
| Volcano: |
1. Large magma chamber
2. Bedrock
3. Conduit (pipe)
4. Base
5. Sill
6. Branch pipe
7. Layers of ash emitted by the volcano
8. Flank |
9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15. Ash cloud |
A volcano is an opening, or rupture, in a planet's surface or crust, which
allows hot, molten rock, ash and gases to escape from below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.
Volcanoes are generally found where tectonic plates pull apart or come together. 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.
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. This melt causes the volcanism and makes the 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 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. The crust is then melted by the heat from the mantle and becomes magma. This is due
to the water content lowering the melting temperature. The magma created here 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
Hotspots are not usually located on the ridges of tectonic plates, but above
mantle plumes, where the convection of 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 hotspot.
Volcanic features
Look inside of a crater.Aerial view of
Puʻu ʻŌʻō taken on 9/10/07
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. Many of these vents give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea.
Other types of volcano include cryovolcanoes (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.
Shield volcanoes
-
Hawaii and Iceland are examples of places where volcanoes
extrude huge quantities of basaltic lava in effusive
eruptions 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
Hawaii, along with other shield volcanoes such as Mauna Kea and Kīlauea. Olympus Mons on Mars is the largest
shield volcano and also tallest known mountain in the solar system. Smaller versions of
shield volcanoes include lava cones, and lava mounds.
Cinder cones
-
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 meters high. Most cinder
cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. This volcano may also
throw out ash and dust (and and sometimes lava, but not very usually) Parícutin in
Mexico and Sunset Crater in Arizona are examples of cinder cones.
Stratovolcanoes
In contrast to pāhoehoe, ʻAʻā (pronounced Ah-ah), is a rough, jagged, lava flow.
-
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. Strato/composite volcanoes are made of cinders, ash and lava. The volcanoes are made by a another
volcano. Cinders and ash pile on top of each other, then lava flows on top and dries and then the process begins again. Classic
examples include Mt. Fuji in Japan, Mount Mayon in the
Philippines, and Mount Vesuvius and Stromboli in
Italy.
Super volcanoes
-
Supervolcano is the popular term for a large volcano that usually has 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 are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park
of western USA, 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
-
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 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.
Subglacial volcanoes
-
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. 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.
Erupted material
Lava composition
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,
1987):
- 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.
- 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:
- Some erupted magmas contain <=45% silica and produce ultramafic lava. Ultramafic
flows, also known as komatiites, are very rare; indeed, very few 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.
Lava texture
Two types of lava are named according to the surface texture: ʻAʻa (pronounced
IPA [ʔaʔa]) and
pāhoehoe (pronounced [paːho͡eːho͡eː]), 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.
Volcanic activity
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.
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 human lifespans,
however, they are not.
Scientists usually consider a volcano to be 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 of extinct. 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.
Notable volcanoes
-
The 16 current Decade Volcanoes are:
-
- Avachinsky-Koryaksky, Kamchatka, Russia
- Colima, Mexico
- Mount Etna, Sicily, Italy
- Galeras, Colombia
- Mauna Loa, Hawaii, USA
- Merapi, Indonesia
- Nyiragongo, Democratic Republic
of the Congo
- Mount Rainier, Washington, USA
|
|