1. Magma chamber
2. Country rock
3. Conduit (pipe)
6. Branch pipe
7. Layers of ash emitted by the volcano
9. Layers of lava emitted by the volcano
11. Parasitic cone
12. Lava flow
15. Ash cloud
A volcano is an opening (or rupture) in the Earth's
surface or crust, which allows hot, usually molten rock, ash, and gases to
escape from deep below the surface. Volcanic activity involving the extrusion
of rock tends to form mountains or features like mountains over a period
Volcanoes are generally found where two to three tectonic
plates pull apart or are coming together. A mid-oceanic ridge, like 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 (like the San Andreas
fault). Volcanoes can also form where there is stretching of the Earth's
crust and where the crust grows thin (called "non-hotspot intraplate volcanism"),
such as in the African Rift Valley or the European Rhine Graben with its
Finally, volcanoes can be caused by "mantle plumes," so-called "hotspots;"
these hotspots can occur far from plate boundaries, such as the Hawaiian Islands.
Interestingly, hotspot volcanoes are also found elsewhere in the solar system,
especially on rocky planets and moons. To learn more about Volcanos on other
planets and moons check out your local library and rent books
or it is as simple as searching the internet.
Learning about volcanos on other planets and moons, may give a lot of insight
on how Volcanos react on earth.
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 down and solidifying. In these places, the crust is very thin and
eruptions occur frequently because of the pull by the tectonic plates. The
main part of the mid-oceanic ridges are at the bottom of the ocean, and most
volcanic activity is submarine. Black smokers are a typical example of this
kind of volcanic activity. Where the mid-oceanic ridge comes above sea-level,
volcanoes like the Hekla on Iceland are formed. Divergent plate boundaries
create new seafloor and volcanic islands.
Convergent plate boundaries
In places where one tectonic plate submerges beneath
another, the crust melts and becomes magma. This surplus amount of magma
generated in one location causes the formation of the volcano. Typical examples
for this kind of volcano are the volcanoes in the Pacific Ring of Fire, and
also Mount Etna and Mount Vesuvius.
Hotspots are not located on the ridges of tectonic plates,
but on top of mantle plumes, where the convection of Earth's mantle creates
a column of hot material that rises until it reaches the crust. 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 extinct after a while and a new volcano
is then being 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 current part of the North American
plate over the hotspot.
In July 2006, volcanoes were discovered that did not
fit in any of the above-mentioned categories, since they are located far
from the plate boundary, but are too small to be the result of a mantle plume. A new theory suggests that submergence
of tectonic plates causes stress all over the plate, which causes the plate
to crack in some places. However, other scientists believe the mantle plume
theory to be incorrect, and consider this discovery a confirmation of their
The most common perception of a volcano is of a conical
mountain, spewing lava and poisonous gases from a crater in its top. This
describes just one of many types of volcano and the features of volcanoes
are much more complicated. The structure and behaviour 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 broken the surface, and ash) and gases (mainly
steam and magmatic gases) can be located anywhere on the landform. Many of
these vents give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaiʻi's Kīlauea.
Other types of volcanoes include cryovolcanos (or ice
volcanoes), particularly on some moons of Jupiter, Saturn and Neptune; and
mud volcanoes, which are formations often not associated with known magmatic
activity. Active mud volcanoes tend to involve temperatures much lower than
those of igneous volcanoes, except when a mud volcano is actually a vent
of an igneous volcano.
Toes of a pāhoehoe advance across a road in Kalapana on the east rift zone
of Kīlauea Volcano in Hawaiʻ
- Main article: Shield volcano
Iceland are examples of places where volcanoes extrude huge quantities of
basaltic lava that gradually build a wide mountain with a shield-like profile.
Their lava flows are generally very hot and very fluid, contributing to long
flows. The largest lava shield on Earth, Mauna Loa, rises over 9,000 m from
the ocean floor, is 120 km in diameter and forms part of the Big Island of
Hawaiʻi. Olympus Mons is the largest
shield volcano on Mars, and is the tallest known mountain in the solar system.
Smaller versions of shield volcanoes include lava cones, and lava
Quiet eruptions spread out basaltic lava in flat layers.
The buildup of these layers form a broad volcano with gently sloping sides
called a shield volcano. Examples of shield volcanoes are the Hawaiian Islands.
Volcanic cones or cinder cones result
from eruptions that throw out mostly small pieces of scoria and pyroclastics
(both resemble cinders, hence the name of this volcano type) that build up
around the vent. These can be relatively short-lived eruptions that produce
a cone-shaped hill perhaps 30 to 400 m high. Most cinder cones erupt only
once. Cinder cones may form as flank vents on larger volcanoes, or occur
on their own. Parícutin in Mexico and Sunset Crater in Arizona are
examples of cinder cones.
