VOLCANO
Volcano is an opening or vent through which molten lava, ash, gases etc comes out from the earth’s interior. Most of the volcanoes are concentrated at convergent and divergent plate boundaries but others, located in the interior of plates are associated with hot spots.
Types of Volcanic Eruptions:
1. Icelandic:
It flows as fissure eruption
The lava is basaltic and has less viscosity
It flows quietly and in large quantities forming plateaus.
There is no violent activity or cone formation.
Ex. Columbia (USA), Deccan plateau (India) etc.
2. Hawaiian:
There can be fissure, crater or caldera eruptions
It forms small domes from which mobile lava and gases erupt
It produces fire fountains and only minor amount of gases.
3. Stambolian:
They form stratocones, i.e. lava comes out and solidifies in a layered structure.
They have rhythm to continuous explosions resulting from spasmodic gas escape.
Sometimes clots of lava are ejected producing bombs and scoria.
It can have periodic more intense activity with outpouring of lava.
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4. Vulcanian:
They are also stratocone volcanoes
The associated lavas are more viscous
Sometimes the lava solidifies over the vent forming a crust resulting into gas pressure buildup leading to violent explosions.
After a long quite period, eruption occurs- ejecting bomb, pumice and ash.
Lava flows from the top of the flank after main explosive eruption.
Dark ash laden clouds, convoluted, cauliflower shaped, rises to moderate heights more or less vertical depositing tephra along the flanks of volcano.
5. Vesuvian:
It throws extremely violent explosive gas charged magma from stratocone vent.
Eruption occurs after a long period of quite or mild activity.
Vent tends to be emptied to considerable depth.
The lava erupts in explosive spray and gas cloud reaches to great height and deposits tephra.
6. Pilian:
It is more violent form of visuvian eruption
Last major phase is uprush of gas that carries clouds rapidly upward in vertical column for miles.
It is narrow at base but expands outward at upper elevations, but the clouds are generally low in tephra.
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Page 33
7. Pelean:
It results from high viscosity lava and delayed explosiveness.
Conduit of stratovolcano usually gets blocked by dome or plug, some gas escapes from lateral opening or by destruction or uplift of plug.
8. Katmaian:
It is a variant of pelean eruption characterized by massive outpourings of fluidized ash flows accompanied by widespread explosive tephra.
Ignimbrites are common end products.
Also hot springs and fumaroles are formed.
Fig: Types of volcanic eruptions
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Page 34
Classification Of Volcanoes:
On the basis of material erupted:
Basalt cone:
Basalt cones are very rare
They are low rather than high because of fluidity of basaltic lava.
Ex- Rangitoto (New Zealand) and Skjaldbreit (Iceland) are most suitable examples.
Basalt dome:
Basalt domes are flat domes formed by fluid basalt lava.
They are less in height and broad.
Hawaiian volcanoes are the best example
Ash and cinder cones:
These are high volcanoes formed by high viscous lava.
Ash and cinder cones are built where eruptions are explosive type with predominance of pyroclastic material.
Growth of an ash or cinder cone begins around a crater.
Composite or strato cone:
They are formed by alternate sheets of lava and pyroclastic material
It is formed by alternate periods of explosive and quite eruptions
Most of the largest volcanoes of the world falls under this category—Mt Fujiyama of Japan , Vesuvian of Italy , Cotapaxi and Chimbrazo of Equador are good examples
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Page 35
On basis of periodicity:
Active volcanoes:
Volcanoes which constantly eject lava , gases , ash ,cinder etc are known as active volcanoes.
There are about 600 active volcanoes in the world, most of them being around pacific “Ring of fire”.
The Stramboli volcano erupts so much fire- that it has been termed as the lighthouse of the Mediterranean.
Dormant volcano:
A volcano which has not erupted for a long time but still has probability of eruption is called dormant volcano.
Mt Kilimanjaro is one such volcano.
Extinct volcano:
A volcano which functioned in long geological past but is no longer active is called extinct volcano.
They are only the remnants of the volcanoes.
In many of the cases crater lakes are formed by filling of water into it.
Acancagua of Andes is a typical example of extinct volcano
Disasters & Disaster Management-IASbaba
Page 36
DISTRIBUTION OF VOLCANOES IN THE WORLD:
The distribution of volcanoes on Earth is a function of plate tectonics and hot spot activity.
