A landscape is the collection of landforms. Volcanic landforms consist of Extrusive and Intrusive landforms.
A. Extrusive Landforms: In this case lava solidifies in contact with air. Here the rate of cooling or solidification is faster and hence mineral grains are fine and not distinguishably visible to naked human eyes.
B. Intrusive Landforms: They are formed due to solidification of lava within the crust In this case lava cools or solidifies slowly, that is why the grains are bigger in size.
Volcanic materials are of two types:
A. Pyroclasts
B. Lava
In general, lava dominates the volcanic scenery. However, a volcanic landform containing no lava is possible, it would have only pyroclast. This is seen in two cases. First, when there is a closure of vent after the pyroclasts have come out, here lava gets solidified in the vent and later converts into the crust.
Second, when a thin layer of lava gets deposited over the pyroclasts and after some time the layer of lava is eroded leading to the exposure of the pyroclasts.
Pyroclasts materials are collectively called Tephra.
A large sized angular pyroclastic material or rock is called Block or Brecia.
A large oval sized lava rocks are called volcanic bombs.
Pea shaped lava rock is called Lapili. They are as small as glass marble.
Bombs and lapili solidify before they fall on the ground. This explains their round shape.
Sometimes, lava acquire vesicular shape and the lava crusts hold gases inside. Such rocks are referred to as Pumice, Scoriae or Cinders. Escaping gases make vesicles and holes in the upper solidified lava crust. Lower lava has not solidified in such structures.
The rock which is composed of a mixture of ash, dust, lapili and cinder is called Tuffs.
Hornitos are small mounds of spatter or driblets.
What are volcanoes:
The English word volcano is derived from a Greek word Vulcan who was a Roman god. According to greek mythology he was believed to be living in the interior of the earth. Going by the mythological sense of the people, volcano can be defined as the fire place of gods. No wonder in Japan, Fujiyama is still worshipped. Pele is the Hawaiian goddess of volcanoes. The term Pele’s Tears, though derived from the goddess Pele, in volcanic jargon it means teardrop shaped pyroclastic $ glassy lava thrown out in volcanic explosions, which have fused and solidified in air. In India too the land of myths, Jwalamukhi festival is related to the nature’s peculiar phenomenon in Himachal Pradesh.
The volcanic explosion, a devastating and terrible phenomenon was considered in ancient times as the expression of the anger of a god or goddess.
Coming to the science of volcano, the Penguin’s Dictionary of Geography defines volcano as a vent in the earth’s crust caused by magma forcing its way to the surface, molten rock or lava is finally , sometimes with explosive force rock fragments and ashes being thrown into the air. The emission of lava or eruptions often cause the volcano to take the form of a conical hill or mountain, the latter is gradually built up of ejected material, which is deposited most thickly round the outlet.
In other words, volcanoes are built by the eruption of molten rock and heated gases under pressure from a relatively small pipe or vent, leading from a magma reservoir at depth. Eruption may take place from the top or sides of the cone.
Structure of Volcano
A volcano is a vent or a group of closely spaced vents through which molten rocks, magma and not gases are ejected out of a deep seated source.
A volcano has three important parts:
Vent: An opening or conduit in the surface of the earth through which volcano material is ejected. A series of vents may form along a major fracture creating a fissure volcano. Volcanoes also develop with central vents and subsidiary vents on the sides of the cone.
Cone: The ejected rock material rushing out of the vent creates a cone like formation at the upper end of the vent. The cone has a steep slope and sometimes takes the form of a mountain.
Crater: A funnel shaped depression at the top or on the sides of a volcanic cone. It may be produced by an explosive eruption or by the collapse of the cone following the withdrawal of underlying lava. Its walls are almost vertical.
Volcanoes are of three types:
Extinct: Such volcanoes that don’t have any realistic possibility of magma upwelling in future are called extinct volcanoes. These volcanoes were active in the geological past. Examples are the Popa mountains in Myanmar, Mt Kilimanjaro in Africa, Mauritius, Madagascar, Malagasy and several other islands in the Indian Ocean. There are thousands of extinct volcanoes which are undergoing erosion by running water and wind and therefore look really extinct.
Dormant: Such volcanoes that have exploded in the recent past and may explode in future are known as dormant volcanoes. It is not easy to differentiate between the extinct and the dormant types of volcanoes. The Vesuvius was considered to be an extinct volcano. But it becomes dangerously active after hundreds of years. Examples are, Fujiyama and Krakatoa.
Active: Such volcanoes that have exploded recently and whose explosions may still be vivid in the public memory. Some of them emit lava after every 15 or twenty minutes. Stromboli is most well known among them. There are almost 300 active volcanoes in the the world. Examples are Mauna Loa, Mt Etna, Mt Vesuvius, Cotopaxi of Ecuador—world’s highest active volcano at 19, 600 feet.
Volcanic landscape is conical. The axis of the formation goes along the vent. Magma or lava particles solidify around the vent of a volcano. After solidification, the magma materials become igneous rocks.
Lava is of two types:
A. Felsic or Acidic
B. Mafic or Basic
Mafic lava has greater fluidity than the felsic lava and its melting point is also lower. Presence of high silica content in the Felsic lava increases its meting point. Acidic lava is of two types: Andesitic and Ignimbrite or rhyolitic lava. Andesitic lava has medium silica content while Ignimbrite lava has high silica content. Ignimbrite rocks are highly acidic. Nuee ardente is an andesitic lava flow. Lahar is also a form of andisitic lava flow. It is volcanic mud flow. Lahars cause accumulations of large quantities of sandy and rocky debris on lower slopes and surrounding areas and may form extensive plains. The plains west of central volcanic mountains of North Island of New Zealand is a good example of Lahar. A mixture of fine debris and water, derived generally from a crater lake of a dormant volcano, bursts out when eruption is resumed forming mud that rushes down the mountain side.
A vent is essentially required for the upwelling of felsic lava while the basic lava can come out through a crack or fracture, the only requirement is that the crack should have continuity with the magma chamber. Latur region in Maharashtra has such cracks.
Silica or acid lava builds high steep sided cones while basic or basaltic lava produces a flatter cone of great diameter. Sometimes the surface of the solidified lava may be smooth but usually it is quite rugged. In the case of both acid and basic lava, the escaping gases make the surface of the lava vesicular and full of small holes.
