eruption(Hmm,British: eruption) IsVolcanoからmagma,Volcanic ashEtc. are relatively rapid^[1]It is to spout out on the surface of the earth or in the water.

Volcanic activity(Kazankatsudo,British: volcanic activity), There are various scales and styles depending on the nature of magma.

Japanese Meteorological AgencyThen,craterA solid matter that exceeds a range of approximately 100-300 m horizontally or vertically is to be recorded as an "eruption".^[2].

Eruption style

List of eruption styles

Eruptions take different forms under different conditions.

VolcanologistClassifies this into typical types and names them.

^[3][4][5]

Format name	English name	image	Violent	Height of plume top	Time scale	The main ejecta	Main terrain formed[6]	Main rocks[7]	Remarks
Steam eruption	phreatic eruption		explosive	About 10km or less	A few hours to a day	Water vapor, water, solids around the crater	Mar	-	Groundwater and water vapor heated by volcanic activity blow off solids on the surface of the earth and are rapidly released, causing a steam eruption or an event that does not contain essence.Steam explosionThat is.Often crater spout typeLaharAccompanied by.
Magma steam eruption	phreatomagmatic eruption		explosive	Several km to 10 km or more	Hours to days	Water vapor ・ Solid matter around the crater ・Volcanic bean stone-Base surge	Mar-Tough ring	basalt~Rhyolite	An event in which groundwater and water vapor heated by volcanic activity blow off solids on the surface of the earth and are rapidly released, and contain some essential substances, is called a phreatomagmatic eruption or phreatomagmatic eruption.Vulcanian eruption (British: Ultravulcanian),Surtsey eruption(British: a Surtseyan), And the one where the volcanic plume reaches the stratosphere is a water vapor plinian eruption (British: Phreatoplinian) Called.
Hawaiian eruption	a Hawaiian		Non-explosive	-	~ Decades	Lava flow	Lava flow	basalt	The sediment is mainly lava flow.If the crater is not dotted but linearFissure eruption(British: fissure eruption).In the case of a compound volcanoShield volcanoTo form.Events with extremely large eruption volume and eruption rateFlood basaltIs said.
Lava dome	lava dome		Non-explosive	-	~ Several years	Lava domePyroclastic flow	Lava dome	RhyoliteDay sight	Felsic magma slowly erupts and accumulates on the crater and grows in a dome shape.Often part or all of it collapses, creating a block-and-ash flow type pyroclastic flow.Those with a flat top are called lava dome.
Strombolian eruption	a Strombolian		explosive Non-explosive	Dozens to hundreds of meters Thousands of meters or less	Few seconds Intermittent	Spater Scoria・ Lava flow	Scoria hill	basalt Andesite	In a narrow sense, a mafic magma volcano releases hot lava fragments from the crater instantaneously and periodically.Volcanic ashRefers to an eruption that is rarely accompanied.In a broad sense, it refers to an eruption in which the height of the eruption is about 1000 m or less and the scoria and lava flows form the scoria cone.
Vulcanian eruption	a Vulcanian		explosive	10km or less	A few seconds to a few minutes	Volcanic debris	Pyroclastic cones / pyroclastic materials	Andesite	SmokeExplosive eruptions occur momentarily and at regular intervals to form.
Semi-Plinian eruption	Subplinian		explosive	10km or less	Hours to days	Volcanic debris	Pyroclastic cones / pyroclastic materials	Andesite / dacite	An eruption in which continuous and explosive eruptions last for hours to days.The plumestratosphereThe eruption rate is low compared to the Plinian eruption.Vesuvius volcano (British: a Vesuvian) Is also called.
Pliny eruption	Plinian		explosive	10km or more	Hours to days	Volcanic debris	Pyroclastic materialPyroclastic plateau	Rhyolite-dacite	A continuous, explosive eruption that reaches the stratosphere for hours to days.Eruption column collapse type pyroclastic flows often occur.Among the plinian eruptions, the one with a particularly large scale (high eruption rate) is a super plinian eruption (British: ultraplinian),Catastrophic eruption(British: super-eruption) And so on.

Magma eruption

An eruption in which magma (essential substance) erupts directly on the surface of the earth is called a magma eruption.

Hawaiian eruption

Kilauea Mountain,Mauna Kea MountainSuch,Island of HawaiiEruption style often seen in volcanoes.LiquidityIs high,volatilityAn eruption caused by magma with few components.爆 発Does not happen, a lot溶岩Flows out at high speed.

Strombolian eruption

イタリア OfStromboli volcanoEruption style often seen in.Magma, which is a little less fluid than the Hawaiian eruption, intermittently repeats small explosions,Scoria,Volcanic bombIs released.LiquidLava flowCan also be seen.

