Essay on Volcanoes: Top 7 Essays on Volcanoes| Disasters | Geography

Here is a compilation of essays on ‘Volcanoes’ for class 7, 8, 9, 10. Find paragraphs, long and short essays on ‘Volcanoes’ especially written for school students.

Essay on Volcanoes

Essay Contents:

1. Essay on the Concept of Vulcanicity
2. Essay on the Components of Vulcanicity
3. Essay on the Types of Volcanoes
4. Essay on the Mechanisms and Causes of Vulcanism
5. Essay on the Hazardous Effects of Volcanic Eruptions
6. Essay on Volcanic Materials
7. Essay on the World Distribution of Volcanoes

Essay # 1. Concept of Vulcanicity:

The terms volcanoes, mechanism of volcanoes and vulcanicity are more or less synonymous to com­mon man but these have different connotations in geology and geography. ‘A volcano is a vent, or opening, usually circular or nearly circular in form, through which heated materials consisting of gases, water, liquid lava and fragments of rocks are ejected from the highly heated interior to the surface of the earth’.

According to A. Holmes and D.L. Holmes (1978) a volcano is essentially a fissure or vent, communicating with the interior, from which flows of lava, fountains of incandescent spray or explosive bursts of gases and volcanic ashes are erupted at the surface.

On the other hand, ‘the term vulcanicity covers all those processes in which molten rock mate­rial or magma rises into the crust or is poured out on its surface, there to solidify as a crystalline or semicrystaline rock’.

Some scientists have also used the term of vulcanism as synonym to the term of vulcanicity. For example, P.G. Worcester (1948) has maintained that ‘vulcanism includes all phenomena connected with the movement of heated material from the interior to or towards the surface of the earth.’

It is apparent from the above definitions of volcano and vulcanicity (vulcanism) that the later (vulcanicity) is a broader mechanism which is related to both the environments, endogenetic and exogenetic. In other words, vulcanicity includes all those processes and mechanisms which are related to the origin of magmas, gases and vapour, their ascent and appear­ance on the earth’s surface in various forms.

It is evident that the vulcanicity has two components which operate below the crustal surface and above the crust. The endogenetic mechanism of vulcanicity includes the creation of hot and liquid magmas and gases in the mantle and the crust, their expansion and upward ascent, their intrusion, cooling and solidification in various forms below the crustal surface (e.g., batholiths, laccoliths, sills, dykes, lopoliths, phacoliths etc.) while the exogenous mechanism includes the process of appearance of lava, volcanic dusts and ashes, fragmen­tal material, mud smoke etc. in different forms e.g., fissure flow or lava flood (fissure or quiet type of volcanic eruption), violent explosion (central type of volcanic eruption), hot springs, geysers, fumaroles, solfatara, mud volcanoes etc. It may be, thus, con­cluded that the vulcanicity is a broader mechanism which includes several events and processes which work below the crust as well as above the crust whereas volcano is a part of vulcanicity (vulcanism).

Essay # 2. Components of Volcanoes:

Volcanoes of explosive type or central eruption type are associated with the accumulated volcanic materials in the form of cones which are called as volcanic cones or simply volcanic mountains. There is a vent or opening, of circular or nearly circular shape, almost in the centre of the summital part of the cone.

This vent is called as volcanic vent or volcanic mouth which is connected with the interior part of the earth by a narrow pipe, which is called as volcanic pipe. Vol­canic materials of various sorts are ejected through this pipe and the vent situated at the top of the pipe. The enlarged form of the volcanic vent is known as volcanic crater and caldera. Volcanic materials include lavas, volcanic dusts and ashes, fragmental materials etc. (fig. 9.1).

Essay # 3. Types of Volcanoes:

There is a wide range of variations in the mode of volcanic eruptions and their periodicity.

Thus, vocanoes are classified on the basis of:

(i) The mode of eruption, and

(ii) The period of eruption and the nature of their activities.

(i) Classification on the Basis of the Nature of Volcanic Eruptions:

Volcanic eruptions occur mostly in two ways viz.:

(i) Violent and explosive type of eruption of lavas, volcanic dusts, volcanic ashes and fragmental materi­als through a narrow pipe and small opening under the impact of violent gases, and

(ii) Quiet type or fissure eruption along a long fracture or fissure or fault due to weak gases and huge volume of lavas.

