Volcanoes
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 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.
Volcanoes are described as active, dormant, or extinct. The soil resulting from decomposition of volcanic materials is extremely fertile, and the ash itself is a good polishing and cleansing agent.
Active volcanoes are not scattered over the Earth randomly; instead, most occur in belts, especially in the island arcs and mountain ranges bordering the Pacific Ocean. The concept of seafloor spreading and, more broadly, the theory of plate tectonics offer a logical explanation for the location of most volcanoes.
Lava is molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure. It solidifies into igneous rock that is also called lava. Before reaching the Earth's surface, the mixture of solid and liquid rock, and gases, is known as magma. Lavas are composed chiefly of silica and the oxides of aluminum, iron, magnesium, calcium, sodium, and potassium. Silica, with soda and potash, predominates in the light-colored, acid felsites; iron oxides, lime, and magnesia, in the dark-colored, basic basalts . Rock froth forms on the upper part of a lava flow if bubbles solidify before the gas can escape. Light-colored, glassy froth is pumice ; dark, cindery or slaggy froth, of a coarser texture than pumice, forms what is known as scoriae.
Lava flows which solidify as a mass of blocks and fragments with a rough surface are called block lava, or aa; those which solidify with a smooth, ropy, billowy surface are known as corded lava, or pahoehoe. Lava can sometimes cover wide regions through great fissures in the earth's surface, as in the ancient Columbia River plateau of the NW United States, where it is spread over 30,000 sq mi (77,700 sq km) and is up to 5,000 ft (1,524 m) deep. Other such regions are found in the Deccan plateau of India, in E Brazil, and in Iceland.
Submarine lavas develop through volcanic activity along the mid-oceanic ridges and plate boundaries, where the mid-oceanic ridges produce more lava than any continental eruptions. Such underwater eruptions also harbor rich fauna unique to the vent area, such as red tube worms and giant clams, whose food supply is based on the hydrogen sulfide abundant in the vent waters.
Unique features include black smokers, or hot springs of mineral-rich water that belch out from the ocean ridge where it is most active. In many instances the reasons for the heat and liquidity of magma, its exact source, and the causes of its rise in the earth are not clearly known, though the volcanic activity is often related to seafloor spreading. Other volcanic areas also lie along colliding plate boundaries and around rising magma hot spots. See plate tectonics.
Volcanoes are found in association with midocean ridge systems, seafloor spreading, and along convergent plate boundaries, such as around the Pacific Ocean's Ring of Fire, the ring of plate boundaries associated with volcanic island arcs and ocean trenches surrounding the Pacific Ocean. Continental volcanoes are also associated with converging plate boundaries, such as the volcanoes of the Cascade Range along the W coast of the United States. Isolated volcanoes also form in the midocean area of the Pacific apparently unrelated to crustal plate boundaries. These sea mounts and volcanic island chains, such as the Hawaiian chain, may form from rising magma regions called hot spots.
shield volcano is a large volcano with shallow-sloping sides. The name derives from a translation of "Skjaldbreiour", an Icelandic shield volcano whose name means "broad shield", from its resemblance to a warrior's shield. Shield volcanoes are formed by lava flows of low viscosity - lava that flows easily. Consequently, a volcanic mountain having a broad profile is built up over time by flow after flow of relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano. Many of the largest volcanoes on Earth are shield volcanoes. The largest in terms of area covered is Mauna Loa of Hawaii; the tallest measured from its base under the ocean, however, is Mauna Kea of Hawaii. All the volcanoes in the Hawaiian Islands are shield volcanoes.
Geographical extent
Shield volcanoes can be so large that they are sometimes considered to be a mountain range, such as the Ilgachuz Range and the Rainbow Range, both of which are located in Canada. These shield volcanoes formed when the North American Plate moved over a hotspot similar to the one feeding the Hawaiian Islands, called the Anahim hotspot. Shield volcanoes can be found in many places around the world, including Australia, Ethiopia, and the Galapagos Islands. The Piton de la Fournaise, on Reunion Island, is one of the most active shield volcanoes on earth, with one eruption per year on average.
Magma
The viscosity of magma as it approaches the surface is dependent on its temperature and composition. Shield volcanoes in the Hawaiian Islands erupt magma as hot as 1,200 degrees C (2,200 degrees F), compared with 850 degrees C (1,560 degrees F) for most continental volcanoes, which are usually composed of acidic lava. Because of the fluidity of the lava, major explosive eruptions do not occur. The most severe explosions occur if water enters a vent, although expanding gases in the magma can produce spectacular fountaining of the low viscosity lava.
