Volcano: Types, Formation, Volcanic Eruption and Volcanic Landforms

Important Terms

Volcano: A volcano is an opening in the earth’s crust through which magma, gases and ash are released to the earth’s surface. Volcano generally erupts either through the vent (E.g. Mt. Fujiyama, Japan) or fissure (The Deccan Plateau, India).

Magma: The molten rock material found in the interior of the earth is called magma.

Lava: when magma reaches the earth’s surface, it is known as lava.

Vent: Vent is an opening or mouth of a volcano.

Fumaroles are the gushing fumes through the gap in the volcano.

Crater is a saucer shaped depression ( funnel-shaped hollow ) in the mouth of a volcano.

Caldera: When the crater is widened, it is called as Caldera.

Volcanic ash consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions.

Pumice is a volcanic rock produced when lava with a very high content of water and gases is discharged from a volcano.

You can also check – Geography Notes for UPSC

The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.

Cause of Volcanic Eruptions or Volcanism

Weak Zones in the Earth Crust: The parts of the earth where two tectonic plates collide against or drift apart from each other are considered very weak. Volcanoes may erupt in such zones, for example, African and Eurasian plates.
Magma Saturated with Gases: The magma, in the interior of the earth, is often found saturated with gases like carbon dioxide, and hydrogen sulphide. These gases together with water vapour make the
magma highly explosive. Magma is forced out as lava on the surface of the earth due to the pressure exerted by these gases.

The mechanism of vulcanism and the volcanic activity are associated with several processes, such as:

• There is a huge temperature difference between the inner layers and the outer layers of the earth due to the differential amount of radioactivity.
• This temperature difference gives rise to convectional currents in the mantle.
• The convection currents in the mantle create convergent and divergent boundaries (weak zones).
• At the divergent boundary, molten, semi-molten and sometimes gaseous material appears on earth at the first available opportunity.
• The earthquakes here may expose fault zones through which magma may escape (fissure type volcano).
• At the convergent boundary, the subduction of denser plate creates magma at high pressure which will escape to the surface in the form of violent eruptions.

Types of Volcanoes

Based on the frequency of eruption, there are three types of volcanoes:

1. Active Volcanoes: Volcanoes which erupt frequently are called active volcanoes. Generally, their vent remains open. Mount Etna of Italy, Cotopaxi in Ecuador are some examples.
2. Dormant Volcanoes: These volcanoes may not have erupted in the recent past but there is a possibility of eruption at any time. In other words, they may lie dormant awaiting active eruption
   anytime. Sometimes gases and steam come out of them. Mt. Vesuvius of Italy and Mt. Fujiyama of Japan are examples.
3. Extinct Volcanoes: These volcanoes have exhausted their energy and have not erupted during the known geological period. The vent of these volcanoes remains closed with solidified lava. Popa in Myanmar and Mt. Kenya in eastern Africa are the examples of extinct volcano.

Classification of Volcanoes

Volcanoes are classified on the basis of the nature of eruption and the form developed at the surface. On the basis of the origin of eruption, volcanoes can be classified into two types. i.e.

1. Eruption through cones,
2. Eruption through fissures.

Major types of volcanoes are as follows:

1. Shield Volcanoes

Shield volcanoes are volcanoes shaped like a bowl or shield in the middle with long gentle slopes made by basaltic lava flows. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason, these volcanoes are not steep. Due to low-viscosity, lava can flow a great distance from a vent.

• They become explosive if somehow water gets into the vent; otherwise, they are characterised by low explosivity.
• Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.
• Shield volcanoes are more common in oceanic than continental settings
• Examples – The Hawaiian volcanoes- Mauna Loa (active shield volcano) and Mauna Kea (dormant shield volcano)

2. Composite Volcanoes

These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground. This material accumulates in the vicinity of the vent openings leading to the formation of layers, and this makes the mounts appear as composite volcanoes.

• These are steep-sided volcanoes composed of many layers of volcanic rocks, usually made from high-viscosity lava, ash and rock debris
• They are also called ‘strato volcanoes’.
• Examples – Mount Vesuvius, Mount Fuji and Mount St. Helens.

3. Caldera

These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity.

