Lecture 5 & 6 : Geography

Topics Covered in this Lecture

  1. Sources of Internal Heat of the Earth
  2. Volcanism and its classification
  3. Causes and Consequences of Volcanic Eruptions
  4. Distribution of Volcanoes
  5. Volcanic Landforms

Topics Covered in this Lecture

  1. Introduction about Earthquake
  2. Measurement of Earthquake
  3. Causes of Earthquake
  4. Classification and Distribution
  5. Consequences and Forecasting of Earthquake

The entire process which occurs during a volcanic eruption is known as Vulcanism. Vulcanism includes both intrusive and extrusive features during a volcanic eruption. Another hand, volcano is just a vent or opening on the surface of the earth through which the volcanic materials erupt or come out.

Vocanic materials

Gaseous/water vaporMagma/LavaPyroclastic material
accounts for 60% of material ejected along volcanic eruptions.is of two types- asic magma and acidic magma.includes broken rock, fragmentes, volcanic boms, volcanic ash and dust particles.
important gases are- Co2, CO, H2So4, Sox, nitrus oxides, methane.basic magma is dominated by iron and has less silica. In Acidic magma, silica dominates (more than 60%).
basic magma is MAFIC, fluid, and flows for greater distance. Acidic magma is light in color, highly viscous, and cools quickly.
Basic magma is associated with volcanic plateau and plains while acidic magma is associated with violent eruptions.

Volcanic landforms

These are of two types-

  1. INTRUSIVE– Found in bedding planes of sedimentary rock. Its types-

    Batholith– It is a huge mass of intrusive igneous rock found at greater depths and are rarely exposed. It usually made of granite. Batholiths form the root of a mountain.

    Lopolith– An intrusive igneous rocks having saucer shape concave to the sky.

    Phacolith– A lens-shaped mass of igneous rock which occupies a top of an anticline and base of a syncline connected by a pipe from below.

    Laccolith- Laccolith is a mound of an igneous rock connected by pipe from below. Laccolith may be exposed due to erosion and weathering.
    For example, Maharashtra plateau region, Satmala hills, Ajanta hills, Balaghat range, Harischandra range.

    Sill- A horizontal mass of igneous rock lying parallel to the bedding planes is sill.

    Dyke- A vertical mass of igneous rocks generally perpendicular to the bedding planes is known as a dyke.

  2. EXTRUSIVE– These are of various types. Some are-

    Volcanic cones/mountains– It is generally made of acidic lava. Lava is viscous and layers upon layers are build to form volcanic mountains.

    It has further divisions:

    a. Ash and cinder made of only broken rocks and volcanic ash. They do not have lava and are small in height.

    b. Composite volcanic cones are the highest of all volcanic mountains which are formed due to the alternate layers of pyroclastic material or lava. The lava acts as a cementing material to give rise to huge volcanic mountains. For example, mt. Kilimanjaro, mt. Fuji, mt. Mayon, and mt. Stromboli.

    c. Parasitic cones are having several branches come from the main pipe resulting in small cones on the top of a large cone is a parasitic cone.

    d. Shield volcano cones formed due to basalt. It has low hills and a vast area. For example, Hawaii and Reunion island.

    Crater or caldera– An opening/depression at the top of the volcanic mountain is known as a caldera. Sometimes lakes are formed in craters known as crater lakes. For example, Lake Fujiyama.

    Volcanic Plateaus- Extremely vast, massive landform made up of basalt associated with fissure eruptions on the land are known as volcanic plateaus.

Geysers and hotsprings

In the regions having volcanic history based on the ejection of water, the eruptions is classified into hostsprings and geysers.

Hotsprings are when steam or water flows smoothly from the vent or a fissure. In hot springs and geysers, water can be colorful. These hostsprings have different bacteria known as cyno bacteria that are responsible for giving the water different colors.

Hotsprings and geysers are formed when water seeps or percolates through upper rocks or surface rocks and comes in contact with magma.

If only superheated steam is given out they are known as fumaroles. In India, the examples are- Manikaran (HP) and Puga valley (Laddakh).

World distribution of volcanoes

Volcanoes are associated with the weaker zones of earth’s crust. These are associated with both folding and faulting major volcanic regions-

  1. Pacific ring of fire– It is associated with convergent plate boundary where the Pacific plate is subducting from all the sides below Eurasian and American plates. Explosive composite and highest volcanoes in the world. Due to explosive volcanoes also experience Tsunami.

  2. Mid-continental belt- Located between Mediterranean sea, Africa, and Indian ocean. This includes the volcanoes of Alpine mt. system, volcanoes of rift valley of Africa but also has volcano free zone of Himalayas. This region includes major mts. such as mt. Stromboli, mt. Etna, mt. Vesuvius, etc.

  3. Mid oceanic belt- It is divergent plate boundary. Hence, this have silent or fissure kind of eruption. As the intensity of earthquakes is less, they are not associated with Tsunami.

  4. Regions of Hotspots- Hotspot is a region on the surface of the earth from where the magma eruption causing volcanoes. But hotspots are now not traditionally associated with plate boundaries. The hotspots can be the regions where the crust is weak or they historically may be the regions of old plate boundaries. The volcanoes of hotspots are intra plate volcanoes. For example, the Reunion hotspots was a powerful volcanic region around 66 million years ago. Due to the movement of Indian peninsula towards north. As the western part of Indian peninsula came under Reunion hotspot. It resulted into formation of Deccan Lava Plateau.

Volcanoes in India

  1. Barren Island (Andaman and Nicobar)
  2. Narcondam Island (Andaman and Nicobar)
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