In difference to pāhoehoe, Aa is a term of Polynesian origin, pronounced
Ah-ah, for rough, jagged, spiny lavaflow
Stratovolcanoes are tall conical mountains composed
of lava flows and other ejecta in alternate layers, the strata that give
rise to the name. Stratovolcanoes are also known as composite volcanoes.
Classic examples include Mt. Fuji in Japan, Mount Mayon in the Philippines,
and Mount Vesuvius and Stromboli in Italy.
Super volcano is the popular term for large volcanoes
that usually have a large caldera and can potentially produce devastation
on an enormous, sometimes continental, scale. Such eruptions would be able
to cause severe cooling of global temperatures for many years afterwards
because of the huge volumes of sulfur and ash erupted. They can be the most
dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone
National Park, Lake Taupo in New Zealand and Lake Toba in Sumatra, Indonesia.
Supervolcanoes are hard to identify centuries later, given the enormous areas
they cover. Large igneous provinces are also considered supervolcanoes because
of the vast amount of basalt lava erupted.
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. 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 volcanos. In due time, they may break the ocean
surface as new islands. Pillow lava is a common eruptive product of submarine
Subglacial volcanoes develop underneath icecaps.
They are made up of flat lava flows atop 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.
Very good examples of this can be seen in Iceland. These volcanoes are also
called table volcanoes, tuyas or (uncommonly) mobergs.
Another way of classifying volcanoes is by the composition
of material erupted (lava), since this affects the shape of the volcano.
Lava can be broadly classified into 4 different compositions (Cas & Wright,
- If the erupted magma contains a high percentage (>63%)
of silica, the lava is called felsic.
- Felsic lavas (or rhyolites) tend to be highly
viscous (not very fluid) and are erupted as domes or short, stubby flows.
Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in
California is an example of a volcano formed from felsic lava and is actually
a large lava dome.
- Because siliceous magmas are so viscous, they
tend to trap volatiles (gases) that are present, which cause the magma to
erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows
(ignimbrites) are highly hazardous products of such volcanoes, since they
are composed of molten volcanic ash too heavy to go up into the atmosphere,
so they hug the volcano's slopes and travel far from their vents during large
eruptions. Temperatures as high as 1,200 °C are known to occur in pyroclastic
flows, which will incinerate everything flammable in their path and thick
layers of hot pyroclastic flow deposits can be laid down, often up to many
meters thick. Alaska's Valley of Ten Thousand Smokes, formed by the eruption
of Novarupta near Katmai in 1912, is an example of a thick pyroclastic flow
or ignimbrite deposit. Volcanic ash that is light enough to be erupted high
into the Earth's atmosphere may travel many kilometres before it falls back
to ground as a tuff.
- If the erupted magma contains 52-63% silica, the
lava is of intermediate composition.
- These "andesitic" volcanoes generally only occur
above subduction zones (e.g. Mount Merapi in Indonesia).
- If the erupted magma contains <52% and >45%
silica, the lava is called mafic (because it contains higher percentages
of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are usually much
less viscous than rhyolitic lavas, depending on their eruption temperature;
they also tend to be hotter than felsic lavas. Mafic lavas occur in a wide
range of settings:
- At mid-ocean ridges, where two oceanic plates
are pulling apart, basaltic lava erupts as pillows to fill the gap;
- Shield volcanoes (e.g. the Hawaiian Islands,
including Mauna Loa and Kilauea), on both oceanic and continental crust;
- As continental flood basalts.
- If the erupted magma contains <=45% silica, the
lava is called ultramafic. Ultramafic flows are very rare; indeed, it is
likely that none have been erupted at the Earth's surface since the Proterozoic,
when the planet's heat flow was higher. They are (or were) the hottest lavas,
and probably more fluid than common mafic lavas.
Two types of lava are erupted according to the surface
texture: ʻAʻa (pronounced IPA [ʔaʔa]) and pāhoehoe (pronounced
both words having Hawaiian origins. ʻAʻa is characterized by a rough, clinkery
surface and is what most viscous and hot lava flows look like. However, even
basaltic or mafic flows can be erupted as ʻaʻa
flows, particularly if the eruption rate is high and the slope is steep.
Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface
and is generally formed from more fluid lava flows. Usually, only mafic flows
will erupt as pāhoehoe, since they often erupt at higher temperatures or
have the proper chemical make-up to allow them to flow at a higher fluidity.
A volcanic fissure and lava channel.