Volcanic activity occurs in three major areas:
1. Along subduction boundaries at continental- oceanic plate convergence or oceanic-oceanic plate convergence.
2. Along sea floor spreading centers on the oceans floor and areas of rifting on continental plates (the rift zone in east Africa)
3. At hot spots where individual plumes of magma rise through the crust eg. Hawaii.
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Page 37
Volcanic Hazards:
A volcanic hazard refers to any potentially dangerous volcanic process (e.g. lava flows, pyroclastic flows, ash). A volcanic risk is any potential loss or damage as a result of the volcanic hazard that might be incurred by persons, property, etc. or which negatively impacts the productive capacity/sustainability of a population. Risk not only includes the potential monetary and human losses, but also includes a population's vulnerability.
The following is a list of volcanic hazards:
Volcanic Earthquakes
Directed Blast
Tephra
Volcanic Gases
Lava Flows
Debris Avalanches, Landslides, and Tsunamis
Pyroclastic Surge
Pyroclastic Flows
Lahars
Hazards Prevention
In recent years, with the eruptions of Mount St. Helens and Mount Pinatubo many advances have been made in the study of volcanoes particularly in eruption prediction. The problem with volcanoes is that, though there may be similarities between volcanoes, every volcano behaves differently and has its own set of hazards. That is why it's important for scientists to study and
Disasters & Disaster Management-IASbaba
Page 38
monitor volcanoes. Many active volcanoes near populated areas have not been sufficiently studied to assess risk.
When scientists study volcanoes, they map past volcanic deposits and use satellites to look at volcanic features, ash clouds, and gas emissions. They also monitor seismic activity, ground deformation, and geomagnetic, gravimetric, and geo-electrical and thermal changes at a volcano. They study and monitor volcanic gases and monitor the temperature, flow rate, sediment transport, and water level of streams and lakes near the volcano.
By studying volcanic deposits, scientists can produce hazard maps. These maps indicate the types of hazards that can be expected in a given area the next time a volcano erupts. Dating of these volcanic deposits helps determine how often an eruption may occur and the probability of an eruption each year. Monitoring of a volcano over long periods of time will indicate changes in the volcano before it erupts. These changes can help in predicting when an eruption may occur.
Disasters & Disaster Management-IASbaba
Volcano is an opening or vent through which molten lava, ash, gases etc comes out from the earth’s interior. Most of the volcanoes are concentrated at convergent and divergent plate boundaries but others, located in the interior of plates are associated with hot spots.
Types of Volcanic Eruptions:
1. Icelandic:
It flows as fissure eruption
The lava is basaltic and has less viscosity
It flows quietly and in large quantities forming plateaus.
There is no violent activity or cone formation.
Ex. Columbia (USA), Deccan plateau (India) etc.
2. Hawaiian:
There can be fissure, crater or caldera eruptions
It forms small domes from which mobile lava and gases erupt
It produces fire fountains and only minor amount of gases.
3. Stambolian:
They form stratocones, i.e. lava comes out and solidifies in a layered structure.
They have rhythm to continuous explosions resulting from spasmodic gas escape.
Sometimes clots of lava are ejected producing bombs and scoria.
It can have periodic more intense activity with outpouring of lava.
Disasters & Disaster Management-IASbaba
Page 32
4. Vulcanian:
They are also stratocone volcanoes
The associated lavas are more viscous
Sometimes the lava solidifies over the vent forming a crust resulting into gas pressure buildup leading to violent explosions.
After a long quite period, eruption occurs- ejecting bomb, pumice and ash.
Lava flows from the top of the flank after main explosive eruption.
Dark ash laden clouds, convoluted, cauliflower shaped, rises to moderate heights more or less vertical depositing tephra along the flanks of volcano.
5. Vesuvian:
It throws extremely violent explosive gas charged magma from stratocone vent.
Eruption occurs after a long period of quite or mild activity.
Vent tends to be emptied to considerable depth.
The lava erupts in explosive spray and gas cloud reaches to great height and deposits tephra.
6. Pilian:
It is more violent form of visuvian eruption
Last major phase is uprush of gas that carries clouds rapidly upward in vertical column for miles.
It is narrow at base but expands outward at upper elevations, but the clouds are generally low in tephra.
Disasters & Disaster Management-IASbaba
Page 33
7. Pelean:
It results from high viscosity lava and delayed explosiveness.
Conduit of stratovolcano usually gets blocked by dome or plug, some gas escapes from lateral opening or by destruction or uplift of plug.
8. Katmaian:
It is a variant of pelean eruption characterized by massive outpourings of fluidized ash flows accompanied by widespread explosive tephra.
