Some coun­tries are fa­mous for images of spew­ing lava and moun­tain­ous de­struc­tion. how­ever, ap­pear­ances can be de­ceiv­ing. Not all volcanoes are the same, reports VHAIRI MACK­IN­TOSH.

Cosmos - - Contents -

Where are the most ac­tive volcanoes in the world? . VHAIRI MACK­IN­TOSH has the an­swer.

VOL­CANIC AC­TIV­ITY. It’s the rea­son why the town of El Rodeo in Gu­atemala is cur­rently un­in­hab­it­able, why the Big Is­land of Hawaii gained 1.5 kilo­me­tres of new coast­line in June, and why Denpasar air­port in Bali has closed twice this year.

But th­ese erup­tions should not be seen as de­struc­tive at­tacks on cer­tain places or the peo­ple that live in them. They have noth­ing to do with even the coun­try that hosts them. They oc­cur in spe­cific re­gions be­cause of much larger-scale pro­cesses orig­i­nat­ing deep within the Earth.

Ac­cord­ing to the United States Ge­o­log­i­cal Sur­vey (USGS), ap­prox­i­mately 1,500 po­ten­tially ac­tive volcanoes ex­ist on land around the globe. Here’s a look at four of the world’s most vol­cani­cally ac­tive spots, and the dif­fer­ent pro­cesses re­spon­si­ble for their erup­tions. As you’ll see, there is no one-size-fits-all vol­cano.


Most vol­canic erup­tions go un­no­ticed. That’s be­cause they hap­pen con­tin­u­ously on the ocean floor where cracks in the Earth’s outer layer, the litho­sphere (com­pris­ing the crust and solid up­per man­tle), form at so-called di­ver­gent plate bound­aries. Th­ese mar­gins form due to con­vec­tion in the un­der­ly­ing man­tle, which causes hot, less dense molten ma­te­rial, called magma, to rise to the sur­face. As it forces its way through the litho­spheric plate, magma breaks the outer shell. Lava, the sur­face-equiv­a­lent of magma, fills the crack and pushes the bro­ken pieces in op­po­site directions.

Vol­can­ism from this ac­tiv­ity cre­ated Ice­land. The coun­try is lo­cated on the Mid-at­lantic Ridge, which forms the seam be­tween the Eurasian and North Amer­i­can plates. Ice­land is one of the few places where this type of spread­ing cen­tre pops above sea level.

How­ever, vol­can­ism on Ice­land also hap­pens be­cause of its lo­ca­tion over a hot spot. Th­ese spots de­velop above ab­nor­mally hot, deep re­gions of the man­tle known as plumes.

Each plume melts the over­ly­ing ma­te­rial and buoy­ant magma rises through the litho­sphere – pic­ture a lava lamp – to erupt at the sur­face.

This vol­canic dou­ble whammy pro­duces both gentle fis­sure erup­tions of basaltic lava as well as stra­to­vol­ca­noes that are char­ac­terised by pe­ri­odic non­ex­plo­sive lava flows and ex­plo­sive, py­ro­clas­tic erup­tions, which pro­duce clouds of ash, gas and de­bris.

In 2010, the two-month erup­tion of the ice-capped Ey­jaf­jal­la­jökull stra­to­vol­cano – the one that no one out­side Ice­land can pro­nounce – at­tracted a lot of me­dia at­ten­tion be­cause the re­sult­ing ash cloud grounded thou­sands of flights across Europe.

In fact, it was a rel­a­tively small erup­tion. It is be­lieved that a ma­jor erup­tion in Ice­land is long over­due. Four other volcanoes are all show­ing signs of in­creased ac­tiv­ity, in­clud­ing the coun­try’s most feared one, called Katla.


More than 197 mil­lion In­done­sians live within 100 km of a vol­cano, with nearly nine mil­lion of those within 10 km. In­done­sia has more volcanoes than any other coun­try in the world. The 1815 erup­tion of its Mount Tamb­ora still holds the record for the largest in re­cent his­tory.

In­done­sia is one of many places lo­cated within the world’s most vol­cani­cally, and seis­mi­cally, ac­tive zone, known as the Pa­cific Ring of Fire. This 40,000 km horse­shoe-shaped re­gion, bor­der­ing the Pa­cific Ocean, is where many tec­tonic plates bang into each other.

In this so-called con­ver­gent plate bound­ary set­ting, the process of sub­duc­tion gen­er­ates vol­can­ism. Sub­duc­tion oc­curs be­cause when two plates col­lide, the higher den­sity plate con­tain­ing oceanic crust sinks be­neath an­other less dense plate, which con­tains ei­ther con­ti­nen­tal crust or younger, hot­ter and there­fore less dense oceanic crust. As the plate de­scends into the man­tle, it re­leases flu­ids that trig­ger melt­ing of the over­rid­ing plate, thus pro­duc­ing magma. This then rises and erupts at the sur­face to form an arc-shaped chain of volcanoes, in­ward of, but par­al­lel to, the sub­duct­ing plate mar­gin.

