11 Volcanoes

Dr.Sayed Zaheen Alam

A volcano is an opening or rupture in the Earth’s surface or crust, which allows hot molten magma, ash and gases to escape from deep below the surface. The term volcano in general refers to the landform built up from the accumulation of lava and pyroclastic debris. Therefore they are very different from other mountains; since they are not formed by folding and crumpling or by uplift and erosion.

Volcanoes are spectacular windows to the Earth’s internal parts. These natural windows allow us to catch a glimpse of the inaccessible depths of the surface of the Earth. Volcanic activity directly provides an explanation that the earth’s interior part must be experiencing high pressure and temperature. There are more than 500 active volcanoes on Earth’s surface, including well-known examples such as Mt. Fuji, Mt. St. Helens, and Mauna Loa.

Skeleton of a volcano

Commonly a cone shaped volcano constitutes a vent, a pipe, a crater, and a cone. The volcanic vent is an opening at the Earth’s surface through which volcanic materials are erupted at the surface of the earth. The shape of the vent may be roughly circular conduits or elongated fissures. The pipe is a passageway in the volcano in which the magma rises through to the surface during an eruption. The crater is a steep bowl-shaped depression at the top of the volcano out of which volcanic materials like, ash, lava, and other pyroclastic materials are released. The molten material beneath the surface of the Earth is called magma. Once it starts moving towards the crust or it reaches the surface, here it is referred to as lava.

Figure: Skeleton of a typical volcano.

Did You Know?

The word “volcano” comes from the little island of Vulcano in the Mediterranean Sea of Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the copy of Vulcan – the blacksmith of the Roman gods.

Why Do Volcanoes Erupt?

For volcanologist it is still a puzzling question that why do volcanoes erupts. Various hypotheses have been proposed. It is assumed that in the beginning Earth was in molten state. Then gradually it radiated its heat and cooled down. Earthquake waves suggests that

Figure: Process of Volcanic Eruption.

and core is hot which melts the rocks and creates the magma. The hot magma comes in contact with water the later becomes too hot and generate steam pressure. The high pressure inside the magma chamber caped by surrounding rock act as strong lid (like the lid of the pressure cooker) which keeps the gas dissolved within the liquid magma. The weaker areas within the crust allow magma and gases to penetrate the rocks and may come out in the form volcanic eruption. For example, when you shake and open a tightly sealed cold drink bottle then the pressure inside the bottle instantly decreases. Once that happens, the gas comes out quickly of its dissolved state and forms bubbles. A volcano may erupt in a similar way, though obviously with much more power and explosion. Before the eruption volcanoes may also give early warning signals by producing a rushing, a hissing and a coughing and puffing sounds. However it is still difficult to predict the volcanic eruption. Nevertheless scientists in order to predict the volcanic eruption monitor seismic activity in volcanic are, the geochemical conditions, change in temperature, topographical change and change in gas composition. The major human adjustment to volcanic activity is evacuation. Education and awareness plays an important role in informing people about the hazards of volcanoes.

Did You Know?`

Sometimes high-viscosity magma comes out from the pipe like toothpaste being squeezed out of a tube. The volcanologists sometimes use the term “toothpaste lava” to denote this eruption.

Distribution of volcanoes and volcanic activity

Most of the world’s volcanoes and volcanic activity can be sighted along the plate boundaries.

The distribution can be classified into one of the following tectonic settings:

(1) Subduction Zones in the Circum Pacific Belt

The zones where one plate goes down under the other due to density difference are the sites of most of the world’s active and explosive volcanoes. The oceanic plate having higher density is subducted under the continental crust. The subducted slab melts under the increasing pressure and temperature to produce magma which comes out through andesitic chain of volcanoes. The volcanoes are mainly situated on the continental side of the trenches.

The figure portrays that the so called “Pacific Ring of Fire” is the collection of volcanoes bordering the Pacific Ocean. This zone is infact a ring of subduction zones. It includes some of the deadliest volcanoes known, such as Pinatubo and Mt. St. Helens. The figure demonstrates that it starts from the Andean region of South America and extends northwards through Central America, Mexico, Western U.S.A. and Canada to Alaska. From Alaska it extends through Aleutian Islands towards the islands off the eastern coast of Asia and passes through Kamchatka, the Kurile Islands, Japan, the Philippines and further south to the New Guinea, Solomon Islands, New Zealand and Antarctica. The volcanic belt of the Indian Ocean which passes through Andamans, Sumatra, Java, Bali, Sunda and Burma meets the Pacific belt near the Malacca Island.

Figure: Pacific Ring of Fire.

