GeographyISC Class 11 · Earth's Interior, Earthquakes and Volcanoes

Vulcanicity: Volcanoes and Their Landforms (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Vulcanicity covers the formation of magma, its rise and eruption or cooling inside the crust. Basic lava is fluid and gives gentle shield volcanoes; acidic lava is viscous and gives explosive cones. Intrusive forms include batholiths, sills and dykes. Volcanoes are numerous at converging plate boundaries because subduction produces gas-rich magma.

Understand Vulcanicity: Volcanoes and Their Landforms

Process and materials. Magma forms in the asthenosphere or at subduction zones, is lighter than the rock around and rises through cracks to magma chambers, then erupts through vents as lava. Products are lava, pyroclasts (dust, ash, cinders or lapilli, bombs) and gases (mainly water vapour with carbon dioxide and sulphur compounds).

Lava properties. Basic (basaltic) lava is low in silica, very hot and fluid; it flows far, erupts gently and forms shields and lava plateaus. Acidic lava is rich in silica, cooler and viscous; it traps gas, erupts explosively and builds steep cones and domes.

Types. By activity: active (Barren Island in the Andamans, Etna, Kilauea), dormant and extinct. By form: shield (Mauna Loa), composite or strato (Fujiyama, Vesuvius), cinder cones, calderas, flood basalt provinces (Deccan Traps) and ridge volcanoes (Iceland).

Forms. Extrusive: cones, craters, calderas, lava plateaus. Intrusive: batholiths (huge granite masses), laccoliths (domes with a level base), lopoliths (saucer-shaped), phacoliths (wavy bodies in folds), sills (horizontal sheets) and dykes (vertical walls cutting across layers).

Converging plates. An oceanic plate subducts; at about 100 km it releases water that lowers the melting point of the overlying mantle; magma forms, rises, is rich in silica and gas and gives explosive composite volcanoes in lines such as island arcs (Japan, Indonesia) and the Andes. This is why most active volcanoes lie in the Ring of Fire. Volcanoes also occur at divergent boundaries and hot spots such as Hawaii.

Vulcanicity: Volcanoes and Their Landforms: key facts

Lava types: Basic: low silica, fluid, gentle, shield | Acidic: high silica, viscous, explosive, steep cone
Intrusive forms: $B a t h o l i t h ∣ L a cco l i t h ∣ L o p o l i t h ∣ P ha co l i t h ∣ S i l l : h or i z o n t a l ∣ D y k e : v er t i c a l$
Volcano activity: $A c t i v e ∣ D or man t ∣ E x t in c t$

How to answer Vulcanicity: Volcanoes and Their Landforms questions

Questions ask for types and forms (4 marks), lava comparison, or reasons for distribution.

1Name the category (extrusive or intrusive; basic or acidic).
2Give shape, lava type and an example for each form.
3For distribution, use the subduction sequence: sinking, water, melting, rising, eruption.
4End with the Ring of Fire or a named example.

Vulcanicity: Volcanoes and Their Landforms in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Honey and water

Suppose one jug holds thin runny syrup and another thick paste, both poured down a slope.

Which one models the lava of a shield volcano?

Try it first, then show the solutionSolution

Thin syrup spreads wide and flat, like fluid basic lava.
Thick paste piles up, like viscous acidic lava building steep cones.

Answer: The runny syrup models shield-volcano lava.

Memory hook: Runny lava, wide shield.

Sheet in layers

Suppose a vertical crack in a cliff is filled with hard dark rock cutting across the sandstone beds.

Name the intrusive form and say how a horizontal version differs.

Try it first, then show the solutionSolution

A vertical wall cutting across layers is a dyke.
A horizontal sheet between layers is a sill.

Answer: A dyke; the horizontal version is a sill.

Memory hook: Dyke = down; sill = sideways.

Tricks to remember Vulcanicity: Volcanoes and Their Landforms

Short memory aids. Say each one aloud once, then use it on a question the same day.

Sill sideways, dyke down
Sill lies flat like a window sill; dyke stands vertical.
Use it when: Intrusive forms.
Silica makes it sticky
More silica, more sticky, more explosive.
Use it when: Lava type.

Vulcanicity: Volcanoes and Their Landforms: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Intrusive form · 1 mark · MCQBoard level

A vertical wall of igneous rock cutting across rock layers is called a:
(a) sill
(b) dyke
(c) laccolith
(d) batholith

Try it first, then show the solutionSolution

Vertical.

Answer: (b) Dyke.

Example 2: Volcano types · 2 marks · Short answerBoard level

Name the three classes of volcano by activity and one example of an active volcano from the chapter.

Try it first, then show the solutionSolution

Active, dormant, extinct.
Active example: Barren Island, Etna or Kilauea.

Answer: Active, dormant, extinct; example Barren Island.