In volcanology, a lava dome or plug dome is a roughly circular mound-shaped protrusion resulting from the slow eruption of felsic lava (usually rhyolite or dacite) from a volcano, or from multiple lava episodes of different magma types. The characteristic dome shape is attributed to high levels of silica in the magma, causing the magma to be quite viscous and thick. The viscosity of the lava prevents it from flowing far from the vent that it extrudes from, causing it to solidify quickly and build on previous volcanic extrusions, creating a dome-like shape.
Domes may reach heights of several hundred meters, and can grow slowly and steadily for months or years. The sides of these structures are composed of unstable rock debris. Due to the possibility of the building up of gas pressure, the dome can experience more explosive eruptions over time. When part of a lava dome collapses while it still contains molten rock and gases, it can produce a pyroclastic flow, one of the most lethal forms of a volcanic event. Other hazards associated with lava domes are the destruction of property, forest fires, and lahars triggered by pyroclastic flows near snow and ice. Lava domes are one of the principal structural features of many stratovolcanoes worldwide.
Characteristics of lava dome eruptions include shallow, long-period and hybrid seismicity, which is attributed to excess fluid pressures in the contributing vent chamber. Other characteristics of lava domes include their spherical dome shape, cycles of dome growth over long periods, and sudden onsets of violent explosive activity. The average rate of dome growth may be used as a rough indicator of magma supply, but it shows no systematic relationship to the timing or characteristic of lava dome explosions.
Some of the world's most famous active lava domes include those at Mount Merapi in central Java of Indonesia, Soufrière Hills in Montserrat, and Mount St. Helens in Washington. Lassen Peak in northern California is one of the largest lava domes in the world and has the distinction of being the only other Cascade volcano besides Mount St. Helens to erupt (1914-1921) in the 20th century.
Shapes of volcanoes include composite cones, or stratovolcanoes, with steep concave sides such as Mt. St. Helens in the W United States; shield cones have gentle slopes and can be relatively large such as the Hawaiian Islands; and cinder cones as Paricutin in Mexico, with steep slopes made of cinderlike materials. Explosive eruptions build up steep-sided cones, while the nonexplosive ones usually form broad, low lava cones. Cones range in height from a few feet to nearly 30,000 ft (9 km) above their base.
Usually the cone has as its apex a cavity, or crater, which contains the mouth of the vent. Such craters are typically less than 1 mi (1.6 km) across, but larger craters, called calderas, ranging in diameter from 3 mi to - in a few instances - 50 mi (5-80 km), are formed by particularly large eruptions.
Craters are circular, bowl-shaped depression on the earth's surface. Simple craters are bowl-shaped with a raised outer rim. Complex craters have a raised central peak surrounded by a trough and a fractured rim.
Many of the largest craters are formed by the impact of meteorites . Impacting at speeds in excess of 10 mi/sec (16 km/sec), a meteorite creates pressures on the order of millions of atmospheres, producing shock waves that blast out a circular hole and often destroy the meteorite. Meteor, or Barringer, Crater, near Winslow, Arizona, c. 34 mi (1 15 km) in diameter and 600 ft (180 m) deep, is probably the best-known crater of this type.
Of the more than 120 impact craters identified on earth, the largest are at Manicouagan, Canada; Vredefort, South Africa; and Chicxulub (off the coast of the Yucatan peninsula), Mexico. Others include Chubb Crater, Quebec; Lake Bosumtwi, Ghana; and Brent Crater, Ontario. Two major impact events have occurred in the 20th cent., both in Siberia. In 1908 in the Tunguska Basin near Lake Baykal one occurred that caused vast destruction of timber from its blast, and the other in 1947 at Sikhote-Alin also caused great damage. Craters that have been obliterated by erosion over thousands of years, leaving only a circular scar on the earth's surface, are called astroblemes .
Craters are also commonly formed at the surface opening, or vent, of erupting volcanoes , particularly of the type called cinder cones, where the lava is extruded rather explosively. Virtually all volcanoes display a crater, called a sink, around the vent; this is believed to be a collapse feature caused by molten lava subsiding as an eruption phase diminishes. Volcanic craters formed in these ways are relatively small, usually less than 1 mi (1.6 km) in diameter, and represent only a small fraction of the cone's diameter at the base.
