Volcanoes
Magma vs. Lava · 5 NCERT Volcano Types · Eruptive Styles · Global & Indian Distribution (28 Micro-Realms)
From Asthenospheric Melt to Surface Edifices
A volcano is a place where gases, ashes, and molten rock material—lava—escape to the ground from the Asthenosphere. Depending on silica viscosity and tectonic setting, eruptions build five major NCERT volcanic forms: broad basaltic Shield Volcanoes, layered Composite Cones, inward-collapsing Calderas, vast Flood Basalt Provinces (Deccan Traps), and 70,000-km Mid-Ocean Ridge Volcanoes.
I. Magma vs. Lava, Silica Rheology & Ejected Materials (NCERT)
Volcano & Conduit
- Vent where lava & gas escape
- Upper mantle (Asthenosphere) source
- Rises through pipe/conduit
Magma vs. Lava (NCERT)
- Magma: Molten rock inside mantle
- Lava: Once it reaches surface
- Degasses upon eruption
Acidic vs. Basic Lava
- Acidic (>65% $\text{SiO}_2$): Viscous, explosive
- Basic (45–52% $\text{SiO}_2$): Fluid, quiet
- Controls cone slope & shape
Pahoehoe, Aa & Pillow Lava
- Pahoehoe: Smooth ropy basalt
- Aa: Rough jagged clinker blocks
- Pillow lava: Submarine quenching
Pyroclastic Debris (Tephra)
- Ash (<2 mm) & Lapilli (2–64 mm)
- Volcanic bombs & angular blocks
- Vesicular pumice & scoria
Volcanic Gases (NCERT)
- Steam ($\text{H}_2\text{O}$) 60–90% dominant
- Nitrogen & Sulphur compounds
- Minor Chlorine, Hydrogen, Argon
Pyroclastic Flows & Lahars
- Nuée ardente: $700^\circ\text{C}$ glowing cloud
- Races downslope at 200+ km/h
- Lahar: Volcanic ash mudflow
II. Types of Volcanoes: The Five NCERT Classifications
1. Shield Volcanoes
- Largest of all volcanoes (barring floods)
- Mostly fluid basalt; not steep
- Hawaiian volcanoes (Mauna Loa)
Shield Water-Vent & Cinder
- Low explosivity unless water enters
- Upward lava fountain at vent
- Develops into a Cinder Cone (NCERT)
2. Composite Volcanoes
- Cooler & more viscous than basalt
- Explosive ash + lava accumulates
- Forms steep symmetric layers
3. Caldera Volcanoes
- Most explosive on Earth (NCERT)
- Collapses on itself; no tall structure
- Huge & shallow magma chamber
4. Flood Basalt Provinces
- Highly fluid lava flows far
- Single flows 50+ m thick (100s km)
- Deccan Traps (Maharashtra plateau)
5. Mid-Ocean Ridge Volcanoes
- 70,000-km oceanic ridge system
- Central axial rift zone
- Experiences frequent eruptions
Plug Domes & Cinder Cones
- Lava Dome: Bulbous rhyolite spine
- Mt. Pelée & Lassen Peak
- Cinder Cone: Parícutin (Mexico)
III. Eruptive Styles, Frequency & Geothermal Features
Active, Dormant & Extinct
- Active: Etna, Stromboli, Barren Is.
- Dormant: Vesuvius, Fuji, Narcondam
- Extinct: Kilimanjaro, Mt. Popa
Effusive vs. Explosive
- Icelandic: Quiet fissure basalt
- Hawaiian: Fluid shield fountains
- Vulcanian & Pelean: Viscous blasts
Strombolian vs. Plinian
- Stromboli: "Lighthouse of Med."
- Plinian: 40-km stratospheric column
- Vesuvius (79 AD) & Pinatubo (1991)
Explosivity Index (VEI 0–8)
- 0 to 8 logarithmic tephra volume
- VEI 0: Hawaiian (<10⁴ m³)
- VEI 8: Supervolcano (>1,000 km³)
Geysers & Hot Springs
- Geyser: Periodic steam-water jet
- Hot Spring: Continuous warm flow
- Puga Valley (Ladakh) & Manikaran
Fumaroles & Solfataras
- Fumarole: Late-stage steam vent
- Solfatara: Sulphur-rich gas vent
- Mofette: Cold $\text{CO}_2$ gas vent
Crater vs. Caldera Lakes
- Crater: Small (<1 km) summit vent
- Caldera: Huge (>2–50 km) collapse
- Lake Toba (Sumatra) & Crater Lake
IV. Global Volcanic Belts & Volcanism in India
Circum-Pacific Ring of Fire
- Subduction trenches around Pacific
- ~75% of world's active volcanoes
- Andes, Cascades, Japan, Philippines
Why No Himalayan Volcanoes?
NO OCEAN SUBDUCTION
- Mid-Continental belt has Etna/Vesuvius
- Himalayas: C–C buoyant collision
- No wet oceanic slab + 70 km thick crust
Mid-Atlantic & African Rift
- Divergent plate boundary belt
- Iceland (Hekla, Laki) & Azores
- East Africa: Kilimanjaro, Nyiragongo
Intraplate Mantle Plumes
- Away from plate boundaries
- Stationary deep D'' thermal plumes
- Hawaii, Réunion, Yellowstone
Barren Island (Active)
- India's ONLY active volcano
- Andaman Sea (138 km NE of Port Blair)
- Sunda–Burma subduction arc
Narcondam & Extinct Sites
- Narcondam (Andaman): Dormant cone
- Dhinodhar Hills (Kutch): Extinct
- Tosham (HR) & Dhosi Hill: Extinct
Baratang Mud Volcanoes
- Baratang Island (Andaman)
- Not true magmatic volcanoes!
- Cold methane gas + mud extrusion
Core Distinctions
• Shield Volcanoes: Largest cones; fluid basalt lava; gentle slopes; low explosivity unless water enters the vent (Hawaiian volcanoes).
• Composite Volcanoes: Cooler and more viscous lava than basalt; explosive eruptions build alternating layers of pyroclastic ash and lava.
• Caldera: Most explosive of Earth's volcanoes; so explosive that when they erupt they collapse on themselves rather than building any tall structure, proving the magma chamber is huge and in close vicinity.
Volcanoes at convergent margins require a wet oceanic slab subducting into the asthenosphere to trigger flux melting. Beneath the Himalayas, buoyant Continent–Continent collision doubles the crust to $70\text{–}75\text{ km}$ thickness, preventing magma from reaching the surface.
3. India's Volcanic Trio (Prelims Favourite)• Barren Island: Only active magmatic volcano in India/South Asia.
• Narcondam Island: Dormant volcano in the Andaman Sea.
• Baratang Island: Mud volcano (cold hydrocarbon gas extrusion, not magma).
The 5 NCERT Volcano Types
Viscosity, Landform & Exemplar Matrix