Free Daily UPSC Mains Practice QuestionsPractice today →
Home › Subjects › Geography › Volcanicity
Topic 13 of 290 Free

Volcanicity

Geography · Learn · Prepare · Practice · Evaluate
1

Learn the Concept

All

Volcanicity (vulcanism) is the set of processes by which molten rock (magma), gases and fragments are forced from the Earth's interior onto or toward the surface. Fix the idea: where molten material cools below the surface it produces intrusive (plutonic) landforms; where it is erupted onto the surface through a volcano it produces extrusive (volcanic) landforms. The style of eruption — gentle (effusive) or violent (explosive) — depends chiefly on the nature of the lava (basic vs acidic). Volcanoes are concentrated along plate boundaries and hotspots, and are both a hazard (eruptions, ash) and a resource (fertile soils, geothermal energy, minerals).

In One Line
  • Magma vs lava: molten rock below the surface (magma) vs on the surface (lava).
  • Lava type decides style: basic (basaltic) = runny, effusive; acidic (silicic) = viscous, explosive.
  • Intrusive forms: batholith, laccolith, lopolith, phacolith, sill, dyke. Extrusive: shield, strato, cinder cone, caldera.
  • Distribution: Ring of Fire, mid-ocean ridges, Mediterranean belt, hotspots.
Glossary (define-on-first-use)
  • Magma / Lava: molten rock below the surface / after it reaches the surface.
  • Effusive vs explosive: quiet outpouring of fluid lava vs violent blasting of viscous lava & gas.
  • Intrusive (plutonic): igneous bodies solidified within the crust.
  • Caldera: a large collapse depression after a violent eruption empties the magma chamber.
  • VEI: Volcanic Explosivity Index — a 0–8 measure of eruption size.

1. Magma, its generation & the materials erupted

  • Causes / magma generation: magma forms by melting of the mantle/crust where pressure falls (decompression at divergent ridges/hotspots), where water lowers the melting point (above subduction zones), or from heat concentration. It rises because it is less dense than surrounding rock.
  • Volcanic materials: lava (molten rock); solid fragments (pyroclasts/tephra) graded by size — fine ash → pea-sized lapilli → large volcanic bombs & blocks; and gases (water vapour, CO₂, SO₂).

2. Lava type & eruption style

  • Basic (basaltic) lava is low in silica, hot, very fluid → flows far, gives gentle (effusive) eruptions and low, broad shield volcanoes (Hawaii) and lava plateaus (Deccan).
  • Acidic (silicic) lava is high in silica, cooler, viscous → traps gas and gives violent (explosive) eruptions and steep composite/strato-volcanoes (Fujiyama) and lava domes.

3. Classification of volcanoes

  • By activity: active (erupting/likely to — e.g. Barren Island, India's only active volcano), dormant (“sleeping” — could revive), extinct (no future eruption expected).
  • By form: shield (broad, gentle, basaltic), composite/strato-volcano (tall, layered lava+ash, explosive), cinder cone (small, steep, of pyroclasts), lava dome (viscous bulge), caldera (collapse basin), and fissure eruptions (lava from long cracks → plateaus).
Volcano types & a strato-volcano's structure Shield (basic, gentle) Cinder cone crater/vent layers (lava+ash) magma chamber Composite / strato (acidic, explosive) Caldera (collapse)
Fig 1 — Shield, cinder cone, composite/strato-volcano (with vent, crater, layers, magma chamber) and caldera.

4. Intrusive landforms

  • Magma solidifying within the crust forms: batholith (huge discordant granite mass, the core of many mountains); laccolith (dome-shaped, arching strata up); lopolith (saucer/basin-shaped); phacolith (lens in the crest/trough of a fold); sill (sheet parallel to bedding); dyke (sheet cutting across bedding); and small bodies — stock and boss.
Intrusive (plutonic) landforms batholith laccolith sill (along bedding) dyke (across bedding) lopolith
Fig 2 — Batholith, laccolith, lopolith, sill (concordant) and dyke (discordant) intrusions.

5. Extrusive landforms & distribution

  • Extrusive forms: lava plateaus & plains (fissure basalt — the Deccan Trap, Columbia Plateau), volcanic cones, craters and calderas (crater lakes — Lonar, Krakatoa).
  • Distribution: ~75% of volcanoes ring the Pacific — the “Pacific Ring of Fire” (subduction zones); others lie along mid-ocean ridges (divergent, submarine), the Mediterranean–Alpine–Himalayan belt, and over hotspots (Hawaii, Iceland).
Global volcano belts Pacific Ringof Fire Mid-Atlantic Ridge Mediterranean–Himalayan belt hotspot
Fig 3 — Volcano distribution: the Pacific Ring of Fire, mid-ocean ridges, the Mediterranean belt and hotspots.

