Volcanic Types
There are three types of volcanoes and each are determined by observing plate movements which formed them. They are hot spot, rift, and subduction.
Simply rift volcanoes form along the boundary of two plates as they tear apart and are usually under water. Hot spots volcanoes are created by hot magma plumes rising to the surface from deep within the mantle. A observable result from hot spot volcanoes are the Hawaiin islands built up from rising and cooling magma overtime. Lastly there's subduction volcanoes that come from collisons between tectonic plates. Subduction are argueably the most important to understand due to there destructive power and abillity to change the fate of the world.

Volcanic Forms

The shape and size of a volcano are determined by:
  • The type of eruption
  • The type of lava which is produced in the eruption
  • The relative amounts of lava and ash which build up to form the volcano spots, hydrothermal vents

Ash and cinder cone volcanoes occur where an explosive eruption hurls small, solid fragments of ash and rock from the volcano’s vent. The ash and rock build up to form volcanoes that are steep-sided but not very tall. This type of volcano is found in the Craters of the Moon area in Idaho, USA. Another example is Paricutin in Mexico.
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Acid lava cone volcanoes are made up of lava which is thick or viscous. This flows very slowly, like treacle, and does not extend very far from the vent. It forms cones that have steep sides. An example is Mount Ngauruhoe in New Zealand which last erupted in 1975.
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Shield volcanoes, like Mauna Loa on Hawaii in the Pacific Ocean, and Piton de la Fournaise on Reunion Island in the Indian Ocean, are made up of basalt-rich lava which is thin, runny and spreads a long way from the vent. As a result, shield volcanoes are very large but have very gently sloping sides. Shield volcanoes are mostly made up of lava and contain very little ash or cinder (approximately 95 per cent lava and 5 per cent ash).
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Composite cone volcanoes, also known as stratovolcanoes, make up more than 60 per cent of all volcanoes on earth. They are usually quite tall. They are formed by a cycle of quiet eruptions of runny lava followed by explosive eruptions of thick lava. Stratovolcanoes have more ash than shield volcanoes. This combination of high ash content and a thick, slow-moving lava means that their sides are much steeper than shield volcanoes. Mount St Helens in the USA, Pinatubo in the Philippines and Fuji in Japan are all examples of composite cone volcanoes.
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Types of Volcanic Eruptions
Volcanoes can be divided according to the explosiveness of their eruptions. The least severe are known as Hawaiian eruptions – these generally produce shield volcanoes. The most severe are called Plinian eruptions – these often involve the collapse of the volcano’s cone and the formation of a caldera. Plinian eruptions were named after the Roman scholar Pliny the Elder, who lost his life, when Mount Vesuvius erupted in AD 79. His nephew, Pliny the Younger, wrote to Tacitus describing the eruption as it began.

external image erupt1.jpgHawaiian

The least violent type of eruption. Large amounts of runny lava erupt and produce large volcanoes with gentle slopes.




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Mild but fairly regular eruptions. Small sticky lava bombs, ash, gas and glowing cinders erupt.


external image erupt3.jpgVulcanian

Violent eruptions shoot out very thick lava and large lava bombs.
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A violent type of eruption. Thick, sticky lava is accompanied by a burning cloud of ash, gas and pumice (a nuee ardente, which is French for ‘fiery cloud’).

external image erupt5.jpgPlinian

The most violent type of eruption. Cinders, gas and ash are flung explosively high into the air. The volcano cone often collapses to form a caldera.

Lava Domes
In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano.
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Image of the rhyolitic lava dome of Chaitén Volcano during its 2008–2009 eruption.

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One of the Mono Craters, an example of a rhyolite dome.

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Lava domes in the crater of Mount St. Helens


Types of Lava Flows


(Fig. 2.46) Pahoehoe lavas. Credit: Dr. Duncan Heron
Pahoehoe lavas are thin. They flow smoothly and are often formed by small volumes of hot, fluidbasalt. The higher the volume of lava emitted the faster the current. Pahoehoe flows move forwards in tongues or lobes and are characterized by a glassy, plastic skin. They may embrace obstacles at a rate of about 50m an hour. When the pahoehoe lava flow cools, it often solidifies to a smooth surface.

Sheet Lavas
Sheet Lavas

(Fig. 2.47) Sheet lavas. Credit: Mr.Mathot
Sheet lavas emerge from fissure systems forming flows commonly ranging between 10m and 30m in thickness. They flow out so fast that vast volumes of basalt are discharged over an enormous area. Featureless lava plateaus are formed. During the eruption of Roza, Oregon, 14 million years ago, 1500km3 of sheet lavas were produced in about a week.

Aa Flows
Aa Flows

(Fig. 2.48) Aa Flows. Credit: Dr. Duncan Heron
Aa flows are emitted from the vent at high rates ranging to 50km an hour, often with much lava fountaining. They are characteristic of viscousmagmas. Aa flows are animated with sporadic bursts of energy. They may push down houses, walls and forests. However, the hallmark of aa lava flows is the very rough surface it produces when it cools and solidifies.

Block Lavas
Block Lavas

(Fig. 2.49) Block Lavas. Credit: Dr. Duncan Heron
Block-lavas are often emerged in a fairly viscous state. They tend to be both stronger and thicker than aa lava flows. The more silicic the magma, the shorter and stubbier is the flow. Block lavas move slowly at a rate ranging from 1 to 5 meter a day. When solidified, they are characterized by often cubic masses with relatively smooth faces. In comparison with aa lava flows the surfaces of block lavas are much less rough and pitted than aa lava flows.
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