Different styles of Volcano

Different styles of Volcano

There are different styles of volcano on the face of Earth and yes the subsurface too.

Volcano Architecture 

Crater eruptions and fissure eruptions come from conduits of different shapes.

Melting in the upper mantle and lower crust produces magma, which rises into the upper crust. Typically, this magma accumulates underground in a magma chamber, a zone of open spaces and/or fractured rock that can contain a large quantity of magma. A portion of the magma may solidify in the magma chamber and transform into intrusive igneous rock, whereas the rest rises through an opening, or conduit, to the Earth’s surface and erupts from a volcano.  The conduit may have the shape of a vertical pipe, or chimney, or may be a crack called a fissure (figure above a and b). At the top of a volcanic edifice, a circular depression called a crater (shaped like a bowl, up to 500 m across and 200 m deep) may develop. Craters form either during eruption as material accumulates around the summit vent, or just after eruption as the summit collapses into the drained conduit.

The formation of volcanic calderas.

During major eruptions, the sudden draining of a magma chamber produces a caldera, a big circular depression up to thousands of meters across and up to several hundred meters deep. Typically, a caldera has steep walls and a fairly flat floor and may be partially filled with ash.

Different shapes of volcanoes.

Geologists distinguish among several different shapes of subaerial (above sea level) volcanic edifices. Shield volcanoes, broad, gentle domes, are so named because they resemble a soldier’s shield lying on the ground (a in figure above). They form when the products of eruption have low viscosity and thus are weak, so they cannot pile up around the vent but rather spread out over large areas. Scoria cones (informally called cinder cones) consist of cone-shaped piles of basaltic lapilli and blocks, generally from a single eruption (b in figure above). Strato-volcanoes, also known as composite volcanoes, are large and cone-shaped, generally with steeper slopes near the summit, and consist of interleaved layers of lava, tephra, and volcaniclastic debris (c in figure above). Their shape, exemplified by Japan’s Mt. Fuji, supplies the classic image that most people have of a volcano; the prefix strato- emphasizes that they can grow to be kilometres high.

Concept of Eruptive Style: Will It Flow, or Will It Blow? 

Kilauea, a volcano on Hawaii, produces rivers of lava that cascade down the volcano’s flanks. Mt. St. Helens, a volcano near the Washington–Oregon border, exploded catastrophically in 1980 and blanketed the surrounding countryside with tephra. Clearly, different volcanoes erupt differently and, as we've noted, successive eruptions from the same stratovolcano may differ markedly in character from one another. Geologists refer to the character of an eruption as eruptive style. Below, we describe several distinct eruptive styles and explore why the differences occur.

Contrasting eruptive styles.

Effusive eruptions 

The term effusive comes from the Latin word for pour out, and indeed that’s what happens during an effusive eruption lava pours out a summit vent or fissure, filling a lava lake around the crater and/or flowing in molten rivers for great distances (a in figure above). Effusive eruptions occur where the magma feeding the volcano is hot and mafic and, therefore, has low viscosity. Pressure, applied to the magma chamber by the weight of overlying rock, squeezes magma upward and out of the vent; in some cases, the pressure is great enough to drive the magma up into a fountain over the vent.

Explosive eruptions 

When pressure builds in a volcano, the eruption will likely yield an explosion. Smaller explosions take place during basaltic eruptions, when gas builds up and suddenly escapes, spattering lava drops and blobs upward these then solidify and fall as tephra. Occasionally, a volcano blows up in a huge explosion. Such catastrophic explosions can be triggered by many causes. For example, if a crack forms in the flank of an island volcano, water will enter the magma chamber and suddenly turn to steam, the expansion of which blasts the volcano apart. Such explosions can also happen in felsic or andesitic volcanoes, if very viscous magma plugs the vent until huge pressure builds inside. If the plug eventually cracks, or the flank of the volcano cracks, the gas inside the volcano suddenly expands, and like a giant shotgun blast, it sprays out the molten contents of the volcano and may cause the volcano itself to break apart. Such explosions, awesome in their power and catastrophic in their consequences, can eject cubic kilometres of debris outward. In some cases, the sudden draining of the magma chamber, and the ejection of debris, causes the remnants of the volcano to collapse and form a caldera.
During a large explosion, the force of the blast shoots debris skyward in a vertical column (b in figure above). But the force can only take the material so high. The huge plumes of ash that rise to stratospheric heights above large explosions do so by becoming turbulent, billowing, convective clouds. This means that the warm mixture of volcanic ash, gas, and air is less dense than the surrounding, cooler air, so the warm mixture rises buoyantly. The resulting plume resembles a mushroom cloud above a nuclear explosion. Coarser-grained ash and lapilli settle from the cloud close to the volcano, whereas finer ash gets carried farther away. Some ash enters high-elevation winds and will be carried around the globe. The denser components collapse downward once they run out of explosive energy, and gravity pulls them back down. This phenomenon, the “collapse” of the column, produces the pyroclastic flows that surge down a volcano’s flanks. What is a pyroclastic flow like? In 1902, the people of St. Pierre, a town on the Caribbean island of Martinique, sadly found out. St. Pierre was a busy port town, about 7 km south of the peak of Mt. Pelée, a volcano. When the volcano began emitting steam and lapilli, residents of the town became nervous and debated about the need to evacuate. Meanwhile, a rhyolite dome grew and obstructed the throat of the volcano. On May 8, the dome suddenly cracked, and the immense pressure that had been building beneath the obstruction was released. In the same way that champagne bursts out of a bottle when you pull out the cork, a cloud of hot ash and pumice lapilli spewed out of Mt. Pelée, and a pyroclastic flow swept 

down Pelée’s flank. Partly riding on a cushion of air, this flow reached speeds of 300 km per hour, and slammed into St. Pierre. Within moments, all the town’s buildings had been flattened and all but two of its 28,000 inhabitants were dead of incineration or asphyxiation. Similar eruptions have happened more recently on the nearby island of Montserrat, but with a much smaller death toll because of timely evacuation (c in figure above).

Relation of eruptive style to volcanic type

Note that the type of volcano (shield, cinder cone, or composite) depends on its eruptive style. Volcanoes that have only effusive eruptions become shield volcanoes, those that generate small pyroclastic eruptions due to fountaining basaltic lava yield cinder cones, and those that alternate between effusive and large pyroclastic eruptions become composite volcanoes (stratovolcanoes). Large explosions yield calderas and blanket the surrounding countryside with ash and/or ignimbrites. Why are there such contrasts in eruptive style? Eruptive style depends on the viscosity and gas contents of the magma in the volcano. These characteristics, in turn, depend on the composition and temperature of the magma and on the environment (subaerial or submarine) in which the eruption occurs. Traditionally, geologists have classified volcanoes according to their eruptive style, each style named after a well known example.

Credits: Stephen Marshak (Essentials of Geology)

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