Vulcanian eruption

Close to Stromboli volcanoVulcanian volcanoEruption style often seen in.Low liquidityviscosityIs highAndesiteOften in the case of quality magma, in recent yearsSakurajima,Mt. AsamayamaEquivalent to the eruption of.With the explosion, volcanic ash,Lapilli,Volcanic blockIs ejected in large quantities.The lava flow becomes semi-solidified (block rubber) and the flow velocity is slow.The name Vulcanian isEnglish Of VIt became the etymology of (volcano).By the way, Japanese volcanoes have the most eruptions.

Play eruption(Plinian eruption)

GrowingLava domeIs destroyed by an explosive eruption such as a Plinian eruption, and a block-and-ash flow type pyroclastic flow is generated.Mount Merapi( 2006),Mount St. Helens( 1980)Such.EtymologyPlay mountain( 1902) Is a Plinian eruption.

Pliny eruption

Roman period OfPompeii,HerculaneumFamous for filling things like 79 OfVesuvius volcanoEruption style.Observed this eruption in detail and left a record for posterityPliniusIt was named after the Plinian eruption.It is basically a large-scale Strombolian eruption.Volcanic ashpumiceConsists ofPlumeIt is,stratosphereTo reach.When this eruption column collapses, a huge pyroclastic flow (medium-scale pyroclastic flow) is generated, causing widespread damage.Mount Fuji(Hoei Great Eruption), Mt. Asamayama (1783 Tenmei eruption), etc.

An eruption between the Plinian eruption and the Strombolian eruption is called a quasi-Plinian eruption, and intermittent pumice ash fall andEmpty shakeRepeat the eruption accompanied by.

Ultra Plinian eruption(Caldera eruption,Catastrophic eruption）

Among the Plinian eruptions, it refers to eruptions of VEI 6 or higher.Although it does not occur very often, it occurs every hundreds of thousands to millions of years and is one of the largest volcanic eruptions. The latest eruption of VEI6 level eruption 1991On Luzon Island, PhilippinesMount PinatuboThe volcano that caused the VEI 7-8 class eruption isLake Toba,Yellowstone,Onikai CalderaAnd so on.Sometimes referred to as a caldera eruption, but even a non-explosive Hawaiian eruptioncalderaMay be formed.

Flood basalt

Flood basalts occur about once every tens of millions of years.The surface of the earth cracks on a large scale, and a large amount of lava is supplied to the surface of the earth in a short period of time.For exampleIndia OfDeccan Plateau OfbasaltThe area is about 1.5 times that of the whole of Japan.About the cause of the occurrence recentlyPlume tectonicsIs being discussed in.

Steam eruption

An eruption in which magma (essential substance) does not erupt on the surface of the earth.

The phenomenon that the water inside the volcanic body is indirectly warmed by magma and erupts without magma.Steam explosionThat is.Although it is an explosive eruption, the scale is not so large and it includes eruptions that erupt volcanic ash, so in JapanSteam eruptionHas been proposed by some volcanologists^[8].

Magma steam eruption

An eruption in which magma and a large amount of water vapor erupt on the surface of the earth.

The phenomenon that water comes into direct contact with magma, causing a steam explosion and ejecting with magma.Magma steam explosionThat is.Explosive eruption.

Effect of ejecta components

The mode of eruption of a volcano will vary greatly depending on the fluidity of the magma and the amount of volatile components at the time of the eruption.In particular, the amount of volatile components affects the explosiveness of magma, and the more volatile components there are, the larger the explosion that blows up volcanic ash and lava.

1. When the fluidity is high and the volatile components escape from the magma, so the volatile components at the time of eruption are low-like the eruption of a volcano on the Big Island of Hawaii, the lava flow continues to flow quietly (Hawaiian eruption).
2. When the fluidity is rather high and the volatile components are difficult to escape from the magma, so the volatile components at the time of eruption are relatively large --1986Mihara(izu-Oshima) Like the early days of the eruption, lava erupts high and wide like a curtain (Strombolian eruption).
3. When the fluidity is low and the volatile components at the time of eruption are low for some reason- Showa ShinzanThere is no big explosion or lava outflow like the eruption of, and a lava dome is formed.
4. If there is a lot of volatile components at the time of eruption due to low fluidity and volatile components cannot escape from magma-it will be an explosive eruption like Mt. Asamayama or Sakurajima (Pliny eruption).

