Volcanism

Volcanism refers to the geological processes through which molten material (magma), gases, water vapour, and rock fragments escape from the Earth’s interior through vents and fissures, reaching the surface as lava. These processes not only shape landforms but also play a crucial role in the internal dynamics of the planet.

At the heart of volcanism is the asthenosphere—a semi-fluid layer of the upper mantle that lies beneath the solid lithosphere. Magma, originating here, rises through the crust and may either erupt explosively or ooze out gently, depending on various geological conditions.

Causes of Volcanism

Volcanism is driven by a combination of thermal, pressure, and chemical dynamics deep within the Earth:

• Geothermal Gradient: There is a steady increase in temperature with depth. This geothermal gradient melts rock under specific pressure conditions.
• Decompression Melting: As pressure decreases near the surface, the melting point of rocks also lowers, enabling magma generation.
• Water and Volatiles: Water percolating from the surface gets superheated, contributing to magma formation. Volatiles (e.g., CO₂, SO₂) reduce the melting point.
• Tectonic Forces: Convergent and divergent boundaries generate fractures, faults, and zones of weakness through which magma can ascend.
• Magma Ascent: Buoyant magma rises due to its lower density and pressure from gas expansion, eventually breaking through to the surface.

Types of Lava

Understanding lava type is crucial to understanding volcano behavior and resulting landforms:

• Basic Lava:
  • Also known as basaltic lava.
  • High temperature and low silica content make it fluid and fast-flowing.
  • Forms broad, gently sloping shield volcanoes (e.g., Hawaiian volcanoes).
  • Non-explosive due to low gas content.
• Acidic Lava:
  • High in silica, making it viscous and slow-moving.
  • Cools and solidifies quickly, forming steep-sided volcanoes.
  • Often leads to violent, explosive eruptions.

Types of Volcanoes

Based on Eruption Style

• Explosive Volcanoes:
  • Characterized by sudden, violent eruptions.
  • Associated with acidic lava and high gas pressure.
  • Forms classic conical shapes and eject pyroclastic material.
  • Found mostly in subduction zones (e.g., Mount St. Helens, Vesuvius).
• Effusive Volcanoes:
  • Lava flows out gently with less or no explosion.
  • Associated with basic lava.
  • Found along rift zones or hotspots.

Based on Periodicity

• Active Volcanoes:
  • Show continuous or periodic eruption activity.
  • Around 600 active volcanoes globally.
  • Example: Stromboli (Italy), Kilauea (Hawaii).
• Dormant Volcanoes:
  • Currently inactive but can erupt again.
  • Example: Mt. Fuji (Japan), Mt. Kilimanjaro (Tanzania).
• Extinct Volcanoes:
  • No longer expected to erupt; have been inactive for thousands of years.
  • Example: Mt. Aconcagua (Argentina).

Volcanic Mountains

These are formed by the accumulation of volcanic material around the vent after successive eruptions. They include lava, ash, cinders, and volcanic bombs. Examples: Mt. Etna (Italy), Mt. Merapi (Indonesia), Mt. Fuji (Japan).

Landforms Associated with Volcanic Activity

Volcanic activity produces two main types of landforms:

Intrusive (Plutonic) Landforms

These form when magma solidifies before reaching the surface:

• Batholiths:
  • Massive, dome-shaped deep-seated intrusions.
  • Composed of granitic rocks, exposed after denudation.
  • Example: Bundelkhand granites in India.
• Laccoliths:
  • Dome-shaped intrusions with a flat base, linked to magma chambers.
  • Found in Karnataka and Telangana.
• Lapoliths:
  • Saucer-shaped intrusions concave upwards.
  • Less common but indicate horizontal movement of magma.
• Phacoliths:
  • Lens-shaped bodies located at the crest of folds.
  • Found in folded mountain regions.
• Sills and Sheets:
  • Horizontal layers of solidified lava within strata.
• Dykes:
  • Vertical intrusions that cut across rock layers.
  • Common in the Deccan Traps of Maharashtra.

Extrusive Landforms

These are formed when magma reaches the surface and cools:

• Shield Volcanoes:
  • Broad, gently sloping structures.
  • Formed by fluid lava flows.
  • Example: Mauna Loa (Hawaii).
• Composite Volcanoes (Stratovolcanoes):
  • Built of alternate layers of lava and ash.
  • Steep-sided and prone to explosive eruptions.
  • Found around the Pacific Ring of Fire.
• Calderas:
  • Large, bowl-shaped depressions formed after a violent eruption and collapse of the volcano summit.
  • Example: Krakatoa (Indonesia).
• Flood Basalts:
  • Extensive flat regions formed by successive lava flows.
  • Example: Deccan Traps (India).
• Mid-Ocean Ridge Volcanoes:
  • Located along divergent boundaries in the ocean.
  • Part of submarine volcanic systems.

Geysers and Hot Springs

• Geysers: Periodically erupting fountains of superheated water and steam due to underground volcanic activity.
  • Example: Old Faithful (USA), Rotorua (New Zealand).
• Hot Springs: Water percolates deep underground, heats up, and returns to the surface.
  • Used for bathing, heating, and therapeutic purposes.

Distribution of Volcanoes

Volcanoes are mostly concentrated along plate boundaries. About 80% occur at convergent boundaries, 15% at divergent boundaries, and a few at intraplate hotspots.

Major Volcanic Belts

• Circum-Pacific Belt (Ring of Fire):
  • Most seismically and volcanically active belt.
  • Surrounds the Pacific Ocean; includes Japan, Indonesia, Alaska, Chile, etc.
• Mid-Continental Belt (Mediterranean Belt):
  • Includes volcanic regions of the Alps, Italy, and Indonesia.
  • Caused by the convergence of African, Indian, and Eurasian plates.
• Mid-Atlantic Belt:
  • Extends along the mid-Atlantic ridge.
  • Includes Iceland and Azores Islands.
• Intra-Plate Volcanoes:
  • Caused by mantle plumes or hotspots.
  • Example: Hawaiian Islands, Yellowstone.

Significance of Volcanism

• Landform Development: Creation of new islands, plateaus (e.g., Deccan), mountains.
• Soil Fertility: Volcanic ash contributes to rich soils.
• Geothermal Energy: Harnessed in volcanic regions like Iceland and New Zealand.
• Mineral Resources: Source of valuable minerals such as sulphur, copper, and pumice.
• Scientific Research: Helps understand Earth’s interior dynamics.

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Subject: Geography

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