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=====Destructive plate margins=====
=====Destructive plate margins=====


These are the most visible and among the most well-known types of volcanoes on earth, forming above the [[subduction zone]]s where (oceanic) plates dive into the mantle. Their magmas are typically ''calc-alkaline'' as a result of their origins in the upper parts of altered ocean plate materials, mixed with sediments, and rise through variable thicknesses of more-or-less continental crust. The denser plate sinks (subducts) under the lighter one and the friction from the melting plate causes magma to force its way out through a crack in the crust. Unsurprisingly, their compositions are much more varied than at constructive margins.
These are the most visible and among the most well-known types of volcanoes on earth, forming above the [[subduction zone]]s where (oceanic) plates dive into the mantle. Their magmas are typically ''calc-alkaline'' as a result of their origins in the upper parts of altered ocean plate materials, mixed with sediments, and rise through variable thicknesses of mo

=====Hotspots=====

[[Image:Lava_flow_at_Krafla,_1984.jpg|thumb|right|250px|[[1984]] Eruption at [[Krafla]], [[Iceland]]]]

[[Hotspot (geology)|Hotspots]] were originally a catch-all for volcanoes that didn't fit into one of the above two categories, but today this refers to a more specific circumstance - where an isolated [[Mantle plume|plume]] of hot mantle material hits the underside of the crust, either ([[oceanic crust|oceanic]] or [[continental crust|continental]]). The mantle plume can lead to a volcanic center that is not obviously connected with a plate margin. The classic example is the [[Hawaiian Islands]], which is generated by a hotspot underneath the oceanic crust of the Pacific. Yellowstone is cited as another classic example; in this case this involves continental crust because it is far inland. Iceland is sometimes cited as a third classical example, but complicated by the coincidence of a hotspot intersecting an [[Mid ocean ridge|oceanic ridge]] constructive margin.

There are debates about the simple "hotspot" concept, since scientists cannot agree on whether the "hot mantle plumes" originate in the upper mantle or in the lower mantle. Meanwhile, field geologists and petrologists see considerable variation in the detailed chemistry of magmas generated by mantle plumes. Additionally, high-resolution [[seismology]] of different hotspots is yielding different pictures of the deep sub-structure of [[Hawaii]] versus [[Iceland]]. There is no detailed consensus about how to interpret these varied results, and it seems plausible that eventually several different sub-types of hotspots may be identified in the future.

==Predicting volcanic eruptions==
Scientists have not yet been able to predict with absolute certainty when a volcanic eruption will take place, but significant progress has been made in recent times. The main world organization for predicting and monitoring volcanic activity is the [[United States Geological Survey]] (USGS). The USGS invests significant resources monitoring and researching volcanos (as well as other geological phenomina).
[[Image:sthelens3.jpg|thumb|right|200px|[[Mount St. Helens]] erupted explosively on [[May 18]], [[1980]] at 8:32 a.m. PDT]]
Volcanologists monitor the following phenomena to help forecast eruptions:

====Seismicity====
Seismic activity (earthquakes and tremors) always occurs as volcanoes awaken and prepare to erupt and are a very important link to eruptions. Some volcanoes normally have continuing low-level seismic activity, but an increase may signal a greater likelihood of an eruption. The types of earthquakes that occur and where they start and end are also key signs. Volcanic seismicity has three major forms: '''short-period earthquake''', '''long-period earthquake''', and '''harmonic tremor'''.

*Short-period earthquakes are like normal fault-generated earthquakes. They are caused by the fracturing of brittle rock as magma forces its way upward. These short-period earthquakes signify the growth of a magma body near the surface and are known as 'A' waves.

*Long-period earthquakes are believed to indicate increased gas pressure in a volcano's plumbing system. They are similar to the [[water hammer|clanging sometimes heard in a house's plumbing system]]. These oscillations are the equivalent of acoustic vibrations in a chamber, in the context of magma chambers within the volcanic dome and are known as 'B' waves.

