Effusive eruption

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An `a`a lava flow from Mauna Loa during its 1984 eruption. Aa channel flow from Mauna Loa.jpg
An ʻaʻā lava flow from Mauna Loa during its 1984 eruption.

An effusive eruption is a type of volcanic eruption in which lava steadily flows out of a volcano onto the ground. There are two major groupings of eruptions: effusive and explosive. [1] Effusive eruption differs from explosive eruption, wherein magma is violently fragmented and rapidly expelled from a volcano. Effusive eruptions are most common in basaltic magmas, but they also occur in intermediate and felsic magmas. These eruptions form lava flows and lava domes, each of which vary in shape, length, and width. [2] Deep in the crust, gasses are dissolved into the magma because of high pressures, but upon ascent and eruption, pressure drops rapidly, and these gasses begin to exsolve out of the melt. A volcanic eruption is effusive when the erupting magma is volatile poor (water, carbon dioxide, sulfur dioxide, hydrogen chloride, and hydrogen fluoride), which suppresses fragmentation, creating an oozing magma which spills out of the volcanic vent and out into the surrounding area. [1] The shape of effusive lava flows is governed by the type of lava (i.e. composition), rate and duration of eruption, and topography of the surrounding landscape. [3]

Types of volcanic eruptions Basic mechanisms of eruption and variations

Several types of volcanic eruptions—during which lava, tephra, and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.

Lava Molten rock expelled by a volcano during an eruption

Lava is molten rock generated by geothermal energy and expelled through fractures in planetary crust or in an eruption, usually at temperatures from 700 to 1,200 °C. The structures resulting from subsequent solidification and cooling are also sometimes described as lava. The molten rock is formed in the interior of some planets, including Earth, and some of their satellites, though such material located below the crust is referred to by other terms.

Explosive eruption Type of volcanic eruption in which lava is violently expelled

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 that expelled lava violently froths into volcanic ash when pressure is suddenly lowered at the vent. Sometimes a lava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can send rocks, dust, gas and pyroclastic material up to 20 km (12 mi) into the atmosphere at a rate of up to 100,000 tonnes per second, traveling at several hundred meters per second. This cloud may then collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.


Time-lapse video of Kilauea flank vent eruption, 2005

For an effusive eruption to occur, magma must be permeable enough to allow the expulsion of gas bubbles contained within it. If the magma is not above a certain permeability threshold, it cannot degas and will erupt explosively. Additionally, at a certain threshold, fragmentation within the magma can cause an explosive eruption. This threshold is governed by the Reynolds Number, a dimensionless number in fluid dynamics that is directly proportional to fluid velocity. Eruptions will be effusive if the magma has a low ascent velocity. At higher magma ascent rates, the fragmentation within the magma passes a threshold and results in explosive eruptions. [4] Silicic magma also exhibits this transition between effusive and explosive eruptions, [5] but the fragmentation mechanism differs. [4] The 1912 Novarupta eruption and the 2003 Stromboli eruption both exhibited a transition between explosive and effusive eruption patterns. [5] [6]

Reynolds number Dimensionless quantity that is used to help predict fluid flow patterns

The Reynolds number is an important dimensionless quantity in fluid mechanics used to help predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow. These eddy currents begin to churn the flow, using up energy in the process, which for liquids increases the chances of cavitation. The Reynolds number has wide applications, ranging from liquid flow in a pipe to the passage of air over an aircraft wing. It is used to predict the transition from laminar to turbulent flow, and is used in the scaling of similar but different-sized flow situations, such as between an aircraft model in a wind tunnel and the full size version. The predictions of the onset of turbulence and the ability to calculate scaling effects can be used to help predict fluid behaviour on a larger scale, such as in local or global air or water movement and thereby the associated meteorological and climatological effects.

Fluid dynamics Sub-discipline of fluid mechanics

In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics and hydrodynamics. Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation.

Velocity rate of change of the position of an object as a function of time, and the direction of that change

The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction of motion. Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.

Basaltic eruptions

Basaltic composition magmas are the most common effusive eruptions because they are not water saturated and have low viscosity. Most people know them from the classic pictures of rivers of lava in Hawaii. Eruptions of basaltic magma often transition between effusive and explosive eruption patterns. The behavior of these eruptions is largely dependent on the permeability of the magma and the magma ascent rate. During eruption, dissolved gasses exsolve and begin to rise out of the magma as gas bubbles. [7] If the magma is rising slowly enough, these bubbles will have time to rise and escape, leaving a less buoyant magma behind that fluidly flows out. Effusive basalt lava flows cool to either of two forms, ʻaʻā or pāhoehoe. [8] This type of lava flow builds shield volcanoes, which are numerous in Hawaii, [9] and is how the island was and currently is being formed.

Basalt A magnesium- and iron-rich extrusive igneous rock

Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.

Shield volcano Low profile volcano usually formed almost entirely of fluid lava flows

A shield volcano is a type of volcano usually composed almost entirely of fluid lava flows. It is named for its low profile, resembling a warrior's shield lying on the ground. This is caused by the highly fluid lava erupted, which travels farther than lava erupted from a stratovolcano, and results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.

