A volcano is an opening in the earth’s crust through which gases, molten rocks materials (lava), pyroclastic fragments, ash, steam etc. are emitted outward in the course of an eruption. Such vents or openings occur in those parts of the earth’s crust where the rock strata are relatively weak. Although volcanism is best known on Earth, there is evidence that it has been important in the development of the other terrestrial planets—Mercury, Venus, and Mars—as well as some natural satellites such as Earth’s Moon and Jupiter’s moon Io.
Magma denotes the molten rocks and related materials seen inside earth. A weaker zone of the mantle called asthenosphere, usually is the source of magma. Once this magma came out to the earth surface through the vent of a volcano, it is called as the lava. Therefore, lava is nothing but the magma on earth surface.
There are two types of lava:
(i) Acidic or Composite or Stratovolcanic lava: These lavas are highly viscous with a high melting point. They are light-colored, of low density, and have a high percentage of silica. They flow slowly and seldom travel far before solidifying. The resultant cone is therefore steep sided. The rapid solidifying of lava in the vent obstructs the flow of the out-pouring lava, resulting in loud explosions, throwing out many volcanic bombs or pyroclasts. Sometimes the lavas are so viscous that they form a spine or plug at the crater like that of Mt. Pelee in Martinique.

(ii) Basic or Basaltic or Shield lava: These are the hottest lavas, about 1,000°C. (1,830°F.) and are highly fluid. They are dark colored like basalt, rich in iron and magnesium but poor in silica. They flow out of volcanic vent quietly and are not very explosive. Due to their high fluidity, they flow readily with a speed of 10 to 30 miles per hour. They affect extensive areas, spreading out as thin sheets over great distances before they solidify (This is how Deccan Traps were formed). The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome.


Causes of Volcanism

The chemical reactions of radioactive substances deep within the interior of the earth generate tremendous amount of heat. Some heat is already present in the form of residual heat (heat captured at the center during earth’s formation) is already present at the earth’s interior. There is a huge temperature difference between the inner layers and the outer layers of the earth due to differential amount of radioactivity. This temperature difference gives rise to convectional currents in the outer core as well as the mantle. The convectional currents in the mantle create convergent and divergent boundaries.

Types of Volcanoes
((i) Active Volcano: Keeps on ejecting volcanic material at frequent intervals
Ex –  Mount Etna in Italy; Mount Stromboli in Italy; Mount Yasur in Vanuatu; Mount Ambrym in Vanuatu; Mount Tinakula in Solomon Islands; Mount Erta Ale in Ethiopia; Mount Manam in Papua New Guinea; Mount Langila in Papua New Guinea; Mount Bagana in Papua New Guinea; Mount Semeru in Indonesia; Mount Merapi in Indonesia; Mount Dukono in Indonesia; Mount Sakura-jima in Japan; Mount Suwanose-jima in Japan; Mount Fuji of Japan; Mount Santa Maria in Guatemala; Mount Pacaya in Guatemala; Mount Sangay in Ecuador; Mount Erebus in Antarctica; Mount Piton de la Fournaise in Reunion; Mount Kilauea in Hawaii; Mount St. Helen in Washington; and Mayon Volcano in the Philippines.
(ii) Dormant Volcano: One in which eruption has not occurred for a long time but can occur any time in future i.e. Barren Island (Andaman), Versuris (Italy)
 (iii) Extinct Volcano: No eruption has occurred in historic times and possibility of future eruption is also remote i.e. Mt. Popa (Myanmar). But we can never be thoroughly sure about them. Vesuvius (Bay of Naples near Italy) and Mt. Krakatau (Sunda straits) were thought to be extinct but both erupted violently.