(2) Divergence zones: volcanoes of the Mid Atlantic Ridge and over the Continents

In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. For example the Mid-Atlantic Ridge separates the North and South American Plate from the Eurasian and African Plate.

The figure demonstrates that, this pulling apart is causing “sea-floor spreading” as new volcanic material is added to the oceanic plates. The spreading sites are the common sites of basaltic lava eruption. On the whole, sea-floor spreading is basically volcanic, but it is a very slow and regular process, without the explosive outbursts of the volcanoes on land. Magma rises through the cracks and leaks out onto the ocean floor like a long, thin, undersea volcano. As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. This process along the divergent boundary has created longest topographic feature in the form of Mid oceanic ridges under the Oceans of the world Most of this activity is out of sight under the oceans which is less hazardous to people.

Figure: Volcanoes of the Mid Atlantic Ridge in divergent zones.

The map shows that over the continents, the divergence zones with fissure type of volcanic eruptions are represented by the East African Rift Valley Zone. This belt extends from Ethiopia to Tanzania. The Kilimanjaro in Tanzania is a well known example of this belt.

Figure: East African Rift Valley Zone.

(3) Intra-plate oceanic volcanism (Hawaiian chain and other oceanic volcanic seamounts)

Intra-plate oceanic volcanism can be represented by a single oceanic volcano, or lines of volcanoes such as the Hawaiian-Emperor seamount chains. They are also popular as hotspots and are located within the tectonic plates instead of plate margins. The map demonstrate that the Hawaiian volcanoes are located well within the Pacific plate rather than near a plate boundary.

Figure: Location of Hawaiian volcanoes within the Pacific plate.

(4) Mid Continental belt and Volcanoes in the Mediterranean region:

This belt is extended from the Mediterranean Alps to the Himalayan Region. Most often visited active volcanoes are found in this belt. Vesuvius and Stromboli are well known example of this belt. Mount Etna in Sicily is Europe’s largest volcano. Its frequent eruptions often attract visitors.

Figure: Etna in Sicily, Europe’s largest volcano.

Products of volcanic eruption

The products of volcanic eruption may be in the form of gas, solid and liquid. They are as follows:

Gases

It is assumed that interactions between the gases contained in molten magma increase pressure and heat within the ascending lavas which results in explosive activity. Therefore, the most explosive eruptions are probably the consequence of gas-rich magmas. Major part of gases consists of hydrogen (H), carbon (C), oxygen (O), sulfur (S) and nitrogen (N). The hydrogen is largely present as water (H2O). Volcanoes also emit methane (CH4) and ammonia (NH3) but in trace amounts. They also have an important effect on the regional and global environment and may contribute greenhouse gases to the atmosphere.

Do You Know?

Volcanologists also use typical gas mask near volcanoes to safe their own life from the hazardous effect of volcanic gases.

Solid

When fragments are blown out by explosive eruptions, soild materials are ejected, known generally as pyroclastic materials or pyroclasts (Pyro = Fire Clast = Fragments). The term tephra covers all the pyroclastic debris that collects through vertical airfall. The finest of these is the dust which can remain suspended in the air for long time and can travel long distances. The size of pyroclasts may be less than 2 millimeters to pieces that weigh several tons.

The horizontal bar diagram shows different types of solid material which comes out from the volcanoes: Very fine dust and sand-sized volcanic ash (2 mm across). If the ash is found in compact form it is known as volcanic tuffs. The material of intermediate size (2 to 64 n mm across) is called Lapilli or little stones which is almost the size of walnuts. The largest pyroclasts (more than 64 mm across) having rounded masses are known as volcanic bombs.

Figure: Relative Size of Solid Volcanic material.

Did you know?

In context to volcano the term ‘Ash’ is a misnomer since it is not the outcome of combustion, but composed of fine particles of lava.

Liquid

The most important product of volcanic eruption is the lava. The lava is the molten magma which reaches the surface. The nature of lava also determines the form of volcanic cone. The lava is classified into three types on the basis of proportion of silica content: basaltic (or basic or mafic), andesitic (or intermediate), and rhyolitic (or silic). The Figure shows that the basaltic lava has less than 50 per cent silica, andesitic has 55 to 65 per cent Silica and the rhyolite contains more than 65 per cent silica. The figure also demonstrates that with the increase in silica in the lava the viscosity of lava also increases.

Figure: Classification of Lava on the Basis Silica Content.

When Lava solidifies it may take a number of forms, some of which have been given Hawaiian names. For example pahoeho type and aa (Pronounced as ah-ah) type. The figure shows that the surface of the former is smooth, wrinkled and resembles coils of rope. In the latter the lava surface appears angular, jagged and blocky. The aa type of lava is the result of gases escaping violently from within the lava and the effects of the drag of the still molten material under the resisting crust. The draining of lava from beneath the solidified crust may give shape to lava tunnels and lava caves.

