![Broken Top rock layers](https://upload.wikimedia.org/wikipedia/commons/4/44/Broken_Top_rock_layers.jpg)
A stratovolcano, also known as a composite volcano, is a
conical
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines conn ...
volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are ...
built up by many layers (strata) of hardened
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
and
tephra
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they rem ...
. Unlike
shield volcano
A shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more v ...
es, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of
explosive eruption
In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such ...
s and
effusive eruption
An effusive eruption is a type of volcanic eruption in which lava steadily flows out of a volcano onto the ground.
Overview
There are two major groupings of eruptions: effusive and explosive. Effusive eruption differs from explosive eruption, ...
s, although some have collapsed summit craters called
caldera
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
s. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often
felsic
In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...
, having high-to-intermediate levels of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(as in
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
,
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
, or
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...
), with lesser amounts of less-viscous
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
magma. Extensive felsic lava flows are uncommon, but have travelled as far as .
Stratovolcanoes are sometimes called composite volcanoes because of their composite stratified structure, built up from sequential outpourings of erupted materials. They are among the most common types of volcanoes, in contrast to the less common shield volcanoes. Two famous examples of stratovolcanoes are
Krakatoa
Krakatoa (), also transcribed (), is a caldera in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The caldera is part of a volcanic island group (Krakatoa archipelago) comprising four islands. Tw ...
in
Indonesia
Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
, known for its
catastrophic eruption in 1883, and
Vesuvius
Mount Vesuvius ( ; it, Vesuvio ; nap, 'O Vesuvio , also or ; la, Vesuvius , also , or ) is a somma-stratovolcano located on the Gulf of Naples
The Gulf of Naples (), also called the Bay of Naples, is a roughly 15-kilometer-wide (9 ...
in
Italy
Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical re ...
, whose
catastrophic eruption in AD 79 buried the
Roman
Roman or Romans most often refers to:
*Rome, the capital city of Italy
*Ancient Rome, Roman civilization from 8th century BC to 5th century AD
*Roman people, the people of ancient Rome
*''Epistle to the Romans'', shortened to ''Romans'', a letter ...
cities of
Pompeii
Pompeii (, ) was an ancient city located in what is now the ''comune'' of Pompei near Naples in the Campania region of Italy. Pompeii, along with Herculaneum and many villas in the surrounding area (e.g. at Boscoreale, Stabiae), was buried ...
and
Herculaneum
Herculaneum (; Neapolitan and it, Ercolano) was an ancient town, located in the modern-day ''comune'' of Ercolano, Campania, Italy. Herculaneum was buried under volcanic ash and pumice in the eruption of Mount Vesuvius in AD 79.
Like the nea ...
. Both eruptions claimed thousands of lives. In modern times,
Mount St. Helens
Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...
in
Washington State
Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washington ...
, USA and
Mount Pinatubo
Mount Pinatubo is an active stratovolcano in the Zambales Mountains, located on the tripoint boundary of the Philippine provinces of Zambales, Tarlac and Pampanga, all in Central Luzon on the northern island of Luzon. Its eruptive history w ...
in the
Philippines
The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no),
* bik, Republika kan Filipinas
* ceb, Republika sa Pilipinas
* cbk, República de Filipinas
* hil, Republ ...
have erupted catastrophically, but with fewer deaths.
The existence of stratovolcanoes on other bodies of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
has not been conclusively demonstrated. One possible exception is the existence of some isolated
massif
In geology, a massif ( or ) is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole. The term also refers to a ...
s on Mars, for example the Zephyria Tholus.
Creation
![Subduction-en](https://upload.wikimedia.org/wikipedia/commons/c/ce/Subduction-en.svg)
Stratovolcanoes are common at
subduction zone
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
s, forming chains and clusters along plate tectonic boundaries where
oceanic crust
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...
is drawn under
continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' be ...
(continental arc volcanism, e.g.
Cascade Range
The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, ...