• In some volcanoes, the magma chamber below the surface may be emptied after volcanic eruptions.
• The volcanic material above the chamber collapses into the empty magma chamber, and the collapsed surface appears like a large cauldron-like hollow (tub shaped) called the caldera.
• When water from rain or melted snow gets accumulated in the caldera, it becomes a caldera lake (in general, the caldera lakes are also called crater lakes).
• Due to their unstable environments, some crater lakes exist only intermittently. Caldera lakes, in contrast, can be quite large and long-lasting.
• For example, Lake Toba (Indonesia) formed after its supervolcanic eruption around 75,000 years ago. It is the largest crater lake in the world.
• Mount Mazama (Cascade Volcanic Arc, USA) collapsed into a caldera, which was filled with water to form Crater Lake (the literal name of the lake formed by the collapse of Mount Mazama is ‘Crater Lake’).

4. Flood Basalt Provinces ( Lava Plateaus )

These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows. There can be a series of flows with some flows attaining a thickness of more than 50 m. Individual flows may extend for hundreds of km.

• Example: Siberian Traps, Deccan Traps, Snake Basin, Icelandic Shield, Canadian Shield.

5. Mid-Ocean Ridge Volcanoes

These volcanoes occur in oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge experiences frequent eruptions. We shall be discussing this in detail in the next chapter.

6. Cinder Cone Volcano

It forms when magma is thrown out to the surface, cooled in to ash and cinders and settled around the mouth of volcano. It is less dangerous than other volcanoes.

7. Lava Dome

Unlike composite and shield volcanoes, lava domes are of significantly smaller structure. They are formed when the lava is too viscous to flow to a great distance. As the lava dome slowly grows, the outer surface cools and hardens as the lava continues to pile within. Eventually, the internal pressure can shatter the outer surface, causing loose fragments to spill down its sides.

Vulcanicity

Vulcanicity includes all those processes in which molten rock material or magma rises to the crust to solidify as crystalline or semi-crystalline rocks. Some scientists use ‘vulcanism’ as a synonym for vulcanicity. Vulcanicity has two components;

1. The endogenetic mechanism: includes the creation of hot and liquid magma and gases in the mantle and the crust, their expansion and upward ascent, their intrusion and cooling and solidification in various forms below the crustal surface.
2. The exogenous mechanism: includes the process of the appearance of lava, volcanic dust and ashes, fragmental materials, mud, smoke, etc., in different forms on the earth’s surface.

Volcanic Materials

There are mainly three types of material which come out in volcanic eruptions, in liquid, solid and gaseous forms.

1. Liquid material: It is molten rock material. When the molten rock material is below the earth’s surface it is called ‘magma’. When it appears on the surface it is called ‘lava’. On the basis of the percentage of silica it is classified into two types
   1. Acidic Lava: It contains a higher percentage of silica. It has got a high melting point. It is thick, fluid and moves slowly.
   2. Basic Lava: It contains less percentage of silica. It has a low melting point. It’s more fluid and can flow over long distances.
2. Solid material: It consists of dust particles and rock fragments. When the material is very fine it is called volcanic dust. The small-sized solid particles are called ash. The solid angular fragments are known as breccias. Sometimes, the lava material thrown into the air solidifies into small fragments before falling on the earth’s surface; it is called volcanic bombs.
3. Gaseous Material: At the time of the volcanic eruption, a dark cloud of smoke can be seen over the crater. On the basis of shape, the cloud is called a cauliflower cloud. Various inflammable gases are found in these clouds. These gases produce flames.

Volcanic Landforms

Volcanic landforms are divided into extrusive and intrusive landforms based on whether magma cools within the crust or above the crust.

• Rocks formed by the cooling of magma within the crust are called Plutonic rocks.
• Rocks formed by the cooling of lava above the surface are called Igneous rocks.
• In general, the term ‘Igneous rocks’ is used to refer to all rocks of volcanic origin.

Extrusive Volcanic Landforms

• Extrusive landforms are formed from material thrown out to the surface during volcanic activity.
• The materials thrown out include lava flows, pyroclastic debris, volcanic bombs, ash, dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.
• These landforms are – Composite, Shield, Lava Plateaus, Caldera, Cinder cone, Lava Dome and 5. Mid-Ocean Ridge.