A single eruption may end in a short time or last for several months or longer.Especially in long-term eruptions, the eruption style may change over time.For example, there are many volatile components at the beginning, and even if lava or volcanic ash is blown up high, the volatile components may decrease in the middle and it may not be possible to blow up volcanic ash.Then, in the latter half of the eruption, the volatile components are released, lava flows out, and the eruption ends.As an example of such changes over time, the example of the large eruption of Tenmei at Mt. Asamayama is shown.

• A large amount of volcanic ash is ejected high in the sky (Tenmei descending pumice) → Pyroclastic flow hits the ground (Agatsuma pyroclastic flow, Kamahara pyroclastic flow) → Lava flows out (demon extruded lava)

Effect of the amount of ejecta

In addition to the effects of the components, the eruption style and the magnitude of damage vary greatly depending on the amount of ejecta and the eruption speed.Here are two examples of extremes with large eruption volumes.

Under the condition 2 above, if the amount of one ejection is an order of magnitude larger, the ejected gas will be地球Cover,Abnormal weatherbyBad cropAnd so on.One example 1783 Ofアイス ランド OfLaki volcanoIn the case of the eruption (Rakagigal fissure eruption), about 130 eruptedcraterThe length of the row was 25 km, and a large amount of lava was ejected.However, the damage caused by lava was minor because the eruption occurred in a remote location.But a lot of toxicVolcanic gas(1 million tonsSulfurous acid gasAnd 800 million tonsHydrogen fluoride) Is released in Iceland家畜50% ofpopulation20% was lost.Also,stratosphereOf volcanic gas origin that has risen to(I.e.(Sulfate mistEtc)Northern hemisphereReach the ground by coveringInsolationIs decreasing worldwidetemperatureHas decreased.In addition, this eruption laterFrench RevolutionIt is said that it was the trigger to cause.In Japan, it overlapped with the influence of the large eruption of Mt. Asamayama (the large eruption of Tenmei) that occurred in the same year.Tohoku regionCaused a huge number of starvation deathsGreat Famine of TenmeiCaused.

Aso Caldera,Aira CalderaEruption

Under the conditions of 4 above, if the amount of one eruption is an order of magnitude larger, the major axis is several kilometers-a dozen kilometers.calderaIt will be a very large-scale eruption that will form.Japanese archipelagoIn the eruption of Aso caldera 9 years ago and Aira caldera (on the north side of Sakurajima)Kinko BayThe eruption of the whole) is known as a typical example.In the Aso caldera eruption, pyroclastic flows covered most of Kumamoto and Oita prefectures and northern Miyazaki prefecture.In addition, in the eruption of Aira Caldera, pyroclastic flowSilas PlateauWas formed.The volcanic ash erupted by these eruptions also accumulated throughout Japan, and a huge caldera was formed at the site where a large amount of magma had escaped.In eruptions that form a large caldera like these, pumice stones are generated over a thickness of several meters to 1 m or more and a radius of several tens of kilometers or more by the pyroclastic flow in one eruption.AccumulationAnd the wide area that erupted at the same timeTephraHowever, it often covers more than half of the Japanese archipelago.The caldera that causes eruptions like these is south of the Aso caldera.Kyushu regionIt is often seen in the Tohoku and Hokkaido regions.

Location of the eruption

The style of eruption of a volcano changes greatly depending on where it erupts, especially the presence of water.

Surtsey eruption(Ultracano eruption)

water surfaceEruptions nearby or in shallow places where magma is undergroundgroundwaterIf you meet, the water will instantlyboilingAnd explosive because it causes volume expansionMagma steam explosionOccurs.Previously it was called the Urtakukano eruption,Sulzey IslandThe eruption was a typical Vulcanian eruption, so it came to be called this.

(English edition）(Glacial bottom eruption)

HugeGlacierIf a volcano erupts under, it will have the same form as a submarine volcano, but the scale of the eruption is large.ice sheetIf you have unraveled, a huge lake under the glacier (Subglacial lake), And the glacier wall can't support the weight of a large amount of water and collapses, causing it to flow to houses and bridges.洪水Occurs.This great floodYork outburopeCall.

Underwater eruption

When it erupts into water, it is sometimes called an underwater eruption.^[9].

Submarine eruption

Submarine volcanoIf something erupts in deep water,Water pressureExplosion did not occur due to the high temperature, and the lava that eruptedseawaterSuddenly chilled inPillow lavaOrHyaloclastiteBecomes

Scale of eruption

Eruption magnitude

As an indicator of the magnitude of the explosionVolcanic Explosivity IndexIs used internationally.

However, the Volcanic Explosivity Index does not represent the amount of energy, so Japanese volcanologistsYukio Hayakawa(1993)^[10]It is,Eruption magnitudeIs proposing.

a formula

However, "m = mass of ejecta (kg)", and in the case of steam explosion, the amount of ejecta including the existing rock body.Also, the volume of collapsed sediments such as debris avalanches is not included.

