Volcanoes

About Volcanoes:

An opening in an earth crust which allows molten rock, gases, and debris to escape to the surface is called volcanoes. The gases which are present in the volcanos are sulfur compounds, nitrogen compounds, and trace amounts of argon, hydrogen, and chlorine. And the phenomenon of eruption of molten rock i.e. magma onto the surface of the Earth or a solid-surface planet or moon is called volcanism

Vent is a break in the surface through lava, pyroclastics and volcanic gases erupt. When thermal convection currents, coupled with gravitational effects of changes on the earth’s surface then causes movement of molten rocks in the mantle. Volcanoes exist along the edges between tectonic plates, massive rock slabs that makeup Earth’s surface. 90% of volcanoes exist within the Ring of Fire along the edges of the Pacific Ocean..

Magma vs Lava: The difference

• Magma is the term used to denote 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.
• The process by which solid, liquid and gaseous material escape from the earth’s interior to the surface of the earth is called as Volcanism.

Factors that lead to volcanism:

• Plate Tectonics: The majority of volcanoes occur where two lithospheric plates converge and one overrides the other, forcing it down into the mantle to be reabsorbed.

• Ocean floor spreading: A major site of active volcanism is along the axis of the oceanic ridge system, where the plates move apart on both sides of the ridge and magma wells up from the mantle.
• Weak Earth Surface: Because of high pressure in the earth’s interior, the magma and gases escape with great velocity as the pressure is released through eruptions where opportunity is provided by weak zones along the earth’s surface.
• Faults: Whenever extreme pressure builds in the mantle, along fault lines an eruption is likely to happen next. The earthquakes, for instance, may expose fault zones through which magma may escape. 
• Magma crystallization: Decreasing temperatures can cause old magma to crystallize and sink to the bottom of the chamber and this movement can force fresh liquid magma up and out – similar to dropping a brick in a bucket of water.
• A decrease in external pressure: A decrease in external pressure can trigger an eruption as it may minimize the volcano’s ability to hold back by increasing the pressures inside the magma chamber
• Plate movement: Volcanism may occur because of plate movement over a “hot spot” from which magmas can penetrate to the surface. Ex. Islands of Hawaii 

The gases and dust particles thrown into the atmosphere during volcanic eruptions have effects on climate.

Major types of volcanoes

• Volcanoes are classified based on the nature of eruption and the form developed at the surface.

Shield Volcanoes

• The Shield volcanoes are the largest of all the volcanoes on the earth, which are not steep.
• These volcanoes are mostly made up of basalt.
• They become explosive if in some way water gets into the vent, otherwise, they are characterized by low-explosivity.
• The lava that is moving upwards does so in a fountain-form and emanates the cone at the vent’s top and then develops into a cinder cone.
• Eg: Hawaiian shield volcanoes

Composite Volcanoes

• Composite Volcanoes are characterized by outbreaks of cooler and more viscous lavas than basalt.
• They are constructed from numerous explosive eruptions.
• Large quantities of pyroclastic material and ashes find their way to the ground along with lava.
• This material gathers near the vent openings resulting in the creation of layers.
• Mayon Volcano in the Philippines, Mount Fuji in Japan, and Mount Rainier in Washington are the major composite volcanoes in the world.
• The major composite volcano chains are Pacific Rim which is known as the “Rim of Fire”.

Caldera

• Calderas are known as the most explosive volcanoes of Earth.
• They are generally explosive.
• When they erupt, they incline to collapse on themselves rather than constructing any structure.
• The collapsed depressions are known as calderas.

Flood Basalt Provinces

• Flood Basalt Province volcanoes discharge highly fluid lava that flows for long distances.
• Many parts of the world are covered by thick basalt lava flows.

Mid-Ocean Ridge Volcanoes

• These volcanoes are found in the oceanic areas.
• There exists a system of mid-ocean ridges stretching for over 70000 km all through the ocean basins.
• The central region of this ridge gets frequent eruptions.

Lava Domes

• When eruption lava is too thick to flow and makes a steep-sided mound as the lava piles up near the volcanic vent then such volcanoes are called lava domes.
• They are formed from slow eruptions of highly viscous lava.
• Sometimes they are also formed within the crater of a previous volcanic eruption like: composite volcano, they can produce violent, explosive eruptions, but their lava cannot flow far from the originating vent.

Further volcanoes are categorised into three categories:

• Active: Volcano which erupted recently and have chances of further eruption are known to be active one.
• Dormant: Volcano which has not erupted for a long time but in future there is possibility of eruption are called dormant one.
• Extinct: Volcano which erupted thousands of years ago and there’s no possibility of an eruption is called extinct.