All Extrusive Volcanic Landforms are discuss above as Classification of Volcanoes.

Intrusive Volcanic Landforms

The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either upon reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust).

The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms.

Batholiths

These are granitic bodies. Batholiths are the cooled portion of magma chambers. A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. They appear on the surface only after the denudational processes remove the overlying materials. They cover large areas, and at times, assume depth that may be several km.

Laccoliths

These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It resembles the surface volcanic domes of the composite volcanoes, only these are located at deeper depths. It can be regarded as the localised source of lava that finds its way to the surface. The Karnataka plateau is spotted with domal hills of granite rocks. Most of these, now exfoliated, are examples of laccoliths or batholiths.

Lapolith

As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called lapolith.

Phacolith

A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of the anticline in the folded igneous strata. Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the phacoliths.

Sills

The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.

Dykes

When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes. These are the most commonly found intrusive forms in the western Maharashtra area. These are considered the feeders for the eruptions that led to the development of the Deccan traps.

Effects of Volcanic Activities

Destructive effects of volcano:

• Showers of cinders and bombs can cause damage to life and properties.
• Sometimes ash can precipitate under the influence of rain and completely cover large areas.
  • It can lower temperatures at a regional or global scale. This could trigger famines on a large scale. E.g. 1815 eruption of Mount Tambora.
• The collapse of the volcanic landforms in seas and oceans cause tsunamis. E.g. 2018 Sunda Strait tsunami.
  • Tsunamis can be generated in large water bodies due to violent eruptions. E.g. 1883 Krakatoa eruption.

Positive Effects of Volcanoes:

• Volcanism creates new landforms.
• Volcanic rocks yield very fertile soil upon weathering and decomposition.
• The Kimberlite rock of South Africa, the source of diamonds, is the pipe of an ancient volcano.
• In the vicinity of active volcanoes, waters in the depth are heated from contact with hot magma giving rise to springs and geysers.
  • 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.
• Mineral resources, particularly metallic ores are brought to the surface by volcanoes. Sometimes copper and other ores fill the gas-bubble cavities.
• 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.

Volcanism Cause Acid Rain and Ozone Destruction

• The volcanic gases pose potential hazard to people, animals; agriculture, while sulphur dioxide gas can lead to acid rain and air pollution.
  • 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.
• Recent eruption of the Hunga Tonga volcano in January 2022 also injected a lot of water vapor into the stratosphere due to this Ozone hole grows to record size.

Distribution of Volcanoes and Earthquakes Across the World

Most known volcanic activity and the earthquakes occur along converging plate margins and mid-oceanic ridges. The major regions of volcanic distributions are as follows.

1. Pacific Ring of Fire

Circum-Pacific region, popularly termed the ‘Pacific Ring of Fire’, has the greatest concentration of active volcanoes. Volcanic belt and earthquake belt closely overlap along the ‘Pacific Ring of Fire’. It
is estimated to include two-thirds ( 70% ) of the world’s volcanoes.

Regions with Active Volcano Along ‘Pacific Ring of Fire’

• The Aleutian Islands into Kamchatka, Japan,
• the Philippines, and Indonesia (Java and Sumatra in particular),
• Pacific islands of Solomon, New Hebrides, Tonga and North Island, New Zealand.
• Andes to Central America (particularly Guatemala, Costa Rica and Nicaragua), Mexico and right up to Alaska.

2. Mid Atlantic Region

The Mid Atlantic Region coasts has comparatively fewer active volcanoes but many dormant or extinct volcanoes, example. St. Helena, Cape Verde Islands and the Canary Islands. But the volcanoes of Iceland and the Azores are active.

3. The Great Rift valley of Africa

In Africa some volcanoes are found along the East African Rift Valley. Kilimanjaro and Mt. Kenya are extinct volcanoes. The only active volcano in West Africa is Mt. Cameroon.

4. Mediterranean Region

Volcanoes of the Mediterranean region are mainly associated with the Alpine folds. Example, Mt. Vesuvius and Mt. Stromboli (known as the Light House of the Mediterranean Sea).

