Volcanoes
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

Most volcanoes (like this in the Chilean Andes) are related to plate tectonics and most of them occur near plate boundaries. Only 5 % of the known world volcanoes are not closely related to plate margins. 539 volcanoes are known to be active. Some 1500 volcanoes have been active since the last ice age. The worst eruption (in historic times) so far occured in Indonesia (Tambora, Sumbawa) on 5 April 1815 with ca. 92,000 casualties (starvation was the major cause of deaths). Mount Tambora is a caldera volcano.

Osorno volcano near Puerto Montt, Chile. 41.10 S, 72.49 W summit elevation 2652 m stratovolcano Eruptions of Osorno Volcano 1869, 1855, 1851, 1837, 1834-35, 1790-91, 1765, 1719, 1644?, 1640?, 1575?

Types of Volcanoes

Fissure Volcanoes

Fissure volcanoes (or linear volcanoes) erupt from giant cracks (fissures) that open in the ground and expel vast quantities of extremely fluid lava (basalt) that spread far and wide to form huge pools that can cover almost everything around. When these pools of lava cool and solidify, the surface remains mostly flat. They occur at hot spots or at spreading ridges where a dome of lave has risen to typically less than 10 km below the surface of the earth.

A fissure typically resides along a fault produced when tensional stresses (from extension in a spreading zone) result in the subsidence of a block of rock - in a so-called graben or rift valley. Today fissure volcanoes are best seen in Iceland (which straddles the Mid-Atlantic Ridge), but can also be observed in Hawaii.

The lava upwelling at oceanic ridges is underwater fissure volcanism.

Flood or Plateau Basalt (subtype of Fissure Volcanoes)

Plateau basalts or flood basalts may be considered a special sort of fissure volcanism. In the past such flood basalt have formed "Large Igneous provinces" (LIPs) like the Deccan Plateau (or Deccan Traps) of west-central India; the Paraná Basin of southern Brazil, Argentina, and Uruguay; the Columbia Plateau of the north-western United States; the Drakensberg Plateau of South Africa; the Siberian Traps; and the central plateau of the North Island, New Zealand.

Because a LIP may in several cases have occurred simultaneously with oceanic anoxic events and extinction events, it has been proposed that the volcanic byproducts of LIP formation may have had a profound and deleterious effect on the global environment. The most important examples are the Deccan traps: end Cretaceous extinction event, the Karoo-Ferrar: end Toarcian extinction event, the Central Atlantic Magmatic province: end Triassic extinction event, and the Siberian traps: end Permian extinction event (the largest in the Phanerozoic). The Siberian Traps are believed to be the world's largest ever lava plateau.

I stress in the past, because a flood basalt eruption has never been witnessed.

Dykes

Dykes (or dikes as they call them in the US) are narrow sheets of volcanic rock formed beneath the earth's surface, and cutting across the bedding or structural planes of the host rock. Thickness can vary from sub-centimeter scale to many meters in thickness and the lateral dimensions can extend over many kilometers. ikes are usually high angle to near vertical in orientation. Dykes occasionally occur in large swarms

Dyke Swarms

tend to be either parallel or radial. Linear swarms may be related to rifting.

Shield Volcanoes

Shield volcanoes are built of countless outpourings of fluid lava flows from a central vent. They are gently sloping (at the most up to 15°) and built almost entirely of low viscosity (easily flowing) basaltic lava flows. If flows are copious and frequent the shield-shaped volcanoes can reach many tens of kilometres in circomference and be more than 2 km high.The eruptions are generally nonexplosive due to the low silica content. Sometimes they have a collapse caldera. Mauna Loa, on Hawaii, is the classic example of a shield volcano. This is the world's tallest structure - 10 km high measured from its base at the seafloor to 4 km above sea level. It has taken a few miollion years to buil it through thousands of lava flows on top of each other.

Cinder Cones

Cinder cones are built of solid fragments of lavas (cinders) ejected from a single vent. The ejected material is deposited as layers that dip away from the crater at the summit. The profile of the cone is determined by the maximum angle at which debris remains stable instead of sliding downhill. They are characterised by explosive liquid lava and are relatively small.

Stratovolcanoes or Composite Volcanoes

Composite volcanoes are often called stratovolcanoes. They are built of layers of lava, ash and volcanic debris. They got the name stratovolcano because they are layered (derived from latin stratum = layer) - but other volcanoes like shield volcanoes and cinder cones are layered as well. They are called composite because the emit lava as well as pyroclasts (solid volcanic fragments) giving rise to alternating layers of lava and pyroclasts. The lavas are relatively viscous (slow flowing), and may eruptions are explosive.

Pyroclasts (" tephra") are any volcanic fragment that was hurled through the air by volcanic activity.

Volcanic Dome

In contrast to basaltic lavas, felsic lavas are so viscous that they can just barely flow. Domes look as though lava had been squeezedout of a vent like toothpaste, with very little lateral spreading. Domes often plug vents, trapping gases. Pressures increase until an explosion occurs, blasting the dome into fragments. This happened in Mt. St. helens in 1980. Commonly adjacent to craters of composite volcanoes.

Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements. The term is derived from using the MA from magnesium and the FIC from the Latin word for iron (ferro), but mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color and have relatively high specific gravities (greater than 3.0). Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro.

Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silica and oxygen, aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich variety) and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color and have specific gravities less than 3.0. The most common felsic rock is granite, which represents the purified end product of the earth's internal differentiation process. Rhyolite has the same chemical composition as granite. Rhyolite is the lava form, while granite is intrusive, which means that it formed beneath the Earth's surface.

Maar

A maar is a low-relief, broad volcanic crater formed by shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table. Maars often fill with water to form a lake.

Caldera

Very large composite volcano collapsed after an explosive period; frequently associated with plug domes.

Tambora, Sumbawa, Indonesia

Mount Tambora's eruption on 10 April 1815, buried the inhabitants of Sumbawa Island under searing ash, gas and rock and is blamed for around 90,000 deaths mostly due to starvation. The eruption was at least four times more powerful than Mount Krakatoa's in 1883. It was the largest eruption in historic time. About 150 cubic kilometers of ash were erupted (about 150 to 200 times more than the 1980 eruption of Mount St. Helens). On a geologic scale a volcano in the Yellowstone area is believed to have exploded with a thousand km³ of pulverized rocks and ashes 640.000 years ago - and even worse about 2.2 million years ago with 2.500.000 km³. On the eruption magnitude scale called the Volcanic Explosivity Index or VEI Mount St. Helens was VEI 5, Krakatau VEI 6, Tambora VEI 7, and Yellowstone VEI 8. Tambora's blast also sent sulfur dioxide 43 km into the air, creating a chemical chain reaction in the atmosphere that caused a year of global cooling that made 1816 "the year without a summer".
Tambora and Krakatau (often called Krakatoa in English) are situated in a volcanic arc. A volcanic (or magmatic arc) is a generally curved linear belt of volcanoes above a subduction zone, and the volcanic and plutonic rocks formed there. The (volcanoes on the) Indonesian islands of Sumatra, Java, Bali, Lombok, Sumawa, Komodo, Flores etc. form such a volcanic arc. Indonesia has some 155 active volcanoes. Most of Indonesia's volcanoes are part of this volcanic arc called the Sunda arc, a 3,000 km long line of volcanoes extending from northern Sumatra to the Banda Sea. Most of these volcanoes are the result of subduction of the Australia Plate beneath the Eurasia Plate.

Yellowstone

Yellowstone has had three very large caldera-forming eruptions in the last 2 million years. These eruptions occurred about 2.0, 1.3, and 0.6 million years ago. Two of those are among the largest eruptions known to have occurred on Earth (each more than 1,000 cubic kilometres). The youngest caldera is an elliptical depression, nearly 80 kilometres long and 50 kilometres wide, that occupies much of Yellowstone National Park (see map). The eruptions have moved from west to east (or rather the North American plate has moved westwards). The volcano is an intraplate volcano (meaning that it is far from plate boundaries) and it is a so-called hot spot volcano, like the Hawaii volcanoes. How hot spots work and if "hot spot" is a relevant term are still under heavy discussions.

Map of the United States showing distribution of airborne ash deposits erupted in the three major caldera-forming eruptive episodes of the Yellowstone Plateua volcanic field. USGS (2007) after Izett and Wilcox (1282)

The wisdom so far is that volcanism on Earth occurs in three tectonic settings:

1. divergent plate boundaries (e.g., mid-ocean ridges),
2. convergent plate boundaries (e.g., island arcs), and
3. hotspots.

In the report "Volcanism in Repsonse to Plate Flexure" a fourth category dubbed "petit spots" was suggested. The authors propose that these small volcanoes erupt along lithospheric fractures in response to plate flexure during subduction. See my blog of 10 August 2006.

In June 1912, Novarupta erupted in the largest volcanic eruption of the twentieth century. Novarupta, meaning "new eruption", is a volcano located on the Alaska Peninsula in the Katmai area, about 470 km southwest of Anchorage. Its eruption of June 6–June 8, 1912, was ten times more powerful than the 1980 eruption of Mount St. Helens and led to the formation of this 841 m volcano. About 15 km³ of volcanic material was ejected during 60 hours beginning on 6 June 1912. At least seven eruptions of comparable size seem to have occurred within 800 km of Anchorage in the last 4,000 years.

Novarupta is situated near the Alaskan-Aleutian plate boundary. This boundary is 4000 km long, and subduction along it has caused some of the largest earthquake ruptures on this planet. The great earthquakes of 1964, 1957 and 1965 are all M9 class events and rank globally among the six largest historic earthquakes.

