Competitive Exams: Volcanic Belts

Various Volcanic Belts

  1. Circum-Pacific Belt (Fire Girdle of the pacific or the fire ring of the Pacific) Island arcs and Festoons E. g. Sakhalin, Kamchatka, Japan, Philippines, Aleutian Island, Hawaii, some Highest volcanic Peaks-Cotopaxi (S. America), Fujiyama (Japan), Shasta, Rainier and Hood (W. Cordillera of N. America), Valley of ten thousand smokes (Alaska), Mt. St. Helens (USA), Kilavea (Hawaii), Mt. Taral, Pinatubo and Mayon (Philippines).

  2. Mid-Continental Belt (Volcanic Zones of convergent continental plate margins) Volcanes of Alpine Mt. Chains, Mediterranean Sea, Agean Sea, Fault zone of Africa (Kilimanjaro, Mem, Elgon, Birunga, Rungwe etc.)

  3. Mid-Atlantic Belt-Hekla (Iceland), Lesser Antilles, Southern Antilles, Azores, St. Helena, Mt. Pelee (Martinique) in Caribbean Sea.

  4. Intra Plate Volcanoes-various chain volcanoes, Columbia plateau, peninsular India, Parana of Brazil and Paraguay. Flood

Basalts and Shield Volcanoes

Geologists postulate that at various points beneath the lithosphere there occur mantle plumes which are isolated columns of heated rock rising slowly within the asthenosphere. Directly above a mantle plume, crystal basalt can be heated to the point of melting and produce a Magma pocket. The site of Magma is called a hot-spot.

Where a mantle plume liese beneath a continental lithospheric plate, the hot spot may generate enormous volumes of basaltic lava that accumulate layer upon layer and is known as Flood Basalts. Hot spots also form above mantle plumes in the oceanic lithosphere. The emerging basalt builds a class of volcanoes known as shield volcanoes.

Volcanic Features (Topography produced by vulcanity)

  1. Extrusive

    1. From Explosive type of Eruption

    2. From Fissure type of Eruption

  2. Intrusive

Features of Explosion

Features formed due to the explosive type of eruption can be divided into Elevated and Depressed forms. Elevated Forms

  • Cinder or ash cones (low height and formed of volcanic dusts)

  • Composites Cones (highest of all cones) -stratification of different materials-Parasite Cones (several branches of pipes come out from the main central pipe) -Basic lava cones (shield cones) (high degree of fluidity of lava produces a long cone with significantly low height)

  • Acid Lava cones (highly viscous lava produces high cones)

  • Lava Domes (similar to shield cones but are larger and more extensive).

  • Lava Plugs (Plugging of volcanic pipes and vents when volcanoes become extinct) Depressed Forms

  • Craters (crater lakes)

  • Calderas

  • Intrusive (these can be concordant or discordant) Concordant are those which have boundaries parallel with beddings of the rocks. Those which cut through the bedding are known as Discordant.

  • Major intrusive features are: Concordant features:

  • Lopoliths-Saucer like bodies concordant to the structure of the rocks and of enormous size (e. g. Duluth (USA). Bushveld (S. Africa) )

  • Discordant features:

  • Batholiths or Bathyliths (very large deep seated discordant intrusions. Largest intrusive bodies)

  • Bosses and stocks-small intrusions similar to batholiths. Bosses have circular whereas stocks have irregular intrusions.

  • Minor intrusive features: Concordant features

  • Sills-thin sheet like intrusions injected between bedding planes.

  • Laccoliths-when viscous magma pushes into overlying strata and is bent upward to form a done.

  • Bysmaliths-Faulted variants of laccoliths

  • Discordant features:

  • Dykes (Vertical instrusions cutting across bedding planes).

  • Volcanic Necks (Eroded remnant of solidified lava which formerly filled the vent of a volcano).

  • Diapirs These are formed from domes when the overlying rocks are ruptured and the intrusive body forces upwards. An earthquake is a ruthless demonstration of the power of the tectonic forces caused by endogenetic thermal conditions.