Index Letusan Gunungapi

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Index Letusan Gunungapi ( Volcano Explosivity Index )

Volcano Explosivity Index (VEI)  atau Index Letusan Gunungapi diperknalkan oleh Chris Newhall dari U.S. Geological Survey dan Steve Self dari Universitas Hawaii pada tahun 1982 sebagai gambaran atau ukuran besaran letusan gunungapi.

Volume produk letusan, tinggi kolam letusan, dan observasi kualitatif ( berkisar dari lemah-mega kolosal ) digunakan untuk magnitude 8 merupakan letusan gunungapi terbesar yang tercatat sejarah.

Skala VEI berkisar dari 0 hingga 8. Skala 1 merupakan 10 kal lebih kuat dari letusan skala 0. Nilai 0 merupakan letusan non explosive ( kurang dari 10.000 meter kubik tephra yang dilontarkan), sedangkan nilai 8 menggambarkan letusan mega-kolosal yang memuntahkan material letusan 1.000.000.000.000 meter kubik dengan tinggi kolam letusan lebih dari 25km. Setiap interval skala menggambarkan peningkatan kriteria letusan.

Abu, bom gunungapi, dan ignimbrit ( memiliki vesikulasi seperti gelembung gas ) dan kesebandingan batuan padat diperhitungkan untuk mengetahui volume total magma yang dikeluarkan pada saat letusan. Salah satu kelemahan VEI adalah tidak mempertimbangkan besar kekuatan semburan letusan. Sehingga sangat sulit untuk mendapatkan data letusan pra sejarah dan letusan yang tidak teramati.

Para peneliti mengkur kekuatan letusan gunungapi dengan menggunakan VEI, dengan mencatat volume material letusan, tinggi kolam letusan, dan lama letusan.

Dalam VEI, ada ketidaksinambungan antara VEI 1 dan 2. Batas bawah volume letusan yang terlemparkan berubah sesuai dengan kelipatan 100 dari 10.000 sehingga 1.000.000 meter kubik, sedangkan kelipatan 10 antar index lebih besar.

Empat puluh tujuh letusan yang memiliki VEI 8 atau lebih pada kisaran umur Ordovisiun-Pleistosen. Empat puluh dua letusan di antaranya terjadi 36 juta tahun yang lalu. Letusan terakhir adalah Taupo Oruanui sekitar 26.500 tahun yang lalu. Selama Holosen ( dalam 10.000 tahun terakhir ) tidak pernah terjadi letusan besar dengan skala VEI 8.

Suatu letusan tunggal dengan volume > 10.000 km3 diketahui terjadi di Fish Canyon ( 27 juta tahun yang lalu ) dengan VEI 9.2.

VEI and ejecta volume correlation

The Volcanic Explosivity Index (VEI) is a relative measure of the explosiveness of volcanic eruptions. It was devised by Chris Newhall of the United States Geological Survey and Stephen Self at the University of Hawaii in 1982.

Volume of products, eruption cloud height, and qualitative observations (using terms ranging from “gentle” to “mega-colossal”) are used to determine the explosivity value. The scale is open-ended with the largest volcanoes in history given magnitude 8. A value of 0 is given for non-explosive eruptions, defined as less than 10,000 m3 (350,000 cu ft) of tephra ejected; and 8 representing a mega-colossal explosive eruption that can eject 1.0×1012 m3 (240 cubic miles) of tephra and have a cloud column height of over 20 km (12 mi). The scale is logarithmic, with each interval on the scale representing a tenfold increase in observed ejecta criteria, with the exception of between VEI 0, VEI 1 and VEI 2.

With indices running from 0 to 8, the VEI associated with an eruption is dependent on how much volcanic material is thrown out, to what height, and how long the eruption lasts. The scale is logarithmic from VEI 2 and up; an increase of 1 index indicates an eruption that is 10 times as powerful. As such there is a discontinuity in the definition of the VEI between indices 1 and 2. The lower border of the volume of ejecta jumps by a factor of 100 from 10,000 to 1,000,000 m3 (350,000 to 35,310,000 cu ft) while the factor is 10 between all higher indices. In the following table, the frequency of each VEI indicates the approximate frequency of new eruptions of that VEI or higher.

CLASSIFICATION

With indices running from 0 to 8, the VEI associated with an eruption is dependent on how much volcanic material is thrown out, to what height, and how long the eruption lasts. The scale is logarithmic from VEI 2 and up; an increase of 1 index indicates an eruption that is 10 times as powerful. As such there is a discontinuity in the definition of the VEI between indices 1 and 2. The lower border of the volume of ejecta jumps by a factor of 100 from 10,000 to 1,000,000 m3 (350,000 to 35,310,000 cu ft) while the factor is 10 between all higher indices. In the following table, the frequency of each VEI indicates the approximate frequency of new eruptions of that VEI or higher.