条件

As a condition to maintain objectivity

1. If eruptions occur at the same time from a point 10 km or more away, each eruption shall be a separate eruption.
2. Eruption M provides an aftereffect period, 3 year when M <1. When M => 3, it is 10 years.Eruptions that occur during this aftereffect period will not be treated as new eruptions unless they exceed the previous M.

Magma eruption amount

The eruptive sediments produced by the eruption include those of the original magma (essential material) and those derived from the volcanic mountain body and bedrock destroyed by the eruption (similar material). It's difficult.

Therefore, what is converted and calculated by approximating the one composed of essential substances derived from magma is called "magma ejection amount".In the unit km³ To DRE: Dnape Rock Equivalent Is additionally written.In other words, it is an index showing the equivalent volume when all ejecta have the same specific gravity as lava.

Even in the essential material, the specific gravity of the ejected sediment is different due to the difference in events such as pyroclastic flow and volcanic ash (pyroclastic fall), and the magma is about 2.5 g / cm.³In contrast, pyroclastic flow and volcanic ash deposits are approximately 1.0 g / cm.³Is said to be^[11]..In other words, there are more deposits in pyroclastic flows and volcanic ash than the amount of eruption expressed by DRE.^{[Note 1]}.

Volcanic ash

Volcanic ashThe diameter of the volcanic rock that accompanies the eruption is 2mmRefers to the crushed ones below^[12].

The main cause of volcanic ash is

• Volcanic eruption^[12]
• A hot rock avalanche flows down a volcanic hillside^[12]
• When lava splashes^[12]

and so on.

The appearance such as the color and size of volcanic ash differs depending on the type of volcano and eruption, and the color varies from light gray to black, and the size varies from pebbles size to fine particles such as cosmetic powder.^[12].

Volcanic ash floating in the air blocks sunlight and deteriorates visibility, and it also becomes charged with electricity due to collision and friction between fine particles, causing lightning and lightning.^[12]..In addition, fine-grained volcanic ash may be swept downwind under the influence of the wind along with large-scale plumes.^[12]..Immediately after formation, the volcanic ash is covered with an acidic film, which, when inhaled by the human body, causes irritating harmful effects on the lungs and eyes, causing health hazards, and if accumulated, it affects the water quality of the surrounding area. It may have an adverse effect, and at the same time, it may have an adverse effect on plants and cause crop failure.^[12]..This film is quickly removed by rainfall^[12].

A large amount of accumulated volcanic ash mixes with the soil in the volcanic area and becomes a fertile topsoil layer.^[12]..The abundance of fertile soil around many volcanic areas is due to the presence of old volcanic ash deposits.^[12].

History of volcanic eruptions

footnote

Annotation

Source

References

• Earthquake Investigation CommitteeHen "Japan Eruption Spirit", 1991.ISBN 4-87044-102-0. -"Earthquake Investigation Committee Report Nos. 86, 87" ( 1918(Published) reproduction.
• National ObservatoryHen "Volcano" "Science chronology 20"Maruzen, 2007, pp. 663-693.ISBN 978-4-621-07902-7.
• Yuzo Kato"Pumice: Message from Submarine Volcano"Yasaka Shobo, 2009,^{[Page number required]}.ISBN 978-4-89694-930-8.
• Shigeo Aramaki, "Classification and mechanism of eruption phenomenon"Volcano. Vol. 2" 1975 Volume 20 TOKUBE p.205-221, two:10.18940 / kazanc.20.TOKUBE_205, Volcanological Society of Japan

Related item

• Volcano(List of volcanoes,List of volcanoes (Japan),Pacific Rim Volcanic Zone,Decade Volcanoes）
• Volcanic Explosivity Index(VEI)
• Volcanic ejecta,Volcanic debris,magma,Magma chamber
• aftermath:Volcanic lightning,Empty shake(Explosion sound)
• Geyser,Mud volcano
• Mass extinction,Volcanic winter

Disaster prevention

• Eruption prediction
• Volcanic disaster forecast map
• List of natural phenomena named by the Japan Meteorological Agency
• International Association of Volcanology and Chemistry of the Earth's Interior,Volcanic eruption prediction liaison meeting,US Geological Survey,National Institute of Geophysics and Volcanoes, Italy

外部 リンク

• Volcano -Meteorological Agency
• Volcano and eruption (PDF) --Gifu Local Meteorological Observatory
• Disaster Prevention Basic Course Disaster Prediction -12. Volcanic Eruption- Disaster Prevention Research Institute