*[[Harmonic tremor]]s are often the result of magma pushing against the overlying rock below the surface. They can sometimes be strong enough to be felt as humming or buzzing by people and animals, hence the name.

Patterns of seismicity are complex and often difficult to interpret; however, increasing seismic activity is a good indicator of increasing eruption risk, especially if long-period events become dominant and episodes of harmonic tremor appear.

In December 2000, scientists at the [[National Center for Prevention of Disasters]] in [[Mexico City]] predicted an eruption within two days at [[Popocatépetl]], on the outskirts of Mexico City. Their prediction used research done by [[Bernard Chouet]], a Swiss vulcanologist working at the [[United States Geological Survey]], into increasing long-period oscillations as an indicator of an imminent eruption. The government evacuated tens of thousands of people; 48 hours later, the volcano erupted as predicted. It was Popocatépetl's largest eruption for a thousand years, yet no one was hurt.

Using a similar method, researchers can detect volcanic eruptions by monitoring infra-sound—sub-audible sound below 20Hz. The IMS Global Infrasound Network, originally set up to verify compliance with nuclear test ban treaties, has 60 stations around the world that work to detect and locate erupting volcanoes.[http://www.inframatics.org/pdf/inframatics_march2003.pdf]

====Gas emissions====
[[Image:Pompeii the last day 1.jpg|thumb|The eruption of Vesuvius in Discovery Channel's ''[[Pompeii: The Last Day|Pompeii]]''.]]
As magma nears the surface and its pressure decreases, gases escape. This process is much like what happens when you open a bottle of soda and carbon dioxide escapes. Sulfur dioxide is one of the main components of volcanic gases, and increasing amounts of it herald the arrival of increasing amounts of magma near the surface. For example, on [[May 13]], [[1991]], an increasing amount of sulfur dioxide was released from [[Mount Pinatubo]] in the [[Philippines]]. On May 28, just two weeks later, sulfur dioxide emissions had increased to 5,000 tonnes, ten times the earlier amount. Mount Pinatubo later erupted on [[June 12]], [[1991]]. On several occasions, such as before the Mount Pinatubo eruption, sulfur dioxide emissions have dropped to low levels prior to eruptions. Most scientists believe that this drop in gas levels is caused by the sealing of gas passages by hardened magma. Such an event leads to increased pressure in the volcano's plumbing system and an increased chance of an explosive eruption.

====Ground deformation====
Swelling of the volcano signals that magma has accumulated near the surface. Scientists monitoring an active volcano will often measure the tilt of the slope and track changes in the rate of swelling. An increased rate of swelling, especially if accompanied by an increase in sulfur dioxide emissions and harmonic tremors is a high probability sign of an impending event. The deformation of [[Mount St. Helens]] prior to the May 18, 1980 eruption was a classic example of deformation, as the north side of the volcano was bulging upwards as magma was building up underneath. But most cases of ground deformation are usually detectable only by sophisicated equipment used by scientists, but they can still predict future eruptions this way.

====Iceberg tremors====
It has recently been published that the striking similarities between [[iceberg]] [[tremors]], which occur when they run aground, and volcanic tremors may help experts develop a better method for predicting [[volcanic eruptions]]. Despite the fact that icebergs have much simpler structures than volcanoes, they are physically easier to work with. The similarities between volcanic and iceberg tremors include long durations and [[amplitude]]s, as well as common shifts in [[frequencies]].
(Source: [http://www.canadiangeographic.ca/magazine/MA06/indepth/science.asp Canadian Geographic] "Singing icebergs")

===Hydrology===
There are 3 main methods that can be used to predict a volcanic eruption through the use of hydrology:-
*The first is the detection of lahars and other debris flows close to their sources. USGS scientists have developed an inexpensive, durable, portable and easily installed system to detect and continuously monitor the arrival and passage of debris flows and floods in river valleys that drain active volcanoes.