In­done­sia marks the junc­tion be­tween many con­verg­ing plates and, thus, the sub­duc­tion pro­cesses and vol­can­ism are com­plex. Most of In­done­sia’s volcanoes, how­ever, are part of the Sun­dra Arc, an is­land vol­canic range caused by the sub­duc­tion of the Indo-aus­tralian Plate be­neath the Eurasian Plate. Vol­can­ism in eastern In­done­sia is mainly caused by the sub­duc­tion of the Pa­cific Plate un­der the Eurasian Plate.

The stra­to­vol­ca­noes that form in con­ver­gent plate bound­ary set­tings are the most dan­ger­ous be­cause they are char­ac­terised by in­cred­i­bly fast, highly ex­plo­sive py­ro­clas­tic flows. One of In­done­sia’s stra­to­vol­ca­noes, Mount Agung, erupted on 29 June for the sec­ond time in a year, spew­ing ash more than two km into the air and ground­ing hun­dreds of flights to the pop­u­lar tourist des­ti­na­tion, Bali.


The June 3 erup­tion of the Gu­atemalan stra­to­vol­cano, Vol­can de Fuego (Vol­cano of Fire), dev­as­tated Gu­atemalans, and the rest of the world, as hor­ri­fy­ing images and videos of peo­ple try­ing to es­cape the quick­mov­ing py­ro­clas­tic flow filled the news.

Like In­done­sia, Gu­atemala’s lo­ca­tion within the Ring of Fire and the sub­duc­tion-re­lated pro­cesses that go along with its lo­ca­tion are re­spon­si­ble for the volcanoes found here. Lo­cated on the other side of the Pa­cific Ocean, vol­can­ism is caused by the sub­duc­tion of the much smaller Co­cos Plate be­neath the North Amer­i­can-caribbean Plate.

Un­like In­done­sia, how­ever, the con­ver­gent bound­ary be­tween th­ese two plates oc­curs on land in­stead of within the ocean. There­fore, the Gu­atemalan arc does not form is­lands but a north­west-south­east trending chain of on­shore volcanoes.

The same process is re­spon­si­ble for the for­ma­tion of the An­des – the world’s long­est con­ti­nen­tal moun­tain range – fur­ther south along the west­ern coast of South Amer­ica. In this case, sub­duc­tion of the Naz­caAntarc­tic Plate be­neath the South Amer­i­can Plate causes vol­can­ism in coun­tries such as Chile and Peru.


When some­one men­tions Hawaii, it’s hard not to pic­ture a vol­cano. But Hawaii’s volcanoes are ac­tu­ally not typ­i­cal. That’s be­cause they are not found on a plate bound­ary. In fact, Hawaii is slap-bang in the mid­dle of the Pa­cific Plate – the world’s largest.

Like Ice­land, Hawaii is also un­der­lain by a hot spot. How­ever, be­cause the Pa­cific Plate is moving to the north­west over this rel­a­tively fixed man­tle anom­aly, the re­sult­ing vol­can­ism cre­ates a lin­ear chain of is­lands within the Pa­cific Ocean. A vol­cano form­ing over the hot spot will be car­ried away, over mil­lions of years, by the moving tec­tonic plate. As a new vol­cano be­gins to form, the older one be­comes ex­tinct, cools and sinks to form a sub­ma­rine moun­tain. Through this process, the is­lands of Hawaii have been form­ing for the past 70 mil­lion years.

The typ­i­cal shield volcanoes that form in this ge­o­log­i­cal set­ting are pro­duced from gentle erup­tions of basaltic lava and are rarely ex­plo­sive. The youngest Hawai­ian shield vol­cano, Ki­lauea, erupted in­tensely on 3 May of this year, and 1,170 de­gree Cel­sius lava has been flow­ing over the is­land and into the ocean ever since. Ki­lauea, which has been con­tin­u­ously ooz­ing since 1983, is re­garded as one of the world’s most ac­tive volcanoes, if not the most.


It may be sur­pris­ing to hear that de­spite the Hi­malayas, like the An­des, be­ing lo­cated on a very ac­tive con­ver­gent plate bound­ary, they are not vol­cani­cally ac­tive. In fact, there are barely any volcanoes at all within the moun­tain range.

This is be­cause the two col­lid­ing plates that are re­spon­si­ble for the for­ma­tion of the Hi­malayas con­tain con­ti­nen­tal crust at the con­ver­gent plate bound­ary, dis­tinct from the oceanic-con­ti­nen­tal or ocean­ic­o­ceanic crustal bound­aries in the Gu­atemalan and In­done­sian cases, re­spec­tively.

As the two col­lid­ing plates have sim­i­lar com­po­si­tions, and there­fore den­si­ties, and both their den­si­ties are much lower than the un­der­ly­ing man­tle, nei­ther plate is sub­ducted. It’s a bit like wood float­ing on wa­ter. As sub­duc­tion causes the litho­spheric par­tial melt­ing that gen­er­ates the magma in con­ver­gent plate bound­ary set­tings, vol­can­ism is not com­mon in con­ti­nent-con­ti­nent col­li­sions.

Un­for­tu­nately, Hi­malayan peo­ple don’t get off that eas­ily though, be­cause dev­as­tat­ing earthquakes go hand-in-hand with this sort of set­ting.

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