Hawaii U.S. state in the United States

Hawaii is a state of the United States of America. It is the only state located in the Pacific Ocean and the only state composed entirely of islands.

Silicic eruptions

Alaskan volcano Novarupta with an effused lava dome at the summit. Novarupta.jpg
Alaskan volcano Novarupta with an effused lava dome at the summit.

Silicic magmas most commonly erupt explosively, but they can erupt effusively. [10] These magmas are water saturated, [11] and many orders of magnitude more viscous than basaltic magmas, making degassing and effusion more complicated. Degassing prior to eruption, through fractures in the country rock surrounding the magma chamber, [12] plays an important role. Gas bubbles can begin to escape through the tiny spaces and relieve pressure, visible on the surface as vents of dense gas. [13] The ascent speed of the magma is the most important factor controlling which type of eruption it will be. For silicic magmas to erupt effusively, the ascent rate must be to m/s, with permeable conduit walls, [4] so that gas has time to exsolve and dissipate into the surrounding rock. If the flow rate is too fast, even if the conduit is permeable, it will act as though it is impermeable [4] and will result in an explosive eruption. Silicic magmas typically form blocky lava flows [14] or steep-sided mounds, called lava domes, because their high viscosity [15] does not allow it to flow like that of basaltic magmas. When felsic domes form, they are emplaced within and on top of the conduit. [16] If a dome forms and crystallizes enough early in an eruption, it acts as a plug on the system, [16] denying the main mechanism of degassing. If this happens, it is common that the eruption will change from effusive to explosive, due to pressure build up below the lava dome. [10]

Silicic is an adjective to describe magma or igneous rock rich in silica. The amount of silica that constitutes a silicic rock is usually defined as at least 63 percent. Granite and rhyolite are the most common silicic rocks.

Country rock (geology)

Country rock is a geological term meaning the rock native to an area, in which there is an intrusion of viscous geologic material, commonly magma, or perhaps rock salt or unconsolidated sediments.

Volcanism phenomena and processes associated with the action of volcanos, geysers and fumaroles

Volcanism is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called a vent. It includes all phenomena resulting from and causing magma within the crust or mantle of the body, to rise through the crust and form volcanic rocks on the surface.

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Volcano A rupture in the crust of a planetary-mass object that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface

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.

Magma Mixture of molten or semi-molten rock, volatiles and solids that is found beneath the surface of the Earth

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 satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma is produced by melting of the mantle and/or the crust at various tectonic settings, including subduction zones, continental rift zones, mid-ocean ridges and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chambers or trans-crustal crystal-rich mush zones. During their storage in the crust, magma compositions may be modified by fractional crystallization, contamination with crustal melts, magma mixing, and degassing. Following their ascent through the crust, magmas may feed a volcano or solidify underground to form an intrusion. While the study of magma has historically relied on observing magma in the form of lava flows, magma has been encountered in situ three times during geothermal drilling projects—twice in Iceland, and once in Hawaii.

Stratovolcano Tall, conical volcano built up by many layers of hardened lava and other ejecta

A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice and ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica, with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).

Lava dome Roughly circular protrusion from slowly extruded viscous volcanic lava

In volcanology, a lava dome or volcanic dome is a roughly 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 earth are lava dome forming. The geochemistry of lava domes can vary from basalt to rhyolite although the majority are of intermediate composition The characteristic dome shape is attributed to high viscosity that prevents the lava from flowing very far. This high viscosity can be obtained in two ways: by high levels of silica in the magma, or by degassing of fluid magma. Since viscous basaltic and andesitic domes weather fast and easily break apart by further input of fluid lava, most of the preserved domes have high silica content and consist of rhyolite or dacite.

Masaya Volcano Volcán Masaya

Masaya is a caldera located in Masaya, Nicaragua, 20 km south of the capital Managua. It is Nicaragua's first and largest national park, and one of 78 protected areas of Nicaragua. The complex volcano is composed of a nested set of calderas and craters, the largest of which is Las Sierras shield volcano and caldera. Within this caldera lies a sub-vent, which is Masaya Volcano sensu stricto. The vent is a shield type composing of basaltic lavas and tephras and includes a summit crater. This hosts Masaya caldera, formed 2,500 years ago by an 8-km³ basaltic ignimbrite eruption. Inside this caldera a new basaltic complex has grown from eruptions mainly on a semi-circular set of vents that include the Masaya and Nindiri cones. The latter host the pit craters of Masaya, Santiago, Nindiri and San Pedro. Observations in the walls of the pit craters indicate that there have been several episodes of cone and pit crater formation.

Cerro Azul (Chile volcano) mountain in Curicó Province Chile

Cerro Azul, sometimes referred to as Quizapu, is an active stratovolcano in the Maule Region of central Chile, immediately south of Descabezado Grande. Part of the South Volcanic Zone of the Andes, its summit is 3,788 meters (12,428 ft) above sea level, and is capped by a summit crater that is 500 meters (1,600 ft) wide and opens to the north. Beneath the summit, the volcano features numerous scoria cones and flank vents.