Types of volcanoes

Volcanologists have developed many schemes to categorize volcanoes into various types. On the basis of Periodicity of Eruption, Nature of Eruption and shape and morphology of volcanoes, the volcanoes can be classified into various types. The most popular classification is based on periodicity of Eruption i.e. active, dormant and dead volcano.

(A) Basis: Periodicity of Eruption

(i) Active volcano- A volcano is generally considered active if it has erupted within recent history. It is generally regarded as active so long as the magma reservoir is present. The map shows location of the recently active volcanoes of the world. It can be observed from the map that most of them are located in the so called “Pacific Ring of Fire belt”. St. Mount Helens in Cascade Range of United States is a well known example of this category. The catastrophic eruption of Mount St. Helens on May 18, 1980, and related mudflows and flooding caused significant loss of life and property damage (USGS).

Figure: Distribution of Recently Active Volcanoes of the World.

(ii) Dormant Volcano- When the volcano has not erupted in recent times but is fresh looking is regarded as dormant. A dormant volcano exhibits no indication for future eruption but they may erupt suddenly and violently causing enormous damage to life and property. Vesuvius volcano is a classic example of this category which erupted in 79 A.D and destroyed the Roman cities of Pompeii and Herculaneum. It again erupted in 1631 A.D. the frequency of eruption again increased in 19th century.

(iii) Extinct- A volcano is considered extinct when it has no recent eruptive history. Impact of erosion can be seen on this type of volcano. They are unlikely to erupt again. For example, Arthur’s Seat in Scotland.

Points to Ponder

It may be hazardous to classify any volcano as dead or dormant volcano, since it may erupt without any indication

(B) Basis: Nature of Eruption

The most commonly used classification by volcanologist is that originally proposed by Lacroix in 1908. There are four principle types of eruptions i.e., Hawaiian, Strombolian, Vulcanian and Plinian. The figure demonstrates that degree of explosiveness, height of eruption and viscosity of magma increases from Hawaiian to Plinian type of volcano.

Figure: Types of Volcanoes on the Basis of Nature of Eruption.

On the basis of shape and morphology volcanoes can be clasiified into shield, Cone and Composite cones, or stratovolcanoes.

(i) Shield volcanoes:

The basaltic lavas are comparatively fluid and spread quietly, easily and widely. The figure shows that the gradual build-up of thousands of these flows slowly constructs broad flat shield volcano. They are named as shield volcanoes because they are shaped like a warrior’s shield. The classic example of a shield volcano is Mauna Loa on Hawaii.

Figure: The internal structure of a typical shield volcano.

Figure: Hawaii shield volcanoes. (Image credit: ©Google earth 2015).

(ii) Cone volcanoes or Cinder Cones

Cinder cones are the most abundant of all volcanoes. Small cones consisting mostly of pyroclastic debris each having a single vent are called cinder cones. When pyroclastic fragments fall and accumulate close to the vent, they may pile up to form a very symmetric cinder cone. The figure shows a classic example of Cinder cones on Lanzarote Island of Canary Islands.

Figure: Cinder cones on Lanzarote Island, Canary Islands, Spain (Image credit: ©Google earth 2015).

(iii) Composite cones, or stratovolcanoes

Figure: Composite cones, or stratovolcanoes

Sometimes pyroclastic material either flows through break in the crater wall or comes out from edges of the cone which forms composite volcanoes or alternatively, stratovolcanoes. The figure shows that they are known as composite volcanoes because they are built up of layers of more than one kind of material. The mix of lava and pyroclastics allows them to grow larger than either cinder cones or volcanic domes. The composite volcanoes have a crater at the summit which generally contains a central vent or a clustered group of vents. Some of the most beautiful volcanic mountains in the world are composite volcanoes, including Mount Cotopaxi in Ecuador and Mount St. Helens in U.S.A.

Features formed

Volcanoes have shaped the earth’s crust since its existence, producing dramatic volcanic cones, volcanic domes, volcanic plateaus, fertile farmlands, islands, Craters and collapsing Calderas on the surface of the Earth. The volcanic activity also creates unique intrusive landforms in the form of Batholiths, Dyke, sills, Laccolith , Lapolith and Phacolith. Some minor features such as Hot Spring, Geysers and Fumaroles are also sites of tourist’s attraction. The basic extrusive and Intrusive volcanic features are as follows:

(A) Extrusive Feature

(i) Craters and Calderas

The crater is a bowl or funnel-shaped depression or cavity usually of volcanic origin. It is usually more or less circular in the plan at the summit of the volcanic mountain. The diameter of crater is commonly less than 1.6 kilometres. Craters may result from either explosive activity or from subsidence. The figure exhibits an areal view of a crater at the summit of Mt St. Helens (USA). It should be noted that craters may also form due to impact of meteorites and mining process but in this case it will not be associated with volcanic activity.

