Environmental Geology-Volcanoes
The name 'Volcano' is given after a small island near Sicily which was named by the ancient Romans. The term volcano is derived from 'Vulcan', the Roman God of fire and metalwork. The study of volcanoes is the significant in the understanding of the earth's interior. Volcanoes emit magma and gases and the volcanic eruption process brings out molten rock material from inside the earth to the earth's surface. These natural processes bring about changes in the earth's landscape. The geologic processes that give rise to volcanoes and volcanic rocks are collectively known as volcanism. Majority of the volcanoes are dormant and there are approximately 1500 active volcanoes. Volcanoes can be explosive and non-explosive. There are other volcanoes that alternate between quiet and explosive, i.e. the volcano at Stromboli, near Sicily. The magma composition is an important factor. Siliceous or felsic magma is more viscous and volatile rich in comparison to magma with less silica, i.e mafic. Hence they erupt explosively when compared to the mafic magmas. Scientists estimate that at least 500 million of the total world population is at risk from volcanoes. Millions of people are vulnerable to the effects of dangerous eruptions. Therefore, it is important to improve our understanding of the volcanoes and how they work. Study of volcanos is known as volcanology and scientist studying volcanoes is called volcanologist.
Let us now learn some definitions of volcanoes and their types.
5.2 Definitions:
Volcano: Volcano may be defined as any landform that releases lava, gas or ashes or has done so in the past. A volcano is a vent or chimney and its surrounding accumulations of volcanic material, which connects a pool of molten rock material (magma) below the Earth's surface with the surface. Volcano may appear on the surface through some fissure called fissure type or through a single opening where it assumes shape of a cone called cone or crater type. Volcanoes are often classified by their recent history of activity as active, dormant or extinct.
Active: A volcano is active when it is erupting intermittently or continuously. Active volcanoes are those that have recently erupted or those that show current volcanic activity, such as the eruption of lava, release of gas or seismic activity i.e. Mt. Helens in the state of Washington.
Dormant: A dormant volcano has not erupted for a long time but can erupt again. These are the volcanoes which show eruptions with a lapse of considerable period.
Extinct: An extinct volcano is that which will never erupt again. Extinct volcanoes show no signs of activity and have not erupted for long periods of time. These volcanoes may now only be the remnants of lava flows or the central volcanic pipe.
Volcanism: It 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 erun through a break in the surface called a vent.
5.3 DISTRIBUTION OF VOLCANOES LCANO
Active: A volcano is active when it is erupting intermittently or continuously. Active volcanoes are those that have recently erupted or those that show current volcanic activity, such as the eruption of lava, release of gas or seismic activity i.e. Mt. Helens in the state of Washington.
Dormant: A dormant volcano has not erupted for a long time but can erupt again. These are the volcanoes which show eruptions with a lapse of considerable period.
Extinct: An extinct volcano is that which will never erupt again. Extinct volcanoes show no signs of activity and have not erupted for long periods of time. These volcanoes may now only be the remnants of lava flows or the central volcanic pipe.
Volcanism: It 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 through a break in the surface called a vent
5.3 DISTRIBUTION OF VOLCANOES
It is important to understand the distribution of volcanoes Raend volcanoes and earthquakes are not randomly distributed; but they tend to be concentrated along limited zones along the plate boundaries (Figure 5.1). According to the theory of plate tectonics, the Earth's outer shell (lithosphere) is made up of seven large and many smaller moving plates. When the plates move, their boundaries can collide, diverge from each other or slide past each other, resulting in geological processes such as earthquakes, volcanoes and mountain formations. Volcanoes normally occur around certain specific places of the earth's crust. They occur in the following areas:
➤ Along subduction boundaries at continental plate-oceanic plate convergence such as at Mt Saint Helens
➤ Along oceanic plate oceanic plate convergence such as at Japan and Philippines
➤ Ring of Fire surrounding Pacific Ocean
➤ Hot spots such as the Hawaiian Islands mantle plumes
➤ Spreading centers mid-ocean ridges such as in Iceland, rift valleys such as at Mt. Kilamanjaro, Africa
5.4 DIFERENT PARTS OF A VOLCANO
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A typical volcano has different parts. Typically, volcanoes are openings in the earth's surface from which molten rock; magma and gases escape. The main parts include the following (Figure 5.2):
1. Vent: It is an opening that serves as an outlet for air, smoke and fumes. It is a pipe-like conduit connecting the magma chamber to the surface.