Volcanic Landforms
(A) Intrusive Landforms

The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or from the inside itself. Depending on the location of the cooling of lava, igneous rocks are classified into two :
(i) Volcanic Igneous rocks (Extrusive igneous rocks):
Cooling of the rock occurs at the surface of the earth.  The examples of extrusive landforms (forming over the surface) are :
 lava plateau, volcanic mountain, etc.
(ii) Plutonic Igneous rocks (Intrusive igneous rocks):
Cooling takes place in the crust and not over the surface. E.g. Granite, Gabbro, Diorite etc. Intrusive igneous rocks are classed into the following types according to their forms.
(i) Batholiths: A large body of magmatic material that cools in the deeper depth in the form of a large dome. These are granitic bodies. They sometimes appear on the earth surface when the denudation processes remove the overlying materials.
(ii) Laccoliths: Large dome shaped intrusive bodies with a level base and pipe-like conduit from below. Resembles a composite volcano structure, but beneath the earth. (Eg: Karnataka Plateau)
(iii) Lapoliths: They are saucer shaped, concave to the sky.
(iv) Phacoliths: Wavy materials which have a definite conduit to source beneath.
(v) Sheets/ sills: They are the near horizontal bodies of intrusive igneous rocks. Thinner ones are called as sheets and while thick horizontal deposits are called sills.
(vi) Dykes: When the lava comes out through cracks and fissures, they solidify almost perpendicular to the ground to form wall like structures called dykes. (Eg: Deccan traps in Maharastra region).

(​B) Extrusive landforms

When the Lava and other volcanic materials are thrown out to the Earth’s surface during volcanic eruptions, the extrusive igneous landforms are formed. It includes volcanic Lava, pyroclastic debris, ash, volcanic bombs, and gases such as Sulphur dioxide, nitrogen compounds and other gases.

(i) Conical vent and fissure vent
The narrow cylindrical vent through which the lava flows out to the earth’s crust during a volcanic activity is known as a conical vent. Conical vents are more common in the composite (or strato volcanic) volcanic features.
The fissure is a narrow linear vent through which the lava comes out to the earth’s crust during a volcanic eruption. The fissure vents are more commonly found in the areas of basaltic volcanism.The fissure vents are often few meters wide, which can be several kilometres long.

(ii) Shield volcanoes 
Shield volcanoes are characterized by gentle upper slopes and a little steeper lower slopes. They are composed of relatively fluid lava flows which have been built over a central vent. Mostly, the low viscosity basaltic lava which is high in fluidity form Shield volcanoes. It leads to the formation of the extrusive igneous rocks.
Shield volcanoes are mostly non-explosive, but they can become explosive if water gets inside the vent.
Shield volcanoes are the largest volcanoes in the world. They extend to greater heights and distances. Examples of Shield volcanic landforms include Mauna Loa volcanoes of Hawaii.
(iii) Cinder cone
A Cinder cone has the features of a steep conical hill with loose pyroclastic fragments which include volcanic clinkers, cinder, volcanic ash (scoria) around the vent.
Cinder cone are made entirely of the loose grainy cinders, and they lack lava. Cinder cone usually has very steep sides along with a small crater on its top. They are small volcanoes.

(iv) Composite Cone
Composite cones (strato-volcanoes) are mainly cone shaped with moderate steep Sites. The andesitic lava, along with the pyroclastic materials and ashes which find their way to the ground gets accumulated in the vicinity of vent openings. This leads to the formation of layers, which makes the volcanic mounts appear as composite volcanoes.
Composite cones are also known as stratovolcanoes. Most common and highest volcanoes have the features of composite cones. For example, Stromboli, the Lighthouse of Mediterranean, mount Fuji etc.

(v) Flood basalt plains (Lava plateaus)
When a very thin and fluid lava comes out to the Earth’s surface, and flow after intervals for long periods of time, spreading to a large area; it produces a layered, undulating- wave-like flat surfaces.
These types of extrusive igneous rocks and landforms are known as flood basalt landforms or Lava plateaus. The Deccan traps of India, Snake basin of USA, Canadian Shield etc. are the examples of Flood basalt provinces.

(vi) Mid-oceanic ridge
The mid-oceanic ridges occur in the underwater oceans. There is a system of 70000 km long mid-oceanic ridges that stretch along all the major ocean basins. The central portion of the mid-oceanic ridges is associated with frequent volcanic eruptions.
The lava which comes out through these eruptions are Basaltic and have less silica content, so they are less viscous. Due to less viscosity, they flow through longer distances and cool slowly. This outpour of lava through volcanic eruptions is responsible for the phenomenon of seafloor spreading.