A caldera is a much larger crater, typically ranging from 3 to 18 mi (5-30 km) in diameter, and represents a considerable fraction of the volcano's basal diameter. In a few instances, however, tremendous volcanic eruptions have left calderas 50 mi (80 km) or so, such as that that forms much of Yellowstone National Park or the basin of Lake Toba, Sumatra, Indonesia. Most calderas are formed by the collapse of the central part of a cone during great eruptions. A few small calderas have been formed by explosive eruptions in which the top of a volcano was blown out. Some volcanic craters are created by a combination of these events. Formed thousands of years ago, the caldera that contains Crater Lake, Oreg., is 6 mi (9.7 km) in diameter. In recent times, caldera-producing eruptions occurred at Krakatoa, Indonesia, in 1883 and Katmai, Alaska, in 1912.
More than 500 volcanoes are known to have erupted on the earth's surface since historic times, and many more have erupted on the ocean floor unobserved by humans. Fifty volcanoes have erupted in the United States, which ranks third, behind Indonesia and Japan, in the number of historically active volcanoes. Of the world's active volcanoes, more than half are found around the perimeter of the Pacific, about a third on midoceanic islands and in an arc along the south of the Indonesian islands, and about a tenth in the Mediterranean area, Africa, and Asia Minor.
Evidence of extraterrestrial volcanic activity also has been found. Space probes have detected the remnants of ancient eruptions on earth's moon, Mars (which has the largest volcano in the solar system, Olympus Mons, 340 mi/550 km across and 15 mi/24 km high), and Mercury; these probably originated billions of years ago, since these bodies are no longer capable of volcanic activity.
Triton (a satellite of Neptune), Io (a satellite of Jupiter), and Venus are the only bodies in the solar system besides earth that are known to be volcanically active. The volcanic processes that occur in the outer portion of the solar system are very different from those in the inner part. Eruptions on earth, Venus, Mercury, and Mars are of rocky material and are driven by internal heat. Io's eruptions are probably sulfur or sulfur compounds driven by tidal interactions with Jupiter. Triton's eruptions are of very volatile compounds, such as methane or nitrogen, driven by seasonal heating from the sun.
Terrestrial volcanic eruptions may take one or more of four chief forms, or phases, known as Hawaiian, Strombolian, Vulcanian, and Peleean. In the Hawaiian phase there is a relatively quiet effusion of basaltic lava unaccompanied by explosions or the ejection of fragments; the eruptions of Mauna Loa on the island of Hawaii are typical. The Strombolian phase derives its name from the volcano Stromboli in the Lipari, or Aeolian, Islands, N of Sicily. It applies to continuous but mild discharges in which viscous lava is emitted in recurring explosions; the ejection of incandescent material produces luminous clouds.
A more explosive volcanic eruption is the Vulcanian, where the magma (lava before emission) accumulates in the upper level of the vent but is blocked by a hardened plug of lava that forms between consecutive explosions. When the explosive gases have reached a critical pressure within the volcano, masses of solid and liquid rock erupt into the air and clouds of vapor form over the crater.
The Peleean, derived from Mt. Pelee, is the most violent, emitting fine ash; hot, gas-charged fragments of lava; and superheated steam in an incandescent 'cloud' that travels downhill at great speed. Eruptions are often accompanied by torrential rains caused by the condensation of steam. The erupted fragments vary in size, including minute particles of volcanic dust and ash, lapilli (cinders or pellets), bombs (rounded or ellipsoidal masses of hardened magma), and huge masses called blocks.
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 Pu'u 'O'o on a flank of Hawaii's Kilauea.
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.
The term mud volcano or mud dome is used to refer to formations created by geo-excreted liquids and gases, although there are several different processes which may cause such activity. Temperatures are much cooler than igneous processes. The largest structures are 10 km in diameter and reach 700 metres in height. About 86% of released gases are methane, with much less carbon dioxide and nitrogen emitted. Ejected materials often are a slurry of fine solids suspended in liquids which may include water (frequently acidic or salty) and hydrocarbon fluids.
A mud volcano may be the result of a piercement structure created by a pressurized mud diapir which breaches the earth's surface or ocean bottom. Temperatures may be as low as the freezing point of ejected materials, particularly when venting is associated with the creation of hydrocarbon clathrate hydrate deposits.
Mud volcanoes are often associated with petroleum deposits and tectonic subduction zones and orogenic belts; hydrocarbon gases are often erupted. They are also often associated with lava volcanoes; in the case of such close proximity, mud volcanoes emit incombustible gases including helium, whereas lone mud volcanoes are more likely to emit methane.
In Azerbaijan, eruptions are driven from a deep mud reservoir which is connected to the surface even during dormant periods, when seeping water still shows a deep origin. Seeps have temperatures up to 2-3 degrees C above the ambient temperature.
