Volcanoes
Introduction
The term volcano is used to refer to a rupture in the crust of a body the size of a planet, for example Earth, allowing hot gases, volcanic ash and lava to escape from inside the body (Gilbert, 1998). The term volcano also refers to the mountain or cone of materials that have escaped from inside the body and accumulated around the rupture through which they escaped. Volcanoes occur on both the sea and land (Lockwood & Hazlett, 2013).
The process of escape of materials from inside the body and their accumulation around the rupture is called volcanic activity. Volcanic activity is believed to have contributed to the formation of continents, oceans and the atmosphere of the earth during the Earth’s infancy (Lockwood & Hazlett, 2013).
A volcano can be active, dormant or extinct, depending on the frequency of its eruption. An active volcano is one that is undergoing the process of eruption. The active volcano does not have to be erupting at the time of its description as active, but needs to have erupted in the recent past (ten thousand years ago or less), and is expected to erupt in a short space of time (Lockwood & Hazlett, 2013). A dormant volcano is a volcano whose eruption occurred earlier than ten thousand years ago, and is expected to re-erupt at some point time in the future (Lockwood & Hazlett, 2013). Finally, and extinct volcano is one that is not expected to re-erupt (Lockwood & Hazlett, 2013).. The classification of volcanoes as active, dormant and extinct are only human definitions of this geographically phenomena – some volcanoes have been observed to have failed to conform them (Lockwood & Hazlett, 2013).
Formation of Volcanoes
The process by which volcanoes form is described using the theory of plate tectonics. According to the theory, the crust of the earth is not a spherical shell; rather, it is composed of rigid plates that float and drift across the surface of the earth relative to each other (Rafferty, 2010). A single plate can be an ocean basin, a continent or a combination of a continent and an ocean basin. However, chances of a plate being a continent alone are rare as it would mean that the boundaries of the plate would be at all the edges of the continent, which, practically, is not possible. Therefore, only ocean basin plates and those that occur as combinations of continents and ocean basins exist. The drifting of the tectonic plates past, toward or away from each other is attributed to convectional current of the semi-molten rock in the Earth’s interior (Rafferty, 2010). The convectional currents carry towards the surface of the earth the heat energy generated by the nuclear activity at the centre of the earth (Rafferty, 2010). The movement of the plate tectonics results in either the divergence or the convergence of plate boundaries, the processes both of which are responsible for volcanic activity.
Volcanic activity is triggered when the pressure from the interior of the earth overcomes the weak boundary that forms to fill the space left by two diverging plates, called divergent plate boundary. When the convectional currents in the interior of the earth causes two tectonic plates to move relatively away from each other, hot molten rock cools and solidifies, forming a new ocean crust (Rafferty, 2010). However, the ocean crust so formed is vulnerable to rupture as it is, unlike the diverged plates, very thin and weak. Consequently, the pressure from the interior of the earth easily adiabatically expands and partially melts the mantle, causing it to rupture. The rupture allows volcanic materials to escape from the interior to the earth’s surface, forming a new ocean crust. In situations where the mid-ocean ridge happens to be located above the sea level, a volcanic island, rather than a new seafloor, is formed.
Another cause of volcanic activity is the convergence of two tectonic plates. When convectional currents cause a continental plate and an oceanic plate to approach each other, the eventually of which is a collision of the two plates. As the oceanic plate is relatively lower than the continental plate, the ocean plate ends up being submerged under the continental plate. The result of this submergence is the formation of a deep ocean trench at the offshore. The submerging oceanic plate releases water. Water lowers the melting point of the overlying rock, causing it to melt into Silicon-rich magma (Rafferty, 2010). Owing to its high Silicon content, the magma so formed is usually too vicious to escape to the earth’s surface in most cases: it cools below the surface of the earth. If the magma manages to escape to the earth’s surface, the volcanic activity results in the formation of a volcano.