• Nearly 20 percent of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and parts of north-west China.
• The volcanism of this broad region is largely the result of convergence between the Eurasian Plate and the northward-moving African Plate.
• This type of volcanism is mainly due to breaking up of the Mediterranean plate into multiple plates due to the interaction of African and Eurasian plate

Volcanoes in India

• There are no volcanoes in the Himalayan region and the Indian peninsula.
• Barren Island (only active volcano in India) in the Andaman and Nicobar Islands became active in the 1990s.
• 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.
• It is now considered an active volcano after it spewed lava and ash in 2017.

Types of lavas

There are two main types of lavas:

1. Basic Lavas: These are the hottest lavas and are highly fluid. They are dark coloured like basalt, rich in iron and magnesium but poor in silica. They flow quietly and are not very explosive. They affect extensive areas, spreading out as thin sheets over great distances before they solidify. The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome.
2. Acid Lavas: These lavas are highly viscous with a high melting point. They are light coloured, of low density and have a high percentage of silica. They flow slowly and seldom travel far before solidifying. The resultant volcano is therefore steep-sided ( stratovolcano ). The rapid cooling of lava in the vent obstructs the flow of the outpouring lava, resulting in loud explosions throwing out many volcanic bombs or pyroclasts.

Geysers, Hot Springs and Fumaroles

• 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.

A fumarole is a vent in the Earth’s surface which emits gases and water vapour. Sometimes the emission is continuous, but in majority of cases emission occurs after intervals. It is widely believed that gases and water vapour are generated due to cooling and contraction of magma after the eruption. Fumaroles are the last signs of the activeness of a volcano.

Geyser	Hot water spring
Steam or water at high pressure, along its path, gets accumulated in small reservoirs, fissures and fractures. Once the pressure exceeds the threshold limit, the steam bursts out to the surface disrupting the water at the mouth. Hence the name geyser.	Steam or water at high pressure smoothly flows to the top through the vent and condense at the surface giving rise to a spring.
Silicate deposits at mouth give them their distinct colours.	Some springs are very colourful because of the presence of cyanobacteria of different colours.
Generally, geysers are located near active volcanic areas. Iceland is famous for its geysers.	Found all across the world

Volcano UPSC

Previous Years UPSC Mains Questions related to Volcanism

Explain the formation of thousands of islands in Indonesian and Philippines archipelagos. (2014)

Volcanism Extra points

• The term ‘volcano’ is derived from the Latin term VULCAN, which is the name of Roman “God of Fire”.
• Mt. Krakatau The greatest volcanic explosion known to humans in August 1883.
• Krakatau is a small volcanic island in the Sunda Straits, between Java and Sumatra.
• Cotopaxi in Ecuador is the world’s highest active volcano.
• A crater lake, in general, could be of volcanic origin (volcanic crater lake, volcanic caldera lake) or due to a meteorite impact (meteor crater or impact crater), or in the crater left by an artificial explosion caused by humans.
• Lonar Lake, also known as Lonar crater (Lonar, Buldhana district, Maharashtra) was created by a meteor impact during the Pleistocene Epoch.

Pseudo volcanic features

• Pseudo volcanic features are certain topographic features that resemble volcanic forms but are of non-volcanic origin. They include meteorite crater, salt plugs, and mud-volcanoes.

Meteorite Craters

• Meteorite craters are impact craters that are formed when a meteorite strikes the surface of the earth creating a huge depression.

Salt plug or salt dome

• A salt plug is formed when underground salt deposits at high pressure become ductile and pierce through the overlying sediments to create a diapir (a dome-like intrusion forced into brittle overlying rocks.
• Salt extrusions may take the form of salt hills which exhibit volcanic crater like features.
• Salt structures are impermeable and can lead to the formation of a stratigraphic trap (an impermeable layer capable of retaining hydrocarbons. Structural traps, in contrast, are cracks in faults and folds that can retain hydrocarbons).

Mud-volcanoes

• A mud volcano or mud dome is a landform created by the eruption of mud, water and gases.
• Mud-volcanoes have a similar shape to other types of volcanoes and contains several cones.
• They are usually found near the subduction zones and hot springs.
• Other mud volcanoes, entirely of a non-volcanic origin, occur near oil-fields where methane and other volatile hydrocarbon gases mixed with mud force their way upward.