A volcanic event occurs when there is a sudden or continuing release of energy caused by near-surface or surface magma movement. The energy can be in the form of earthquakes, gas-emission at the surface, release of heat (geothermal activity), explosive release of gases (including steam with the interaction of magma and surface of ground water), and the non-explosive extrusion or intrusion of magma(岩漿). An event could be non-destructive without release of solids or magmatic liquid, or if there is anything to destroy, could be destructive with voluminous lava flows or explosive activity. Destruction usually refers to the works of mankind (buildings, roads, agricultural land, etc.).
A volcanic event can include
(1) an eruptive pulse(短暫) (essentially an explosion with an eruption plume, but also non-explosive surges of lava. A pulse may last a few seconds to minutes,
(2) an eruptive phase(階段) that may last a few hours to days and consist of numerous eruptive pulses that may alternate between explosions and lava surges, and
(3) a single eruption or eruptive episode(間斷), composed of several phases, that may last a few days, months or years (Fisher and Schmincke, 1984). Paricutin(帕里庫亭火山), Mexico was in eruption for nine years. Stromboli, Italy has been in eruption for over 2000 years.
Common eruptions types are Plinian, Hawaiian, Strombolian(斯屈波里島), and Vulcanian. Gas-only eruptions are not so common.
At the present time, carbon dioxide gas is seeping upward through Mammoth Mountain, a composite volcano on the edge of Long Valley Caldera(噴火口形盤層). The carbon dioxide leaks are occurring at several places around the volcano. Long Valley and Mammoth Mountain are being watched by the U.S. Geological Survey. It is not known whether or not the CO2 leaks could be a precursor to a volcanic eruption.
<1>Eruption Columns
Some people concluded(推斷) that a pyroclastic(火山爍岩) flow develops around the base of a collapsing(倒塌) eruption column, deflates, and then moves outward across the landscape under its own momentum. In their model, the momentum that a pyroclastic flow acquires is proportional to the height from which the eruption column collapses.
The conclusion that momentum is the main cause of transport of pyroclastic flows influenced the "energy line" concept of Sheridan (1979). Sheridan explained that the slope of the energy line as proposed by Hsu (1975) for avalanche runout, traces the potential flow head from the top of the gas-thrust region of an eruption column to the distal toe of a flow along the line of transport. However, only a tiny fraction of the total fragmental component reaches the top of the gas-thrust part of an eruption column. Most of the fragmental material(斷岩的物質) that falls back is located between the top of the gas-thrust and the ground surface. The total momentum acquired cannot be a single mass number that attains a particular height, but a summation of all the fragmental mass and the different heights to which they attain. Thus, the calculated momentum(動量) value is exaggerated(誇大).
<2>Hawaiian and Strombolian Eruptions
Hawaiian(夏威夷) eruptions consist of basaltic, highly fluid lavas of low gas content, that produce effusive lava flows and some pyroclastic debris. Thin, fluid lava flows can gradually build up large broad shield volcanoes. Most Hawaiian eruptions start from fissures, commonly beginning as a line of lava fountains that eventually concentrate at one or more central vents. Most of the vesiculating lava falls back in a still molten condition, coalesces and moves away as lava flows. If fountains are weak, most lava will quietly well out of the ground and move away from a vent as a lava flow. Much lava in shield volcanoes is transmitted through tubes enclosed within lava flows. Small spatter cones and, in some instances, basaltic pumice cones such as at Kilauea Iki, may form around vents. Pyroclastic material occurs as bombs, ranging downward in size through lapilli-sized clasts of solidified liquid spatter commonly called cinders, to small volumes of glassy Pele's tears and Pele's hair.
Strombolian(斯屈波里島) eruptions, named after Stromboli Volcano, Italy, are discrete(有區別) explosions separated by periods of less than a second to several hours. They give rise to ash columns and abundant ballistic debris. Ejecta consist of bombs, scoriaceous lapilli(火山爍) and ash(灰燼).