Competitive Exams: Volcanicity

Covers all processes in which molten rock material or magma rises into the crust or is poured out on its surface and solidifies. Three main processes:

  1. Generation of Magma

  2. Intrusion of masses of magma

  3. Extrusion of molten material on to the surface. Magma-A molten silicate material which is a combination of liquid, solid and gas. Its generation is a result of complex interaction of increase in temperature, decrease in pressure and addition of water (water increase the melting point of most silicates). The volcanic material is known as MAGMA below the surface and separates into lava, gases, vapor, ashes and fragmented material as it emerges on the surface. Generally, the molten materials are called magma below and lavas above the surface. Fragmental or Pyroclastic materials are thrown during explosive types of eruption. On the basis of size pyroclastic materials can be represented as: Volcanic Dust < Volcanic Ash< Lapillis < Volcanic Bombs

Constituent of Magma

  • Two most important constituents: Silica (Si) and water-Other elements-Na, Si, K, Al, Fe, Ca, Mg, and gases. Steam and Vapour-69 − 90% of the total gases.

  • Steam and Vapour comprises phreatic and the magmatic vapor. Phreatic vapor comes from the water in the Phreatic Zone.

  • Magmatic vapor comprises carbon dioxide, nitrogen oxide, sulphur dioxide, hydrogen and carbon monoxide etc. Other Compound-Sulphurated Hydrogen, Hydrochloric acid, Volatile chlorides of Iron, Potassium and other metallic matter.

Types of Volcanoes

  1. They are classified on the basis of:

    1. Mode of eruption

    2. Period of eruption and the nature of their activities.

  2. On the basis of the mode of eruption, Volcanoes can be sub-classified as:

    1. Central Eruption type of Explosive Eruption Type (through a central pipe or small opening accompanied with violent and explosive gases).

    2. Fissure Eruption type or Quiet Eruption type (e. g. Mid oceanic ridges). This type can further be divided into following types:

      1. Lava flood or Lava flow (highly fluid lava)

      2. Mud Flow

      3. Fumaroles (of gases issuing from a small hole)

Central Type of Eruption

Central eruption type of volcanoes can further be divided into following types:

  • HAWAIAN (quiet and nonviolent) -having long glassy threaded red lava known as Pele's hair (e. g. Kilavea in Hawaii).

  • STROMBOLIAN (erupts with moderate intensity) -Eruption comprises of lava, pumice, scoria and bombs etc

  • VULCANIAN (erupts with great force and intensity) -Highly viscous lava ash laden with volcanic clouds.

  • PELEEAN (most violent and explosive type) -Domes are formed due to eruption and successive eruption blows off these domes.

  • VESUVIUS extremely violent-Enormous volume of explosive gases is given off:

  • PLINIAN (most destructive).

Periodicity of Eription

On the basis of periodicity of eruptions, they can be sub-classified as follows:

  • ACTIVE: Constantly ejects lavas, ashes etc (e. g. Etna and Stromboli. Stromboli is known as the light house of the Mediterranean.).

  • DORMANT: Which are quiet for some time and may suddenly erupt (E. g. Vesuvius).

  • EXTINCT: These do not have any indications of future eruptions.

World Distribution

Distribution of volcanoes in the world can be explained through classification into fol-lowing systems:

  1. Linear Volcanoes e. g. Hawaiian-Emperor-Seamount Chain (Pacific Ocean). Line-Tuamotu Chain (Pacific Ocean) and Austral-Marshall-Gilbert Chain (Pacific Ocean).

  2. Chain Volcanoes e. g. Andes, Cascade Mountains (USA)

  3. Cluster Volcanoes: e. g. Madeira, Galapagos, Canaries, Azores, Mauritius and Reunion etc.

  4. Ridge Volcanoes: e. g. Mid oceanic ridges and rises

  5. Arc volcanoes: e. g. Kurile, Kamchatka, Japan, Philippines, Sulawesi (Celebes), New Guinea, Solomon Islands, New Caledonia, New Zealand, Java, Bali, and Sumatra, Lesser Antilles, Scotia Tyrrhenian and Aegean Seas and Aleutian Is.