Approximately 1,100 mud volcanoes have been identified on land and in shallow water. It has been estimated that well over 10,000 may exist on continental slopes and abyssal plains. Mud volcanoes are frequently associated with earthquake zones. Many scientists suggest monitoring gas emissions and activity of mud volcanoes, because they can be suitable to predict strong earthquakes.
A supervolcano or super volcanic eruption is a volcanic eruption which is orders of magnitude greater than any volcano in historic times (generally accepted to be greater than 200 cubic kilometres). This kind of eruption is typically sufficient to cause a long-lasting change to weather (such as the triggering of an ice age) sufficient to threaten the extinction of species, and cover huge areas with lava and ash.
Though there is no well-defined minimum explosive size for a "supervolcano," there are at least two types of volcanic eruption that have been identified as supervolcanoes: large igneous provinces and massive eruptions.
Supervolcanoes were seen on other planets via the Voyager program craft on the moons of Jupiter and Saturn. However, this kind of volcano on earth was not discovered until long after the Voyager had gone on to their interstellar missions. The outer Solar System volcanoes were mostly cryovolcanoes, not magma volcanoes. Theoretically a supervolcano could be a cryovolcano, but none has been found within the solar system.
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 and Valles Caldera in New Mexico (both western United States), 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, but are non-explosive.
A stratovolcano, also called a composite volcano, is a tall, conical volcano composed of many layers of hardened lava, tephra, and volcanic ash. These kinds of volcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from them is viscous, and cools and hardens before spreading very far. The source magma of this rock is classified as felsic, having high to intermediate levels of silica (as in rhyolite, dacite, or andesite). This is in contrast to less viscous mafic magma that forms shield volcanoes (such as Mauna Loa in Hawaii), which have a wide base and more gently sloping profile. Although stratovolcanoes are sometimes called composite volcanoes, volcanologists prefer to use the term stratovolcano to distinguish among volcanoes because all volcanoes of any size have a composite (layered) structure - they are built up from sequential outpourings of eruptive materials. Stratovolcanoes are one of the most common types of volcanoes.
Submarine volcanoes are underwater fissures in the earth's surface from which magma can erupt. They are estimated to account for 75% of annual magma output. The vast majority are located near areas of tectonic plate movement, known as mid-ocean ridges. Although most are located in the depths of oceans, some also exist in shallow water, which can spew material into the air during an eruption. Hydrothermal vents, sites of abundant biological activity, are commonly found near submarine volcanoes.
The presence of water can greatly alter the characteristics of a volcanic eruption and the explosions made by these eruptions. For instance, the increased thermal conductivity of water causes magma to turn into glass much more quickly than in a terrestrial eruption. Below ocean depths of about 2243 meters where the pressure exceeds 218 atmospheres, the critical pressure of water, it can no longer boil; it becomes a supercritical fluid. Without boiling sounds, deep-sea volcanoes are difficult to detect at great distances using hydrophones.
The lava formed by submarine volcanoes is quite different from terrestrial lava. Upon contact with water, a solid crust forms around the lava. Advancing lava flows into this crust, forming what is known as pillow lava.
Scientists still have much to learn about the location and activity of underwater volcanoes. The Kolumbo underwater volcano in the Aegean Sea was discovered in 1650 when it burst from the sea and erupted, killing 70 people on the nearby island of Santorini. More recently, NOAA's Office of Ocean Exploration has funded missions to explore submarine volcanoes. Most notably, these have been the Ring of Fire missions to the Mariana Arc in the Pacific Ocean. Using Remote Operated Vehicles, scientists studied underwater eruptions, ponds of molten sulfur, black smoker chimneys and even marine life adapted to this deep, hot environment.
Many submarine volcanoes are usually found as seamounts. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of 1,000 - 4,000 meters depth. They are defined by oceanographers as independent features that rise to at least 1,000 meters above the seafloor. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. An estimated 30,000 seamounts occur across the globe, with only a few having been studied. However, some seamounts are also unusual. For example, while the summits of seamounts are normally hundreds of meters below sea level, the Bowie Seamount in Canada's Pacific waters rises from a depth of about 3,000 meters to within 24 meters of the sea surface.
A subglacial volcano is a volcanic form produced by subglacial eruptions or eruptions beneath the surface of a glacier or ice sheet which is then melted into a lake by the rising lava. Today they are most common in Iceland and Antarctica; older formations of this type are found also in British Columbia and Yukon Territory, Canada.