Besides, the upper surface cools early and forms a crust while lava continues to flow beneath the surface for quite some time. This results into the opening up cracks in the crust and the parts thus broken are transported downward by the moving lava.
Thus, the surface formed by the lava deposits is normally rough and irregular and full of cracks and holes into which rainwater can easily penetrate. The two contrasting surface features or structures may develop owing to differential lava flow at different levels of the volcanic materials in motion. They are known by their Hawaiian names, Aa for angular vesucular scoriaceous surface and Pahoehoe or Ropy for smoothly twisted, convolute surface which develosps on hotter and more fluid lava.
An extreme case of pahoehoe lava occurs when the hot fluid lava either erupts under water. In this case, blobs or lobes of lava upto a diameter of one metre is formed, which has tough but flexible skins (upper surface) and is piled up like sandbags or pillows while their interiors are still in molten state. This is called pillow lava. It is an evidence of sub-aqueous eruption and is commonly found on ocean floor.
Volcanic gases are composed of 60- to 90 % of steam. Other components of volcanic gases are carbon di oxide, nitrogen and sulphur di oxide, and small quantities of hydrogen, carbon mono oxide, sulphur and chlorine.
Solfatara: the stage of volcano when it emits only steam and gases is called the solfatar stage even after the eruption of lava and ashes has ceased. Sulphur content is an important feature of Solfatara stage which is named after the Solfatara volcano near Naples.
TYPES OF CENTRAL ERUPTION:
Based on the nature and intensity of eruption and on the composition of the expelled materials, central eruption is classified under six categories:
1. Hawaiian: Eruption is peaceful here. Hawaiian activity is essentially effusive. Lava is of thin basal variety. There is little or no tephra in the Hawaiian type of eruption. Sometimes, fountains of basaltic spray rise up with gases and when the wind is strong. These lava pieces are stretched i8nto long shiny threads known as Pele’s Hair in the Hawaiian islands—named after Pele, the goddess of fire in the islands. Examples are basalt plateau of Columbia and Iceland.
2. Strombolian: Moderate explosive action is found in this type of eruption. Scoriae and bombs are formed in the strombolian type. Normally eruption is intermittent but may be continuous) and fountains of lava are ejected at regular, rhythmic intervals varying from a few minutes to about an hour and flows are unusual. Stromboli is an island in the Lipri group of islands north of Sicily in the Mediterranean Sea. The lava fountain activity of Stromboli, reflected at night as a red glow on the downside of a towering steam plume has caused the volcano to be known as lighthouse of the Mediterranean. This type of eruption took place in Heckila, Iceland in 1947-48.
3. Vulcanian: The type is named after the Vulcano located in Lipri Islands. Vulcnian activity is explosive. In vulcanian type of eruption, lava solidifies and seals the mouth of the crater in between the two explosions or eruptions. Magma materials of all size are thrown with predominance of ash and dust. Eruption gives appearance of a huge cauliflower…as seen from distance. 1883 explosion of Krakatoa is considered Vulcanian type which was also followed by vesuvian type of explosion. But, that Krakatoa explosion is considered a special type of vulcanian explosion called Phreatic Eruption because it was caused by groundwater or sea water entering the magma chamber and flashing into steam.
4. Vesuvian: It is violent in nature. Lava comes out first from lateral cracks and then through main vent as the gases keep on accumulating in the main vent. Example is 1883 Krakatoa explosion. An eruption of this type was first recorded by Pliny in 79 AC during Vesuvius explosion hence sometimes it is also called Plinian type. Some consider the Vesuvian and Vulcanian types essentially the same and Vesuvius explosion of 79 AD is often cited as an example of Vulcanian eruption.
5. Pelean: It is violently explosive eruption. At the time of first eruption, the dense lava solidifies and closes the mouth of the crater and a dome is formed there. After some time it is broken and lava comes out along the hill slope and then an extremely dense mass of highly charged gaseous lava mixed with magma materials and ash flows down the slope like avalanches. These have been called Nuees ardentes or glowing clouds. Nuee ardente is soundless in spite of the fact that it has the velocity of winds in a hurricane. Example is 1902 explosion of Mt Pelee in West Indies. Nuee ardente is an andesitic lava flow.
An explosion of Pelean type differs from that of Vulcanian type in that the very hot gas and lava mixture is not thrown upwards but spreads downslope as nuee ardente.
6. Mud Volcano: In areas where petroleum or gas is found, sand and clay accompanies the gas emitting out in those areas. If water is also there, sand and clay form mud. The sand and clay is deposited at the vent and form a cone. The example is Bog-Boga in Baku area near Caspean Sea.
DISTRIBUTION OF VOLCANIC LANDFORMS IN THE WORLD
There are three well known zones of volcanic landforms:
A. Mid-oceanic Ridge: Upwelling Zone
B. Subduction Zone
C. Intra-plate volcanic/ seismic zone
At the mid-oceanic ridges mafic lava upwells which widely spreads over the sea floor leading to volcanic landform.
At the subduction zone felsic lava upwells which leads to formation of Plutonic, that is, Intrusive landforms. Though there may also be volcanic, that is, extrusive landforms having felsic lava in the same region. Examples are, Mt Etna, Alaska plateau, Columbia plateau, plateau of Peru, Reunion Islands in the Indian Ocean. In the Mediterranean Sea, all islands are made up of acidic lava.
Intra-plate volcanism is found in the interior of a plate. At present a number of active volcanoes are in the intra-plate region. Examples are, Mt Kilimanjaro: the highest volcanic mountain in Africa, Mt Kenya: the second highest in Africa- both are of acidic nature; Deccan Lava plateau (formed due to basic or fissure eruption): it is the highest lava plateau in the world, Parana plateau of Brazil: the second highest lava plateau in the world, Columbia plateau: the third highest lava plateau and it is the youngest lava plateau dating early Pleistocene age. Parana plateau of Brazil and Deccan plateau of India are contemporary in age.
Volcanic Landforms are of two types:
A. Associated to central eruption
B. Associated to fissure eruption
CENTRAL VOLCANIC LANDFORMS:
Central volcanic eruption occurs through the vent and in general acid lava erupts through this and so some typical landforms are produced.
