Geology And Geophysics

What causes a Volcano to Erupt

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Volcanoes are places where the magma or molten rock, gases and pyroclastic debris erupt through the crust of a planet or moon. When the gas and heat are sufficient to eject magma through the crust of a planet, an eruption occurs.

The word volcano comes from the Roman god of fire, Vulcan, whose forge was believed to be in the active volcano Vulcano in the Italian Lipari Islands.

The simplest volcanoes are simple cracks in the crust from which lava flows. Composite volcanoes are complex structures, can be as large as mountains and have a large crater at the peak.

Six types of volcanic eruptions exist and are named for the history and type of volcano:

1. Fissure eruptions: Fissures or cracks appear where crust plates pull apart. Magma erupts through the fissures and echelons, parallel cracks branching from the fissure. Iceland was created by basaltic magma erupting from fissures. Iceland is home to the Laki Flow, the largest recorded lava flow in history occurring in 1783. Fissure eruptions are also called Icelandic eruptions.

2. Hawaiian eruptions: The most quiescent of eruptions, Hawaiian eruptions are characterized by effusive emissions of highly fluid basalt lava with very low gas content, with less than half of the pyroclastic material (lava cinders) than all others. These long lasting eruptions form shield volcanoes. Kupaianaha volcano erupted smooth pahoehoe lava from 1986 until early 1992.

3. Strombolian eruptions: The volcano-island Stromboli, between Italy and Sicily, has been erupting irregularly every twenty minutes for the last century. Its episodic lightshow gives it the nickname "Lighthouse of the Mediterranean". Although more noisy than Hawaiian eruption, Strombolians are no more dangerous. The eruption resembles a firework display in its expulsion of pyroclastic debris.

4. Vulcanian eruptions: Gaining its name from the forge of Vulcan, this eruption is characterized by a series of loud blasts and high lava expulsion, some as high as 10 km. The magma is expelled by explosive gas pressure inside the conduit of the volcano. Most eruptions last from a few minutes to a few hours. Japan and New Guinea both have active Vulcanian volcanoes.

5. Vesuvian (or Plinian) eruptions: Named for Pliny the Younger's account of the 79 AD eruption of Mount Vesuvius which buried Pompeii in lava and Herculaneum in ash, these powerful volcanoes send plumes of ash into the stratosphere (45 km). These stratavolcano eruptions can last hours or days. The giant plume and umbrella-like cloud over the volcano cover large areas with an even thickness of pumice and ash.

6. Hydrovolcanic eruption: These eruptions are the result of the interaction of magma with water-ground water or surface water. Sometimes called Surtseyan eruptions after the 1963 eruption off of Iceland, these eruptions are similar to an aquatic Strombolian eruption. Responsible for the formation of most volcanic islands, these eruptions are not limited to underwater. Alaska (1977) and the Azores (1957) have experienced groundwater interaction with magma eruptions.

One of the most destructive, although arguably beautiful, forces on Earth, volcano eruptions birth islands, change landscapes and continue to change the face of our planet.

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