*Pre-eruption sediment may be picked up by a river channel surrounding the volcano that shows that the actual eruption may be imminent. Most sediment is transported from volcanically disturbed watersheds during periods of heavy rainfall.

*Volcanic deposit that may be placed on a river bank can easily be eroded which will dramatically widen or deepen the river channel. Therefore, monitoring of the river channels width and depth can be done to predict a future volcanic eruption.

===Remote Sensing===
Remote sensing is the detection by a satellite’s sensors of electromagnetic energy that is absorbed, reflected, radiated or scattered from the surface of a volcano or from its erupted material in an eruption cloud.
*Scientists can monitor the unusually cold eruption clouds from volcanoes using data from two different thermal wavelengths to enhance the visibility of eruption clouds and discriminate them from meteorological clouds
*Sulphur dioxide can also be measured by remote sensing at some of the same wavelengths as ozone. TOMS (Total Ozone Mapping Spectrometer) can measure the amount of sulphur dioxide gas released by volcanoes in eruptions
*The presence of new significant thermal signatures or 'hot spots' may indicate new heating of the ground before an eruption, represent an eruption in progress or the presence of a very recent volcanic deposit, including lava flows or pyroclastic flows.

====Local Predictions====
The eruption of Mt. Nyiragongo on the 17th January 2002 was predicted a week earlier by a local expert who had been watching the volcanoes for years. He informed the local authorities and a UN survey team was dispatched to the areas, however it was declared safe. Unfortunately when the volcano erupted, 40% of the city of Goma was destroyed and many people’s livelihoods were destroyed.
He claimed that he had noticed small changes in the local relief and had monitored the eruption of a much smaller volcano 2 years earlier. Since he knew that these two volcanoes were connected by a small fissure he knew that Mt. Nyiragongo would erupt soon.

====Early Warning for Lahars====
A team of US scientists discovered a method of predicting lahars. Their method was developed by analysing rocks on Mt. Rainier in Washington. The warning system depends on noting the differences between fresh rocks and older ones. Fresh rocks are poor conductors of electricity and become hydrothermicaly altered by water and heat. Therefore, if they know the age of the rocks, and therefore the strength of them, they can predict the pathways of a lahar.

=== Predicting future eruptions of Mt. Etna ===
British geologists have developed a method of predicting future eruptions of Mt. Etna. They have discovered that there is a time lag of 25 years between events that happen below the surface and events that happen on the surface, i.e. a volcanic eruption. The careful monitoring of deep crust events can help predict accurately what will happen in the years to come. So far they have predicted that between 2007 and 2015, volcanic activity will be half of what it was in 1987.

=== Monitoring of Sakurajima, Japan ===
Sakurajima is possibly one of the most monitored areas on earth. The Sakurajima Volcano lies near Kagoshima City, which has a population of 500,000 people. Both the Japanese Meteorological Agency (JMA) and Kyoto University's Sakurajima Volcanological Observatory (SVO) monitors the volcanoes activity. Since 1995, Sakurajima has only erupted from its summit with no release of lava.

Monitoring techniques:-

*Likely activity is signalled by swelling of the land around the volcano as magma below begins to build up. At Sakurajima, this is marked by a rise in the seabed in Kagoshima Bay – tides levels rise as a result

*As magma begins to flow, melting and splitting base rock can be detected as volcanic earthquakes. At Sakurajima, they occur two to five kilometres beneath the surface. An underground observation tunnel is used to detect volcanic earthquakes more reliably.

*Groundwater levels begin to change, the temperature of hot springs may rise and the chemical composition and amount of gases released may alter. Temperature sensors are placed in bore holes which are used to detect ground water temp. Remotes sensing is used on Sakurajima since the gases are highly toxic – the ratio of HCl gas to SO<sub>2</sub> gas increases significantly shortly before an eruption.