1. Cinder cone: These are smallest of the volcanoes, built entirely of pieces (pyroclasts) of solidified lava thrown from a central vent. They form where a high proportion of gas in the molten rock causes it to froth into a bubbly mass and to be ejected from a vent with great violence. The froth breaks up into small gragments which solidify as they are ejected and fall as solid particles near the vent. Finer particles are rained down on the crater, and form a cone in a circle around the crater. The slope of wall ranges from 260 to 300….in other words, cinder cone is a concave structure as a whole and its slope is 30-40 degrees. Cinder cone rarely grow to more than 150 to 300 metres in height. Growth is rapid. Monte Nuovo near Naples grew to a height of 120 metres in the first week of its existence. Puraricutin in Mexico reached a height of 300 metres in three months. Cinder cones usually occur in groups, often many dozens in an area of a few tens of square miles. They sometimes show an alignment parallel with fault lines in the underlying rock. Pushpagiri in Karnataka, Mt Kenya and Mt Kilimanjaro are dominated by pyroclasts. Barren Island in the Indian Ocean is also an example of cinder cone. Pushkar Lake is situated in a cinder cone. Lake Pushkar is an old caldera wherein the conical head has been massively eroded leading to the exposure of pyroclasts. (Image: cinder cone)
2. Acid Dome: In this case pyroclasts are covered by solidified thin layer of rhyolite. Acid domes are also called cumulo-domes or endogenous domes which rise into a crater. Examples are, Reunion Island and also islands in the Auvergne region of France. (image)
3. Basalt Dome: Sometimes basalts came out through the vent when the crustal layer is thin enough. Here concentration of pyroclasts is very low. It is basically a plateau like structure having a raised central portion which is occupied by a lake. Basalt domes and shield volcanoes are essentially the same structure, the difference being that of size only. The basaltic domes grade upwards to become shield volcanoes. Basaltic domes are exogenous domes- piled around a central vent. Examples are, Shield Island in Hawaii, Mauna Loa and Mauna Kea, Tahiti Island and Banks Peninsula in New Zealand.
4. Composite Cone: Composite cones are the characteristics of dormant volcanoes. Composite cone has alternate layers of pyroclasts and solidified lava. These are found in old cone and are known as strato-volcanoes. Most of the world’s great volcanoes are composite cones. The steep sided form is governed by the angle at which the cinder and ash stand, whereas the lava layers provide strength and bulk to the volcano. Height upto several thousand feet and slope of 20 to 30 degrees are characteristics. The slope is less than what is found in the case of cinder cones. The crater may change form rapidly, both from demolition of the upper part and from new accumulation. Examples are Fujiyama- it has three well defined such layers, Vesuvius and Stromboli in Italy, Popokatepatal in Mexico, Mt Hood in Oregon, Mt Shasta and Mt Ranier in USA, Cotopaxy in Ecuador, Mayon in Philippines. (Image)
5. Parasite Cone: Parasite cone is also the characteristics of dormant volcanoes. In this case the lava does not find passage through the main vent or the parent vent due to thick deposition and solidification of lava during erstwhile volcanic activity. Os, the lava tends to move towards subsidiary vents (owing to less pushing gaseous force) along the cracks or the fracture in the crust. Lava come out to the slope side of the parent volcano through some cracks, but at the other cracks may not give route to any amount of lava. The lateral volcano is called parasite or satellite cone. The lateral crack is a heterogeneous layer. Presence of a parent vent is the essential condition for the formation of parasite cone. Also, parasite cones would only develop if the length of the lateral vent is shorter than the main vent and the rocks are brittle. Examples are, Mt Etna- highest in Europe and has 230 satellite cones, Fusiyama has 45 satellite cones, Mt Egmont in New Zealand. Composite and Parasite cones are found on the landforms. (Image)
6. Crater and Caldera: Just above the volcanic vent there is usually a rounded bowl or funnel shaped structure which is called crater. Its diameter is normally small. The following are the causes of crater formation:
A. Violent throw off of the volcanic material
B. Process of solidification of the volcanic materials around the vent or the ring crater and their compactness promote crater formation.
C. Erosion of materials from around the vent.
In the crater depression small lakes are formed which are called maare or maars. Maar is a German word that refers to a small, near circular sheet of water situated in the explosion vent, the result of an eruption that blown a hole in the surface of the rocks, surrounded by a low crater ring of fragments of the country rock but accompanied by no extrusion of igneous rocks.
Craters can be seen in Eiffel volcanic region of Germany due to violent explosion in the western part of the rift valley of Africa. Caldera is an extensive rounded volcanic depression whose diameter is greater than that of a crater. Crater is due to geological process and when it is eroded massively it becomes caldera, that is, for caldera formation erosion is required but it can also be formed by an explosion. A huge caldera with a diameter of four miles was formed at the time of the Krakatoa explosion in 1883, the caldera of Mt Katmai in Alaska, the Crater lake of Oregon is located in the caldera in USA-it has a diameter of six miles and is of Pleistocene origin. On the top of the Barisan Highland in the northwest Sumatra, there is a huge caldera in which Loke Toba is situated. Lonar Lake, lake Nicaragua and Lake Titikaka are the other examples of caldera.
7. Nested Caldera: Also called Cone in cone. It is also associated with dormant volcano. In this case there are two or more than two calderas situated one above the other and each caldera possesses a lake. Example is Fusiyama which has three such lakes.
8. Plug Dome: Plug refers to a more or less cylindrical mass of acid lava, occupying the vent of a dormant or extinct volcano. Here acid lava is generally exposed by denudation. This landform is produced due to greater erosion within the caldera exposing the dykes which is relatively harder. In this case, the surrounding rocks are removed and the dyke remains stranded. This landform is not precisely a dome but similar to dome topography and hence this is known as plug dome. It is also called tholoid or volcanic neck. It is the intrusive feature of Plutons. Examples are found in Nicaragua in central America, in crater dome of Mount Pelee, Matinique in West Indies, the Arthur’s Seat, the site of Edinburgh in Scotland, the plugs of Auvergne in France, Hopi Buttes in north east Arizona (in Colorado plateau region) and also in the Castle Rock (near Edinburgh).
