Definition
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface.
Basics
A volcano is a mountain where lava (very hot, molten rock) comes from a magma chamber under the ground. Most volcanoes have a crater at the top. Materials which poured out from it usually include lava, steam, gaseous compounds of sulphur, ash and broken rock pieces. Lava is called magma before it has come out of the volcano. Volcanoes are also found on planets other than Earth, like the Olympus Mons on Mars.
The word Volcano is thought to come from Vulcano, a volcanic island in the Aeolian Islands of Italy. Vulcano's name itself is taken from Vulcan, the name of a god of fire in Roman mythology. The study of volcanoes is called vulcanology, which is sometimes spelled volcanology.
How volcanoes are formed: Volcanoes are usually made when two tectonic plates move toward each other. When these two plates meet, one of them (usually the Oceanic Plate) goes under the Continental Plate. Afterwards, it melts and forms magma (inside the magma chamber), and the pressure builds up until the magma bursts through the Earth's crust.
Some Famous Volcanos
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- Kilauea (Hawaii, USA)
- Krakatoa (Rakata, Indonesia)
- Mauna Loa (Hawaii, USA)
- Mauna Kea (Hawaii, USA)
- Mount Baker (Washington, USA)
- Mount Edziza (British Columbia, Canada)
- Mount Etnioa (Sicily, Italy)
- Mount Erebus (Ross Island, Antarctica)
- Mount Hood (Oregon, USA)
- Mount Fuji (Honshu, Japan)
- Mount Rainier (Washington, USA)
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- Mount Ruapehu (Nth Island, New Zealand)
- Mount Shasta (California, USA)
- Mount St. Helens (Washington DC, USA)
- Novarupta (Alaska, USA)
- Olympus Mons (Mars (planet))
- Popocatépetil (Mexico-Puebla state line, Mexico)
- Surtsey (Surtsey island, Iceland)
- Santorini (Santorini island, Greece)
- Tambora (Sumbawa, Indonesia)
- Teide (Tenerife, Canary Islands, Spain)
- Vesuvius (Bay of Naples, Italy)
- Yellowstone (Wyoming, USA)
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Topics of Interest
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface. The word volcano is derived from the name of Vulcano island off Sicily which in turn, was named after Vulcan, the Roman god of fire.
Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.
Volcanoes can be caused by mantle plumes. These so-called hotspots, for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.
Plate tectonics and hotspots
Divergent plate boundaries: At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leads to adiabatic expansion, and the partial melting of the mantle causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.
Convergent plate boundaries: Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.
Hotspots are not usually located on the ridges of tectonic plates, but above mantle plumes, where the convection of the Earth's mantle creates a column of hot material that rises until it reaches the crust, which tends to be thinner than in other areas of the Earth. The temperature of the plume causes the crust to melt and form pipes, which can vent magma. Because the tectonic plates move whereas the mantle plume remains in the same place, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the hotspot. The Hawaiian Islands are thought to be formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate currently above the hot spot.
Volcanic features: The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit. This describes just one of many types of volcano, and the features of volcanoes are much more complicated. The structure and behavior of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has escaped to the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform.
Submarine volcanoes are common features on the ocean floor. Some are active and, in shallow water, disclose their presence by blasting steam and rocky debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them prevents the explosive release of steam and gases, although they can be detected by hydrophones and discoloration of water because of volcanic gases. Pumice rafts may also appear. Even large submarine eruptions may not disturb the ocean surface. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather steep pillars over their volcanic vents as compared to above-surface volcanoes. They may become so large that they break the ocean surface as new islands. Pillow lava is a common eruptive product of submarine volcanoes. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals.
Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lavas on the top collapse, leaving a flat-topped mountain. Then, the pillow lavas also collapse, giving an angle of 37.5 degrees. These volcanoes are also called table mountains, tuyas or (uncommonly) mobergs. Very good examples of this type of volcano can be seen in Iceland, however, there are also tuyas in British Columbia. The origin of the term comes from Tuya Butte, which is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near the boundary with the Yukon Territory.
Scientific classification of volcanoes
Active - Eruption in historic times - Historical record - 500 years - C14 dating - 10,000 years - Local seismic activity - Oral / folkloric history
Potentially Active - Solfataras / Fumaroles - Geologically young (possibly erupted < 10,000 years and for calderas and large systems - possibly < 25,000 years). - Young-looking geomorphology (thin soil cover/sparse vegetation; low degree of erosion and dissection; young vent featuresl; +/- vegetation cover). - Suspected seismic activity. - Documented local ground deformation - Geochemical indicators of magmatic involvement. - Geophysical proof of magma bodies. - Strong connection with subduction zones and external tectonic settings.
Inactive - No record of eruption and its form is beginning to change by the agents of weathering and erosion via formation of deep and long gullies.
A popular way of classifying magmatic volcanoes is by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant, and those that have not erupted in historical times called extinct. However, these popular classifications—extinct in particular—are practically meaningless to scientists. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above.
Effects of volcanoes
There are many different types of volcanic eruptions and associated activity: phreatic eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g., rhyolite), effusive eruption of low-silica lava (e.g., basalt), pyroclastic flows, lahars (debris flow) and carbon dioxide emission. All of these activities can pose a hazard to humans. Earthquakes, hot springs, fumaroles, mud pots and geysers often accompany volcanic activity.
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, halocarbons, organic compounds, and volatile metal chlorides.
Lava is molten rock expelled by a volcano during an eruption. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at temperatures from 700 °C to 1,200 °C (1,300 °F to 2,200 °F). Although lava is quite viscous, with about 100,000 times the viscosity of water, it can flow great distances before cooling and solidifying, because of both its thixotropic and shear thinning properties.
Volcanoes on other planetary bodies
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. Europa, the smallest of Jupiter's Galilean moons, also appears to have an active volcanic system, except that its volcanic activity is entirely in the form of water, which freezes into 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.
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. 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. It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.
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).
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 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 tremors
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.
Gas emissions
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 sophisticated 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 amplitudes, as well as common shifts in frequencies. (Source: 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 January 17, 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 area; however, it was declared safe.
Unfortunately, when the volcano erupted, 40% of the city of Goma
was destroyed along with many people's livelihoods. The expert claimed
that he had noticed small changes in the local relief and had monitored
the eruption of a much smaller volcano two 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 analyzing 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 hydrothermically 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 volcano's 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 – tide 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 SO2 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, signaling 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.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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