*As an eruption approaches, tiltmetre systems measure minute movements of the mountain. Data is relayed in real-time to monitoring systems at SVO

*Seismometers detect earthquakes which occur immediately beneath the crater, signalling the onset of the eruption. They occur 1 to 1.5 seconds before the explosion

*With the passing of an explosion, the tiltmeter system records the settling of the volcano.

==Effects of volcanoes==
[[Image:Volcanic injection.jpg|thumb|250px|right|Volcanic "injection"]]
[[Image:Mauna Loa atmospheric transmission.png|thumb|250px|Solar radiation reduction due to volcanic eruptions]]
[[Image:TOMS SO2 time nov03.png|thumb|250px|Sulfur dioxide emissions by volcanoes.]]
[[Image:SO2 Galapagos 20051101.jpg|thumb|250px|right|Average concentration of sulfur dioxide over the Sierra Negra Volcano ([[Galapagos Islands]]) from October 23-November 1, 2005]]
There are many different kinds of volcanic activity and eruptions:
* [[phreatic eruptions]] (steam-generated eruptions)
* explosive eruption of high-[[silica]] [[lava]] (e.g., [[rhyolite]])
* effusive eruption of low-silica lava (e.g., [[basalt]])
* [[pyroclastic flow]]s
* [[lahar]]s (debris flow)
* [[carbon dioxide]] emission
All of these activities can pose a hazard to humans.

Volcanic activity is often accompanied by [[earthquake]]s, [[hot spring]]s, [[fumarole]]s, [[mud pot]]s and [[geyser]]s. Low-magnitude earthquakes often precede eruptions.

The concentrations of different volcanic gases can vary considerably from one volcano to the next. [[Water vapor]] is typically the most abundant volcanic gas, followed by [[carbon dioxide]] and [[sulfur dioxide]]. Other principal volcanic gases include [[hydrogen sulfide]], [[hydrogen chloride]], and [[hydrogen fluoride]]. A large number of minor and trace gases are also found in volcanic emissions, for example: [[hydrogen]], [[carbon monoxide]], and volatile metal chlorides.

Large, explosive volcanic eruptions inject water vapor (H<sub>2</sub>O), carbon dioxide (CO<sub>2</sub>), sulfur dioxide (SO<sub>2</sub>), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock and pumice) into the stratosphere to heights of 10-20 miles above the Earth's surface. The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H<sub>2</sub>SO<sub>4</sub>), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the Earth's [[albedo]] - its reflection of radiation from the Sun back into space - and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years. The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon pollution, generates chlorine monoxide (ClO), which destroys [[ozone]] (O<sub>3</sub>). As the aerosols grow and coagulate, they settle down into the upper troposphere where they serve as nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) are dissolved in water droplets in the eruption cloud and quickly fall to the ground as acid rain. The injected ash also falls rapidly from the stratosphere; most of it is removed within several days to a few weeks. Finally, explosive volcanic eruptions release the greenhouse gas carbon dioxide and thus provide a deep source of [[carbon]] for biogeochemical cycles.

Gas emissions from volcanoes are a natural contributor to [[acid rain]].

Volcanic activity releases about 130 to 230 teragrams (145 million to 255 million [[short ton]]s) of [[carbon dioxide]] each year.

Volcanic eruptions may inject [[Particulate|aerosols]] into the [[Earth's atmosphere]]. Large injections may cause visual effects such as unusually colorful sunsets and affect global climate mainly by cooling it.

Volcanic eruptions also provide the benefit of adding nutrients to soil through the weathering process of volcanic rocks. These fertile soils assist the growth of plants and various crops.

==Past beliefs==
[[Image:Kircherearthfires.jpg|thumb|left|Kircher's model of the [[Earth]]'s internal fires, from ''Mundus Subterraneus'']]
Before it was understood that most of the Earth's interior is molten, various explanations existed for volcano behavior. For decades after awareness that compression and radioactive materials may be heat sources, their contributions were specifically discounted. Volcanic action was often attributed to chemical reactions and a thin layer of molten rock near the surface.