During the eruption, the heat of the lava from the subglacial volcano melts the overlying ice. The water quickly cools the lava, resulting in pillow lava shapes similar to those of underwater volcanoes. When the pillow lavas break off and roll down the volcano slopes, pillow breccia, tuff breccia, and hyaloclastite form. The melted water may be released from below the ice as happened in Iceland in 1996 when the Grimsvötn caldera erupted, melted 3 cubic km ice and gave rise to a large glacial lake outburst flood.
The shape of subglacial volcanoes tends to be quite characteristic and unusual, with a flattened top and steep sides supported against collapse by the pressure of the surrounding ice and meltwater. If the volcano eventually melts completely through the ice layer, then horizontal lava flows are deposited, and the top of the volcano assumes a nearly-level form. However, if significant amounts of lava are later erupted subaerially, then the volcano may assume a more conventional shape. The more distinctly flat-topped, steep-sided subglacial volcanoes are called tuyas, named after Tuya Butte in northern British Columbia by Canadian geologist Bill Mathews in 1947. In Iceland, such volcanoes are also known as table mountains.
On January, 2008, the British Antarctic Survey (Bas) scientists led by Hugh Corr and David Vaughan, reported (in the journal Nature Geoscience) that 2,200 years ago, a volcano erupted under Antarctica ice sheet (based on airborne survey with radar images). The biggest eruption in the last 10,000 years, the volcanic ash was found deposited on the ice surface under the Hudson Mountains, close to Pine Island Glacier.
A cryovolcano is, literally, an icy volcano. Cryovolcanoes form on icy moons, and possibly on other low-temperature astronomical objects (e.g., Kuiper belt objects). Rather than molten rock, these volcanoes erupt volatiles such as water, ammonia or methane. Collectively referred to as cryomagma or ice-volcanic melt[1], these substances are usually liquids and form plumes, but can also be in vapour form. After eruption, cryomagma condenses to a solid form when exposed to the very low surrounding temperature.
The energy required to melt ices and produce cryovolcanoes usually comes from tidal friction. It has also been suggested that translucent deposits of frozen materials could create a sub-surface greenhouse effect that would accumulate the required heat.
It is hypothesized that the Kuiper belt object Quaoar has exhibited cryovolcanism in the past. In this case, the source of energy would be radioactive decay.
Ice volcanoes were first observed on Neptune's moon Triton during the Voyager 2 flyby in 1989.[1] Plumes above the limb of Enceladus feeding Saturn's E Ring. These appear to emanate from the "Tiger Stripes" near the south pole
On November 27 2005 Cassini photographed geysers on the south pole of Enceladus.
Indirect evidence of cryovolcanic activity was later observed on several other icy moons of our solar system, including Europa, Titan, Ganymede, and Miranda. Cassini has observed several features thought to be cryovolcanoes on Titan. Such volcanism is now believed to be a significant source of the methane found in Titan's atmosphere.
Notable eruptions within historic times have been those of Vesuvius, in Italy (AD 79, 1906, and other times); Tambora, in Indonesia, where between 30 and 50 cu mi (125-210 cu km) of molten and shattered rock were blown into the air (1815); Krakatoa, near Java, material from which was sent 17 mi (27 km) into the atmosphere (1883); Paricutin, in Mexico, the volcano that began in a cornfield (1943); Hibok Hibok, on Camiguin island in the Philippines, which killed 84 people (1948); Besymianny, in Kamchatka, where 2 cu mi (8 cu km) of material were hurled into the air (1956); the peak of Tristan da Cunha, whose eruption caused the entire settlement to be evacuated (1961); Agung, in Bali, which killed 1,100 people (1963); Mt. St. Helens in Washington, which exploded with an energy equivalent to 10 million tons of TNT, killing 35, with 25 missing (1980); El Chichon in Mexico, which expelled about 500 million tons of ash and gas (1982); and Mt. Pinatubo in the Philippines, which killed over 500 people and ejected over 2 cu mi (8 cu km) of material (1991).
Other notable volcanoes are Cotopaxi and Chimborazo (Ecuador), Iztaccihuatl and Popocatepetl (Mexico), Lassen Peak and Katmai (United States), and Etna (Sicily).
Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 ft (4,170 m) above sea level and over 29,000 ft (8,850 m) above the ocean floor; from its base below sea level to its summit, Mauna Loa is taller than Mt. Everest.
In 1963 the birth of the volcanic island Surtsey near Iceland was observed. In November of that year events began with a submarine eruption along the Mid-Atlantic Ridge. Eruption followed eruption until June, 1967, by which time the island stood 492 ft (150 m) above sea level and covered an area of almost 2 sq mi (3 sq km). The island has not grown since the last eruption, and it is presently volcanically quiet.
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