VOLCANOES

VOLCANOES

A volcano is a vent in the earth’s crust from which molten rock material (magma), explosive bursts of gases, and volcanic ashes erupt.

Forms of volcanoes

• Shield Volcanoes: Largest of all the volcanoes, but are not steep. They 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 from the cone at the vent’s top and then develops into a cinder cone.
  • Example: Hawaiian shield volcanoes.
• Dome volcanoes (lava dome): Steep-sided mounds are formed when the viscous lava reaches the Earth’s surface, cannot flow away readily, and accumulates around the vent.
  • Example: Galeras lava dome, Colombia.
• Composite Volcanoes: They 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.
  • Example: Mayon Volcano in the Philippines, Mount Fuji in Japan, and Mount Rainier in Washington.

• Caldera: They are the most explosive volcanoes on Earth. When they erupt, they tend to collapse on themselves rather than construct any structure. The collapsed depressions are known as calderas.
  • Example: Crater lake, Newberry.

• Flood Basalt Provinces: They discharge highly fluid lava that flows for long distances. Many parts of the world are covered by thick basalt lava flows.
  • Example: Columbia river plateau.

Causes of Volcanism

• Trapped heat: The chemical reactions of radioactive substances deep within the interior of the earth generate tremendous amounts of heat. Some heat is already present in the form of residual heat at the earth’s interior.
• Temperature gradient: There is a huge temperature difference between the inner layers and the outer layers of the earth due to a differential amount of radioactivity.
• Convection currents: They are generated in the outer core as well as the mantle due to the temperature gradient. Convection currents in the mantle create convergent and divergent boundaries.
• At the divergent boundary: Molten, semi-molten, and sometimes gaseous material appears on earth, usually at the plate margin. The earthquakes may expose fault zones through which magma may escape.
  • Example: Fissure type volcano.

• At the convergent boundary: Subduction of denser plates creates magma at high pressure which will escape to the surface. Because of high pressure, the magma and gases escape with great velocity as the pressure is released through eruptions.

DISTRIBUTION OF VOLCANOES

1. Across the World

• Pacific Ring of Fire (Circum-Pacific region): It is estimated to include two-thirds of the world’s volcanoes.
  • Result of plate tectonics: Movement, collision, and destruction of lithospheric plates under and around the Pacific Ocean have created a nearly continuous series of subduction zones, where volcanoes are created and earthquakes occur.
  • Convergent plate boundaries: Consumption of oceanic lithosphere at these convergent plate boundaries has formed oceanic trenches, volcanic arcs, back-arc basins, and volcanic belts.
• Mid-Atlantic belt: A few basaltic volcanoes of fissure eruption type also occur along the mid-oceanic ridge, where seafloor spreading is in progress. They have comparatively few active volcanoes but many dormant or extinct volcanoes.
  • Example: St. Helena, Cape Verde Islands, and Canary Islands, etc. But the volcanoes of Iceland and the Azores are active.
• Great Rift region: The only active volcano of West Africa is Mt. Cameroon. There are some volcanic cones in Madagascar, but active eruptions have not been known so far. In Africa, some volcanoes are found along the East African Rift Valley.
  • Example: Mt. Kilimanjaro and Mt. Kenya, both probably extinct.

• 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. The West Indian islands have experienced some violent explosions in recent times.
  • Example: Mt. Pelee.
• Mid-Continental belt: Volcanoes of the Mediterranean region, stretching from Spain to the Caucasus, are largely the result of convergence between the Eurasian Plate and the northward-moving African Plate.
  • Example: Vesuvius, Stromboli (Light House of the Mediterranean), and those of the Aegean islands.
• Intra-plate volcanoes (Hot spot volcanoes): They are scattered in the inner parts of plates away from the margins. They occur in the middle of plate boundaries where magma exits from weaknesses in the earth’s surface.
  • Example: Hawaiian Islands.

2. Volcanoes in India

• Mainland India: There are no volcanoes in the Himalayan region or in the Indian peninsula.
• Barren Island: Located 135 km northeast of Port Blair, it became active in 1991 and 1995. After its activity in the nineteenth century, it passed through a mild solfataric stage as evidenced by the sublimations of sulphur on the walls of the crater.
• Narcondam Island: Located about 150 km northeast of Barren Island, it is probably extinct. Its crater wall has been completely destroyed.