[[Society of Jesus|Jesuit]] [[Athanasius Kircher]] (1602-1680), witnessed eruptions of [[Aetna]] and [[Stromboli]], then visited the crater of [[Vesuvius]] and published his view of an Earth with a central fire connected to numerous others caused by the burning of [[sulfur]], [[bitumen]] and [[coal]].

==See also==
===Lists===
*[[List of volcanoes]]
*[[List of famous volcanic eruption deaths]]
*[[Volcanic Explosivity Index]] (includes list of large eruptions)
*[[List of deadliest natural disasters]]

===Types===
*[[Supervolcano]]
*[[Prehistoric volcano]]
*[[Black smoker]] (deep sea vent)

===Specific locations===
*[[Iceland hotspot]]
*[[Pacific Ring of Fire]]
*[[Io (moon)|Io]]
*[[Triton (moon)]]

===Volcano-related phenomena===
*[[Volcanic gas]]
*[[Magma]]
*[[Lava]]
*[[Tsunami]]
*[[Lahar]]

===General topics===
*[[Geomorphology]]
*[[Earth science]]

===People===
*[[Haroun Tazieff]] (famous volcanologist)

==References==
[[Image:Vol.escape.jpg|thumb|140px|An evacuation Route sign, by [[Mount Rainier]], in case of eruption or [[lahar]].]]
*Macdonald, Gordon A., and Agatin T. Abbott. (1970). <cite>Volcanoes in the Sea</cite>. University of Hawaii Press, Honolulu. 441 p.
*Ollier, Cliff. (1988). <cite>Volcanoes</cite>. Basil Blackwell, Oxford, UK, ISBN 0-631-15664-X (hardback), ISBN 0-631-15977-0 (paperback).

==Further reading==
<!--
Note: This is an Icelandic name, it has no surname part so it's inappropriate to list it as Sigurðsson, Haraldur.
-->
* [[Haraldur Sigurðsson]], ed. (1999) ''Encyclopedia of Volcanoes''. Academic Press. ISBN 012643140X. This is a reference aimed at geologists, but many articles are accessible to non-professionals.

==External links==
{{commons|Volcano}}
*[http://volcanoes.usgs.gov/Products/Pglossary/pglossary.html Glossary of Volcanic Terms from USGS]
*[http://volcano.und.nodak.edu/vwdocs/glossary.html Volcanic and Geologic Terms] from [http://volcano.und.nodak.edu/ Volcano World]
*[http://news.bbc.co.uk/1/hi/sci/tech/3183047.stm Television program (BBC) on the prediction of Popocatepetl's 2000 eruption]
*[http://www.volcano.si.edu Smithsonian Global Volcanism Program]
*[http://www.geology.sdsu.edu/how_volcanoes_work Explore the geologic causes of an eruption]
*[http://science.howstuffworks.com/volcano.htm/printable ''How Volcanoes Work'' by Tom Harris]
*[http://www.geology.sdsu.edu/how_volcanoes_work/ How Volcanoes Work] - Educational resource on the science and processes behind volcanoes, intended for university students of geology, volcanology and teachers of earth science.
*[http://www.geonet.org.nz/volcanocam.html '''Volcano Cam''' Geonet's live pictures of 4 of New Zealand's volcanoes]
*[http://www.discover-indo.tierranet.com/volcano01.htm Indonesian Volcanoes] Discover some of the larger Indonesian volcanos.
*[http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Labs/VolcanicMaterials/ Volcanic Materials Identification]
*[http://www.projectshum.org/NaturalDisasters/volcano.html Natural Disasters - Volcano] Great research site for kids.