Different Types of Volcanic Eruption:

Based on various factors like chemistry of magma, temperature, viscosity, volume, presence of groundwater, and water and gas content volcanic eruption are of various types. They are:

• Hydrothermal eruption: Eruption which does not include ash and not magma. They are caused because of heat in the hydrothermal system.
• Phreatic eruption: It is a kind of eruption which is caused when the magma interacts with the water. Such eruptions magma and ash are not present.
• Phreatomagmatic eruption: It is a kind of eruption which takes place when there is the interaction between the newly formed magma and water.
• Strombolian and Hawaiian eruption: Hawaiian type of eruption has fire fountains in them whereas the Strombolian eruption has explosions due to lava fragments in them.
• Vulcanian eruption: Such eruptions last only for a short duration and can reach up to a height of 20 km.
• Subplinian and Phinian eruptions: Sub Plinian eruptions reach up to 20 km height whereas eruptions which reach up to 20-35 km height are called as Phinian eruption.

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.

1. 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.
2. 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)
3. Lapoliths: They are saucer shaped, concave to the sky.
4. Phacoliths: Wavy materials which have a definite conduit to source beneath.
5. 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.
6. 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).

Volcanoes in India

• There are no volcanoes in the Himalayan region or the Indian peninsula.
• Barren Island (only active volcano in India) in the Andaman and Nicobar Islands became active in the 1990s.
• It is now considered an active volcano after it spewed lava and ash in 2017.
• The other volcanic island in Indian territory is Narcondam, about 150 km north-east of Barren Island; it is probably extinct. Its crater wall has been destroyed.

Destructive Effects of Volcanism

• Showers of cinders and bombs can cause damage to life. E.g. the eruption of Mount Vesuvius in 79 AD.
• Tsunamis can be generated in large water bodies due to violent eruptions. E.g. 1883 Krakatoa eruption.
• The collapse of the volcanic landforms in seas and oceans cause tsunamis. E.g. 2018 Sunda Strait tsunami.
• The ash from a larger eruption dispersing over a large area can lower temperatures at a regional or global scale. This could trigger famines on a large scale. E.g. 1815 eruption of Mount Tambora.
• In Hawaiian type eruption, a single flow spreads widely over open slopes or down the valleys as lava rivers engulfing entire cities.
• Lahars (a violent type of mudflow or debris flow) can bury entire cities in a matter of minutes causing a high number of causalities. E.g. 1985 eruption of Nevado del Ruiz volcano.
• The sudden collapse of lava domes can cause violent volcanic flows that destroy everything on their path. E.g. the 1902 eruption of Mount Pelée.
• Powerful winds drive the gas plume higher into the atmosphere and carry it to a greater distance disrupting air travel (this happened in 2010 when a stratovolcano in Iceland erupted and disrupted air travel over entire Europe for weeks).
• A supervolcanic super-eruption can cause a small-scale extinction event. E.g. The Toba eruption (Indonesia) triggered a dramatic global winter 74,000 years ago.

Volcanism – Acid Rain, Ozone Destruction

• The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulphur dioxide, carbon dioxide, and hydrogen fluoride.
• Locally, sulphur dioxide gas can lead to acid rain and air pollution downwind from a volcano.
• Globally, large explosive eruptions that inject a tremendous volume of sulphur aerosols into the stratosphere can lead to lower surface temperatures and promote depletion of the Earth’s ozone layer.

Positive Effects of Volcanism

• Volcanism creates new fertile landforms like islands, plateaus, volcanic mountains etc. E.g. Deccan traps.
• The volcanic ash and dust are very fertile for farms and orchards.
• Volcanic rocks yield very fertile soil upon weathering and decomposition.
• Although steep volcano slopes prevent extensive agriculture, forestry operations on them provide valuable timber resources.
• Mineral resources, particularly metallic ores are brought to the surface by volcanoes. Sometimes copper and other ores fill the gas-bubble cavities.
• The famed Kimberlite rock of South Africa, the source of diamonds, is the pipe of an ancient volcano.
• In the vicinity of active volcanoes, waters in depth are heated from contact with hot magma giving rise to springs and geysers.
• The heat from the earth’s interior in areas of volcanic activity is used to generate geothermal electricity. Countries producing geothermal power include USA, Russia, Japan, Italy, New Zealand and Mexico.
• The Puga valley in Ladakh region and Manikaran (Himachal Pradesh) are promising spots in India for the generation of geothermal electricity.
• Geothermal potential can also be used for space heating.
• As scenic features of great beauty, attracting a heavy tourist trade, few landforms outrank volcanoes.
• At several places, national parks have been set up, centred around volcanoes. E.g. Yellowstone National Park.
• As a source of crushed rock for concrete aggregate or railroad ballast and other engineering purposes, lava rock is often extensively used.

Geysers and Hot Springs

• Water that percolated into the porous rock is subjected to intense heat by the underlying hard rock which is in contact with hot magma in the mantle or the lower part of the crust.
• Under the influence of intense heat, the water in the capillaries and narrow roots in the porous rock undergoes intense expansion and gets converted to steam resulting in high pressure.
• When this steam or water at high pressure finds a path to the surface through narrow vents and weak zones, appear at the surface as geysers and hot water springs.

Source: NCERT Geography

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