,
Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
,
Campania
Campania (, also , , , ) is an administrative Regions of Italy, region of Italy; most of it is in the south-western portion of the Italian peninsula (with the Tyrrhenian Sea to its west), but it also includes the small Phlegraean Islands and the i ...
) or another oceanic plate (
island arc
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...
volcanism, e.g.
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
,
Philippines
The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no),
* bik, Republika kan Filipinas
* ceb, Republika sa Pilipinas
* cbk, República de Filipinas
* hil, Republ ...
,
Aleutian Islands
The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi language, Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a cha ...
). The magma forming stratovolcanoes rises when water trapped both in hydrated minerals and in the porous
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
rock of the upper oceanic crust is released into
mantle rock of the
asthenosphere
The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...
above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering", and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. The water freed from the rock lowers the
melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...
of the overlying mantle rock, which then undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, and pools temporarily at the base of the
lithosphere
A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
. The magma then rises through the
crust, incorporating silica-rich crustal rock, leading to a final
intermediate composition In igneous petrology, an intermediate composition refers to the chemical composition of a rock that has 5263 wt% SiO2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite, dacite, and trachyandes ...
. When the magma nears the top surface, it pools in a
magma chamber
A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upw ...
within the crust below the stratovolcano.
The processes that trigger the final eruption remain a matter of research. Possible mechanisms include:
* Magma differentiation, in which the lightest, most silica-rich magma and volatiles such as water, halogens, and sulfur dioxide accumulate in the uppermost part of the magma chamber. This can dramatically increase pressures.
*
Fractional crystallization of the magma. When anhydrous minerals such as
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
crystallize out of the magma, this concentrates volatiles in the remaining liquid, which can lead to ''second boiling'' that causes a gas phase (carbon dioxide or water) to separate from the liquid magma and raise magma chamber pressures.
* Injection of fresh magma into the magma chamber, which mixes and heats the cooler magma already present. This could force volatiles out of solution and lower the density of the cooler magma, both of which increase pressure. There is considerable evidence for magma mixing just before many eruptions, including magnesium-rich olivine crystals in freshly erupted silicic lava that show no reaction rim. This is possible only if the lava erupted immediately after mixing since olivine rapidly reacts with silicic magma to form a rim of pyroxene.
* Progressive melting of the surrounding
country rock
Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal s ...
.
These internal triggers may be modified by external triggers such as
sector collapse
A sector collapse is the collapse of a portion of a volcano due to a phreatic eruption, an earthquake, or the intervention of new magma. Occurring on many volcanoes, sector collapses are generally one of the most hazardous volcanic events, and will ...
,
earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s, or
interactions with groundwater. Some of these triggers operate only under limited conditions. For example, sector collapse (where part of the flank of a volcano collapses in a massive landslide) can trigger eruption only of a very shallow magma chamber. Magma differentiation and thermal expansion also are ineffective as triggers for eruptions from deep magma chambers.
Whatever the precise mechanism, the pressure in the magma chamber increases to a critical point where the roof of the magma chamber fractures, and the contents of the magma chamber are provided with a path to the surface through which to erupt.
Hazards
![Etna from 2900m](https://upload.wikimedia.org/wikipedia/commons/c/c9/Etna_from_2900m.jpg)
In
recorded history
Recorded history or written history describes the historical events that have been recorded in a written form or other documented communication which are subsequently evaluated by historians using the historical method. For broader world hist ...
, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations.
Subduction-zone stratovolcanoes, such as
Mount St. Helens
Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...
,
Mount Etna
Mount Etna, or simply Etna ( it, Etna or ; scn, Muncibbeḍḍu or ; la, Aetna; grc, Αἴτνα and ), is an active stratovolcano on the east coast of Sicily, Italy, in the Metropolitan City of Catania, between the cities of Messina a ...
and
Mount Pinatubo
Mount Pinatubo is an active stratovolcano in the Zambales Mountains, located on the tripoint boundary of the Philippine provinces of Zambales, Tarlac and Pampanga, all in Central Luzon on the northern island of Luzon. Its eruptive history w ...