Thus, on the basis of the nature and intensity of eruptions volcanoes are divided into two types e.g.:

(1) Central eruption type or explosive eruption type, and

(2) Fissure eruption type or quiet eruption type.

(1) Volcanoes of central eruption type:

Central eruption type or explosive eruption type of volcanoes occurs through a central pipe and small opening by breaking and blowing off crustal surface due to violent and explosive gases accumulated deep within the earth. The eruption is so rapid and violent that huge quantity of volcanic materials consisting of lavas, volcanic dusts and ashes, fragmental materials etc., are ejected upto thousands of metres in the sky.

These materials after falling down accumulate around the volcanic vent and form volcanic cones of various sorts. Such volcanoes are very destructive and are disastrous natural hazards.

Explosive volcances are further divided into 5 sub-types on the basis of difference in the intensity of eruption, variations in the ejected volcanic material and the period of the action of volcanic events as given below:

(i) Hawaiin type of volcanoes:

Such volcanoes erupt quietly due to less viscous lavas and non-violent nature of gases. Rounded blisters of hot and glowing mass/boll of lavas (blebs of molten lava) when caught by a strong wind glide in the air like red and glowing hairs. The Hawaiin people consider these long glassy threads of red molten lava as Pele’s hair (Pele is the Hawaiin goddess of fire).

Such volcanoes have been named as Hawaiin type because of the fact that such eruptions are of very common occurrence on Hawaii island. The eruption of Kilavea volcano of the southern Hawaii island in 1959-60 continued for seven days (from November 14 to 20, 1959) when about 30 mil­lion cubic metres of lavas poured out.

The intermittent eruptions continued upto December 21, 1959, when the volcano became dormant. It again erupted on January 13, 1960 and about 100 million cubic metres of lavas were poured out of one kilometre long fissure.

(ii) Strombolian type of volcanoes:

Such volca-noes, named after Stromboli volcano of Lipari island in the Mediterranean Sea, erupt with moderate inten­sity. Besides lava, other volcanic materials like pum­ice, scoria, bombs etc. are also ejected upto greater height in the sky. These materials again fall down in the volcanic craters. The eruptions are almost rhythemic or nearly continuous in nature but sometimes they are interrupted by long intervals.

(iii) Vulcanian type of volcanoes:

These are named after Vulcano of Lipari island in the Mediterranean Sea. Such volcanoes erupt with great force and inten­sity. The lavas are so viscous and pasty that these are quickly solidified and hardened between two eruptions and thus they crust over (plug) the volcanic vents.

These lava crusts obstruct the escape of violent gases during next eruption. Consequently, the violent gases break and shatter the lava crusts into angular fragments and appear in the sky as ash-laden volcanic clouds of dark and often black colour assuming a convoluted or cauli­flower shape (fig. 9.2c).

(iv) Peleean type of volcanoes:

These are named after the Pelee volcano of Martinique Island in the Caribbean Sea. These are the most violent and most explosive type of volcanoes. The ejected lavas are most viscous and pasty. Obstructive domes of lava are formed above the conduits of the volcanoes. Thus, every successive eruption has to blow off these lava domes. Consequently, each successive eruption oc­curs with greater force and intensity making roaring noise.

The most disastrous volcanic eruption of Mount Pelee on May 8,1902 destroyed the whole of the town of St. Pierre killing all the 28,000 inhabitants leaving behind only two survivors to mourn the sad demise of their brethren. Such type of disastrous violent erup­tions are named as nuee ardente meaning thereby ‘glowing cloud’ of hot gases, lavas etc., coming out of a vocanic eruption.

The nuee ardente spread laterally out of the mountain (Mount Pelee) with great speed which caused disastrous avalanches on the hillslopes which plunged down the slope at a speed of about 100 kilometres per hour. The annihilating explosive erup­tion of Krakatoa volcano in 1883 in Krakatoa Island located in Sunda Strait between Java and Sumatra is another example of violent volcanic eruption of this type.