(vii) crater
Volcanic activity often creates craters. Some volcanic craters are deep and have steep sides. Others are wide and shallow.  A crater is not the same thing as a caldera. Craters are formed by the outward explosion of rocks and other materials from a volcano. Calderas are formed by the inward collapse of a volcano’s magma chamber. Craters are usually much smaller features than calderas, and calderas are sometimes considered giant craters. Craters at the top of volcanoes are called summit craters. Summit craters are where volcanic material is at or near the Earth’s surface. Volcanoes may have one summit crater, such as Mount Fuji in Japan. Or they may have several. Mount Etna, in Italy, has four.

(viii) Caldera
A caldera is formed by the collapse of a volcano into itself, making it a large, special form of volcanic crater. A caldera collapse is usually triggered by the emptying of the magma chamber beneath the volcano, as the result of a large volcanic eruption.
When the Lava ceases to flow after the volcanic eruption, the creator of volcanoes turns into a lake, which is known as Caldera lake. The rainwater and snowmelt often get accumulated in these enclosed depressions leading to the formation of lakes.
Lonar in Maharashtra, Krakatoa in Indonesia, and Lake Caldera in southern Oregon etc. are the examples of Caldera lakes.

Distribution of Volcanoes across the World
If we look at the global distribution of volcanoes we see that volcanism occurs four principal settings.
(i) Along divergent plate boundaries, such as Oceanic Ridges or spreading centers.
(ii) In areas of continental extension (that may become divergent plate boundaries in the future).
(iii) Along converging plate boundaries where subduction is occurring.
(iv) And, in areas called "hot spots" that are usually located in the interior of plates, away from the plate margins. 
Since the 16th century, around 480 volcanoes have been reported to be active. Of these, nearly 400 are located in and around the Pacific Ocean and 80 are in the mid-world belt across the Mediterranean Sea, Alpine-Himalayan belt and in the Atlantic and Indian Oceans. The belts of highest concentration are Aleutian-Kurile islands arc, Melanesia and New Zealand-Tonga belt. Only 10 per cent to 20 per cent of all volcanic activity is above sea and terrestrial volcanic mountains are small when compared to their submarine counterparts. Most known volcanic activity and the earthquakes occur along converging plate margins and mid-oceanic ridges. There is a strikingly close agreement between volcanic and earthquake zones of the earth.
Volcanic Explosivity Index
The volcanic explosivity index (VEI) ranks eruptions based on combined intensity and magnitude. The higher the VEI, the more explosive the eruption.  Height of the eruption plume, volume of explosively ejected material, and duration of eruption are the criteria for assessing VEI.  Of the 3,300 historic eruptions, 42% lasted less than a month. A significant number (33%) of eruptions lasted from 1-6 months. A few volcanoes (16), such as Stromboli and Mount Etna of Italy, have erupted continuously for over 20 years.  Unfortunately, of 252 explosive eruptions, 42% erupted most violently in the first day of activity.

Negative effects of volcanic eruptions

Volcanic eruptions are a highly damaging natural disaster and are highly destructive in nature. volcanic eruptions have been responsible for the destruction of whole cities and towns by the advancing lava. Violent earthquakes are associated with volcanic eruptions which have often caused damage to life and property. The mudflows of volcanic ashes which get saturated by rainfall can bury the nearby areas. The earthquakes are associated with volcanism and in coastal areas, they can cause tsunamis which have often caused the large destruction of life and property. Different gases released from volcanic eruptions such as carbon dioxide, hydrogen fluoride, Sulphur Dioxide etc are hazardous to human life and environment. The volcanic gases such as Sulphur Dioxide etc have also been responsible for causing acid rain. Large volcanic eruptions inject a large number of Sulphur aerosols in the stratosphere which can lead to the lowering of surface temperature and increase in the depletion of Ozone layer. The release of SO2 from volcanic eruptions has been responsible for lowering of earth’s temperature leading to crop failures and famines.
Positive impacts of the volcanic eruption

Volcanic eruptions are responsible for the formation of new landforms such as islands, plateaus, Volcanic Islands, and mountains etc. The volcanic lava, ash and dust are very fertile for the cultivation of different plants. The weathering of Volcanic rocks leads to the formation of fertile soil.