, typically erupt with explosive force: the magma is too stiff to allow easy escape of volcanic gases. As a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty magma. Following the breaching of the vent and the opening of the crater, the magma degasses explosively. The magma and gases blast out with high speed and full force.
Since 1600
CE, nearly 300,000 people have been killed by volcanic eruptions.
Most deaths were caused by
pyroclastic flow
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...
s and
lahar
A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
Lahars are extreme ...
s, deadly hazards that often accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fine ash, fragmented lava, and superheated gases that can travel at speeds over . Around 30,000 people were killed by pyroclastic flows during the 1902 eruption of
Mount Pelée
Mount Pelée or Mont Pelée ( ; french: Montagne Pelée, ; gcf, label=Antillean Creole, Montann Pèlé, meaning "bald mountain" or "peeled mountain") is an active volcano at the northern end of Martinique, an island and French overseas departm ...
on the island of
Martinique
Martinique ( , ; gcf, label=Martinican Creole, Matinik or ; Kalinago: or ) is an island and an overseas department/region and single territorial collectivity of France. An integral part of the French Republic, Martinique is located in th ...
in the Caribbean.
In March to April 1982, three explosive eruptions of
El Chichón
El Chichón, also known as Chichonal, is an active volcano in Francisco León, Chiapas, Francisco León, north-western Chiapas, Mexico. El Chichón is part of a geologic zone known as the Chiapanecan Volcanic Arc. El Chichón is a complex of dome ...
in the State of
Chiapas
Chiapas (; Tzotzil language, Tzotzil and Tzeltal language, Tzeltal: ''Chyapas'' ), officially the Free and Sovereign State of Chiapas ( es, Estado Libre y Soberano de Chiapas), is one of the states that make up the Political divisions of Mexico, ...
in southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people.
Two
Decade Volcanoes
The Decade Volcanoes are 16 volcanoes identified by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) as being worthy of particular study in light of their history of large, destructive volcanic eruption, ...
that erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed an ash cloud into the air and produced huge pyroclastic surges and
lahar
A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
Lahars are extreme ...
floods that devastated a large area around the volcano. Pinatubo, located in
Central Luzon
Central Luzon ( pam, (Reyun ning) Kalibudtarang Luzon, pag, (Rehiyon na) Pegley na Luzon, tgl, (Rehiyon ng) Gitnang Luzon, ilo, (Rehion/Deppaar ti) Tengnga ti Luzon), designated as Region III, is an administrative region in the Philippines, ...
just west-northwest from
Manila
Manila ( , ; fil, Maynila, ), officially the City of Manila ( fil, Lungsod ng Maynila, ), is the capital of the Philippines, and its second-most populous city. It is highly urbanized and, as of 2019, was the world's most densely populate ...
, had been dormant for 6 centuries before the 1991 eruption, which ranks as one of the largest eruptions in the 20th century.
Also in 1991, Japan's
Unzen Volcano
is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki on the island of Kyushu, Japan's southernmost main island.
In 1792, the collapse of one of its several lava domes triggered a megatsunami ...
, located on the island of Kyushu about east of Nagasaki, awakened from its 200-year slumber to produce a new
lava dome
In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions on ...
at its summit. Beginning in June, the repeated collapse of this erupting dome generated ash flows that swept down the mountain's slopes at speeds as high as . Unzen is one of more than 75 active volcanoes in Japan; an eruption in 1792 killed more than 15,000 people—the worst volcanic disaster in the nation's history.
The
eruption of Mount Vesuvius in 79
Of the many eruptions of Mount Vesuvius, a major stratovolcano in southern Italy, the best-known is its eruption in 79 AD, which was one of the deadliest in European history. The eruption of Mount Vesuvius in 79 AD is one of the best-known in h ...
completely smothered the nearby ancient cities of
Pompeii
Pompeii (, ) was an ancient city located in what is now the ''comune'' of Pompei near Naples in the Campania region of Italy. Pompeii, along with Herculaneum and many villas in the surrounding area (e.g. at Boscoreale, Stabiae), was buried ...
and
Herculaneum
Herculaneum (; Neapolitan and it, Ercolano) was an ancient town, located in the modern-day ''comune'' of Ercolano, Campania, Italy. Herculaneum was buried under volcanic ash and pumice in the eruption of Mount Vesuvius in AD 79.