(v) Visuvious type of volcanoes:

These are more or less similar to Vulcanian and Strombolian type of volcanoes, the difference lies only in the intensity of expulsion of lavas and gases. There is extremely vio­lent expulsion of magma due to enormous volume of explosive gases.

Volcanic materials are thrown up to greater height in the sky. The ejected enormous vol­ume of gases and ashes forms thick clouds of ‘cauli­flower form.’ The most destructive type of eruption is called as Plinian type because of the fact that such type of eruption was first observed by Plini in 79 A.D.

(2) Fissure eruption type of volcanoes:

Such vol­canoes occur along a long fracture, fault and fissure and there is slow upwelling of magma from below and the resultant lavas spread over the ground surface. The speed of lava movement depends on the nature of magma, volume of magma, slope of ground surface and temperature conditions. The Laki fissure eruption of 1783 in Iceland was so quick and enormous that huge volume of lavas measuring about 15 cubic kilometers was poured out from a 28-km long fissure. The lava flow was so enormous that it travelled a distance of 350 kilometres.

(ii) Classification on the Basis of Periodicity of Erup­tions:

Volcanoes are divided into 3 types on the basis of period of eruption and interval period between two eruptions of a volcano e.g.:

(i) Active volcanoes,

(ii) Dormant volcanoes, and

(iii) Extinct volcanoes.

(i) Active Volcanoes:

Active volcanoes are those which constantly eject volcanic lavas, gases, ashes and fragmental ma­terials. It is estimated that there are about more than 500 volcanoes in the world. Etna and Stromboli of the Mediterranean Sea are the most significant examples of this category. Stromboli Volcano is known as Light House of the Mediterranean because of continuous emission of burning and luminous incandescent gases.

Most of the active volcanoes are found along the mid- oceanic ridges representing divergent plate margins (constructive plate margins) and convergent plate margins (destructive plate margins represented by eastern and western margins of the Pacific Ocean). The latest eruption took place from Pinatubo volcano in June 1991 in Philippines. Mayon of Philippines re-erupted in Feb. 2000.

(ii) Dormant Volcanoes:

Dormant volcanoes are those which become quiet after their eruptions for some time and there are no indications for future eruptions but suddenly they erupt very violently and cause enormous damage to human health and wealth.

Visuvious volcano is the best example of dormant volcano which erupted first in 79 A.D., then it kept quiet upto 1631 A.D., when it suddenly exploded with great force. The subsequent eruptions occurred in 1803, 1872, 1906, 1927, 1928, and 1929.

(iii) Extinct volcanoes:

The volcanoes are con­sidered extinct when there are no indications of future eruption. The crater is filled up with water and lakes are formed. It may be pointed out that no volcano can be declared permanently dead as no one knows, what is happening below the ground surface.

Essay # 4. Mechanisms and Causes of Vulcanism:

As stated earlier the volcanic eruptions are asso­ciated with weaker zones of the earth surfaces repre­sented by mountain building at the destructive or convergent plate margins and fracture zones repre­sented by constructive or divergent plate boundaries at the splitting zones of mid-oceanic ridges and the zones of transform faults represented by conservative plate boundaries.

The mechanism of vulcanicity (vulcanism) and volcanic eruptions is closely associated with sev­eral interconnected processes such as:

(i) Gradual in­crease of temperature with increasing depth at the rate of 1°C per 32 m due to heat generated from the disintegration of radioactive elements deep within the earth.

(ii) Origin of magma because of lowering of melting point caused by reduction in the pressure of overlying superincumbent load due to fracture caused by splitting of plates and their movement in opposite direction.

(iii) Origin of gases and vapour due to heat­ing of water which reaches underground through per­colation of rainwater and melt-water (water derived through the melting of ice and snow).

(iv) The ascent of magma forced by enormous volume of gases and vapour, and

(v) Finally the occurrence of volcanic eruptions of either violent explosive central type or quiet fissure type depending upon the intensity of gases and vapour and the nature of crustal surface.