Volcanic eruptions are also the source of mineral resources. They bring useful and important minerals resources to the surface of Earth. For example, the diamond mines of the kimberlite rocks of South Africa, are actually the part of an ancient volcano.

The areas surrounded by the active volcanoes give rise to the formation of springs and geysers. These springs and geysers can even be used for the generation of geothermal electricity. The Yellowstone National Park of USA generates electricity from the geothermal electricity. The Puga Valley of Ladakh in India is also a promising spot for geothermal electricity.
The landforms formed by the volcanic eruptions are also great tourist spots and have a great natural beauty. For example, the Yellowstone National Park of USA is a great tourist spot.
Apart from these, Volcanic rocks are also used as raw materials for various building and Engineering purposes etc.
                                                                           Related Terms
Cinder cones: Cinder cones are of low height and are formed of volcanic dust and ashes etc pyroclastic material. Falling under the influence of gravity, these particles accumulate around the vent, in a large pile of tephra. The form of a cinder cone is very distinctive, with steep straight sides and a crater (depression) at the top of the hill.
Composite cones: A composite cone results when formative eruptions are sometimes effusive and sometimes explosive. Composite cones are therefore composed of a combination-that is, they represent a composite-of lava flow and pyroclastic materials. They are also called stratovolcanoes because they are constructed of layers (strata) of pyroclastics and lava. They are formed due to deposition of alternate layers lava and fragmental material wherein lava acts as cementing material.
Shield volcanoes: When numerous successive basaltic lava flow occur in a given region they can eventually pile up into the shape of a large mountain called a shield volcano.
Plug Domes: Where extremely viscous silica-rich magma has pushed up into the vent of a volcanic cone without flowing beyond it, it forms a plug dome.
Calderas: A caldera is a large, basin shaped depression formed at the volcanic mouth. It forms when summit material on a volcanic mountain collapses inward after an eruption or other loss of magma. A caldera may fill with rainwater. 
all fragmental material produced by a volcanic eruption regardless of composition, size or emplacement mechanism. Often characterized based on size. Ash, cinders, bombs, blocks.
Size Classification Ash:
(i) particles < 2mm in diameter, rock, glass, minerals
(ii) Lipilli: or cinders, 2mm – 64 mm in diameter, vesicular texture
(iii) Bombs: > 64 mm in diameter, molten projectile
(iv) Blocks: > 64 mm in diameter, solid projectile 
Volcanic Cones: The most well known volcanic landform features are the volcanic cones themselves. Mount Taranaki and Mount Ngauruhoe are classic symmetrical cones formed of alternating layers of lava and airborne debris (ash and scoria).
Crater Lakes: Eruption craters sometimes become filled with water to form lakes such as the Emerald Lakes of Tongariro and the crater lakes of Ruapehu and White Island. These crater lakes are quite rare globally, although we have some fine examples in New Zealand. 
Lava flows: Lava flows solidify into lobe shaped features with very rough jagged surfaces and a layered cross section. Where they were flowing quite quickly, they may have developed side ridges or ‘levees’ that look similar to the artificial ‘stop banks’ made to prevent rivers from flooding. These superb examples of young lava flows can be seen in the Mangatepopo Valley at the start of the Tongariro Crossing. Different stages of revegetation on the different flows indicate their relative ages.
Red rocks are common around volcanoes and geothermal areas, and also found as layers within older greywacke sediments, such as at Red Rocks on the south coast of Wellington. They are evidence of the effect of hot geothermal fluids interacting with iron to create iron oxide (rust).
Calderas: Calderas are the devastated remains of the most violent of all volcano types. So much material was erupted from them that the emptied, cooled and contracted magma chamber below the volcano collapses in on itself to create an enormous ring shaped depression. Lake Taupo and Lake Rotorua are examples of calderas. Widespread pumice deposits from their past eruptions can be found in many parts of the North Island as well as in offshore drill cores.

Tuesday, 04th Sep 2018, 06:51:13 PM

Add Your Comment:
Post Comment