6. Volcanoes & society

  • Hazards: lava flows, ash fall, pyroclastic flows, lahars (mudflows), gases, and triggered tsunamis — measured by the Volcanic Explosivity Index (VEI, 0–8).
  • Benefits: fertile volcanic soils (the black soil of the Deccan), geothermal power (Iceland, Puga in Ladakh), mineralisation (ores from magmatic/hydrothermal activity), building materials and tourism.
Basic (basaltic)Acidic (silicic)
SilicaLowHigh
ViscosityLow (fluid)High (sticky)
EruptionEffusive (gentle)Explosive (violent)
FormShield / lava plateauStrato-volcano / dome
Key Facts to Remember
  • Magma (below) vs lava (surface); pyroclasts: ash < lapilli < bombs.
  • Basic = fluid, effusive, shield; acidic = viscous, explosive, strato.
  • Intrusive: batholith, laccolith, lopolith, phacolith, sill (parallel), dyke (across).
  • Distribution: Ring of Fire (~75%), mid-ocean ridges, Mediterranean belt, hotspots.
  • India: Barren Island (only active); Deccan Traps (fissure basalt); Lonar crater lake.
Connect the Dots
  • Plate tectonics: volcanoes at divergent, convergent & hotspot settings (earlier leaf).
  • Earthquakes: volcanic earthquakes; same belts (next leaf).
  • Soils (Biogeography): fertile volcanic (black) soils.
  • Indian geology (Paper II): Deccan Traps, Barren Island.
2

Prepare for the Exam — Prelims

Prelims

Tested on basic vs acidic lava, volcano forms, intrusive landforms (sill vs dyke), the Ring of Fire, and Indian examples (Barren Island, Deccan).

High-probability areas
  • Basic = fluid/effusive/shield; acidic = viscous/explosive/strato.
  • Sill = parallel to bedding; dyke = cuts across.
  • Batholith, laccolith, lopolith, phacolith.
  • Ring of Fire = ~75% of volcanoes (subduction).
  • Barren Island = India's only active volcano; Lonar crater.
Common traps
  • Caldera = collapse (bigger than a crater).
  • Laccolith arches strata up; lopolith is saucer-shaped.
  • Deccan Traps = fissure (basic) eruption.
  • Pyroclast size: ash < lapilli < bombs.
Revision tip: Magma generation → lava type (basic/acidic) → eruption style → intrusive/extrusive forms → Ring of Fire & hotspots.
2

Prepare for the Exam — Mains

Mains

Optional Paper I / GS Paper I. The examiner rewards linking lava type to eruption/landform and mapping distribution to plate settings.

Likely question types
  • Classify volcanoes and the landforms they produce. (15)
  • Discuss the global distribution of volcanoes. (15)
  • Distinguish between intrusive and extrusive landforms. (10)
  • Examine volcanoes as both hazard and resource. (10)
Dimensions to cover
  • Magma, materials, lava type.
  • Classification (activity/form).
  • Intrusive & extrusive forms.
  • Distribution & society.
Value-additions
  • Deccan Traps; Barren Island; Lonar.
  • Ring of Fire & hotspots.
  • VEI; geothermal (Puga).
3

How to Write the Answer

Mains
Worked example — decoding the question

“The nature of lava determines the form of a volcano.” Discuss. (15)
Link basic/acidic lava to effusive/explosive eruptions and to shield/strato forms.

Introduction · ~20%
Define volcanicity; note lava viscosity as the key.
Body · ~60%
Basic (low silica, fluid) → effusive → shield volcanoes & lava plateaus (Deccan, Hawaii). Acidic (high silica, viscous) → explosive → strato-volcanoes & domes (Fuji). Add gases/pyroclasts, caldera formation, and distribution by plate setting. Use volcano-type diagram.
Conclusion · ~20%
Lava chemistry, set by tectonic setting, shapes the volcano.
10-mark: ~150 words · ~7 min
15-mark: ~250 words · ~12 min
Common mistakes
  • Confusing sill and dyke.
  • Calling the Deccan an explosive eruption.
  • Mixing crater and caldera.
Model opening line: “A volcano wears the shape of its lava — runny basalt builds gentle shields, sticky silica builds explosive cones.”
4

Prelims Questions

Prelims

30 interactive MCQs — previous-year questions are included and tagged with their year. Select an answer for instant feedback and an explanation; your score is tracked and you can review wrong answers at the end.

Practice set
Question 1 of 30
0/0
Score
5

Mains Questions

Mains

15 mains questions — previous-year questions are included and tagged. Write a full answer, then click Evaluate My Answer for paragraph-level feedback. Reveal the model structure after you attempt.

15 marks · ~250 words · ~11 min
Classify volcanoes and describe the landforms they produce.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Discuss the global distribution of volcanoes in relation to plate tectonics.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Distinguish between intrusive and extrusive volcanic landforms.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Examine volcanoes as both a hazard and a resource.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
'The nature of lava determines the form of a volcano.' Discuss.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Describe the intrusive landforms of vulcanism.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Explain the formation of a caldera.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Discuss the causes and process of magma generation.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Describe the extrusive landforms of vulcanism with examples.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Explain the significance of the Deccan Traps.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Examine the relationship between eruption style and lava type.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Discuss the Pacific Ring of Fire.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Describe the volcanic activity of India.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

15 marks · ~250 words · ~11 min
Analyse the benefits and hazards of volcanic activity.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

10 marks · ~150 words · ~7 min
Explain the Volcanic Explosivity Index.
Evaluate My Answer
Model answer structure

Intro: define the key term and give a one-line context hook.
Body 1: conceptual / historical basis with a named fact, date or example.
Body 2: evidence and analysis — sites, inscriptions, coins, archaeologists.
Body 3: limitations or the counter-view (balanced).
Conclusion: a specific, reasoned judgment / way forward.

← Mountain Building / Orogenesis Earthquakes →