2. Crater: It is a depression or a bowl-shaped geological formation at the summit of a volcano. It is connected by a vent or pipe to the magma chamber below.
3. Fumaroles: They are secondary vents on the flank of a volcano which emit steam and other
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4. Throat: Entrance of a volcano. The part of the conduit that ejects lava and volcanic ash.
5. Conduit: It is an underground passage through which the magma flows.
6. Magma Chamber: It is a reservoir of magma in the earth's crust where the magma may reside temporarily on its way from the upper mantle to the earth's surface.
7. Caldera: It is a crater more than 1 km in diameter, formed at the summit of a volcano when lava is drained from an underground magma chamber, causing the summit of the volcano to be unsupported, and to collapse.
8. Pit crater: They are collapse features on the flanks or summit of a volcano that are smaller than the main caldera at the summit of a volcano.
9. Lava Flow: This is sheet of molten or solidified lava.
10. Sill: This is an intrusive body of magma that pushes its way between layers of sediments.
11. Ash Cloud: It is a cloud of volcanic gases, ash and dust that form during an explosive volcanic eruption.
12. Secondary Vent: It is vent that connects to the secondary cone for the lava to flow.
When fractures occur within the earth's crust, then the pressure on the magma becomes less, and the magma is pushed up to the surface through the crack. When magma reaches the earth's surface it is referred to as lava. Successive eruption of hot, molten matter forms a cone-shaped mountain of lava. The process produces igneous rocks but may also help form metamorphic rocks due to the heat along with the eruptions. The viscosity of the magma is controlled by its acidic, intermediate or the basic nature. If the lava is acidic and rich in silica, it will harden and set quickly. Such volcanoes erupt violently and are called Composite Volcanoes. Some examples are Mount Etna in Sicily, Mount St. Helens in the USA and Mount Vesuvius in Italy. Basic lava which is low in silica but rich in iron content flows freely and sets slowly. Such volcanoes are called Shield Volcanoes. Some examples are those in Hawaii. Stratovolcanoes are built over many thousands of years after dozens of eruptions. Volcanic eruptions result in bringing new rock material from the interior of the earth to the earth's surface. These rocks help in the formations of mountains and islands. Further, the gases and volatiles emitted have contributed to the earth's atmosphere and oceans.
5.6 KINDS OF VOLCANIC ERUPTIONS
As discussed earlier in the chapter, volcanic eruptions can be of two types: explosive and non-explosive eruptions. Let us now learn about this in the following paragraphs.
1. Explosive Eruptions: They occur due the presence of to high gas content or volatiles in highly
viscous magmas. During these eruptions magma is fragmented into clots of liquid that cools down and become sand-sized particles. These solid particles become pyroclasts and tephra or volcanic ash. Other fragments released include blocks and bombs named according to the angular and aerodynamic shapes. Lapilli are those that consist of mostly gas bubbles. They can also be released resulting in a low density highly vesicular rock fragment called pumice. Volcanic gas clouds and ash can rise up to 45 km into the atmosphere.
2. Non-explosive Eruptions: These eruptions occur when the magmas are less viscous and contain less volatile matter. When the viscosity is low these eruptions start with fire fountains as a result of release of dissolved gases. When the gas content is low and the viscosity is high then lava will produce a volcanic dome. Lava flows result on the surface of earth. When the volcanic eruption is submarine then pillow lavas are produced.
Magma composition, temperature and viscosity
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Viscosity is the resistance to flow and it depends primarily on the composition of the magma and its temperature. This parameter is of utmost importance in determining the eruptive behaviour of magmas. The magmas with higher silica content have higher viscosity than those with lower silica contents. Granitic or sialic magma is more viscous than basaltic or mafic magma. The shapes of volcanoes also depend on the viscosity of the magma. For example, basaltic lava forms flat shield volcanoes. Gases are not easily released from viscous magmas. Here, the pressure builds up and leads to violent explosions. These are typical of strato-volcanoes.
Characteristics of Magma: They control the kind of eruption that would take place.
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a) Magma types: Types of magma are determined by the chemical composition of the magma and three broad general types are recognized:
Basaltic magma contain SiO2 45-55 wt%, are high in Fe, Mg, Ca, and low in K, Na
Andesitic magma contain SiO2 55-65 wt%, and are intermediate in Fe, Mg, Ca, Na, K
Rhyolitic magma contain SiO2 65-75%, are low in Fe, Mg, Ca, and are high in K, Na
b) Volatile content in Magmas: The explosive character of magmas is due to the presence of gases.