At times, eruption occurs through cracks and fractures in the rock structure. In this case basic lava is the only material that upwells in huge quantity. Felsic lava does not have enough fluidity to pass through small cracks. The upwelled basic lava spreads over a vast area due to high fluid nature of the moving magma. This process leads to the formation of large basaltic landforms. Deccan plateau of India and Columbia plateau. All the major plateaux of the world are basaltic.
Radial dykes may sometimes be laid bare by deep erosion of a volcanic neck. A classic example is Shiprock in New Mexico where volcanic neck with prominent radiating dykes are seen in north east New Mexico. Radiating dykes are also found in the Glasshouse Mountains of Queensland, north of Brisbane.
Plate Tectonics and Volcanism:
In 1968 Morgan outlined the hypothesis of plate tectonics. He divided the earth’s surface into twenty plates which are moving relative to one another above the weaker semiplastic asthenosphere as a result, it is believed, of thermal convection currents within the mantle. Le Pichon simplified the concepts of plate tectonics by dividing the earth’s surface into six major, and a few minor or small ones.
The phenomenon of volcanism takes place due to destructive plate activities along the margins of the converging plates.As the oceanic crust is forced downwards into the subduction zone, it becomes molten and forced back to the surface of the earth as a chain of volcanoes.
DISTRIBUTION OF VOLCANOES
Volcanoes in Philippines
There are about 200 hundred volcanoes in the Philippines archipelago, 21 of which are active; to name a few, Mt Isarog (1900 metres) situated 280 kilometres southeast of Manila, erupted 600 years ago; Mr Natib 1200 (metres), 40 kilometres south of Mt Pinatubo is supposed to have ejected steam and therefore mild tremors were recorded by the scientists.
Eruption in Japan
Mt Unzen erupted on June 3, in 1991, killing 38 geologists who were studying the invisible activities going on within the heart of the volcano. The evacuation of nearby inhabitants was already accomplished on the warnings issued by the same group of 38 scientists, who themselves fell victims to the fury of the nature.
Eruption in Andaman Islands
A team engaged in routine inspection of light houses in the Andaman Islands sighted thick smoke spewing out of the Barren Island volcano 125 kilometres from port Blair on April 10 in 1991.
The advancing lava front had already covered a sizeable portion of the island, rendering the light houses non functional and making it virtyually impossible to use any of the landing sites.
The volcano had erupted 200 years ago. The lava thrown out on Barren Island is chemically different from the lava spewed during its previous eruption. According to geologists, the volcano activity on Barren Island has been in three phases. Eruption of submarine lava gave birth to the island 50,000 thousand years ago, creating a joint volcanic cone that covered the whole island. In the second phse, the cone was later blown off and the ejected debris was deposited on the cauldron. In the last phase, a large spatter cone has developed at the mouth of the eruption as a result of which, two volcanic cones are now visible.
The eruption on about 30th April in 1991 originated not in the older crater, but in a vent alongside it.
Narcondum Island, the other volcanic island in the region has shown no sign of becoming active. According to the geological survey of India, Nacondum Islands has no recorded history of eruption.
Two volcanic islands in the Andamans are in the Pacific Volcanic belt which extends to Japan and Philippines.
WORLD DISTRIBUTION
The zones along which earthquake, volcanic or mountain building activity is currently taking place coincide with junctions between plates.
1. Island Festoons of the Pacific: Two third of world’s active volcanoes of the world and thousands of inactive volcanoes are found in this belt. The volcanic belt surrounds the pacific ocean. It is also known as the Ring or Circle of Fire. Among the important volcanic areas of this circle of fire are Aleutian Islands, Alaskan sub-continent, Guatemala, northern and southern Andes. The Circle of Fire contacts with the Atlantic belt at two places: A. Lesser Antilles Vocanic belt. B. The one spot situated in the volcanic belt of South Antilles and which joins Patagonia with Grahamland.
2. Alpine Himalayan Belt: The belt starts from Maderia and the Canary Islands and passes through the Vesuvious, Lipri Island, the Etna and the Aegian volcanoes of the Mediterranian Sea. The belt continues further through Caucasus, Armenia, Iran and reaches Balochistan. After crossing the Himalayas the belt crosses Yunan, Myanmar, Andamans and ends in Indonesian Islands.
3. African Rift Valley: It starts from Bay of Guinea to Red Sea passing through the middle of Africa. The most active volcano in this belt is Cameroon. Mt Kilimanjaro and Kenyan volcanic mountains are beyond the rift valley but Algon is within the belt.
4. Other Areas: Mid Atlantic Cape Verde is famous among them. Volcanoes of Indian Ocean are Comoro, Mauritius and Reunion situated close to Madagascar. There are many volcanoes close to Antarctica.
Environmental Aspects of Volcanoes:
Heavy loss of life and property occurs when volcanoes erupt. About 50 active volcanoes erupt each year. About 360 million people live in the shadows of volcanoes that may explode at any time. Geologists watch only mountains they believe to be still active. Some of the world’s worst disasters have resulted from the eruption of volcanoes thought to be extinct, for example, Vesuvius in 79 AD. In 1985 eruption of Nevado del Ruiz in Columbia killed 22,000 people.
Volcanic ash may have a beneficial effect upon productivity of soil where the ash fall is relatively light. Eruption of Sunset crater near Flagstaff, Arizona in 1065 AD spread a layer of sandy volcanic ash over Barren reddish soil of the surrounding region and caused it to become highly productive because of the moisture conserving effect of the ash, which acted as mulch in the semi-arid climate.
Steep slopes provide valuable timber resources e.g. San Francisco. A scenic feature of great beauty, attracting a heavy tourist trade, few landforms outranks volcanoes. National Parks have formed due to volcanic activities, e.g. Mt Rainier, Mt Lassen and Crater Lake.
The gas-bubble cavities in some ancient lava have become filled with copper or other ores. The famed Kimberlite rock of South Africa sources of diamonds is the pipe of an ancient Volcano.
As a source of crushed rock for concrete aggregate or railroad ballast and other engineering purposes, lava rock is often extensively used. Thus the ancient layers that make up the watching ridges of northern New Jersey have in places been vertically leveled in quarrying operations continued over several decades.