Like the nea ...
with thick deposits of
pyroclastic surge
A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but it has a lower density or contains a much higher ratio of gas to rock, which makes i ...
s and
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s. Although the death toll is estimated between 13,000 and 26,000 remains, the exact number is still unclear. Vesuvius is recognized as one of the most dangerous volcanoes, due to its capacity for
powerful explosive eruptions combined with the high population density of the surrounding
Metropolitan Naples area (totaling about 3.6 million inhabitants).
Ash
![Ashfall from Pinatubo, 1991](https://upload.wikimedia.org/wikipedia/commons/6/6f/Ashfall_from_Pinatubo%2C_1991.jpg)
Apart from possibly affecting the climate, volcanic clouds from explosive eruptions also pose a serious hazard to aviation safety.
For example, during the 1982 eruption of
Galunggung
Mount Galunggung (Indonesian: ''Gunung Galunggung'', formerly spelled ''Galoen-gong'') is an active stratovolcano in West Java, Indonesia, around southeast of the West Java provincial capital, Bandung (or around to the east of the West Java t ...
in
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
,
British Airways Flight 9
British Airways Flight 009, sometimes referred to by its callsign Speedbird 9 or as the Jakarta incident, was a scheduled British Airways flight from London Heathrow to Auckland, with stops in Bombay, Kuala Lumpur, Perth, and Melbourne.
On 24 ...
flew into the ash cloud, suffering temporary engine failure and structural damage. During the past two decades, more than 60 airplanes, mostly commercial airliners, have been damaged by in-flight encounters with volcanic ash. Some of these encounters have resulted in the power loss of all engines, necessitating emergency landings. Luckily, to date no crashes have happened because of jet aircraft flying into volcanic ash.
Ashfall
The Ashfall Fossil Beds of Antelope County in northeastern Nebraska are rare fossil sites of the type called lagerstätten that, due to extraordinary local conditions, capture an ecological "snapshot" in time of a range of well-preserved fossil ...
s are a threat to health when inhaled and ash is also a threat to property with enough accumulation. An accumulation of is sufficient to cause most buildings to collapse. Dense clouds of hot volcanic ash, caused by the collapse of an
eruptive column or by being laterally expelled from the partial collapse of a volcanic edifice or lava dome during explosive eruptions, can generate devastating pyroclastic flows or surges, which can sweep off everything in their paths.
Lava
![Mayon 0052](https://upload.wikimedia.org/wikipedia/commons/7/73/Mayon_0052.jpg)
Lava flows from stratovolcanoes are generally not a significant threat to humans and animals because the highly
viscous
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inter ...
lava moves slowly enough for everyone to flee out of the path of flow. The lava flows are more of a threat to property. However, not all stratovolcanoes erupt viscous and sticky lava.
Nyiragongo
Mount Nyiragongo ( ) is an active stratovolcano with an elevation of in the Virunga Mountains associated with the Albertine Rift. It is located inside Virunga National Park, in the Democratic Republic of the Congo, about north of the town of ...
is very dangerous because its
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
has an unusually low silica content, making it quite fluid. Fluid lavas are typically associated with the formation of broad shield volcanoes such as those of Hawaii, but Nyiragongo has very steep slopes down which lava can flow at up to . Lava flows could melt down ice and glaciers that accumulated on the volcano's crater and upper slopes, generating massive
lahar
A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
Lahars are extreme ...
flows. Rarely, generally fluid lava could also generate massive lava fountains, while lava of thicker viscosity can solidify within the vent, creating a
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
which can result in highly explosive eruptions.
Volcanic bombs
Volcanic bombs are extrusive
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
s ranging from the size of books to small cars, that are explosively ejected from stratovolcanoes during their climactic eruptive phases. These "bombs" can travel over away from the volcano, and present a risk to buildings and living beings while shooting at very high speeds (hundreds of kilometers/miles per hour) through the air. Most bombs do not themselves explode on impact, but rather carry enough force to have destructive effects as if they exploded.
