Rajasthan

Appearance

Volcanoes ​

1. Definition and Mechanism ​

  • A volcano is an opening or vent in the earth’s crust through which molten rock (lava), rock fragments, ash, steam, and gases are emitted.

  • These emissions originate from the hot interior of the Earth, escaping through lines of weakness in the crust.

  • Volcanoes are a result of intense internal heat and pressure.

  • Magma is the hot molten rock beneath the crust; once it reaches the surface, it is called lava.

  • The tremendous force of magma and trapped gases breaks through the crust, leading to eruptions.

  • This process is known as vulcanism – the release of solid, liquid, and gaseous materials from within the Earth to its surface.

2. Volcanic Materials ​

  • Materials ejected during eruptions include:

    • Lava flows

    • Pyroclastic debris

    • Volcanic bombs

    • Ash and dust

    • Gases: nitrogen compounds, sulphur compounds, chlorine, hydrogen, and argon.

3. Sources of Information About Earth's Interior ​

  • Volcanic eruptions provide direct access to magma from deep inside the Earth, enabling laboratory analysis.

  • However, it remains difficult to determine the exact depth of magma origin.

4. Classification of Volcanoes ​

A. Based on Nature of Eruption and Form ​

  1. Shield Volcanoes

    • Largest volcanoes (e.g., Hawaiian volcanoes).

    • Composed mostly of fluid basalt lava.

    • Have gentle slopes and low explosivity unless water interacts with the vent.

    • Lava fountains can form cinder cones at the vent.

  2. Composite Volcanoes

    • Erupt cooler, more viscous lava, often explosively.

    • Emit lava, ash, and pyroclastic material.

    • Result in layered structures around vent areas.

  3. Caldera

    • Most explosive type.

    • The eruption is so violent that the volcano collapses into itself, forming a depression.

    • Indicates a large, shallow magma chamber.

  4. Flood Basalt Provinces

    • Erupt highly fluid lava over large areas.

    • Form thick basalt sheets (up to 50 m thick, hundreds of km wide).

    • Example: Deccan Traps, India.

  5. Mid-Ocean Ridge Volcanoes

    • Found along the mid-ocean ridges, over 70,000 km long.

    • Characterized by frequent eruptions at the central ridge axis.

B. Based on Frequency of Eruption ​

  1. Active Volcanoes

    • Erupt frequently or recently.

    • Examples: Stromboli, Krakatoa, Mayon, Mauna Loa, Barren Island (India).

  2. Dormant Volcanoes

    • Erupted in historical times but are currently inactive.

    • Examples: Vesuvius (Italy), Cotopaxi (South America).

  3. Extinct Volcanoes

    • No record of eruption in human history.

C. Based on Mode of Eruption ​

  1. Central Type Volcanoes

    • Erupt through a single vent or hole.

    • Form domes or conical hills.

    • Often violent due to sudden release of gases and lava.

    • Examples: Vesuvius, Fuji-Yama.

  2. Fissure Type Volcanoes

    • Magma flows from elongated cracks, not central vents.

    • Produces horizontal lava sheets or dome-shaped structures.

    • Example: Deccan Traps (lava plateaus).

D. Based on Fluidity of Lava ​

  1. Basic Lava Volcanoes

    • Rich in metallic minerals, low melting point, high fluidity.

    • Lava spreads in thin, extensive sheets.

    • Forms shields or domes.

    • Example: Hawaiian shield volcanoes.

  2. Acid Lava Volcanoes

    • Rich in silica, high melting point, high viscosity.

    • Lava solidifies quickly, forming steep cones.

    • Associated with higher land features and steep slopes.

5. Volcanic Landforms ​

A. Extrusive Landforms ​

  • Formed by volcanic materials accumulating on the surface.

  • Common Features:

    • Volcanic cones with craters

    • Shield volcanoes, composite cones, cinder cones, calderas, flood basalt provinces

    • Seamounts (underwater volcanoes that don’t reach the surface)

    • Volcanic mountains (e.g., Mauna Loa, Mount Popa, Vesuvius, Cotopaxi, Fuji Yama)

    • Volcanic plateaus (e.g., Maharashtra Plateau, Snake River Plateau in the USA)

B. Intrusive Landforms ​

  • Formed when lava cools within the crust, solidifying into igneous rocks.

  1. Lacoliths – Dome-shaped, intrusive bodies with flat bases. Found in Karnataka (domal granite hills).

  2. Lapoliths – Saucer-shaped bodies (concave upward).

  3. Phacoliths – Wavy igneous bodies at anticline tops or syncline bases.

  4. Sills and Sheets – Horizontal layers of solidified lava (sills = thicker; sheets = thinner).

  5. Dykes – Vertical, wall-like structures of solidified lava. Common in Western Maharashtra.

  6. Batholiths – Massive underground magma bodies, forming mountain cores.

6. Global Distribution of Volcanoes ​

  • Around 500 volcanoes globally.

  • Found mainly in three belts:

  1. Circum-Pacific Belt (Ring of Fire)

    • Most active zone.

    • Includes Andes, Alaska, Aleutian Islands, Japan, Philippines, Indonesia, New Zealand.

  2. Mid-World Mountain Belt

    • Runs from Alps to Himalayas, merging into the Ring of Fire.

    • Second largest volcanic zone.

  3. African Rift Valley Belt

    • Third in rank.

    • Mostly extinct volcanoes; Mt. Cameroon is an exception (active).

  • Volcanoes are linked with tectonic activity – especially folding and faulting.

  • Found along coastal ranges, islands, and mid-ocean ridges.

  • Interior continental areas are mostly volcano-free.

  • Patterns resemble those of earthquake zones.

7. Hazards from Volcanoes ​

  • Volcanic eruptions cause sudden and drastic surface changes.

  • Mudflows (lahars) form when volcanic debris mixes with rainwater, causing destructive floods.

    • Common in Andes and Rockies.

  • Landslides are often triggered by volcanic activity.

  • Volcanoes can cause volcanic earthquakes, especially in active belts.

8. Volcanoes in India ​

  • Barren Island in the Andaman & Nicobar Islands is India’s only active volcano.

  • Deccan Traps: A massive flood basalt formation covering much of Maharashtra Plateau.

  • Mount Popa (in neighboring Myanmar) is another regional example.