- as T-waves, released by submarine earthquakes associated with submarine volcanic eruptions. The reason for this is that land-based seismometers cannot detect sea-based earthquakes below a magnitude of 4, but acoustic waves travel well in water and long periods of time. A system in the North Pacific, maintained by the United States Navy and originally intended for the detection of submarines, has detected an event on average every 2 to 3 years.[40]
The most common underwater flow is pillow lava, a circular lava flow named after its unusual shape. Less common are glassy, marginal sheet flows, indicative of larger-scale flows. Volcaniclastic sedimentary rocks are common in shallow-water environments. As plate movement starts to carry the volcanoes away from their eruptive source, eruption rates start to die down, and water erosion grinds the volcano down. The final stages of eruption caps the seamount in alkalic flows.[41] There are about 100,000 deepwater volcanoes in the world,[42] although most are beyond the active stage of their life.[41] Some exemplary seamounts are Loihi Seamount, Bowie Seamount, Davidson Seamount, and Axial Seamount.
Subglacial
Main article: Subglacial eruption
Subglacial eruptions are a type of volcanic eruption characterized by interactions between lava and ice, often under a glacier. The nature of glaciovolcanism dictates that it occurs at areas of high latitude and high altitude.[43] It has been suggested that subglacial volcanoes that are not actively erupting often dump heat into the ice covering them, producing meltwater.[44] This meltwater mix means that subglacial eruptions often generate dangerous jökulhlaups (floods) and lahars.[43]A diagram of a Subglacial eruption. (key: 1. Water vapor cloud 2. Crater lake 3. Ice 4. Layers of lava and ash 5. Stratum 6. Pillow lava 7. Magma conduit 8. Magma chamber 9. Dike) Click for larger version.
The study of glaciovolcanism is still a relatively new field. Early accounts described the unusual flat-topped steep-sided volcanoes (called tuyas) in Iceland that were suggested to have formed from eruptions below ice. The first English-language paper on the subject was published in 1947 by William Henry Mathews, describing the Tuya Butte field in northwest British Columbia, Canada. The eruptive process that builds these structures, originally inferred in the paper,[43] begins with volcanic growth below the glacier. At first the eruptions resemble those that occur in the deep sea, forming piles of pillow lava at the base of the volcanic structure. Some of the lava shatters when it comes in contact with the cold ice, forming a glassy breccia called hyaloclastite. After a while the ice finally melts into a lake, and the more explosive eruptions of Surtseyan activity begins, building up flanks made up of mostly hyaloclastite. Eventually the lake boils off from continued volcanism, and the lava flows become more effusive and thicken as the lava cools much more slowly, often forming columnar jointing. Well-preserved tuyas show all of these stages, for example Hjorleifshofdi in Iceland.[45]
Products of volcano-ice interactions stand as various structures, whose shape is dependent on complex eruptive and environmental interactions. Glacial volcanism is a good indicator of past ice distribution, making it an important climatic marker. Since they are imbedded in ice, as ice retracts worldwide there are concerns that tuyas and other structures may destabalize, resulting in mass landslides. Evidence of volcanic-glacial interactions are evident in Iceland and parts of British Columbia, and it is even possible that they play a role in deglaciation.[43]
Glaciovolcanic products have been identified in Iceland, the Canadian province of British Columbia, the U.S. states of Hawaii and Alaska, the Cascade Range of western North America, South America and even on the planet Mars.[43] Volcanoes known to have subglacial activity include:
- Mauna Kea in tropical Hawaii. There is evidence of past subglacial eruptive activity on the volcano in the form of a subglacial deposit on its summit. The eruptions originated about 10,000 years ago, during the last ice age, when the summit of Mauna Kea was covered in ice.[46]
- In 2008, the British Antarctic Survey reported a volcanic eruption under the Antarctica ice sheet 2,200 years ago. It is believed to be that this was the biggest eruption in Antarctica in the last 10,000 years. Volcanic ash deposits from the volcano were identified through an airborne radar survey, buried under later snowfalls in the Hudson Mountains, close to Pine Island Glacier.[44]
- Iceland, well known for both glaciers and volcanoes, is often a site of subglacial eruptions. An example an eruption under the Vatnajökull ice cap in 1996, which occurred under an estimated 2,500 ft (762 m) of ice.[47]
- As part of the search for life on Mars, scientists have suggested that there may be subglacial volcanoes on the red planet. Several potential sites of such volcanism have been reviewed, and compared extensively with similar features in Iceland:[48]
-
-
Viable microbial communities have been found living in deep (-2800 m) geothermal groundwater at 349 K and pressures >300 bar. Furthermore, microbes have been postulated to exist in basaltic rocks in rinds of altered volcanic glass. All of these conditions could exist in polar regions of Mars today where subglacial volcanism has occurred.
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Phreatic eruptions
Main article: Phreatic eruption
Phreatic eruptions (or steam-blast eruptions) are a type of eruption driven by the expansion of steam. When cold ground or surface water come into contact with hot rock or magma it superheats and explodes, fracturing the surrounding rock[49] and thrusting out a mixture of steam, water, ash, volcanic bombs, and volcanic blocks.[50] The distinguishing feature of phreatic explosions is that they only blast out fragments of pre-existing solid rock from the volcanic conduit; no new magma is erupted.[51] Because they are driven by the cracking of rock strata under pressure, phreatic activity does not always result in an eruption; if the rock face is strong enough to withstand the explosive force, outright eruptions may not occur, although cracks in the rock will probably develop and weaken it, furthering future eruptions.[49]Diagram of a phreatic eruption. (key: 1. Water vapor cloud 2. Magma conduit 3. Layers of lava and ash 4. Stratum 5. Water table 6. Explosion 7. Magma chamber)
Often a precursor of future volcanic activity,[52] phreatic eruptions are generally weak, although there have been exceptions.[51] Some phreatic events may be triggered by earthquake activity, another volcanic precursor, and they may also travel along dike lines.[49] Phreatic eruptions form base surges, lahars, avalanches, and volcanic block "rain." They may also release deadly toxic gas able to suffocate anyone in range of the eruption.[52]
Volcanoes known to exhibit phreatic activity include:
- Mount St. Helens, which exhibited phreatic activity just prior to its catastrophic 1980 eruption (which was itself Plinian).[50]
- Taal Volcano, Philippines, 1965.[51]
- La Soufrière of Guadeloupe (Lesser Antilles), 1975-1976 activity.[51]
- Soufrière Hills volcano on Montserrat, West Indies, 1995–2012.
- Poás Volcano, has frequent geyser like phreatic eruptions from its crater lake.
- Mount Bulusan, well known for its sudden phreatic eruptions.
- Mount Ontake, all historical eruptions of this volcano have been phreatic including the deadly 2014 eruption.
See also
References
- Heiken, G. & Wohletz, K. Volcanic Ash. University of California Press. p. 246.
- "VHP Photo Glossary: Effusive Eruption". USGS. 29 December 2009. Retrieved 3 August 2010.
- "Volcanoes of Canada: Volcanic eruptions". Geological Survey of Canada. Natural Resources Canada. 2 April 2009. Retrieved 3 August 2010.
- "How Volcanoes Work: Hawaiian Eruptions". San Diego State University. Retrieved 2 August 2010.
Wednesday, 8 June 2016
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