NDA Geography · Earth's Structure, Landforms and Geological Time

Volcanoes and Igneous Activity

Magma rising from the mantle either erupts at the surface to build volcanoes and extrusive rock, or cools at depth into intrusive bodies like batholiths — the starting point of the rock cycle.

All Earth's Structure, Landforms and Geological Time notes NDA Geography strategy

Why this matters

5 PYQs, mostly MODERATE. The reliable earners are: the volcano shapes (especially the explosive composite cone), the named volcanic deposits (tuff, lapilli), and the intrusive igneous forms (batholith, laccolith). 'Igneous' simply means 'born of fire' — formed from cooled magma or lava.

Concept 1 of 3: Types of volcano

A volcano's shape is set by how runny its lava is. Thin, runny basaltic lava spreads far and builds a low, wide SHIELD (Hawaii). Thick, sticky lava traps gas and erupts EXPLOSIVELY, piling up steep COMPOSITE cones of lava and ash in layers (Fuji, Vesuvius, Mount Ibu). Small piles of cinders around a vent are CINDER cones.

Definition

Shield volcano — gentle slopes, runny basaltic lava, quiet effusive eruptions (Mauna Loa, Hawaii).
Composite / stratovolcano — steep cone built of alternating lava and ash; EXPLOSIVE eruptions throwing out pyroclastic material that accumulates in layers near the vent (Fuji, Vesuvius, Mount Ibu in Indonesia).
Cinder cone — small, steep heap of cinders around a single vent.
Volcanic / lava dome — thick lava piled over the vent without flowing far.

Type	Lava / eruption	Shape	Example
Shield	Runny basalt · effusive	Low, wide	Mauna Loa
Composite	Sticky · explosive, pyroclastic	Steep cone, layered	Fuji, Mount Ibu NDA 2026 — composite = explosive + pyroclastic + layers near the vent.
Cinder cone	Cinders around a vent	Small, steep	Paricutin

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2026 · Q105Moderate

Example 1 · Earth's Structure, Landforms and Geological Time · Volcanoes and Igneous Activity

Which of the following are associated with composite volcanoes? I. They result in explosive eruptions. II. They eject pyroclastic materials. III. Pyroclastic materials and ashes accumulate near the vent, forming layers.

Explosive = composite, not shield

Shield volcanoes are the QUIET ones (runny lava). If a question stresses explosive eruptions and pyroclastic ash, the answer is the composite cone.

Drill 2 more on types of volcano

Concept 2 of 3: Volcanic ejecta and deposits

An erupting volcano throws out fragments of every size — fine ash, pea-sized lapilli, and large bombs. When that loose ash is later carried by running water and laid down in layers, it hardens into a soft rock called tuff. Knowing the names of these deposits handles the 'what is this volcanic material' questions.

Definition

Pyroclastic material — fragments blasted out in an explosive eruption.
Ash (finest) → lapilli (pea- to walnut-sized) → bombs / blocks (largest).
Tuff — rock formed when volcanic ash is carried by running water and deposited as a sedimentary layer, then cemented.

Material	What it is
Ash	Finest volcanic dust
Lapilli	Pea- to walnut-sized fragments
Tuff	Hardened, water-deposited volcanic ash NDA 2025 — ash carried by water and deposited as a layer becomes tuff.

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

NDA · Apr 2025 · Q104Moderate

Example 2 · Earth's Structure, Landforms and Geological Time · Volcanoes and Igneous Activity

Which one of the following is formed when volcanic ash is carried by running water and is deposited as a sedimentary layer?

Concept 3 of 3: Intrusive igneous bodies

Not all magma reaches the surface. Much of it cools slowly underground, freezing into shaped bodies called intrusions. The biggest and deepest is the BATHOLITH — a vast dome of granite at the roots of mountains. Smaller intrusions (laccolith, lopolith, phacolith, sill, dyke) take their names from their shape and position.

Definition

Batholith — the LARGEST, deepest intrusive body; a huge dome of magma cooled deep in the crust (the 'lowermost/innermost' intrusion). Granite forms here.
Laccolith — a mushroom-shaped dome that arches the overlying strata up.
Lopolith — a saucer-shaped (downward-sagging) intrusion.
Phacolith — a lens-shaped intrusion in the crest/trough of a fold.
Sill (horizontal sheet) and dyke (vertical sheet) — thin tabular intrusions.

Intrusion	Shape / position
Batholith	Largest, deepest dome of magma NDA 2021 & 2023 — the large, deep-seated magma dome is the batholith.
Laccolith	Mushroom dome arching strata up
Lopolith	Saucer-shaped (sagging)
Phacolith	Lens in a fold crest/trough
Sill / Dyke	Horizontal / vertical sheet

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

NDA · Apr 2021 · Q123Moderate

Example 3 · Earth's Structure, Landforms and Geological Time · Volcanoes and Igneous Activity

A large body of magmatic material that cools in the deeper depth of the Earth's crust and develops in the form of large domes is known as

Batholith vs Laccolith

Both are domes, but batholith is the giant, deep-seated one, while a laccolith is a smaller, shallower mushroom that pushes the overlying layers upward.

Drill 1 more on intrusive igneous bodies

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Types of volcano3 rows

Type	Lava / eruption	Shape	Example
Shield	Runny basalt · effusive	Low, wide	Mauna Loa
Composite	Sticky · explosive, pyroclastic	Steep cone, layered	Fuji, Mount Ibu NDA 2026 — composite = explosive + pyroclastic + layers near the vent.
Cinder cone	Cinders around a vent	Small, steep	Paricutin

Volcanic ejecta and deposits3 rows

Material	What it is
Ash	Finest volcanic dust
Lapilli	Pea- to walnut-sized fragments
Tuff	Hardened, water-deposited volcanic ash NDA 2025 — ash carried by water and deposited as a layer becomes tuff.

Intrusive igneous bodies5 rows

Intrusion	Shape / position
Batholith	Largest, deepest dome of magma NDA 2021 & 2023 — the large, deep-seated magma dome is the batholith.
Laccolith	Mushroom dome arching strata up
Lopolith	Saucer-shaped (sagging)
Phacolith	Lens in a fold crest/trough
Sill / Dyke	Horizontal / vertical sheet

Watch out for (2)

Explosive = composite, not shield
→ Types of volcano
Batholith vs Laccolith
→ Intrusive igneous bodies

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