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Introduction to volcanic eruption types Eruptions can be effusive with flowing rivers of lava or (gentle) or explosive with rapidly occuring eruptions which generate large volumes of ash. Fluid lava eruptions tend to be more effusive with some lava fountains and are generaly found in hot spot or rifting settings. The rock type erupted here is normally basalt which is sourced from the mantle. Occasionally basalt will be erupted in subduction zone settings ie. Stromboli and Etna. In this situation the magma will have ben injected into the base of the volcanic system from the mantle and had little chance to pond in a magma chammber before erupting. Lava fountains can occur. Explosive eruptions tend to occur at subduction zones where lava is more viscous and contains more gas and steam. As magma rises within the crust its chemical composoition changes. Silica is added from the surrounding crust which makes it more viscous, water and gas form bubbles .Within the magma chamber the melt cools and starts to crystallise making it less fluid. All this stickiness makes for an explosive mix-bit like blowing bubbles in thick milk shake! Exceptions are where hotspots occur under continental crust which cause giant caldera forming volcanoes or supervolcanoes. Supervolcanoes put other eruptions in the shade. They erupt incredibly violently for days on end and change the climate of the whole world by filling the atmosphere with ash and dust. We don't want one to go off. |
  USGS Volcano types diagram |
Shield Volcanoes Shield volcanoes have gentle slopes and large diameters. They are formed by the accumulation of low viscosity basalt lava flows. Because the flows have low viscousity they flow a long way. Lava tubes form which enable molten lava to flow long distances from the vent. They form where large volumes of basalt lava are erupted over along period of time from one location such as over a hot spot/mantle plume. They are called shield volcanoes ecause they look like a warriors shield. The most famous and largest shield volcanoes in the world are those in Hawaii. Mauna Loa is the largest volcano on earth , Mauna Kea is 16000m tall from base to summit thats greate than Everest.Kilauea is still active and pours lava into the sea everyday.Alll located on Big Island/Hawaii. To the top / Uk shop / USA shop |
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Composite / Stratvolcano Composite and Stratovolcanoes are one and the same. They are the volcano diagram that we all drew at school with layers of ash and lava. They are normaly located above subduction zones and erupt explosively. Viscous lava, ash,tephra and pyroclastic flows build up a steep sided cone with a crater(s) at the summit. The voclano can erupt a variety of lava types from black basalt, through andsite and sticky dacite to the eruptive equivalent of granite- rhyolite. This variety of lavas is due to cooling and crystallisation of the magma within the volcano changing the composition of the melt when it is erupted. The volcano may have smaller parasitic cones. Domes of lava may form in the summit craters. Mt.St.Helens and Mt.Rainier in the Cascades are both Stratovolcanoes. To the top / Uk shop / USA shop |
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Cinder Cones Cinder cones are compsed of rock fragments which have been ejected from a vent possibly as molten lava , cooled on their descent and dropped downwind of a volcanic vent. Often made of scoria and tephra ( small volcanic rock fragments) they from quickly perhaps over a few days. They can 100 of metres tall. The rock fragments are cooled quickly and glassy in texture. They require low viscosity lava to form such as basalt and can be found in Iceland and on Mt.Etna. To the top / Uk shop / USA shop |
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Lava Domes Lava domes are composed of highly viscous lavas with high silica contents such as dacite or rhyolite. They are typically found in the craters of stratovolcanoes such as Mt.St. Helens or Soufriere Hills. The lava is exuded rather than erupted and is squeezed out like thick toothpaste. If they become unstable and collapse pyroclastic flows can be produced or tall eruption clouds. Lassen peak in the cascade range is the worlds largest lava dome at over 10 000 feet, and is made of dacite. To the top / Uk shop / USA shop |
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Supervolcano Caldera A supervolcano eruption is rare and only recently recognised. A supervolcano usually forms over a hot spot and under thick crust. They often appear on the surface as a large depression called a caldera which can be 10 s of km across. The depression is sometimes filled with a lake as at Taupo. The caldera is formed as the magma inside is violently blown out of the magma chamber below. Initially this is from the edges then the whole caldera floor collapses, leaving a flat bottomed depressio surrounded by steep sides. Eruptions are incredibly violent and dramatic and although volcanoligists and TV crews would love to see one the rest of the world can wait!! Yellowstone has been much talked about in the media as due for an eruption but there are no signs of this just yet. Hooray! A supervolcano has not erupted within the modern technological age. The most recent eruption that gets close is the 1815 eruption of Tambora in Indonesia wich ejcted 100 km3 of material. Previous to this the last true supervolcano eruption was Taupo New Zealand 26 000 years ago whcih ejected 800km3 of material. Both these eruptions dramativally effected the worlds climate and food availability. To the top / Uk shop / USA shop |
