噴火過程における揮発性物質の役割(＜特集＞火山学の『夢』を語る)

抄録

Magma, which buoyantly ascends from its source region to the upper crust, will be trapped at level of neutral buoyancy (LNB) in the upper crust, where density of magma and crust is equal. Since magma is gravitationally stable in the magma reservoir at LNB, decrease in magma density is necessary to restart magma ascent from the magma reservoir. Processes, which can cause density decrease of magma, may include vesiculation due to crystallization, bubble concentration at roof of a magma reservoir, and supply of bubbles from newly injected magmas. Once conduit is open to the surface, balance of load pressures of magma column and crust may control continuation of eruption. Since density of magma decreases effectively with ascent by vesiulation, driving force for eruption is quite dependent on volatile content in a magma. Eruption will terminate when volatile content in the magma decreases enough to lose the driving force, or when eruption conduit is closed by wall collapse. Since H₂O content in silicic magma is larger than mafic magma, it is likely that the former mechanism for termination of eruption is more important in mafic systems, but the latter is more important in silicic systems. Volatile content in an erupting magma is not only dependent on an original content, but also on volatile (including bubbles) accumulation and escape processes in a magma reservoir and conduit system. Volatile loss from ascending magma through a conduit has been proposed as a responsible mechanism to bifurcate an explosive plinian eruption and quiet lava-dome eruption from a volatile-rich magma. Separated two-phase flow of mixture of bubbles and magma can create intermittent emission of bubble-rich and bubble-poor magmas, such as strombolian eruption. Excess degassing is observed during eruption and quiescent stage of volcanic activity. The excess degassing from a quiescent volcano is characterized by continuous, intensive and high-temperature degassing. Those features of the degassing suggest that the degassing occurs from a convecting magma column. Excess degassing during plinian and lava-dome eruption suggests volatile oversaturation in a magma reservoir. Since observation of the excess degassing during eruption is so common, it is likely that bubble accumulation in a reservoir, in particular bubble supply from newly injected basaltic magma, provides driving force to start the eruption. Magma reservoir formed because magma is stable in the reservoir. Therefore, processes in a magma reservoir, such as vesiculation due to magma crystallization or magma mixing, is likely to be a key to understand initiation processes of volcanic eruption. Since magma inevitably loses heat during storage in a magma chamber emplaced in cool crust, crystallization is essential consequence of magma storage in a magma reservoir. However, crystallization process in a magma chamber is still poorly constrained and required further investigations. Although magma mixing could be a simple consequence of eruption from a stratified magma chamber, common observation of evidences of magma mixing not only in products of andesitic eruption but also in silicic ones suggests that magma mixing is a necessary process for the beginning of eruption. If basaltic magma is saturated with CO₂-rich bubbles, injection of basaltic magma beneath a silicic magma can cause bubble supply to the silicic magma to initiate eruption.