A third way – unrelated to tectonic plates – in which volcanoes form is one that results from the formation of volcanic sites or volcanic hotspots by mantle plumes. The hypothesis around the so-called mantle plumes is that they are columns of high temperature materials rising from the boundary between the core and the mantle of the earth, in a confined place causes melting in high proportions (Gilbert, 1998). The materials are therefore able to escape to the earth’s surface. The so-formed types of volcanoes tend to be dormant because tectonic plates move across them, covering the plumes for a long period of time, and advances over them after a very long period of time (over ten thousand years), eventuality reactivating them.
The Nature of Volcanic Eruptions
The nature of volcanic eruptions varies from one volcano to another. The variation in eruption is attributed the difference in the content of magma associated with every volcano. If the Silica and gas content of the magma in question is substantially low, then the magma is runny and therefore easily forces its way out of the interior of the earth (Lopes, 2005). The resultant lava easily flows and spreads on the surface of the earth. On the other hand, according to Lopes, if the Silica and gas content of the magma under consideration is high, the resultant volcanic activity is characterized with violent explosions (2005). However, sometimes the magma is too thick and viscous that it ends up blocking the rupture or volcanic vent, resulting in a failed volcanic activity; of course unless the pressure is in the interior of the earth is high enough to overcome the blockage, in which case the resulting explosion is very violent.
Volcanic eruptions are geologically classified into four chief phases: Hawaiian, Strombolian, Vulcanian and Peleean eruptions (Lopes, 2005). In the Hawaiian form, the magma is very low in Silica and gas; therefore, runny lava escapes out in the form of a fountain – the eruption is not explosive. In the Strombolian phase, the lava is thick and viscous but successfully escapes to the surface. However, the lava is too thick to form a defined fountain; rather, it is ejected continuously, accompanied with mild explosions. The dome, which is formed in this phase, is showered with molten drizzle by ash and lava. When the magna fails to reach the earth, and, therefore, ends up blocking the volcanic vent instead, the phase is described as Vulcanian. If, in the Vulcanian phase, the pressure builds up to a level high enough to unblock the volcanic vent. A very highly explosive eruption occurs, hurling out tons of semi-molten lava skywards, forming a cloud of vapor over the crater. More violent than the Vulcanian phase is the Pelean phase. Pelean eruptions are characterized with outbursts of explosions that generate a dense mixture of very hot rock and gas, known as pyroclastic flow. A steep sided dome is formed by the thick viscous lava. Pelean eruption typically continues for a couple of years, sometime after which the dome may collapse (Lopes, 2005).
Volcanic eruptions are often cause heavy rains. The steam, a component of a volcanic eruption, rises into the atmosphere and condenses into clouds. The clouds later fall to earth in the form of rain. The rain is usually contaminated with Carbon Dioxide, Fluorine, Radon and Hydrogen Sulfide, all of which are also products of volcanic eruption.
A consequence of the escape of material from the interior of the earth in a volcanic eruption is the formation of a void in the space initially occupied by the escaped material. This leaves the central part of the volcanic cone unsupported and, thus, the wall of the vent and the crater become susceptible to collapse. If they collapse, a caldera (a large round depression) is created across the volcano’s summit.
Features of Volcanoes
The typical notion of a volcano as a cone-shaped mountain emitting gases and lava from a crater at its peak is only one of the many variations of volcanoes. Volcanic eruptions result in a variety of features, the most common ones of them being lava domes, shield volcanoes, composite cones, cinder cones and tuyas (Greeen & Short, 2012).
Lava domes result from gradual eruptions of highly thick and viscous lava. The lava is too thick and viscous to flow significantly far away from the vent of the volcano. A lava dome looks like toothpaste protruding from the vent. Continued volcanic activity increases the mass of the protrusion. The growth of the dome exerts pressure on the surrounding crust, which, as a result, crumbles to form a hip of rock fragment around the base (Greeen & Short, 2012). Lava domes can form in craters or away from craters.
As its name implies, a shield volcano is a wide volcanic feature with a profile that resembles a shield. Shield volcanoes are formed when thin and runny lava erupts and, owing to its very low viscosity, flow a substantial distance from the vent (Greeen & Short, 2012). Shield volcanoes occur in oceans more than on continents as magma of low viscosity is characteristically low in Silica content.