Theory of plate tectonics now very well explains the mechanism of vulcanism and volcanic eruptions. In fact, volcanic eruptions are very closely associated with the plate boundaries. It may be pointed out that the types of plate movements and plate boundaries also determine the nature and intensity of volcanic erup­tion. Most of the active fissure volcanoes are found along the mid-oceanic ridges which represent splitting zones of divergent plate boundaries (fig. 9.5).

Two plates move in opposite directions from the mid-oceanic ridges due to thermal convective currents which are originated in the mantle below the crust (plates). This splitting and lateral spreading of plates creates fractures and faults (transform faults) which cause pressure release and lowering of melting point and thus materials of upper mantle lying below the mid-oceanic ridges are melted and move upward as magmas under the impact of enormous volume of accumulated gases and vapour.

This rise of magmas along the mid-oceanic ridges (constructive or divergent plate bounda­ries) causes fissure eruptions of volcanoes and there is constant upwelling of lavas. These lavas are cooled and solidified and are added to the trailing ends of divergent plate boundaries and thus there is constant creation of new basaltic crust.

The volcanic eruptions of Iceland and the islands located along the mid- Atlantic ridge are caused because of sea-floor spread­ing and divergence of plates. It is obvious that diver­gent or constructive plate boundaries are always asso­ciated with quiet type of fissure flows of lavas because the pressure release of superincumbent load due to divergence of plates and formation of fissures and faults is a slow and gradual process.

It is apparent from the above discussion that the mid-oceanic ridges, representing splitting zones, are associated with active volcanoes wherein the supply of lava comes from the upper mantle just below the ridge because of differential melting of the rocks into tholeiitic basalts.

Since there is constant supply of basaltic lavas from below the mid-oceanic ridges and hence the volcanoes are active near the ridges but the supply of lavas decreases with increasing distance from the mid- oceanic ridges and therefore the volcanoes become inactive, dormant and extinct depending on their dis­tances from the source of lava supply, e.g., mid-oceanic ridges.

This fact has been validated on the basis of the study of the basaltic floor of the Atlantic Ocean and the lavas of several islands. It has been found that the islands nearer to the mid-Atlantic Ridge have younger lavas whereas the islands away from the ridge have older lavas. For example, the lavas of Azores islands Situated on either side of the mid-Atlantic Ridge are 4- million years old whereas the lavas of Cape Verde Island, located far away from the said ridge, are 120- million years old.

Destructive or convergent plate boundaries are associated with explosive type of volcanic eruptions. When two convergent plates collide along Benioff zone (subduction zone), comparatively heavier plate margin (boundary) is subducted beneath comparatively lighter plate boundary. The subducted plate margin, after reaching a depth of 100 km or more in the upper mantle, is melted and thus magma is formed.

This magma is forced to ascend by the enormous volume of accumulated explosive gases and thus magma appears as violent volcanic eruption on the earth’s surface. Such type of volcanic eruption is very common along the destructive or convergent plate boundaries which represent the volcanoes of the Circum-Pacific Belt and the Mid-Continental Belt.

The volcanoes of the island arcs and festoons (off the east coast of Asia) are caused due to subduction of oceanic crust (plate) say Pacific e below the continental plate, say Asiatic plate near Japan Trench.

Essay # 5. Hazardous Effects of Volcanic Eruptions:

Volcanic eruptions cause heavy damage to human lives and property through advancing hot lavas and fallout of volcanic materials; destruction to human structures such as buildings, factories, roads, rails, airports, dams and reservoirs through hot lavas and fires caused by hot lavas; floods in the rivers and climatic changes.

A few of the severe damages wrought by volcanic eruptions may be summarized as given below:

(1) Huge volumes of hot and liquid lavas mov­ing at considerably fast speed (recorded speed is 48 km per hour) bury human structures, kill people and ani­mals, destroy agricultural farms and pastures, plug rivers and lakes, burn and destroy forest etc. The great eruption of Mt. Loa on Hawaii poured out such a huge volume of lavas that these covered a distance of 53 km down the slope.

