Volcanoes emit larger amounts of volcanic gas than can be dissolved in the volume of erupted magma during a variety of volcanic processes, including explosive and effusive eruption and noneruptive continuous degassing. Degassing of unerupted magma with a much larger volume than that of erupted magma caused such a large degassing; erupted magma represents only a small portion of the magma that drives volcanic activity. Evaluation of the magma-gas differentiation process causing the excess degassing is necessary to understand eruption processes, magma chamber evolution, and crustal growth by magma intrusion. Three mechanisms are proposed to explain various degassing modes, including eruption of bubble-accumulated magma, degassing of a convecting magma column, and permeable gas transportation from a deep magma chamber. Examples of large degassing in excess of the erupted magma are common in subduction zone volcanism but are rare in rift- and hot spot–associated volcanism.