A. Extrusive Landforms: In this case lava solidifies in contact with air. Here the rate of cooling or solidification is faster and hence mineral grains are fine and not distinguishably visible to naked human eyes.
B. Intrusive Landforms: They are formed due to solidification of lava within the crust In this case lava cools or solidifies slowly, that is why the grains are bigger in size.
Volcanic materials are of two types:
A. Pyroclasts
B. Lava
In general, lava dominates the volcanic scenery. However, a volcanic landform containing no lava is possible, it would have only pyroclast. This is seen in two cases. First, when there is a closure of vent after the pyroclasts have come out, here lava gets solidified in the vent and later converts into the crust.
Second, when a thin layer of lava gets deposited over the pyroclasts and after some time the layer of lava is eroded leading to the exposure of the pyroclasts.
Pyroclasts materials are collectively called Tephra.
A large sized angular pyroclastic material or rock is called Block or Brecia.
A large oval sized lava rocks are called volcanic bombs.
Pea shaped lava rock is called Lapili. They are as small as glass marble.
Bombs and lapili solidify before they fall on the ground. This explains their round shape.
Sometimes, lava acquire vesicular shape and the lava crusts hold gases inside. Such rocks are referred to as Pumice, Scoriae or Cinders. Escaping gases make vesicles and holes in the upper solidified lava crust. Lower lava has not solidified in such structures.
The rock which is composed of a mixture of ash, dust, lapili and cinder is called Tuffs.
Hornitos are small mounds of spatter or driblets.
What are volcanoes:
The English word volcano is derived from a Greek word Vulcan who was a Roman god. According to greek mythology he was believed to be living in the interior of the earth. Going by the mythological sense of the people, volcano can be defined as the fire place of gods. No wonder in Japan, Fujiyama is still worshipped. Pele is the Hawaiian goddess of volcanoes. The term Pele’s Tears, though derived from the goddess Pele, in volcanic jargon it means teardrop shaped pyroclastic $ glassy lava thrown out in volcanic explosions, which have fused and solidified in air. In India too the land of myths, Jwalamukhi festival is related to the nature’s peculiar phenomenon in Himachal Pradesh.
The volcanic explosion, a devastating and terrible phenomenon was considered in ancient times as the expression of the anger of a god or goddess.
Coming to the science of volcano, the Penguin’s Dictionary of Geography defines volcano as a vent in the earth’s crust caused by magma forcing its way to the surface, molten rock or lava is finally , sometimes with explosive force rock fragments and ashes being thrown into the air. The emission of lava or eruptions often cause the volcano to take the form of a conical hill or mountain, the latter is gradually built up of ejected material, which is deposited most thickly round the outlet.
In other words, volcanoes are built by the eruption of molten rock and heated gases under pressure from a relatively small pipe or vent, leading from a magma reservoir at depth. Eruption may take place from the top or sides of the cone.
Structure of Volcano
A volcano is a vent or a group of closely spaced vents through which molten rocks, magma and not gases are ejected out of a deep seated source.
A volcano has three important parts:
Vent: An opening or conduit in the surface of the earth through which volcano material is ejected. A series of vents may form along a major fracture creating a fissure volcano. Volcanoes also develop with central vents and subsidiary vents on the sides of the cone.
Cone: The ejected rock material rushing out of the vent creates a cone like formation at the upper end of the vent. The cone has a steep slope and sometimes takes the form of a mountain.
Crater: A funnel shaped depression at the top or on the sides of a volcanic cone. It may be produced by an explosive eruption or by the collapse of the cone following the withdrawal of underlying lava. Its walls are almost vertical.
Volcanoes are of three types:
Extinct: Such volcanoes that don’t have any realistic possibility of magma upwelling in future are called extinct volcanoes. These volcanoes were active in the geological past. Examples are the Popa mountains in Myanmar, Mt Kilimanjaro in Africa, Mauritius, Madagascar, Malagasy and several other islands in the Indian Ocean. There are thousands of extinct volcanoes which are undergoing erosion by running water and wind and therefore look really extinct.
Dormant: Such volcanoes that have exploded in the recent past and may explode in future are known as dormant volcanoes. It is not easy to differentiate between the extinct and the dormant types of volcanoes. The Vesuvius was considered to be an extinct volcano. But it becomes dangerously active after hundreds of years. Examples are, Fujiyama and Krakatoa.
Active: Such volcanoes that have exploded recently and whose explosions may still be vivid in the public memory. Some of them emit lava after every 15 or twenty minutes. Stromboli is most well known among them. There are almost 300 active volcanoes in the the world. Examples are Mauna Loa, Mt Etna, Mt Vesuvius, Cotopaxi of Ecuador—world’s highest active volcano at 19, 600 feet.
Volcanic landscape is conical. The axis of the formation goes along the vent. Magma or lava particles solidify around the vent of a volcano. After solidification, the magma materials become igneous rocks.
Lava is of two types:
A. Felsic or Acidic
B. Mafic or Basic
Mafic lava has greater fluidity than the felsic lava and its melting point is also lower. Presence of high silica content in the Felsic lava increases its meting point. Acidic lava is of two types: Andesitic and Ignimbrite or rhyolitic lava. Andesitic lava has medium silica content while Ignimbrite lava has high silica content. Ignimbrite rocks are highly acidic. Nuee ardente is an andesitic lava flow. Lahar is also a form of andisitic lava flow. It is volcanic mud flow. Lahars cause accumulations of large quantities of sandy and rocky debris on lower slopes and surrounding areas and may form extensive plains. The plains west of central volcanic mountains of North Island of New Zealand is a good example of Lahar. A mixture of fine debris and water, derived generally from a crater lake of a dormant volcano, bursts out when eruption is resumed forming mud that rushes down the mountain side.
A vent is essentially required for the upwelling of felsic lava while the basic lava can come out through a crack or fracture, the only requirement is that the crack should have continuity with the magma chamber. Latur region in Maharashtra has such cracks.
Silica or acid lava builds high steep sided cones while basic or basaltic lava produces a flatter cone of great diameter. Sometimes the surface of the solidified lava may be smooth but usually it is quite rugged. In the case of both acid and basic lava, the escaping gases make the surface of the lava vesicular and full of small holes.
