A volcanic event occurs when there is a sudden or continuing release of energy caused by movement of near-surface or surface magma. Energy can be in the form of earthquakes, surface gas emissions, release of heat (geothermal activity), non-explosive extrusion or intrusion of magma, and the explosive release of gases, including steam caused by the interaction of magma and surface ground water.
A volcanic event can include:
- An eruptive pulse, essentially an explosion with an eruption plume, but also non-explosive surges of lava that may last from a few seconds to minutes.
- An eruptive phase that may last from a few hours to days, consisting of numerous eruptive pulses that may alternate between explosions and lava surges.
- A single eruption or eruptive episode, composed of several phases, that may last a few days, months, or years. A volcano in Paricutin, Mexico, erupted for nine years while one in Stromboli, Italy, has been erupting for more than 2,000 years.
Some volcanoes, such as domes and basaltic scoria cones, may form completely within a few weeks or months. Others, such as shield volcanoes and composite volcanoes, may take 10 million years or more before volcanism completely dies out.
Types of Eruptions
Volcanic eruptions and eruptive phases are traditionally classified according to a wide range of criteria. Many have been given names from volcanoes where a certain type of behavior was first observed or most commonly occurs.
Common eruptions types are Plinian, Hawaiian, Strombolian, and Vulcanian.
TopPlinian Eruptions
Widely dispersed sheets of pumice and ash are derived from high eruption columns that result from high-velocity, voluminous, gas-rich eruptions, commonly lasting for several hours to about four days. These are called Plinian for Pliny the Younger who described the famous 3-day eruption of Vesuvius, in 79 AD, during which the towns of Pompei and Herculaneum were buried by several feet of pyroclastic material. The energy and characteristics of a Plinian eruption depends on the gas content of the magma, exit pressure, viscosity, vent radius and shape, and volume of magma erupted.Hawaiian Eruption
Hawaiian eruptions consist of basaltic, highly fluid lava with low-gas content that produces 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. Lava in shield volcanoes is also transmitted through tubes enclosed within the lava flows. Small spatter cones and, in some instances, basaltic pumice cones, such as the one at Kilauea Iki on Hawaii's Big Island, may form around vents. Pyroclastic material occurs as bombs, ranging downward in size through lapilli-sized clasts of solidified liquid spatter, known as cinders, to small volumes of glassy material called Pele's tears and Pele's hair.Strombolian Eruptions
Strombolian eruptions, named after the Stromboli volcano in Italy, are discrete 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 and ash.Vulcanian Eruptions (hydrovolcanic)
Vulcanian eruptions are from hydrovolcanic processes. Many volcanologists use the term Vulcanian for highly explosive, short-lived eruptions that produce black, ash- and steam-laden eruption columns witnessed during the 1888-90 eruptions of Vulcano, a small volcano in the Eolian Islands in Italy.
Volcano
Source(s):
Volcanoes by Fisher, Richard V
Volcanoes by Fisher, Richard V