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Fissures and Rifts Fissures and rifts are elongate cracks in the earths surface.Fissures are smaller than rifts. They typically form where the crust is under tension and is pulled apart allowing molten rock to pour out. Basalt is the predominate rock type found and fountains or curtains of "fire" can spectacularly erupt. Eruptions can pour out large volumes of basalt and eventually focus on one or two main vents. Iceland has had spectacular fissure eruptions because the crust is under extensional tension due to the rifting or spreading as the North Atlantic and European plates pull apart. Rifts are on a larger scale and involve great extensional tensions in the crust. Successful rifting can create new oceans and split apart contients. The NW coast of Scotland has plenty of volcanics which erupted when the North Atlantic ocean was formed. To the top / Uk shop / USA shop |
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Hawaiian Hawaiian eruptions are characterised by the long flowing rivers of basaltic lava which gradully build up into vast shield volcanoes. Lava fountains, fissure eruptions and lava flows are the main characteristics. eruptions are effussive rather than explosive and are generally much much safer. Basalt can form two distinctive lava flows one called "pahoehoe" in Hawaiin or rope like and "aa aa" which popular myth says is the sound you make if you walk on it bare foot-its very sharp. Pahoehoe is very fluid and flows like custard or treacle, it can form channels which then roof over to form lava tubes. Aa aa is a slower moving flow with a steep front and is blocky and jagged. To the top / Uk shop / USA shop |
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Strombolian Strombolian eruptions are characterized by the intermittent explosion or fountaining of basaltic lava from a single vent or crater. Each episode is caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly. The lava fragments generally consist of partially molten volcanic bombs that become rounded as they fly through the air. Stromboli in the aeolain islands of Italy is on eof the worlds most reliable and cintinually active volcanoes- worth the walk! To the top / Uk shop / USA shop |
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Phreatoplinian and Surtseyan Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits). The intense heat of such material (as high as 1,170° C for basaltic lava) may cause water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks, and bombs. Surtesy erupted off the coast of Iceland between 1963 and 1967. To the top / Uk shop / USA shop |
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Pelean The pelean style of eruption is charcterised by dast moving pyroclastic flows or surges whcih incinerate everything i their path. Large volumes of dust,ash.rock and gas are ejected from the volcano then career down slopes as Nuee Ardente or pyroclastic flows. The style is named after Mt.Pelee in the Carribbean whci erupted in 1902 killing 28 000 people. www.mount-pelee.com To the top / Uk shop / USA shop |
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Vulcanian Vulcanian eruptions are noisy and dramatic and are named after Vulcano in the Aeolian Islands. A phreatomagmatic (steam and lava) eruption occurs first, then a black explosive cloud as the volcano vent is cleared then a final whiter column of ash and gas. Lava bombs are not rounded and are typically made of viscous andesite and dacite which could explain their ragged shapes. A final viscous lava effusion may occur. Stromboli and Soufriere Hills have both shown this eruption style. To the top / Uk shop / USA shop |
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Plinian and Ultraplinian Plinian eruptions are named after Pliny the Younger who wrote a detailed accoount of the eruption of Vesuvius in AD79. He likened the tall eruption cloud to an umbrella pine- tall slender trunk and large spreading top. Eruption clouds can extend well above 11km into the atmosphere and produce vast quantities of ash and gas.When the column looses its upward momentum it collapses back down the slopes of the volcano creating pyroclastic flows. Ash and pumice fall may be deep enough to bury homes,collapse roofs and turn day to night. Lahars may form where ash and water mix burying everything in their path. Pompeii and Hercalaneum were famously buried in AD79 and the last eruption of Vesuvius was in 1944. Mt. Pinatubo, in the Phillipines,was the worlds last dramatic plinian erption in 1991. Due to the proximity of an american airbase the voclano wa closely monitored and 1000s of locals evacuated to safety. Although the lahars and refugee camps caused many fatalities. To the top / Uk shop / USA shop |
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 Hawaiian |
 Strombolian |
 Phreatoplinian and Surtseyan |
 Pelean |
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 Plinian and Ultraplinian |
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Hawaii |
Yellowstone Supervolcano | Mt .Etna Europes largest volcano |
Mt.St.Helens Washington USA |
Stromboli Lighthouse of The Med. |