Composite cones, also called Stratovolcanoes, are high conical volcanic mountains composed strata of layers of lava-flow and other ejected materials such as ash and cinders. Stratovolcanoes are formed from different structures from different volcanic eruptions of the same volcano (Greeen & Short, 2012). The lava spreads on top of ash and cinder, on which it cools and solidifies. The process recurs in the next eruption and happens again in the next eruption.
A cinder cone is a cone-shaped steep hill made of cinders, volcanic ash or volcanic clinkers that have formed around the vent of a volcano. The material that make up a cinder cone form from the breakage into small fragments and solidification of gas charged lava that has been blown into the air either in a fountain or in an explosive eruption. The slope of the cone ranges between thirty and forty degrees and most cinders cones have at their peak a crater in the form of a bowl (Green & Short, 2012).
A tuya is a flat-topped mountain that forms when the icecaps covering flat lava (the lava that flows on top of a pile of lava and palagonite) melts. The melting of the icecaps makes the overflowing lava collapse, leaving in its place a flat-topped mountain called a tuya.
Problems Caused by Volcanoes
Volcanic activity poses a threat to humans and the ecosystem around which the volcanic activity take place. A variety of materials are released into the atmosphere and onto the environment in the event of a volcanic eruption: Silica (Rhyolite and Basalt), Carbon Dioxide, Sulfides, steam, Chloride and Flourides (Sheets & Grayson, 2013). Volcanic eruptions are also associated with springs, mud-spots, fumaroles and earthquakes (Sheets & Grayson, 2013).
Gases from the interior of the earth are release into the atmosphere in a volcanic eruption. Some of the gases are Sulfur Dioxide, Carbon Dioxide, Hydrogen Fluoride, Carbon Dioxide and water vapor. Ash is also emitted alongside the gases. The addition of these gases into the atmosphere appreciably contributes to global warming (Sheets & Grayson, 2013).
The gases released into the atmosphere cause acid rain. Sulfur Dioxide, Carbon Dioxide, Hydrogen Fluoride and Hydrogen Chloride mix with water vapor in the atmosphere to form their associated acids, the result of which is the formation of acid rain (Sheets & Grayson, 2013). Release ash also falls from the atmosphere for a number of days (Sheets & Grayson, 2013). This ash can pose a threat to aviation. For example, the particle may be melted by a jet aircraft’s high temperature and end up sticking on the blades of its turbines, changing their shape and, thus, compromising the operation of the aircraft, and even potentially causing an accident. The gases and ash also cause respiratory complications to humans and animals.
Violent volcanic eruptions can cause a direct and immediate destruction to the surrounding ecosystem and even the death of humans. The lava that is usually released in an eruption is usually extremely high in temperature, high enough to kill humans and animals by just burning if not evacuated in time. Further, the lava is usually released in very high quantities and, therefore, end up falling and spreading over vast areas of land, burying the surrounding plants and animals. For instance, the violent volcanic eruption that led to the formation of the Siberian Traps is associated with the extinction of ninety percent of the species that lived at the time of its eruption (Sheets & Grayson, 2013).
References
Green, J., & Short, N. M. (Eds.). (2012). Volcanic landforms and surface features: a
photographic atlas and glossary. Springer Science & Business Media.
Lockwood, J. P., & Hazlett, R. W. (2013). Volcanoes: global perspectives. John Wiley & Sons.
Lopes, R. M. (2005). The volcano adventure guide. Cambridge University Press.
Rafferty, J. P. (2010). Plate Tectonics, Volcanoes, and Earthquakes. The Rosen Publishing
Group.
Sheets, P. D., & Grayson, D. K. (Eds.). (2013). Volcanic activity and human ecology. Elsevier.
Wallace, P. J. (2005). Volatiles in subduction zone magmas: concentrations and fluxes based on
melt inclusion and volcanic gas data. Journal of Volcanology and Geothermal Research, 140(1), 217-240.