Lahar
Lahars (from a
Javanese term for volcanic mudflows) are mixtures of volcanic debris and water. Lahars usually come from two sources: rainfall or the melting of snow and ice by hot volcanic elements, such as lava. Depending on the proportion and temperature of water to volcanic material, lahars can range from thick, gooey flows that have the consistency of wet concrete to fast-flowing, soupy floods.
As lahars flood down the steep sides of stratovolcanoes, they have the strength and speed to flatten or drown everything in their paths. Hot ash clouds, lava flows and pyroclastic surges ejected during
1985 eruption of
Nevado del Ruiz
The Nevado del Ruiz (), also known as La Mesa de Herveo ( en, Mesa of Herveo, the name of the nearby town) is a volcano on the border of the departments of Caldas and Tolima in Colombia, about west of the capital city Bogotá. It is a strat ...
in
Colombia
Colombia (, ; ), officially the Republic of Colombia, is a country in South America with insular regions in North America—near Nicaragua's Caribbean coast—as well as in the Pacific Ocean. The Colombian mainland is bordered by the Car ...
melted snow and ice atop the high Andean volcano. The ensuing lahar flooded the city of
Armero
Armero is a municipality in the Tolima Department, Colombia. According to the National Department of Statistics of Colombia, 12,852 lived in the town in 2005. Its median temperature is 27 °C. It was founded in 1895, but was not officia ...
and nearby settlements, killing 25,000 people.
Effects on climate and atmosphere
![Paluweh2013labeled](https://upload.wikimedia.org/wikipedia/commons/5/54/Paluweh2013labeled.jpg)
As per the above examples, while the Unzen eruptions have caused deaths and considerable local damage in the historic past, the impact of the June 1991 eruption of Mount Pinatubo was global. Slightly cooler-than-usual temperatures were recorded worldwide, with brilliant sunsets and intense sunrises attributed to the
particulates
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
; this eruption lofted particles high into the
stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
. The
aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
s that formed from the
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
(SO
2),
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
(CO
2), and other gases dispersed around the world. The SO
2 mass in this cloud—about 22 million tons—combined with water (both of volcanic and atmospheric origin) formed droplets of sulfuric acid, blocking a portion of the sunlight from reaching the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
and ground. The cooling in some regions is thought to have been as much as 0.5 °C (0.9 °F).
An eruption the size of Mount Pinatubo tends to affect the weather for a few years; the material injected into the stratosphere gradually drops into the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, where it is washed away by rain and cloud precipitation.
A similar, but extraordinarily more powerful phenomenon occurred in the cataclysmic April 1815 eruption of
Mount Tambora
Mount Tambora, or Tomboro, is an active stratovolcano in West Nusa Tenggara, Indonesia. Located on Sumbawa in the Lesser Sunda Islands, it was formed by the active subduction zones beneath it. Before 1815, its elevation reached more than ...
on
Sumbawa
Sumbawa is an Indonesian island, located in the middle of the Lesser Sunda Islands chain, with Lombok to the west, Flores to the east, and Sumba further to the southeast. Along with Lombok, it forms the province of West Nusa Tenggara, but there ...
island in
Indonesia
Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
. The Mount Tambora eruption is recognized as the most powerful eruption in recorded history. Its eruption cloud lowered global temperatures by as much as 3.5 °C (6.3 °F).
In the year following the eruption, most of the Northern Hemisphere experienced sharply cooler temperatures during the summer. In parts of Europe, Asia, Africa, and North America, 1816 was known as the "
Year Without a Summer
The year 1816 is known as the Year Without a Summer because of severe climate abnormalities that caused average global temperatures to decrease by . Summer temperatures in Europe were the extreme weather, coldest on record between the years of 1 ...
", which caused a considerable agricultural crisis and a brief but bitter famine, which generated a series of distresses across much of the affected continents.
List
See also
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References
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Volcanic landforms