Besides, the upper surface cools early and forms a crust while lava continues to flow beneath the surface for quite some time. This results into the opening up cracks in the crust and the parts thus broken are transported downward by the moving lava.
Thus, the surface formed by the lava deposits is normally rough and irregular and full of cracks and holes into which rainwater can easily penetrate. The two contrasting surface features or structures may develop owing to differential lava flow at different levels of the volcanic materials in motion. They are known by their Hawaiian names, Aa for angular vesucular scoriaceous surface and Pahoehoe or Ropy for smoothly twisted, convolute surface which develosps on hotter and more fluid lava.
An extreme case of pahoehoe lava occurs when the hot fluid lava either erupts under water. In this case, blobs or lobes of lava upto a diameter of one metre is formed, which has tough but flexible skins (upper surface) and is piled up like sandbags or pillows while their interiors are still in molten state. This is called pillow lava. It is an evidence of sub-aqueous eruption and is commonly found on ocean floor.
Volcanic gases are composed of 60- to 90 % of steam. Other components of volcanic gases are carbon di oxide, nitrogen and sulphur di oxide, and small quantities of hydrogen, carbon mono oxide, sulphur and chlorine.
Solfatara: the stage of volcano when it emits only steam and gases is called the solfatar stage even after the eruption of lava and ashes has ceased. Sulphur content is an important feature of Solfatara stage which is named after the Solfatara volcano near Naples.
TYPES OF CENTRAL ERUPTION:
Based on the nature and intensity of eruption and on the composition of the expelled materials, central eruption is classified under six categories:
1. Hawaiian: Eruption is peaceful here. Hawaiian activity is essentially effusive. Lava is of thin basal variety. There is little or no tephra in the Hawaiian type of eruption. Sometimes, fountains of basaltic spray rise up with gases and when the wind is strong. These lava pieces are stretched i8nto long shiny threads known as Pele’s Hair in the Hawaiian islands—named after Pele, the goddess of fire in the islands. Examples are basalt plateau of Columbia and Iceland.
2. Strombolian: Moderate explosive action is found in this type of eruption. Scoriae and bombs are formed in the strombolian type. Normally eruption is intermittent but may be continuous) and fountains of lava are ejected at regular, rhythmic intervals varying from a few minutes to about an hour and flows are unusual. Stromboli is an island in the Lipri group of islands north of Sicily in the Mediterranean Sea. The lava fountain activity of Stromboli, reflected at night as a red glow on the downside of a towering steam plume has caused the volcano to be known as lighthouse of the Mediterranean. This type of eruption took place in Heckila, Iceland in 1947-48.
3. Vulcanian: The type is named after the Vulcano located in Lipri Islands. Vulcnian activity is explosive. In vulcanian type of eruption, lava solidifies and seals the mouth of the crater in between the two explosions or eruptions. Magma materials of all size are thrown with predominance of ash and dust. Eruption gives appearance of a huge cauliflower…as seen from distance. 1883 explosion of Krakatoa is considered Vulcanian type which was also followed by vesuvian type of explosion. But, that Krakatoa explosion is considered a special type of vulcanian explosion called Phreatic Eruption because it was caused by groundwater or sea water entering the magma chamber and flashing into steam.
4. Vesuvian: It is violent in nature. Lava comes out first from lateral cracks and then through main vent as the gases keep on accumulating in the main vent. Example is 1883 Krakatoa explosion. An eruption of this type was first recorded by Pliny in 79 AC during Vesuvius explosion hence sometimes it is also called Plinian type. Some consider the Vesuvian and Vulcanian types essentially the same and Vesuvius explosion of 79 AD is often cited as an example of Vulcanian eruption.
5. Pelean: It is violently explosive eruption. At the time of first eruption, the dense lava solidifies and closes the mouth of the crater and a dome is formed there. After some time it is broken and lava comes out along the hill slope and then an extremely dense mass of highly charged gaseous lava mixed with magma materials and ash flows down the slope like avalanches. These have been called Nuees ardentes or glowing clouds. Nuee ardente is soundless in spite of the fact that it has the velocity of winds in a hurricane. Example is 1902 explosion of Mt Pelee in West Indies. Nuee ardente is an andesitic lava flow.
An explosion of Pelean type differs from that of Vulcanian type in that the very hot gas and lava mixture is not thrown upwards but spreads downslope as nuee ardente.
6. Mud Volcano: In areas where petroleum or gas is found, sand and clay accompanies the gas emitting out in those areas. If water is also there, sand and clay form mud. The sand and clay is deposited at the vent and form a cone. The example is Bog-Boga in Baku area near Caspean Sea.
DISTRIBUTION OF VOLCANIC LANDFORMS IN THE WORLD
There are three well known zones of volcanic landforms:
A. Mid-oceanic Ridge: Upwelling Zone
B. Subduction Zone
C. Intra-plate volcanic/ seismic zone
At the mid-oceanic ridges mafic lava upwells which widely spreads over the sea floor leading to volcanic landform.
At the subduction zone felsic lava upwells which leads to formation of Plutonic, that is, Intrusive landforms. Though there may also be volcanic, that is, extrusive landforms having felsic lava in the same region. Examples are, Mt Etna, Alaska plateau, Columbia plateau, plateau of Peru, Reunion Islands in the Indian Ocean. In the Mediterranean Sea, all islands are made up of acidic lava.
Intra-plate volcanism is found in the interior of a plate. At present a number of active volcanoes are in the intra-plate region. Examples are, Mt Kilimanjaro: the highest volcanic mountain in Africa, Mt Kenya: the second highest in Africa- both are of acidic nature; Deccan Lava plateau (formed due to basic or fissure eruption): it is the highest lava plateau in the world, Parana plateau of Brazil: the second highest lava plateau in the world, Columbia plateau: the third highest lava plateau and it is the youngest lava plateau dating early Pleistocene age. Parana plateau of Brazil and Deccan plateau of India are contemporary in age.
Volcanic Landforms are of two types:
A. Associated to central eruption
B. Associated to fissure eruption
CENTRAL VOLCANIC LANDFORMS:
Central volcanic eruption occurs through the vent and in general acid lava erupts through this and so some typical landforms are produced.