[[Category:Landforms]]
[[Category:Plate tectonics]]
[[Category:Igneous rocks]]
[[Category:Volcanology]]
[[Category:Geological processes]]
[[Category:Volcanoes|*]]
[[Category:Geological hazards]]
[[Category:Climate forcing agents]]

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Revision as of 16:59, 15 May 2006

the bombing of iraq would be the best hting ever ball sack after the eruption of May 18, 1980]]

Volcanoes elsewhere in the solar system

File:Olympus mons 1998.jpg
Olympus Mons (Latin, "Mount Olympus") is the tallest known mountain in our solar system, located on the planet Mars.

The Earth's Moon has no large volcanoes, but does have many volcanic features such as maria (the darker patches seen on the moon), rilles and domes.

The planet Venus has a surface that is 90% basalt, indicating that volcanism played a major role in shaping its surface. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Lava flows are widespread and forms of volcanism not present on Earth occur as well. Changes in the planet's atmosphere and observations of lightning, have been attributed to ongoing volcanic eruptions, although there is no confirmation of whether or not Venus is still volcanically active.

There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth:

These volcanoes have been extinct for many millions of years, but the European Mars Express spacecraft has found evidence that volcanic activity may have occurred on Mars in the recent past as well.

Jupiter's moon Io is the most volcanically active object in the solar system, due to tidal interaction with Jupiter. It is covered with volcanoes that erupt sulfur, sulfur dioxide and silicate rock, and as a result, Io is constantly being resurfaced. Its lavas are the hottest known anywhere in the solar system, with temperatures exceeding 1,800 K (1,500 °C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io [1]. Europa, the smallest of Jinto ice on the frigid surface. This process is known as cryovolcanism, and is apparently most common on the moons of the outer planets of the solar system.

Ice volcanoes on Enceladus

In 1989 the Voyager 2 spacecraft observed ice volcanoes (cryovolcanism) on Triton, a moon of Neptune and in 2005 the Cassini-Huygens probe photographed fountains of frozen particles erupting from Saturn's moon Enceladus [2]. The ejecta may be composed of water, liquid nitrogen, dust, or methane compounds. Cassini-Huygens also found evidence of a methane-spewing cryovolcano on the Saturnian moon Titan, which is believed to be a significant source of the methane found in its atmosphere. [3] It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.

Volcanology

Volcano formation

Diagram of a destructive margin causing earthquakes and a volcanic eruption

Like most of phenomena occurring in the earth's interior, the movements and dynamics of magma are poorly understood. However, it is known that an eruption may follow movement of magma upwards into the solid layer (the earth's crust) beneath a volcano and occupying a magma chamber. Eventually, magma in the chamber is forced upwards and flows out across the planet's surface as lava, or the rising magma can heat water in the surrounding landform and change the water into steam, creating great pressure. As a result, explosive eruptions can occur. Such explosive eruptions can produce a wide range of volcanic debris, such as volcanic ash (also known as tephra), volcanic bombs, which can be large enough to kill people and animals. Eruptions can vary from effusive to extremely explosive.

Many volcanoes are formed at destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust - this process is called subduction. As the oceanic crust is subducted, it descends into the mantle where temperatures are generally higher than near the surface of the planet. Increases in temperature and pressure with depth cause water trapped in the descending oceanic crust to escape from minerals in the crust. This process is called dehydration, commonly occurs at depths of about 100 km (62 miles) and can also be a source of very deep earthquakes due to an associated change in volume of the dehydrating rock mass (such as the 2001 Nisqually Earthquake in Washington State, USA). The water that escapes from the dehydrating oceanic crust migrates into the surrounding mantle which has a different composition than the descending crust. At ambient conditions in the mantle at 100km depth, water will induce partial melting of the mantle. This melt is less dense than the surrounding mantle and will consequently rise though the mantle to the overlying crust. As the magma (melt) rises through the crust it may melt and assimilate some of the surrounding crust, it may cool and begin to grow crystals, and it may exsolve gas.

