A volcano is an opening in Earth’s crust that allows molten rock from beneath the crust to reach the surface. This molten rock is called magma when it is beneath the surface and lava when it erupts or flows from a volcano. Along with lava, volcanoes also release gases, ash, and rock.
Three basic types of magma:
- Mafic or Basaltic— SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
- Intermediate or Andesitic— SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
- Felsic or Rhyolitic— SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na.
Based on the activity , Volcanoes can be classified as following:-
Active Volcano – An active volcano to volcanologists is a volcano that has shown eruptive activity within recorded history. Thus an active volcano need not be in eruption to be considered active.
- Currently there are about 600 volcanoes on Earth considered to be active volcanoes.
- Each year 50 to 60 of volcanoes actually erupt.
- Extinct Volcano – An extinct volcano is a volcano that has not shown any historic activity, is usually deeply eroded, and shows no signs of recent activity. How old must a volcano be to be considered extinct depends to a large degree on past activity.
Dormant Volcano – A dormant volcano (sleeping volcano) is somewhere between active and extinct. A dormant volcano is one that has not shown eruptive activity within recorded history, but shows geologic evidence of activity within the geologic recent past. Because the lifetime of a volcano may be on the order of a million years, dormant volcanoes can become active volcanoes all of sudden. These are perhaps the most dangerous volcanoes because people living in the vicinity of a dormant volcano may not understand the concept of geologic time, and there is no written record of activity. These people are sometimes difficult to convince when a dormant volcano shows signs of renewed activity.
Based on the morphology,viscosity and type of eruptions volcanoes can be classified as following:-
- Shield volcano:-They are formed by running lava and spread to a larger space. The magma coming out is known as basaltic magma.
- Composite volcano:-They are also called strato-volcanoes.They are explosive in natureThe magma is known as andesitic magma.
- Caldera volcano:-It is hugely explosive in nature which makes the volcano collapse and results in a depression on the surface.
Volcanoes distribution is
- Along divergent plate boundaries, such as Oceanic Ridges or spreading centers.:-Active volcanism is currently taking place along all of oceanic ridges, but most of this volcanism is submarine volcanism. One place where an oceanic ridge reaches above sea level is at Iceland, along the Mid-Atlantic Ridge. Here, most eruptions are basaltic in nature, but, many are explosive strombolian types or explosive phreatic or phreatomagmatic types.
- In areas of continental extension (that may become divergent plate boundaries in the future).:-
- Along converging plate boundaries where subduction is occurring.:-All around the Pacific Ocean, is a zone often referred to as the Pacific Ring of Fire, where most of the world’s most active and most dangerous volcanoes occur. The Ring of Fire occurs because most of the margins of the Pacific ocean coincide with converging margins along which subduction is occurring
- And, in areas called “hot spots” that are usually located in the interior of plates, away from the plate margins.:-Volcanism also occurs in areas that are not associated with plate boundaries, in the interior of plates. These are most commonly associated with what is called a hot spot. Hot spots appear to result from plumes of hot mantle material upwelling toward the surface, independent of the convection cells though to cause plate motion.
Hazards/ Effects of Volcanism
- Lava Flows – lava flows are common in Hawaiian and Strombolian type of eruptions, the least explosive. Although they have been known to travel as fast as 64 km/hr, most are slower and give people time to move out of the way. Thus, in general, lava flows are most damaging to property, as they can destroy anything in their path.
Pyroclastic Flows – Pyroclastic flows are one of the most dangerous aspects of volcanism. They cause death by suffocation and burning. They can travel so rapidly that few humans can escape.
- Ash falls – Although tephra falls blanket an area like snow, they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow an cause the collapse of roof. They and can affect areas far from the eruption. Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation. Tephra falls can cause loss of agricultural activity for years after an eruption.
- Mudflows (Lahars) As discussed above, mudflows can both accompany an eruption and occur many years after an eruption. They are formed when water and loose ash deposits come together and begin to flow. The source of water can be derived by melting of snow or ice during the eruption, emptying of crater lakes during an eruption, or rainfall that takes place any time with no eruption.
Debris Avalanches, Landslides, and Debris Flows – Volcanic mountains tend to become oversteepened as a result of the addition of new material over time as well due to inflation of the mountain as magma intrudes. Oversteepened slopes may become unstable, leading to a sudden slope failure that results in landslides, debris flows or debris avalanches. Debris avalanches, landslides, and debris flows do not necessarily occur accompanied by a volcanic eruption. There are documented cases of such occurrences where no new magma has been erupted.
- Flooding – Drainage systems can become blocked by deposition of pyroclastic flows and lava flows. Such blockage may create a temporary dam that could eventually fill with water and fail resulting in floods downstream from the natural dam. Volcanoes in cold climates can melt snow and glacial ice, rapidly releasing water into the drainage system and possibly causing floods. Jokaulhlaups occur when heating of a glacier results in rapid outburst of water from the melting glacier.
Tsunami – Debris avalanche events, landslides, caldera collapse events, and pyroclastic flows entering a body of water may generate tsunami. During the 1883 eruption of Krakatau volcano, in the straits of Sunda between Java and Sumatra, several tsunami were generated by pyroclastic flows entering the sea and by collapse accompanying caldera formation. The tsunami killed about 36,400 people, some as far away from the volcano as 200 km.
- Volcanic Earthquakes – Earthquakes usually precede and accompany volcanic eruptions, as magma intrudes and moves within the volcano. Although most volcanic earthquakes are small, some are large enough to cause damage in the area immediately surrounding the volcano, and some are large enough to trigger landslides and debris avalanches, such as in the case of Mount St. Helens.
- Atmospheric Effects- Fined grained ash and sulfur gases expelled into the atmosphere reflect solar radiations and cause cooling of the atmosphere. CO2 released by volcanoes can cause warming of the atmosphere.