VEI Ejecta volume (bulk) Classification Description Plume Frequency Tropospheric
injection
Stratospheric
injection[2]
Examples
0 < 104 m3 Hawaiian Effusive < 100 m continuous negligible none
KīlaueaPiton de la FournaiseErebus
1 > 104 m3 Hawaiian / Strombolian Gentle 100 m – 1 km daily minor none
Stromboli (since Roman times), Nyiragongo (2002), Raoul Island (2006)
2 > 106 m3 Strombolian / Vulcanian Explosive 1–5 km every two weeks moderate none
Unzen (1792), Cumbre Vieja (1949), Galeras (1993), Sinabung (2010)
3 > 107 m3 Vulcanian / Peléan/Sub-Plinian Catastrophic 3–15 km 3 months substantial possible
Lassen Peak (1915), Nevado del Ruiz (1985), Soufrière Hills (1995), Nabro (2011), Ontake (2014)
4 > 0.1 km3 Peléan / Plinian/Sub-Plinian Cataclysmic > 10 km (Plinian or sub-Plinian) 18 months substantial definite
Laki (1783), Mayon (1814), Pelée (1902), Sakurajima (1914), Galunggung (1982), Eyjafjallajökull (2010), Calbuco (2015)
5 > 1 km3 Peléan/Plinian Paroxysmic > 10 km (Plinian) 12 years substantial significant
Vesuvius (79), Fuji (1707), Tarawera (1886), Agung (1963), St. Helens (1980), Hudson (1991), Puyehue (2011)
6 > 10 km3 Plinian / Ultra-Plinian Colossal > 20 km 50–100 yrs substantial substantial
Laach Lake Volcano (c. 12,900 BC), Veniaminof (c. 1750 BC), Lake Ilopango (535), Huaynaputina (1600), Krakatoa (1883), Santa Maria(1902), Novarupta (1912), Pinatubo (1991)
7 > 100 km3 Ultra-Plinian Super-colossal > 20 km 500–1,000 yrs substantial substantial
Aira Caldera (22,000 BC), Kikai Caldera (4,300 BC), Cerro Blanco (c. 2300 BC), Thera (c. 1620 BC), Taupo (180), Baekdu (946), Samalas(1257), Tambora (1815)
8 > 1000 km3 Ultra-Plinian Mega-colossal > 20 km > 50,000 yrs[3][4] vast vast
La Garita (26,300,000 BC), Cerro Galán (2,200,000 BC), Huckleberry Ridge Tuff (2,100,000 BC), Yellowstone (630,000 BC), Whakamaru(in TVZ) (254,000 BC),[5] Toba (74,000 BC), Taupo (25,360 BC)

 

About 40 eruptions of VEI 8 magnitude within the last 132 million years (Mya) have been identified, of which 30 occurred in the past 36 million years. Considering the estimated frequency is > 50,000 years,[3] there are likely many such eruptions in the last 132 Mya that are not yet known. Based on incomplete statistics, other authors assume that at least 60 VEI 8 eruptions have been identified.[6][7] The most recent is Lake Taupo‘s Oruanui eruption, 25,360 years ago, which means that there have not been any Holocene eruptions with a VEI of 8.[8]

There have been at least 10 eruptions of VEI 7 in the last 10,000 years. There are also 58 plinian eruptions, and 13 caldera-forming eruptions, of large, but unknown magnitudes. By 2010, the Global Volcanism Program of the Smithsonian Institution had catalogued the assignment of a VEI for 7,742 volcanic eruptions that occurred during the Holocene (the last 11,700 years) which account for about 75% of the total known eruptions during the Holocene. Of these 7,742 eruptions, about 49% have a VEI of ≤ 2, and 90% have a VEI ≤ 3.[9]

LIMITATION :

Under the VEI, ashlavalava bombs, and ignimbrite are all treated alike. Density and vesicularity (gas bubbling) of the volcanic products in question is not taken into account. In contrast, the DRE (dense-rock equivalent) is sometimes calculated to give the actual amount of magma erupted. Another weakness of the VEI is that it does not take into account the power output of an eruption, which makes the VEI extremely difficult to determine with prehistoric or unobserved eruptions.

Although VEI is quite suitable for classifying the explosive magnitude of eruptions, the index is not as significant as sulphur dioxide emissions in quantifying their atmospheric and climatic impact, as a 2004 paper by Georgina MilesRoy Grainger and Eleanor Highwood points out.

“Tephra, or fallout sediment analysis, can provide an estimate of the explosiveness of a known eruption event. It is, however, not obviously related to the amount of SO2emitted by the eruption. The Volcanic Explosivity Index (VEI) was derived to catalogue the explosive magnitude of historical eruptions, based on the order of magnitude of erupted mass, and gives a general indication as to the height the eruptive column reached. The VEI itself is inadequate for describing the atmospheric effects of volcanic eruptions. This is clearly demonstrated by two eruptions, Agung (1963) and El Chichón (1982). Their VEI classification separates them by an order of magnitude in explosivity, although the volume of SO2 released into the stratosphere by each was measured to be broadly similar, as shown by the optical depth data for the two eruptions.”[10]

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