Ignimbrites are common end products.
Also hot springs and fumaroles are formed.
Fig: Types of volcanic eruptions
Disasters & Disaster Management-IASbaba
Page 34
Classification Of Volcanoes:
On the basis of material erupted:
Basalt cone:
Basalt cones are very rare
They are low rather than high because of fluidity of basaltic lava.
Ex- Rangitoto (New Zealand) and Skjaldbreit (Iceland) are most suitable examples.
Basalt dome:
Basalt domes are flat domes formed by fluid basalt lava.
They are less in height and broad.
Hawaiian volcanoes are the best example
Ash and cinder cones:
These are high volcanoes formed by high viscous lava.
Ash and cinder cones are built where eruptions are explosive type with predominance of pyroclastic material.
Growth of an ash or cinder cone begins around a crater.
Composite or strato cone:
They are formed by alternate sheets of lava and pyroclastic material
It is formed by alternate periods of explosive and quite eruptions
Most of the largest volcanoes of the world falls under this category—Mt Fujiyama of Japan , Vesuvian of Italy , Cotapaxi and Chimbrazo of Equador are good examples
Disasters & Disaster Management-IASbaba
Page 35
On basis of periodicity:
Active volcanoes:
Volcanoes which constantly eject lava , gases , ash ,cinder etc are known as active volcanoes.
There are about 600 active volcanoes in the world, most of them being around pacific “Ring of fire”.
The Stramboli volcano erupts so much fire- that it has been termed as the lighthouse of the Mediterranean.
Dormant volcano:
A volcano which has not erupted for a long time but still has probability of eruption is called dormant volcano.
Mt Kilimanjaro is one such volcano.
Extinct volcano:
A volcano which functioned in long geological past but is no longer active is called extinct volcano.
They are only the remnants of the volcanoes.
In many of the cases crater lakes are formed by filling of water into it.
Acancagua of Andes is a typical example of extinct volcano
Disasters & Disaster Management-IASbaba
Page 36
DISTRIBUTION OF VOLCANOES IN THE WORLD:
The distribution of volcanoes on Earth is a function of plate tectonics and hot spot activity.
Volcanic activity occurs in three major areas:
1. Along subduction boundaries at continental- oceanic plate convergence or oceanic-oceanic plate convergence.
2. Along sea floor spreading centers on the oceans floor and areas of rifting on continental plates (the rift zone in east Africa)
3. At hot spots where individual plumes of magma rise through the crust eg. Hawaii.
Disasters & Disaster Management-IASbaba
Page 37
Volcanic Hazards:
A volcanic hazard refers to any potentially dangerous volcanic process (e.g. lava flows, pyroclastic flows, ash). A volcanic risk is any potential loss or damage as a result of the volcanic hazard that might be incurred by persons, property, etc. or which negatively impacts the productive capacity/sustainability of a population. Risk not only includes the potential monetary and human losses, but also includes a population's vulnerability.
The following is a list of volcanic hazards:
Volcanic Earthquakes
Directed Blast
Tephra
Volcanic Gases
Lava Flows
Debris Avalanches, Landslides, and Tsunamis
Pyroclastic Surge
Pyroclastic Flows
Lahars
Hazards Prevention
In recent years, with the eruptions of Mount St. Helens and Mount Pinatubo many advances have been made in the study of volcanoes particularly in eruption prediction. The problem with volcanoes is that, though there may be similarities between volcanoes, every volcano behaves differently and has its own set of hazards. That is why it's important for scientists to study and
Disasters & Disaster Management-IASbaba
Page 38
monitor volcanoes. Many active volcanoes near populated areas have not been sufficiently studied to assess risk.
When scientists study volcanoes, they map past volcanic deposits and use satellites to look at volcanic features, ash clouds, and gas emissions. They also monitor seismic activity, ground deformation, and geomagnetic, gravimetric, and geo-electrical and thermal changes at a volcano. They study and monitor volcanic gases and monitor the temperature, flow rate, sediment transport, and water level of streams and lakes near the volcano.
By studying volcanic deposits, scientists can produce hazard maps. These maps indicate the types of hazards that can be expected in a given area the next time a volcano erupts. Dating of these volcanic deposits helps determine how often an eruption may occur and the probability of an eruption each year. Monitoring of a volcano over long periods of time will indicate changes in the volcano before it erupts. These changes can help in predicting when an eruption may occur.
Disasters & Disaster Management-IASbaba
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