Encyclopedia
A
volcano is a
geological landform on the surface of the
Earth where magma from the planet's interior erupts to the surface. Roughly defined, a volcano consists of a magma chamber,
pipes and vents. The magma chamber is where magma from deep within the planet accumulates, while pipes are channels that lead to surface vents, openings in the volcano's surface through which
lava is ejected during an eruption. Some volcanoes produce material which builds up into a large
mountain-like structure, other types of volcano form different landforms.
Formation
Volcanoes are generally found where two or more
tectonic plates diverge or
converge. The
mid-oceanic ridges, like the
Mid-Atlantic Ridge, are typical examples of divergent tectonic plates where volcanoes are formed, whereas the
Pacific Ring of Fire is a typical example of volcanic activity on convergent tectonic plates. Where two tectonic plates slide past one another volcanic activity is generally not found.
Volcanic activity can also occur due to mantle plumes, the so-called hotspots, which occur at locations far from plate boundaries; hotspot volcanoes are also found elsewhere in the
solar system, especially on its rocky planets and moons. In July 2006, a new type of volcano was discovered, which was called a petitspot, to accentuate the difference with volcanoes formed by mantle plumes.
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 due to 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.
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
Hotspots are not located on the ridges of tectonic plates, but on top of mantle plumes, where due to the
convection of
Earth's
mantle a column of hot material 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.
Petitspots
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 ideas.
Shape
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 and gases can be located anywhere on the landform. Many of these vents give rise to smaller cones such as
Puu Oo on a flank of
Hawaii's
Kilauea.
Other types of volcanoes include
cryovolcanos , 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
- Main article: Shield volcano
Hawaii and
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 Hawaii.
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 mounds.
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.
Cinder cones
Volcanic cones or
cinder cones result from eruptions that throw out mostly small pieces of
scoria and
pyroclastics 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. Paricutín in
Mexico and
Sunset Crater in
Arizona are examples of cinder cones.
Stratovolcanoes
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 volcanoes
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
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 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. Very good examples of this can be seen in Iceland. These volcanoes are also called table volcanoes, tuyas or mobergs.
Erupted material
Lava composition
Another way of classifying volcanoes is by the
composition of material erupted , since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions :
- If the erupted magma contains a high percentage of silica, the lava is called felsic.
- Felsic lavas tend to be highly viscous 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 that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows 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 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:
- 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 the hottest lavas, and probably more fluid than common mafic lavas.
Lava texture
Two types of lava are erupted according to the
surface texture: Aa and
pahoehoe , both words having
Hawaiian origins. Aa 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 aa flows, particularly if the eruption rate is high and the slope is steep. Pahoehoe 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 pahoehoe, since they often erupt at higher temperatures or have the proper chemical makeup to allow them to flow at a higher fluidity.
Volcanic activity
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 explained above.
Surprisingly, 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 but do not actually erupt.
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 , 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 , and rapid rates of ground uplift, many scientists consider it to be an active volcano.
Notable volcanoes
On Earth
- Main article: List of volcanoes
The 16 current
Decade Volcanoes are:
- Avachinsky-Koryaksky, Kamchatka, Russia
- Colima, Mexico
- Mount Etna, Italy
- Galeras, Colombia
- Mauna Loa, Hawaii, USA
- Merapi, Indonesia
- Nyiragongo, Democratic Republic of the Congo
- Mount Rainier, Washington, USA
| - Sakurajima, Japan
- Santamaria/Santiaguito, Guatemala
- Santorini, Greece
Greece lies at the juncture of Europe [i], Asia [i], and Africa [i]. ...
|
Elsewhere in the solar system
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 , 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.. 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 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
