptions can eject liquid and semisolid lava as well as solid fragments of volcanic or nonvolcanic rock that have been carried along by the rising magma before eruption. Very violent explosive eruptions are called Plinian eruptions, after Roman naturalist Pliny the Elder. These eruptions can last for several hours to days and eject a large amount of pyroclastic material. Some volcanoes can produce much more energetic eruptions that eject materials farther from the vents because of their andesitic and dacitic composition.