Tungurahua

Tungurahua is one out of a dozen of volcanoes in the world emitting steam, ashes or lava at present (25 November 2006). The eruptions started on Thursday 16 August 2006. About 10 villages have been damaged by the eruptions. At least 6 people were killed and ca. 20.000 ha of pasture and crops were destroyed around the volcano.

Tungurahua is the largest volcano in Ecuador. It is situated 120 km South of the capital Quito. It is an andesitic-dacitic stratovolcano also known as the "The Black Giant." It has a 183 m wide crater. The summit is 5023 m high. Its geographical location is 1.467°S and 78.442°W. This year's eruptions have been the strongest since 1999.

Tungurahua is the name given by the Quechua Indians for the volcano meaning "Throat of Fire."

There are two main ridges of volcanoes in Ecuador know as the Western and Eastern Cordilleras. Formed by the subduction zone (where the Nazca plate subducts under the South American plate), they are separated by the Inter Andean Valley, which is home to Quito, the capital of Ecuador. Tungurahua is in the Eastern Cordillera.

Andesite is a fine grained volcanic rock (or lava) that erupts from volcanoes at destructive plate margins (where one plate of the Earth's surface moves beneath another), including the Andes, from which it gets its name. Its colour is medium dark.

Dacite is a volcanic rock (or lava) that characteristically is light in colour. It may be considered a quartz-bearing variety of andesite (lighter in colour due to the higher content of quarts). Dacite is primarily associated with andesite and trachyte and forms lava flows, dikes, and sometimes massive intrusions in the centres of old volcanoes. The word dacite comes from Dacia, a Roman province found between the Danube River and Carpathian Mountains (now modern Romania) where the rock was first described.

Lahar

USGS Photograph of lahar on Mt. Saint Helens taken on March 21, 1982, by Tom Casadevall.

A lahar is a special type of mudflow, namely a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. The word lahar is derived from Indonesian.

Volcanoes treated in Olelog

Americas

Novarupta, Alaska
Novarupta, 20th Century's Most Powerful Volcanic Eruption
Friday, 6 October 2006
Largest eruption in the 20th century
Tuesday 3. January 2006

El Reventador, Ecuador
Volcanic plumes
Wednesday, 15 March 2006

Reventador is Spanish for "one that explodes". The volcano is an active stratovolcano located in a remote area of the higher Amazon jungle 90 km east of Quito. El Reventador has a history of explosive eruptions, lava flows and lahars. A Lahar is a flow of pyroclastic (volcanic) material mixed with water. A lahar is often produced when a snow-capped volcano erupts and hot pyroclastics melt a large amount of snow or ice. Lahar is by the way an Indonesian word for a mudflow.

Volcano Fuego, one of Central America's most active volcanoes, is one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks.

Volcán de Fuego ("Volcano of Fire") in Guatemala
Wednesday, 28. December 2005, 15:44:37

Subduction of the Cocos Plate beneath the Caribbean Plate produces the Central American arc.

Mount St. Helens
Tuesday, 29. November 2005, 10:30:10

The Ilamatepec volcano or Santa Ana (El Salvador)
Monday, 3. October 2005, 08:56:16
Sunday, 2. October 2005, 09:15:39

Subduction of the Cocos Plate beneath the Caribbean Plate produces the Central American arc.

Tungurahua, Ecuador
Saturday, 25 November 2006

Some volcanoes in Peru

Africa

Nyamuragira (also known as Nyamulagira) is a broad low-angle shield volcano located 14 km northwest of the steep-sided Nyiragongo volcano and about 25 km north of Lake Kivu in the East African Rift Valley - in the Virunga Mountains of the Democratic Republic of the Congo. Nyamuragira is Africa's most active volcano. The 3058 m high summit of Nyamuragira is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Its last known eruption was in 2004. Historical lava flows extend down the flanks more than 30 km from the summit, reaching as far as Lake Kivu.

Geographical location: 1.408°S, 29.20°E. Nyamuragira and Nyiragongo together, are responsible for nearly two-fifths of Africa's historical eruptions. Nyamurigira erupts every one to two years.

Lake Kivu is by the way one of three known volcanic lakes in the world that contain high dissolved volumes of CO₂ in their deeper waters. Could this lead to a disaster like at Lake Nyos in Cameroon in August 1986?

The Western Rift, also called the Albertine Rift, is edged by some of the highest mountains in Africa, including the Virunga Mountains. The Virunga Mountains are world famous for their mountain gorillas. Edward (former lake Amin), Kivu and Tanganyika are 3 of the so-called Rift Valley lakes.

In the rift zone extensional deformation occurs because the underlying mantle is rising from below and stretching the overlying continental crust. Upwelling mantle may melt to produce magmas, which then rise to the surface (in volcanoes), often along faults produced by the extensional deformation.

Mount Arteale in the Afar region
Sunday, 9 October 2005

Asia

Tavurvur, Papua New Guinea
Sunday, 8 October 2006

	Mayon, Phillipines 22 August 2006
Image by Earth Observatory

Krakatau and Anak Krakatau, Indonesia
Monday, 14 November 2005

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