Volcanic gas

Volcanic gases are gases given off by active volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action.

Quetrupillán mountain in Cautín Province Chile

Quetrupillán is a stratovolcano located in the La Araucanía Region of Chile. It is situated between Villarrica and Lanín volcanoes, within Villarrica National Park. Geologically, Quetrupillán is located in a tectonic basement block between the main traces of Liquiñe-Ofqui Fault and Reigolil-Pirihueico Fault.

Ollagüe mountain shared by Bolivia and Chile

Ollagüe or Ullawi is a massive andesite stratovolcano in the Andes on the border between Bolivia and Chile, within the Antofagasta Region of Chile and the Potosi Department of Bolivia. Part of the Central Volcanic Zone of the Andes, its highest summit is 5,868 metres (19,252 ft) above sea level and features a summit crater that opens to the south. The western rim of the summit crater is formed by a compound of lava domes, the youngest of which features a vigorous fumarole that is visible from afar.

Submarine eruption type of volcanic eruption

Submarine eruptions are those volcano eruptions which take place beneath the surface of water. These occur at constructive margins, subduction zones and within tectonic plates due to hotspots. This eruption style is far more prevalent than subaerial activity. For example, it is believed that 70 to 80% of the Earth's magma output takes place at mid-ocean ridges.

Mount Edziza volcanic complex mountain in Canada

The Mount Edziza volcanic complex is a large and potentially active north-south trending complex volcano in Stikine Country, northwestern British Columbia, Canada, located 38 kilometres (24 mi) southeast of the small community of Telegraph Creek. It occupies the southeastern portion of the Tahltan Highland, an upland area of plateau and lower mountain ranges, lying east of the Boundary Ranges and south of the Inklin River, which is the east fork of the Taku River. As a volcanic complex, it consists of many types of volcanoes, including shield volcanoes, calderas, lava domes, stratovolcanoes, and cinder cones.

Ceboruco volcano in central western Mexico

Ceboruco is a dacitic stratovolcano located in Nayarit, Mexico, northwest of the Trans-Mexican Volcanic Belt. The largest eruption, the Jala Plinian eruption, was around 930 AD ±200, VEI 6, releasing 11 cubic kilometres (2.6 cu mi) of tephra. The most recent and best documented eruption from Ceboruco lasted from 1870–1875, with fumarole activity lasting well into the 20th century. The mountain features one large caldera, created during the Jala eruption, with a smaller crater nested inside that formed when the Dos Equis lava dome collapsed during the Coapales eruption around 1100 AD. Within both of these craters, are several explosive volcanic features, including scoria deposits, lava domes, and pyroclastic domes, or cinder cone volcanoes.

Lateral eruption type of volcanic eruption

A lateral eruption, also called a flank eruption or lateral blast if explosive, is a volcanic eruption that takes place on the flanks of a volcano instead of at the summit. Lateral eruptions are typical at rift zones where a volcano is breaking apart. Since it is easier for molten rock to flow laterally out the sides of weak flanks, the flank gives way before magma is pushed up through a conduit that feeds magma to the summit. These features are commonly found at shield volcanoes and produce basaltic lava flows and cinder cones.

Cerro Chao

Cerro Chao is a lava flow complex associated with the Cerro del León volcano in the Andes. It is the largest known Quaternary silicic volcano body and part of the most recent phase of activity in the Altiplano–Puna volcanic complex.

Cerro Chascon-Runtu Jarita is a complex of lava domes located inside, but probably unrelated to, the Pastos Grandes caldera. It is part of the more recent phase of activity of the Altiplano-Puna volcanic complex.


Ciomadul is a volcano in Romania, and is known as Csomád in Hungarian. It is in the Carpathians, close to the towns of Băile Tușnad and Bixad. It is part of a volcanic chain known as the Călimani (Kelemen) – Gurghiu (Görgényi) – Harghita (Hargita) chain and lies at its southeastern end. Ciomadul consists of several lava domes with two embedded explosion craters known as Mohos and St. Ana, the latter of which contains a crater lake, Lake Sfânta Ana. The dominant volcanic rock at Ciomadul is potassium-rich dacite.

Ubinas volcano in Peru

Ubinas is a stratovolcano in the Moquegua Region of southern Peru, 60 kilometres (37 mi) east of the city of Arequipa. Part of the Central Volcanic Zone of the Andes, it is 5,672 metres (18,609 ft) above sea level. The volcano's summit is cut by a 1.4-kilometre (0.87 mi) wide and 150-metre (490 ft) deep caldera, which itself contains a smaller crater. Below the summit, Ubinas has the shape of an upwards-steepening cone with a prominent notch on the southern side. The gently sloping lower part of the volcano is also known as Ubinas I and the steeper upper part as Ubinas II; they represent different stages in the geologic history of Ubinas.

Lava balloon Floating bubble of lava

A lava balloon is a gas-filled bubble of lava that floats on the sea surface. It can be up to several metres in size. When it emerges from the sea, it is usually hot and often steaming. After floating for some time it fills with water and sinks again.


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