The huge carter-like depression is called Caldera. The diameter of a caldera is usually several times that of a crater. The figure demonstrates the formation of caldera due repeated volcanic eruption. The caldera may also form due to coalescence of several small craters. The

The collapsing of summit of the volcano due to development of underground cavity may also form caldera. The Figure shows formation of caldera and caldera lake on the on the northern Kuriles Islands of Russia. It exhibits that the roof of former volcano has collapsed and created a caldera in which a younger volcano starts to form from the old vent within the caldera lake. In the world the Buldir caldera between the islands of the Aleutian island chain is the largest known caldera.

Figure: On the northern Kuriles Islands (Russia) a roof of former volcano collapsed and created a caldera in which a younger volcano starts to form from the old vent within the caldera lake (Image credit: ©Google earth 2015).

(ii) Volcanic Plateaus and Plains

On the surface of the Earth many extensive fairly leveled lava plateaus have been built by fissure eruption. They have completely covered up the surrounding region. The figure shows that the Deccan lava plateau covers almost 6, 50,000 square kilometers of India’s geographical area.

Figure: Deccan lava plateau.

The basaltic plains in comparison to lava plateau contain much thinner and less extensive accumulation of lava sheets. They are common throughout the world.

(B) Intrusive Features

Intrusive igneous landforms are formed due to cooling and crystallization of magmas beneath the surface. The intrusive landforms can be exposed at the earth’s surface by erosion of overlying rock. Therefore they provide important information about internal structure earth. Some of the important intrusive volcanic landforms forms are as follows:

(i) Batholiths

A batholith (from Greek bathos, depth + lithos, rock) represents huge mass of intrusive (plutonic) igneous rock which accumulates in the crust. It forms the root or heart of the volcanic mountain. The batholiths may be up to 100 kilometers wide and are exposed at the surface only after considerable denudation of the overlying mass. The figure shows that similar to batholiths intrusions but smaller in size are bosses or stocks. The northern part of isle of Arran (UK) contains the conspicuous example of stock.

(ii) Dyke

The figure demonstrates that dyke is a igneous intrusion that cuts across the bedding of the country rock through near vertical fissures. Hundreds of parallel dykes can be traced in North-Western Scotland, especially in the Islands of Mull and Arran. The denudation processes can expose the comparatively harder Dyke to from ‘walls’ or Cliff across the beaches. Sometimes a zone of dykes may surround a circular or dome-shaped intrusion in more or less arcuate from; these are known as ring-dykes (Monkhouse, 1970).

Figure: Dyke.

(iii) Sills

The figure shows that sill is the tabular or sheet-like intrusive body formed when magma is injected along sedimentary bedding surfaces. They are usually formed from low viscosity magma. The Great Whin Sill situated in Great Britain is a well known example.

Figure: Sill.

(iv) Laccoliths

The figure shows that in this type magma collects as a mushroom-shaped mass that arches the overlying strata upward. Laccoliths are formed due to injection of magma along the bedding planes of the horizontally bedded sedimentary rocks. They cover large areas, The Karnataka plateau is spotted with domal hills of granite rocks. Most of these, now exfoliated, are examples of laccoliths or batholiths.

Figure: Laccolith.

(c) Minor Volcanic Features

Geysers and Hot Spring

Sometimes water at depth is heated beyond 100° C or superheated by a body of magma or hot rock. The water may come out with great force along with the mixture of steam and diluted minerals. Such sites in the volcanic region are called geysers. The word geyser comes from the Icelandic word meaning “to gush”. The Figure shows that the ‘Old Faithful’ geyser in Yellowstone National Park, U.S.A is one of the most famous geysers in the world which erupts faithfully at regular interval of about 65 minutes or so. This geyser shoots water between 30 to 50 metres into the air. Minerals dissolved in the hot water are often precipitated around the vents because temperature and pressure drop suddenly as water or steam enters the atmosphere. This produces spectacular deposits of travertine (chemically precipitated calcium carbonate and other minerals). The heated water may also flow quietly generally along the fault zones in the form of hot spring. Such hot springs are quite common in volcanic region.

Thank You

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References

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Useful Websites

http://pubs.usgs.gov/gip/volc/types.html
http://pubs.usgs.gov/gip/volc/intro.html