Mount St. Helens shortly after the eruption of May 18, 1980
A popular way of classifying magmatic volcanoes goes
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
There is no real consensus among volcanologists on how
to define an "active" volcano. The lifespan of a volcano can vary from months
to several million years, making such a distinction sometimes meaningless
when compared to the lifespans of humans or even civilizations. For example,
many of Earth's volcanoes have erupted dozens of times in the past few thousand
years but are not currently showing signs of eruption. Given the long lifespan
of such volcanoes, they are very active. By our lifespans, however, they
are not. Complicating the definition are volcanoes that become restless (producing
earthquakes, venting gasses, or other non-eruptive activities) but do not
Scientists usually consider a volcano active
if it is currently erupting or showing signs of unrest, such as unusual earthquake
activity or significant new gas emissions. Many scientists also consider
a volcano active if it has erupted in historic time. It is important to note
that the span of recorded history differs from region to region; in the Mediterranean,
recorded history reaches back more than 3,000 years but in the Pacific Northwest
of the United States, it reaches back less than 300 years, and in Hawaii,
little more than 200 years. The Smithsonian Global Volcanism Program's definition
of 'active' is having erupted within the last 10,000 years.
Dormant volcanoes are those that are not currently
active (as defined above), but could become restless or erupt again. Confusion
however, can arise because many volcanoes which scientists consider to be
active are referred to as dormant by laypersons
or in the media.
Extinct volcanoes are those that scientists consider
unlikely to erupt again. Whether a volcano is truly extinct is often difficult
to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes
measured in millions of years, a caldera that has not produced an eruption
in tens of thousands of years is likely to be considered dormant instead
For example, the Yellowstone Caldera in Yellowstone
National Park is at least 2 million years old and hasn't erupted violently
for approximately 640,000 years, although there has been some minor activity
relatively recently, with hydrothermal eruptions less than 10,000 years ago
and lava flows about 70,000 years ago. For this reason, scientists do not
consider the Yellowstone Caldera extinct. In fact, because the caldera has
frequent earthquakes, a very active geothermal system (i.e., the entirety
of the geothermal activity found in Yellowstone National Park), and rapid
rates of ground uplift, many scientists consider it to be an active volcano.
- Main article: List of volcanoes
The 16 current Decade Volcanoes are:
- Avachinsky-Koryaksky, Kamchatka, Russia
- Colima, Mexico
- Mount Etna, Italy
- Galeras, Colombia
- Mauna Loa, Hawaiʻi, USA
- Merapi, Indonesia
- Nyiragongo, Democratic Republic of the
- Mount Rainier, Washington, USA
- Sakurajima, Japan
- Santamaria/Santiaguito, Guatemala
- Santorini, Greece
- Taal Volcano, Philippines
- Teide, Canary Islands, Spain
- Ulawun, Papua New Guinea
- Mount Unzen, Japan
- Vesuvius, Italy
Elsewhere in the solar system
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 and no volcanic
activity, although recent evidence suggests it may still possess a partially
molten core. However, the Moon
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. They include
Arsia Mons, Ascraeus Mons, Hecates Tholus, Olympus Mons, and Pavonis Mons.
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.
Galileo orbiter reveals volcanic activity on Jupiter's moon Io.
Jupiter's moon Io is the most volcanically active object
in the solar system because of 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 cryovolcanos
(ice volcanoes) on Triton, a moon of Neptune, and in 2005 the Cassini-Huygens
probe photographed fountains of frozen particles erupting from Enceladus,
a moon of Saturn. 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.
Effects of volcanoes
Solar radiation reduction from volcanic eruptions
Sulfur dioxide emissions by volcanoes.
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 flows, lahars (debris flow) and carbon dioxide
emission. All of these activities can pose a hazard to humans. Volcanic activity
is often accompanied by earthquakes, hot springs, fumaroles, mud pots and
geysers. 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 sulphur dioxide. Other
principal volcanic gases include hydrogen sulphide, 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.
Large, explosive volcanic eruptions inject water vapor
(H2O), carbon dioxide (CO2), sulfur dioxide (SO2),
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 sulphur dioxide to sulphuric acid (H2SO4),
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 sulphate
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 (O3).
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 tons) of carbon dioxide each year. Volcanic
eruptions may inject 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. Volcanic
eruptions can also create new islands, as the magma dries on the water.
Volcano is thought to derive from Vulcano, a
volcanic island in the Aeolian Islands of Italy whose name in turn originates
from Vulcan, the name of a god of fire in Roman mythology. The study of volcanoes
is called volcanology, sometimes spelled vulcanology.
The Roman name for the island Vulcano has contributed
the word for volcano in most modern European languages.
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. Many ancient accounts claim that divine intervention was the actual
cause of volcanic eruptions.
One early idea counter to this, however, was Jesuit
Athanasius Kircher (1602-1680), who 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.