1. Cinder cone: These are smallest of the volcanoes, built entirely of pieces (pyroclasts) of solidified lava thrown from a central vent. They form where a high proportion of gas in the molten rock causes it to froth into a bubbly mass and to be ejected from a vent with great violence. The froth breaks up into small gragments which solidify as they are ejected and fall as solid particles near the vent. Finer particles are rained down on the crater, and form a cone in a circle around the crater. The slope of wall ranges from 260 to 300….in other words, cinder cone is a concave structure as a whole and its slope is 30-40 degrees. Cinder cone rarely grow to more than 150 to 300 metres in height. Growth is rapid. Monte Nuovo near Naples grew to a height of 120 metres in the first week of its existence. Puraricutin in Mexico reached a height of 300 metres in three months. Cinder cones usually occur in groups, often many dozens in an area of a few tens of square miles. They sometimes show an alignment parallel with fault lines in the underlying rock. Pushpagiri in Karnataka, Mt Kenya and Mt Kilimanjaro are dominated by pyroclasts. Barren Island in the Indian Ocean is also an example of cinder cone. Pushkar Lake is situated in a cinder cone. Lake Pushkar is an old caldera wherein the conical head has been massively eroded leading to the exposure of pyroclasts. (Image: cinder cone)
2. Acid Dome: In this case pyroclasts are covered by solidified thin layer of rhyolite. Acid domes are also called cumulo-domes or endogenous domes which rise into a crater. Examples are, Reunion Island and also islands in the Auvergne region of France. (image)
3. Basalt Dome: Sometimes basalts came out through the vent when the crustal layer is thin enough. Here concentration of pyroclasts is very low. It is basically a plateau like structure having a raised central portion which is occupied by a lake. Basalt domes and shield volcanoes are essentially the same structure, the difference being that of size only. The basaltic domes grade upwards to become shield volcanoes. Basaltic domes are exogenous domes- piled around a central vent. Examples are, Shield Island in Hawaii, Mauna Loa and Mauna Kea, Tahiti Island and Banks Peninsula in New Zealand.
4. Composite Cone: Composite cones are the characteristics of dormant volcanoes. Composite cone has alternate layers of pyroclasts and solidified lava. These are found in old cone and are known as strato-volcanoes. Most of the world’s great volcanoes are composite cones. The steep sided form is governed by the angle at which the cinder and ash stand, whereas the lava layers provide strength and bulk to the volcano. Height upto several thousand feet and slope of 20 to 30 degrees are characteristics. The slope is less than what is found in the case of cinder cones. The crater may change form rapidly, both from demolition of the upper part and from new accumulation. Examples are Fujiyama- it has three well defined such layers, Vesuvius and Stromboli in Italy, Popokatepatal in Mexico, Mt Hood in Oregon, Mt Shasta and Mt Ranier in USA, Cotopaxy in Ecuador, Mayon in Philippines. (Image)
5. Parasite Cone: Parasite cone is also the characteristics of dormant volcanoes. In this case the lava does not find passage through the main vent or the parent vent due to thick deposition and solidification of lava during erstwhile volcanic activity. Os, the lava tends to move towards subsidiary vents (owing to less pushing gaseous force) along the cracks or the fracture in the crust. Lava come out to the slope side of the parent volcano through some cracks, but at the other cracks may not give route to any amount of lava. The lateral volcano is called parasite or satellite cone. The lateral crack is a heterogeneous layer. Presence of a parent vent is the essential condition for the formation of parasite cone. Also, parasite cones would only develop if the length of the lateral vent is shorter than the main vent and the rocks are brittle. Examples are, Mt Etna- highest in Europe and has 230 satellite cones, Fusiyama has 45 satellite cones, Mt Egmont in New Zealand. Composite and Parasite cones are found on the landforms. (Image)
6. Crater and Caldera: Just above the volcanic vent there is usually a rounded bowl or funnel shaped structure which is called crater. Its diameter is normally small. The following are the causes of crater formation:
A. Violent throw off of the volcanic material
B. Process of solidification of the volcanic materials around the vent or the ring crater and their compactness promote crater formation.
C. Erosion of materials from around the vent.
In the crater depression small lakes are formed which are called maare or maars. Maar is a German word that refers to a small, near circular sheet of water situated in the explosion vent, the result of an eruption that blown a hole in the surface of the rocks, surrounded by a low crater ring of fragments of the country rock but accompanied by no extrusion of igneous rocks.
Craters can be seen in Eiffel volcanic region of Germany due to violent explosion in the western part of the rift valley of Africa. Caldera is an extensive rounded volcanic depression whose diameter is greater than that of a crater. Crater is due to geological process and when it is eroded massively it becomes caldera, that is, for caldera formation erosion is required but it can also be formed by an explosion. A huge caldera with a diameter of four miles was formed at the time of the Krakatoa explosion in 1883, the caldera of Mt Katmai in Alaska, the Crater lake of Oregon is located in the caldera in USA-it has a diameter of six miles and is of Pleistocene origin. On the top of the Barisan Highland in the northwest Sumatra, there is a huge caldera in which Loke Toba is situated. Lonar Lake, lake Nicaragua and Lake Titikaka are the other examples of caldera.
7. Nested Caldera: Also called Cone in cone. It is also associated with dormant volcano. In this case there are two or more than two calderas situated one above the other and each caldera possesses a lake. Example is Fusiyama which has three such lakes.
8. Plug Dome: Plug refers to a more or less cylindrical mass of acid lava, occupying the vent of a dormant or extinct volcano. Here acid lava is generally exposed by denudation. This landform is produced due to greater erosion within the caldera exposing the dykes which is relatively harder. In this case, the surrounding rocks are removed and the dyke remains stranded. This landform is not precisely a dome but similar to dome topography and hence this is known as plug dome. It is also called tholoid or volcanic neck. It is the intrusive feature of Plutons. Examples are found in Nicaragua in central America, in crater dome of Mount Pelee, Matinique in West Indies, the Arthur’s Seat, the site of Edinburgh in Scotland, the plugs of Auvergne in France, Hopi Buttes in north east Arizona (in Colorado plateau region) and also in the Castle Rock (near Edinburgh).