The relative importance of these processes depends on the composition, amount and ascent rate of the magma. If the magma reaches the surface, it will generate a volcanic eruption. The style of the eruption will depend on the composition and gas content of the magma. The type of volcano will depend on the type of magma that usually erupts at that location over a long period of time, and the viscosity of the magma. High concentrations of silica are associated with high viscosity (thicker, goopier magma) and will form steep sided volcanoes. Volcanic arcs forming near subduction zones, on the edges of continental plates, usually form high-silica melt which create steep sided stratovolcanoes due to the high viscosity of the melt. For example, Mount St. Helens is found inland from the margin between the oceanic Juan de Fuca Plate and the continental North American Plate. Other examples of chains of stratovolcanoes include the Andes, the Cascade range, and the Aleutian Islands.

Shiprock, New Mexico a volcanic neck in the distance, with radiating dike on its south side. Photo credit: USGS Digital Data Series

A volcano is often stereotyped as a mountain sending forth from its summit great clouds of smoke with flames. The truth is that a volcano seldom emits either smoke or flame, although various combinations of hydrogen, carbon, oxygen, and sulfur do sometimes ignite. What is mistaken for smoke consists of vast volumes of fine dust (called volcanic ash), mingled with steam and other vapors, chiefly sulfurous. Most of what appears to be flames is the glare from the erupting materials, glowing because of their high temperature; this glare reflects off the clouds of dust and steam, resembling fire.

Perhaps the most conspicuous part of a volcano is the crater, a basin of a roughly circular shape, formed by a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form.

In some volcanoes, smaller cones or vents may form lower down the principal volcano, along rift zones or fractures. Such features are known as flank vents, flank cones or flank craters.

As a volcano becomes extinct and becomes eroded, solidified lava is often less easily eroded than volcanic ash and as a result, create interesting landforms. Solidified lava filled fractures called dikes often remain. The main vent may remain behind as a volcanic neck. Shiprock in New Mexico, United States is a fine example of these features.

Tectonic environments of volcanoes

Volcanoes can principally be found in three tectonic environments.

Hotspot and types of plate boundaries.
Constructive plate margins

These are by far the most common volcanoes on the Earth. They are also the least frequently seen, because most of their activity takes place beneath the surface of the oceans. Along the whole of the mid-ocean ridge are irregularly spaced surface eruptions, and more frequent sub-surface intrusions without surface expression. The large majority of these are only known because of earthquakes as part of the eruptions, or occasionally if passing shipping happens to notice unusually high water temperatures, unusual rumbling, or chemical precipitates in the seawater. In a few places, midoceanic ridge activity has led to volcanoes reaching to the surface - Saint Helena and Tristan da Cunha are examples - allowing them to be studied in some detail. But most activity takes place at considerable water depths. Iceland is also on a ridge, but has different characteristics than a simple volcano.

It could be argued that the volcanoes of the Great Rift Valley system of East Africa are modified constructive margin volcanoes. However the modifications caused by the presence of thick continental crust are very substantial, and the magmas produced are often very different from the typically very homogenous MORB (Mid-Ocean Ridge Basalt) that makes up the huge majority of constructive margin volcanoes. But still, some MORB lavas are known to have erupted on land, such as in the Afar Triangle, which makes up the northern end of the African Rift Valley. In fact, the Afar Triangle is a chance to see seafloor spreading on dry land, as many parts of it actually lie below sea level and due to the combination of mountain ranges cutting it off from the Red Sea and the fiercely hot and arid climate, it has largely dried up with extensive salt flats. Erta Ale is probably the best known volcano in this region, and is well known for its semipermanent lava lake activity.

Destructive plate margins

These are the most visible and among the most well-known types of volcanoes on earth, forming above the subduction zones where (oceanic) plates dive into the mantle. Their magmas are typically calc-alkaline as a result of their origins in the upper parts of altered ocean plate materials, mixed with sediments, and rise through variable thicknesses of mo

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