Example 3: Lava comparison · 4 marks · Long answerHarder board level

Compare basic and acidic lava on silica content, flow, eruption and landform.

Try it first, then show the solutionSolution

Basic: low silica; acidic: rich in silica.
Basic is fluid and flows far; acidic is viscous and traps gas.
Basic erupts gently; acidic erupts explosively.
Basic builds shields and lava plateaus; acidic builds steep cones and domes.

Answer: Basic lava is fluid, gentle and forms shields; acidic is viscous, explosive and forms steep cones.

Example 4: Intrusive landforms · 4 marks · Long answerHarder board level

Describe three intrusive volcanic landforms with their shape.

Try it first, then show the solutionSolution

Batholith: huge granite mass.
Laccolith: dome with a level base; lopolith: saucer-shaped.
Sill: horizontal sheet; dyke: vertical wall cutting across layers.

Answer: Batholith (huge mass), laccolith (dome) and sill (horizontal sheet), with dyke vertical.

Example 5: Subduction reasoning · 5 marks · Case-basedHOTS / competency

Case: A map shows a curved chain of volcanic islands parallel to a deep trench. (a) Name the plate boundary. (b) Explain why the magma is rich in gas and silica. (c) Explain why such volcanoes are explosive and give two examples of such arcs or chains.

Try it first, then show the solutionSolution

(a) A converging boundary where an oceanic plate subducts.
(b) The sinking plate releases water at about 100 km, melting mantle rock to produce silica-rich, gas-rich magma.
(c) Viscous silica-rich lava traps gas and erupts explosively; Japan and Indonesia island arcs, and the Andes.

Answer: (a) Converging boundary (b) water lowers melting point (c) viscous lava explodes; Japan, Indonesia, Andes.

Where students lose marks in Vulcanicity: Volcanoes and Their Landforms

The slips examiners see most often, and what to write instead.

Mistake: Confusing sill and dyke
The orientations are swapped.
Wrong working
Dyke is horizontal
Right working
Dyke is vertical; sill is horizontal
Do this: Sill is horizontal; dyke is vertical.
Usually costs: 1 mark.
Mistake: Saying acidic lava is runny
Silica effect misunderstood.
Do this: Acidic lava is viscous and traps gas.
Usually costs: 1 mark.
Mistake: Giving no example
Types are listed without volcanoes.
Do this: Add Mauna Loa, Fujiyama or Deccan Traps.
Usually costs: 1 mark.

Exam tips

Use the pair basic/acidic in every lava answer.
Draw a sketch for batholith, sill and dyke if space allows.
Quote an example for each volcano type.
Do not claim volcanoes occur only at converging boundaries.

Practice questions on Vulcanicity: Volcanoes and Their Landforms

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Mauna Loa is an example of a:
(a) composite volcano
(b) shield volcano
(c) cinder cone
(d) caldera
Hint
Fluid basalt.
Answer
(b) Shield volcano.
Q2 · 3 marks · Short answerHarder board level
Describe the materials thrown out by a volcano.
Hint
Lava, pyroclasts, gases.
Answer
Lava; pyroclasts such as dust, ash, cinders and bombs; and gases, mainly water vapour with carbon dioxide and sulphur compounds.
Q3 · 3 marks · Short answerHarder board level
Why are the Deccan Traps called a flood basalt province?
Hint
Fluid basalt over a wide area.
Answer
They are built of fluid basic lava that spread over a very wide area and formed a lava plateau.
Q4 · 4 marks · Long answerHarder board level
Explain why most active volcanoes lie in the Ring of Fire.
Hint
Subduction sequence.
Answer
The Pacific is ringed by converging boundaries; oceanic plates subduct, release water that lowers the melting point of mantle rock, and the gas-rich magma rises to erupt, forming chains such as island arcs and the Andes.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A dormant volcano has steep slopes of ash and lava layers and a violent history. (a) Name its type. (b) Say what lava it is likely to produce. (c) Explain why it could still be dangerous.
Hint
Strato-volcano.
Answer
(a) Composite or strato-volcano, like Fujiyama or Vesuvius. (b) Viscous acidic lava. (c) Dormant volcanoes may erupt again.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Example of a shield volcano?Show answer
Mauna Loa.
2.Example of a composite volcano?Show answer
Fujiyama or Vesuvius.
3.Which pyroclast is largest?Show answer
Volcanic bombs.
4.Horizontal intrusive sheet?Show answer
Sill.
5.Active volcano in the Andamans?Show answer
Barren Island.

Vulcanicity: Volcanoes and Their Landforms: frequently asked questions

Are all volcanoes at plate edges?

No, hot spots such as Hawaii lie within plates.

What is a caldera?

A huge depression formed after explosive collapse.

Why does acidic lava erupt violently?

It is viscous and traps gas.