At times, eruption occurs through cracks and fractures in the rock structure. In this case basic lava is the only material that upwells in huge quantity. Felsic lava does not have enough fluidity to pass through small cracks. The upwelled basic lava spreads over a vast area due to high fluid nature of the moving magma. This process leads to the formation of large basaltic landforms. Deccan plateau of India and Columbia plateau. All the major plateaux of the world are basaltic.
Radial dykes may sometimes be laid bare by deep erosion of a volcanic neck. A classic example is Shiprock in New Mexico where volcanic neck with prominent radiating dykes are seen in north east New Mexico. Radiating dykes are also found in the Glasshouse Mountains of Queensland, north of Brisbane.
Plate Tectonics and Volcanism:
In 1968 Morgan outlined the hypothesis of plate tectonics. He divided the earth’s surface into twenty plates which are moving relative to one another above the weaker semiplastic asthenosphere as a result, it is believed, of thermal convection currents within the mantle. Le Pichon simplified the concepts of plate tectonics by dividing the earth’s surface into six major, and a few minor or small ones.
The phenomenon of volcanism takes place due to destructive plate activities along the margins of the converging plates.As the oceanic crust is forced downwards into the subduction zone, it becomes molten and forced back to the surface of the earth as a chain of volcanoes.
DISTRIBUTION OF VOLCANOES
Volcanoes in Philippines
There are about 200 hundred volcanoes in the Philippines archipelago, 21 of which are active; to name a few, Mt Isarog (1900 metres) situated 280 kilometres southeast of Manila, erupted 600 years ago; Mr Natib 1200 (metres), 40 kilometres south of Mt Pinatubo is supposed to have ejected steam and therefore mild tremors were recorded by the scientists.
Eruption in Japan
Mt Unzen erupted on June 3, in 1991, killing 38 geologists who were studying the invisible activities going on within the heart of the volcano. The evacuation of nearby inhabitants was already accomplished on the warnings issued by the same group of 38 scientists, who themselves fell victims to the fury of the nature.
Eruption in Andaman Islands
A team engaged in routine inspection of light houses in the Andaman Islands sighted thick smoke spewing out of the Barren Island volcano 125 kilometres from port Blair on April 10 in 1991.
The advancing lava front had already covered a sizeable portion of the island, rendering the light houses non functional and making it virtyually impossible to use any of the landing sites.
The volcano had erupted 200 years ago. The lava thrown out on Barren Island is chemically different from the lava spewed during its previous eruption. According to geologists, the volcano activity on Barren Island has been in three phases. Eruption of submarine lava gave birth to the island 50,000 thousand years ago, creating a joint volcanic cone that covered the whole island. In the second phse, the cone was later blown off and the ejected debris was deposited on the cauldron. In the last phase, a large spatter cone has developed at the mouth of the eruption as a result of which, two volcanic cones are now visible.
The eruption on about 30th April in 1991 originated not in the older crater, but in a vent alongside it.
Narcondum Island, the other volcanic island in the region has shown no sign of becoming active. According to the geological survey of India, Nacondum Islands has no recorded history of eruption.
Two volcanic islands in the Andamans are in the Pacific Volcanic belt which extends to Japan and Philippines.
WORLD DISTRIBUTION
The zones along which earthquake, volcanic or mountain building activity is currently taking place coincide with junctions between plates.
1. Island Festoons of the Pacific: Two third of world’s active volcanoes of the world and thousands of inactive volcanoes are found in this belt. The volcanic belt surrounds the pacific ocean. It is also known as the Ring or Circle of Fire. Among the important volcanic areas of this circle of fire are Aleutian Islands, Alaskan sub-continent, Guatemala, northern and southern Andes. The Circle of Fire contacts with the Atlantic belt at two places: A. Lesser Antilles Vocanic belt. B. The one spot situated in the volcanic belt of South Antilles and which joins Patagonia with Grahamland.
2. Alpine Himalayan Belt: The belt starts from Maderia and the Canary Islands and passes through the Vesuvious, Lipri Island, the Etna and the Aegian volcanoes of the Mediterranian Sea. The belt continues further through Caucasus, Armenia, Iran and reaches Balochistan. After crossing the Himalayas the belt crosses Yunan, Myanmar, Andamans and ends in Indonesian Islands.
3. African Rift Valley: It starts from Bay of Guinea to Red Sea passing through the middle of Africa. The most active volcano in this belt is Cameroon. Mt Kilimanjaro and Kenyan volcanic mountains are beyond the rift valley but Algon is within the belt.
4. Other Areas: Mid Atlantic Cape Verde is famous among them. Volcanoes of Indian Ocean are Comoro, Mauritius and Reunion situated close to Madagascar. There are many volcanoes close to Antarctica.
Environmental Aspects of Volcanoes:
Heavy loss of life and property occurs when volcanoes erupt. About 50 active volcanoes erupt each year. About 360 million people live in the shadows of volcanoes that may explode at any time. Geologists watch only mountains they believe to be still active. Some of the world’s worst disasters have resulted from the eruption of volcanoes thought to be extinct, for example, Vesuvius in 79 AD. In 1985 eruption of Nevado del Ruiz in Columbia killed 22,000 people.
Volcanic ash may have a beneficial effect upon productivity of soil where the ash fall is relatively light. Eruption of Sunset crater near Flagstaff, Arizona in 1065 AD spread a layer of sandy volcanic ash over Barren reddish soil of the surrounding region and caused it to become highly productive because of the moisture conserving effect of the ash, which acted as mulch in the semi-arid climate.
Steep slopes provide valuable timber resources e.g. San Francisco. A scenic feature of great beauty, attracting a heavy tourist trade, few landforms outranks volcanoes. National Parks have formed due to volcanic activities, e.g. Mt Rainier, Mt Lassen and Crater Lake.
The gas-bubble cavities in some ancient lava have become filled with copper or other ores. The famed Kimberlite rock of South Africa sources of diamonds is the pipe of an ancient Volcano.
As a source of crushed rock for concrete aggregate or railroad ballast and other engineering purposes, lava rock is often extensively used. Thus the ancient layers that make up the watching ridges of northern New Jersey have in places been vertically leveled in quarrying operations continued over several decades.
