Volcanism: Andesitic & Basaltic Lava, Distribution of Volcanoes

PMF IAS Environment
  • A volcano is a vent or a fissure in the crust from which lava (molten rock), ash, gases, rock fragments erupt from a magma chamber below the surface. Volcanism is the phenomenon of eruption of molten rock, pyroclastics and volcanic gases to the surface through a vent.

Causes of Volcanism

  • 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.

Volcanism Lava types

  • Magma is composed of molten rock and is stored in the Earth’s crust. Lava is magma that reaches the surface through a volcano vent.

Andesitic or Acidic or Composite or Stratovolcanic lava

Volcano (Medium69.Cette William Crochot, via Wikimedia Commons)

  • 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 volcanic cone is therefore stratified (hence the name stratovolcano) and 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 lava plug at the crater like that of Mt. Pelée in Martinique (an island in the Lesser Antilles, Caribbean Islands).
  • Andesitic lava flow occurs mostly along the destructive boundaries (convergent boundaries).

Lava Plug at the crater

Basic or Basaltic or Shield lava

  • These are the hottest lavas, about 1,000 °C and are highly fluid.
  • They are dark coloured 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.
  • The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome.
  • Shield type lava flow is common along the constructive boundaries (divergent boundary).

Distribution of Earthquakes and Volcanoes across the World

  • Most known volcanic activity and the earthquakes occur along converging plate margins and mid-oceanic ridges.
  • It is said that nearly 70 per cent of earthquakes occur in the Circum-Pacific belt.
  • Another 20 per cent of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and parts of north-west China.
  • 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 the sea, and terrestrial volcanic mountains are small when compared to their submarine counterparts.

Volcanism along the 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’.
Regions with active volcanism 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.
The 5 countries with the most volcanoes (Source)
  1. United States – 173 (most of them are in Alaska)
  2. Russia – 166
  3. Indonesia – 139
  4. Iceland – 130
  5. Japan – 112

Other regions

Along the Atlantic coast
  • In contrast, the Atlantic coasts have comparatively few active volcanoes but many dormant or extinct volcanoes, e.g. St. Helena, Cape Verde Islands and the Canary Islands etc.
  • But the volcanoes of Iceland and the Azores are active.
Great Rift region
  • In Africa, some volcanoes are found along the East African Rift Valley, e.g. Mt. Kilimanjaro and Mt. Kenya.
The West Indian islands
  • The Lesser Antilles (Part of West Indies Islands) are made up mainly of volcanic islands, and some of them still bear signs of volcanic liveliness.

Mediterranean volcanism

  • Volcanoes of the Mediterranean region are mainly associated with the Alpine folds, e.g. Vesuvius, Stromboli (Light House of the Mediterranean) and those of the Aegean islands.
  • A few continue into Asia Minor (Mt. Ararat, Mt. Elbruz).
  • 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

Volcanism (Volcanos) 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.

Extinct, Dormant and Active volcanoes

  • Barren Island in the Andaman and Nicobar Islands, Anak Krakatoa are active volcanoes
  • Mount Kilimanjaro (it has three volcanic cones), is a dormant stratovolcano in Tanzania.
  • Mount Kenya is an extinct stratovolcano.
  • The Barren Island in the Andaman and Nicobar Islands of India which was thought to be extinct erupted recently.
  • Before a volcano becomes extinct, it passes through a waning stage during which steam and other hot gases and vapours are exhaled. These are known as fumaroles or solfataras.

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.

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

Usually, a carter like structure is created at the mouth.

  • Almost all the world’s geysers are confined to three major areas: Iceland, New Zealand and Yellowstone Park of U.S.A.

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