Enormous Laki Lava flow of 1783 A.D. travelled a distance of 350 km engulfing two churches, 15 agricultural farms and killing 24 per cent of the total population of Iceland. The cases of Mt. Pelee eruption of 1902 in Martinique Island (in Carib­bean Sea) (total death 28,000) and St. Helens eruption of 1980 (Washington, USA) are representative exam­ples of damages done by lava movement. The thick covers of green and dense forests on the flanks of Mt. St. Helens were completely destroyed due to severe forest fires kindled by hot lavas.

(2) Fallout of immense quantity of volcanic materials including fragmental materials (pyroclastic materials), dusts and ashes, smokes etc. covers large ground surface and thus destroys crops, vegetation and buildings, disrupts and diverts natural drainage sys­tems, creates health hazards due to poisonous gases emitted during the eruption, and causes killer acid rains.

(3) All types of volcanic eruptions, if not pre­dicted well in advance, causes tremendous losses to precious human lives. Sudden eruption of violent and explosive type through central pipe does not give any time to human beings to evacuate themselves and thus to save themselves from the clutches of death looming large over them. Sudden eruption of Mt. Pelee on the Island of Martinique, West Indies in the Caribbean Sea, on May 8, 1902 destroyed the whole of St. Pierre town and killed all the 28,000 inhabitants leaving behind only two survivors to mourn the sad demise of their brethren.

The heavy rainfall, associated with volcanic eruptions, mixing with falling volcanic dusts and ashes causes enormous mudflow or ‘lahar’ on the steep slopes of volcanic cones which causes sudden deaths of human beings. For example, great mud flow created on the steep slopes of Kelut volcano in Japan in the year 1919 killed 5,500 people.

(4) Earthquakes caused before and after the volcanic eruptions generate destructive tsunamis seis­mic waves which create most destructive and disas­trous sea waves causing innumerable deaths of human beings in the affected coastal areas. Only the example of Krakatoa in 1883 would be sufficient enough to demonstrate the disastrous impact of tsunamis which generated enormous sea waves of 30 to 40 m height which killed 36,000 people in the coastal areas of Java and Sumatra.

(5) Volcanic eruptions also change the radiation balance of the earth and the atmosphere and thus help in causing climatic changes. Greater concentration of volcanic dusts and ashes in the sky reduces the amount of insolation reaching the earth’s surface as they scat­ter and reflect some amount of incoming shortwave solar radiation. Dust veils, on the other hand, do not hinder in the loss of heat of the earth’s surface through outgoing long-wave terrestrial radiation.

The ejection of nearly 20 cubic kilometres of fragmental materials, dusts and ashes upto the height of 23 km in the sky during the violent eruption of Krakatoa volcano on August 27, 1883 formed a thick dust veil in the strato­sphere which caused a global decrease of solar radia­tion received at the earth’s surface by 10 to 20 per cent.

(6) A group of scientists believes that volcanic eruptions and fallout of dusts and ashes cause mass extinction of a few species of animals. Based on this hypothesis the mass extinction of dinosaurs about 60 million years ago has been related to increased world­wide volcanic activity. Acid rains accompanied by volcanic eruptions cause large-scale destruction of plants and animals.

Essay # 6. Volcanic Materials:

Volcanic materials discharged during eruptions include gases and vapour, lavas, fragmental materials and ashes.

(i) Vapour and Gases:

Steam and vapour consti­tute 60 to 90 per cent of the total gases discharged during a volcanic eruption.

Steam and vapour include:

(i) Phreatic vapour, and

(ii) Magmatic vapour whereas volcanic gases include carbon dioxide, nitrogen ox­ides, sulphur dioxide, hydrogen, carbon monoxide, etc.

Besides, certain compounds are also ejected with the volcanic gases e.g., sulphurated hydrogen, hydrochlo­ric acid, volatile chlorides of iron, potassium and other metallic matter.

(ii) Magma and Lava:

Generally, molten rock materials are called magmas below the earth’s surface while they are called lavas when they come at the earth’s surface.

Lavas and magmas are divided on the basis of silica percentage into two groups e.g.:

(i) Acidic magma (higher percentage of silica, and

(ii) Basic lava (low percentage of silica).

Lavas and magmas are also classified on the basis of light and dark coloured minerals into:

(i) Felsic lava, and

(ii) Mafic lava.

