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An effusive eruption is a type of
volcanic eruption Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are oft ...
in which
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 ...
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, wherein
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 ...
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 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 lava domes, each of which vary in shape, length, and width. 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. The shape of effusive
lava flows 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 ...
is governed by the type of lava (i.e.
composition Composition or Compositions may refer to: Arts and literature *Composition (dance), practice and teaching of choreography *Composition (language), in literature and rhetoric, producing a work in spoken tradition and written discourse, to include v ...
), rate and duration of eruption, and topography of the surrounding landscape. 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 Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
. 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.
Silicic 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. Silicic is the grou ...
magma also exhibits this transition between effusive and explosive eruptions, but the fragmentation mechanism differs. The 1912
Novarupta Novarupta (meaning "newly erupted" in Latin) is a volcano that was formed in 1912, located on the Alaska Peninsula on a slope of Trident Volcano in Katmai National Park and Preserve, about southwest of Anchorage, Alaska, Anchorage. Formed durin ...
eruption and the 2003 Stromboli eruption both exhibited a transition between explosive and effusive eruption patterns.


Basaltic eruptions

Basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...
ic 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. 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 Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...
lava flows cool to either of two forms,
ʻaʻā 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 ...
or
pāhoehoe 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 ...
. This type of lava flow builds shield volcanoes, which are, for example, numerous in
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...
, and is how the island was and currently is being formed.


Silicic eruptions

Silicic 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. Silicic is the grou ...
magmas most commonly erupt explosively, but they can erupt effusively. These magmas are water saturated, 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, 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. 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 10−5 to 10−2 m/s, with permeable
conduit Conduit may refer to: Engineering systems * Conduit (fluid conveyance), a pipe suitable for carrying either open-channel or pressurized liquids * Electrical conduit, a protective cover, tube or piping system for electric cables * Conduit curre ...
walls, 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 and will result in an explosive eruption. Silicic magmas typically form blocky lava flows or steep-sided mounds, called
lava domes 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 erupti ...
, because their high viscosity 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. If a dome forms and crystallizes enough early in an eruption, it acts as a plug on the system, 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.


References

{{DEFAULTSORT:Effusive Eruption Volcanic eruption types Articles containing video clips