In the Earth, at depths, all magmas contain gas dissolved in the liquid form. When the magma rises to the surface of the Earth, the gas forms a separate vapour phase due to decrease in pressure. The composition of the gases in magma include: mostly water vapour, carbon dioxide, trace amounts of sulfur, chlorine, and fluorine. The felsic or rhyolitic magmas have higher gas contents as compared to the mafic or basaltic magmas. This is because the amount of gas present in magma is related to the chemical composition of the magma. For example, basaltic lava forms flat shield volcanoes. Gases are not easily released from viscous magmas. Here, the pressure the pressur builds up and leads to violent explosions. These are typical of strato-volcanoes.
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c) Magma temperature: They are
observations and scientific can be different. are characterized kewaysureme by different temperatures. Geological field suggest that the eruption temperature of various magmas
basaltic magma can range between 1000 to 1200°C;
andesitic magma can range between 800 to 1000°C; and
rhyolitic magma can range between 650 to 800°C.
5.8 PRODUCTS OF VOLCANIC ERUPTIONS
The products of volcanic eruptions include the following: Lava flows, pyroclastic debris, ash, dust, gases and Nuee ardentes. In addition to the gases that come out of volcanoes, large amounts of solid materials are also ejected. The smaller particles of dust reach the highest levels in the atmosphere. For strong vertical eruptions, dust can be propelled up into the stratosphere, above 17 kilometers.
such heights, the suspended dust particles can block solar radiation, effectively by heating up the stratosphere while the lower troposphere cools causing short term effects.
1) Solid: The solid material of different sizes is ejected out of volcano during eruptions. They are called as 'pyroclastic material'. These materials of varying sizes and grades are known differently based on their size, viz. Volcanic bombs, Lapilli or cinder, Volcanic Ash and volcanic dust.
2) Liquid: The most important product of volcanic eruption is magma It is molten rock material consisting of mineral melt. It is. On average the temperature of magma ranges between 1000° and 1200° C. Molten rock material below the surface is called 'magma and when it comes on the surface it is called 'Lava'. It can be felsic (viscous) or basic (less viscous or fluid).
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3) Gases: Water vapours form one of the most important part constituting of the total volcanic gases. These water vapours may come from the original magma or due to boiling of underground water in the volcanic area. Volcanic gas samples contain water (70%), carbon dioxide (15%) and Nitrogen (5%) along with sulphur dioxide, hydrogen sulfide, carbon monoxide, hydrogen. hydrochloric acid, and methane, chlorine and argon 5.9 HAZARDS ASSOCIATED WITH VOLCANOES VOLCANOES
Do you think there can be hazards accompanied with volcanic eruptions? Yes, there are. So volcanic hazards are classified into primary and secondary hazards:
1. Primary volcanic hazards
Lava flows are usually not very dangerous. They move slowly and can damage property. Again there can be a few exceptions. For example, at Zaire in 1977 the Nyiragongo volcano had a side fissure which drained the lava lake in the main crater very rapidly. It resulted in the lava squirting out at 60 miles per hour. This volcanic activity caused approximately 1000 fatalities.
Violent Eruptions and Pyroclastic Activity: They are the most dangerous as they can travel rapidly. Hot pyroclastic flows result in suffocation, burns and death.
Ash falls: They characterized by mud ash and gases. An example is the 79 AD eruption of Mt. Vesuvius causing about 20,000 fatalities.
Nuee Ardente: They are fast moving flows of gas and magma, in a volcanic avalanche. These can be deadly due to fast flows and destructions. An example to cite is the 1902 eruption of La Soufriere, St. Vincent, which killed 1500. Nuce ardentes consist of clouds of volcanic gases and pyroclastic debris that avalanche down the side of a volcano. They can reach speeds of almost 200 km/hr. A nuce ardente from Mount Pelee, on the Caribbean island of Martinique, killed 28,000 inhabitants and destroyed the town of St. Pierre in 1902.
Gas emissions: They accompany every volcano. Some of the gases include: Hydrogen chloride, Hydrogen sulfide, Hydrogen fluoride, and Carbon dioxide. The eruption that occurred at Iceland in 1783 at Mt. Laki had a massive flux of sulfuric and fluorine gas, which killed great numbers of livestock. This resulted in a famine leading to 10,000 fatalities.
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