Basaltic or mafic lava is characterized by maxi­mum fluidity. Basaltic lava spreads on the ground surface with maximum flow speed (from a few kilome­tres to 100 kilometres per hour, average How speed being 45 to 65 km per hour) due to high fluidity and low viscosity. Basaltic lava is the hottest lava (1,000° to 1,200 C).

Lava flow is divided into two types on the basis of Hawaiin language e.g.:

(i) Pahoehoe, and

(ii) Aa Aa lava flow or block lava flow.

Pahoehoe lava has high fluidity and spreads like thin sheets. This is also known as ropy lava. On the other hand aa aa lava is more viscous. Pahoehoe lava, when solidified in the form of sacks or pillow, is called pillow lava.

(iii) Fragmental or Pyroclastic Materials:

Fragmental or pyroclastic materials thrown during explosive type of eruption are grouped into three categories:

(i) Essential materials include con­solidated forms of live lavas. These are also known as tephra which means ash. Essential materials are unconsolidated and their size is upto 1 mm.

(ii) Acces­sory materials include dead lavas,

(iii) Accidental materials include fragmental materials of crustal rocks.

On the basis of size pyroclastic materials are grouped into:

(i) Volcanic dust (finest particles),

(ii) Volcanic ash (2 mm in size),

(iii) Lapilli (of the size of peas) and

(iv) Volcanic bombs (6 cm or more in size), which are of different shapes viz. ellipsoidal, discoidal, cuboidal, and irregularly rounded.

The dimension of average volcanic bombs ranges from the size of a base-ball or basket-ball to giant size. Sometimes the volcanic bombs weigh 100 tonnes in weight and are thrown upto a distance of 10 km.

Essay # 7. World Distribution of Volcanoes:

Like earthquakes, the spatial distribution of volcanoes over the globe is well marked and well understood because volcanoes are found in a well-defined belt or zone (fig. 9.3). Thus, the distributional pattern of volcanoes is zonal in character.

If we look at the world distribution of volcanoes it appears that the volcanoes are associated with the weaker zones of the earth’s crust and these are closely associated with seismic events say earthquakes. The weaker zones of the earth are represented by folded mountains (western cordillera of North America, Andes, mountains of East Asia and East Indies) with the exceptions of the Alps and the Himalayas, and fault zones.

Volcanoes are also associated with the meeting zones of the continents and oceans. Occurrences of more volcanic eruptions along coastal margins and during wet season denote the fact that there is close relationship between water and volcanic eruption. Similarly, volcanic eruptions are closely associated with the activities of mountain building and fracturing.

Based on plate tectonics, there is close rela­tionship between plate margins and vulcanicity as most of the world’s active volcanoes are associated with the plate boundaries. About 15 per cent of the worlds’ active volcanoes are found along the construc­tive plate margins or divergent plate margins (along the mid-oceanic ridges where two plates move in opposite directions) whereas 80 per cent volcanoes are associ­ated with the destructive or convergent plate boundaries (where two plates collide). Besides, some volcanoes are also found in intraplate regions e.g., volcanoes of the Hawaii Island, fault zones of East Africa etc.

Like earthquakes, there are also three major belts or zones of volcanoes in the world viz.:

(i) Circum-Pacific belt,

(ii) Mid-continental belt, and

(iii) Mid-oceanic ridge belt (fig. 9.3).

(i) Circum-Pacific belt:

The circum-Pacific belt, also known as the ‘volcanic zones of the convergent oceanic plate margins’, includes the volcanoes of the eastern and western coastal areas of the Pacific Ocean (or the western coastal margins of North and South Americas and the eastern coastal margins of Asia), of island arcs and festoons off the east coast of Asia and of the volcanic islands scattered over the Pacific Ocean.

This volcanic belt is also called as the fire girdle of the Pacific or the fire ring of the Pacific. This belt begins from Erebus Mountain of Antarctica and runs north­ward through Andes and Rockies mountains of South and North Americas to reach Alaska from where this belt turns towards eastern Asiatic coast to include the volcanoes of island arcs and festoons (e.g., Sakhalin, Kamchatka, Japan, Philippines etc.).

The belt ulti­mately merges with the mid-continental belt in the East Indies. Most of high volcanic cones and volcanic mountains are found in this belt. Most of the volcanoes are found in chains e.g., the volcanoes of the Aleutian Island, Hawaii Island, Japan etc.

About 22 volcanic mountains are found in group in Ecuador wherein the height of 15 volcanic mountains is more than 4560 m AMSL. Cotopaxi is the highest volcanic mountain of the world (height being 19,613 feet). The other signifi­cant volcanoes are Fuziyama (Japan), Shasta, Rainier and Hood (western cordillera of North America), a valley of ten thousand smokes (Alaska), Mt St. Helens (Washington, USA), Kilavea (Hawaiiland), Mt. Taal, Pinatubo and Mayon (re-eruption in Feb. 2000) of Philippines etc.

Here volcanic eruptions are primarily caused due to collision of American and Pacific plates and due to subduction of Pacific plate below the Asiatic plate.

(ii) Mid-continental Belt:

Mid-continental belt is also known as ‘the vol­canic zones of convergent continental plate mergins’. This belt includes the volcanoes of Alpine mountain chains and the Mediterranean Sea and the volcanoes of fault zone of eastern Africa. Here, the volcanic erup­tions are caused due to convergence and collision of Eurasian plates and African and Indian plates.

The famous volcanoes of the Mediterranean Sea such as Stromboli, Visuvious, Etna etc. and the volcanoes of Aegean Sea are included in this belt. It may be pointed out that this belt does not have the continuity of volcanic eruptions as several gaps (volcanic-free zones) are found along the Alps and the Himalayas because of Compact and thick crust formed due to intense folding activity. The important volcanoes of the fault zone of eastern Africa are Kilimanjaro, Meru, Elgon, Birunga, Rung we etc.

(iii) Mid-Atlantic Belt:

Mid-Atlantic belt includes the volcanoes mainly along the mid-Atlantic ridge which represents the splitting zone of plates. In other words, two plates diverge in opposite directions from the mid-oceanic ridge. Thus, volcanoes mainly of fissure eruption type occur along the constructive or divergent plate mar­gins (boundaries).

The most active volcanic area is Iceland which is located on the mid-Atlantic ridge. This belt begins from Hekla volcanic mountain of Iceland where several fissure eruption type of volca­noes are found. It may be pointed out that since Iceland is located on the mid-Atlantic ridge representing the splitting zone of American plate moving westward and Eurasian plate moving eastward, and hence here is constant upwelling of magmas along the mid-oceanic ridge and wherever the crust becomes thin and weak, fissure flow of lava occurs because of fracture created due to divergence of plates.

The Laki fissure eruption of 1783 A.D. was so quick and enormous that huge volume of lavas measuring about 15 cubic kilometres was poured out from 28-km long fissure. Recently, Hekla and Helgafell volcanoes erupted in the year 1974 and 1973 respectively. Other more active volcanic areas are Lesser Antilles, Southern Antilles, Azores, St. Helena etc.

The dreadful and disastrous eruption of Mount Pelee occurred on May 8,1902 in the town of St. Pierre on the Martinique Island of West Indies in the Caribbean Sea. All the 28,000 inhabitants, except two persons, were killed by the killer volcanic eruption.

(iv) Intra-Plate Volcanoes:

Besides the aforesaid well defined three zones of volcanoes, scattered volca­noes are also found in the inner parts of the continents. Such distributional patterns of volcanoes are called as intraplate volcanoes, the mechanism of their eruption is not yet precisely known. Fig. 9.4 depicts the location of volcanoes of Pacific plate where one branch of volcanoes runs from Hawaii to Kamchatka.

Vulcanicity also becomes active in the inner parts of continental plates. Massive fissure eruption occurred in the north­western parts of North America during Miocene period when 1,00,000 cubic kilometres of basaltic lavas were spread over an area of 1,30,000 km² to form Columbian plateau. Similarly, great fissure flows of lavas covered more than 5,00,000 km² areas of Peninsular India. Parana of Barazil and Paraguay were formed due to spread of lavas over an area of 7,50,090 km².