Intrusive volcanic landforms

Detail

Volcanic rocks vs plutonic rocks

• Magma is molten rock below the ground. When it comes out on the surface, it is called lava.
• Volcanic rocks form when lava cools at the surface. Cooling is fast, so the crystals are small (e.g. basalt).
• Plutonic rocks form when magma cools inside the crust. Cooling is slow, so the crystals are large (e.g. granite).
• A pluton is a deep-seated igneous intrusion, meaning a body of magma that cooled deep underground [5].
• Intrusive volcanic landforms are the shapes magma takes when it cools inside the crust (NCERT Figure 3.4).
• Magma pushes into the rock above it, called the country rock (the older rock that already surrounds the intrusion).
• Magma cools and hardens underground. Nothing shows at the surface yet.

• Denudation (the wearing away of land by weathering and erosion) slowly removes the cover rock. The hard intrusive body then stands out as a hill, dome or ridge.

• Concordant vs discordant: this is the main way intrusions are classified.

• Concordant bodies lie parallel to the rock layers, like sills and laccoliths [4].
• Discordant bodies cut across the rock layers, like dykes and stocks [4].

Batholith

• Definition: a very large body of magma that cooled deep in the crust into a large dome. It covers a wide area and goes several kilometres deep.
• It is mostly granite. It is a cooled magma chamber: the underground store of magma that once fed volcanoes above it.
• Far below the surface, a large magma chamber cools slowly. It forms small plutons and large batholiths [5].
• A stock is the small form of the same thing. It is a plutonic body, or the leftover of a volcano's vent, that covers less than 100 km² (40 square miles) [5].
• It shows at the surface only after a lot of erosion. It then appears as large exposed granite masses.

Laccolith

• Definition: a large dome-shaped intrusive body with a flat (level) base. A pipe-like conduit (a feeding channel) joins it to the magma source below.
• NCERT compares it to a composite volcanic dome that formed underground instead of on the surface.
• It is concordant like a sill. It differs from a sill because it has a flat floor and a domed, mushroom-shaped roof [4].
• How it forms:
• Magma rises through a pipe and spreads between rock layers.
• The magma is thick and cannot spread far sideways, so it pushes the upper layers up into a blister-shaped dome [4].

• Erosion later removes the arched cover and exposes the dome.

• India: the domal granite hills of the Karnataka plateau are laccoliths or batholiths. They are now exfoliated. Exfoliation means the rock peels off in curved sheets, like onion skins, because it heats and cools again and again (see geomorphic-processes).

Lopolith (NCERT spelling: "lapolith")

• Definition: a saucer-shaped intrusive body that is concave to the sky: its middle sags downward, like a dish.
• How it forms:
• Rising magma reaches a weak plane between rock layers.
• It moves sideways along that plane and spreads widely.

• Its weight makes the floor sink, so the body sags into a saucer.

• World example: the Bushveld Complex, South Africa.

• It is a lopolith of about 66,000 km², with an exposed thickness of about 8 km [4].
• It formed about 2 billion years ago in the northern Kaapvaal craton. A craton is an old, stable core of a continent [2].
• It is one of the largest differentiated igneous bodies on Earth. Differentiated means the magma separated into layers of different minerals as it cooled [2].
• Its layers include dunite, norite, anorthosite and ferrodiorite [4].
• It holds most of the world's resources of chromite and the platinum-group elements (PGEs) [3]. It also has vanadium, iron, titanium and nickel [2][4].
• One layer is about 1 m thick and almost pure chromite (the ore of chromium). It runs for tens of kilometres [4].

Phacolith

• Definition: a wavy, lens-shaped intrusive mass found at the base of synclines or the top of anticlines in folded igneous country.
• Syncline: a downfold (a U-shaped bend in rock layers).

• Anticline: an upfold (an arch-shaped bend in rock layers).

• A conduit joins it to a magma chamber below. NCERT notes that this link needs field checking.

• How it forms: magma moves into the weak spaces at the bends of folds. It hardens in the curved shape of the fold, so it bends with the rock layers.

Sill and sheet

• Definition: a near-horizontal intrusive body that lies between rock beds.
• Thick body = sill.

• Thin body = sheet.

• How it forms: magma forces its way between layers of country rock and hardens parallel to them [4]. In short, sills form when molten material squeezes between horizontal layers [5].

• It is concordant.

Dyke

• Definition: a wall-like intrusive body that sets almost perpendicular (upright) to the ground.
• How it forms:
• Magma fills cracks and fissures in the crust.
• It cools in that vertical crack [5].

• It becomes a vertical wall of hard rock. When the softer rock around it wears away, the dyke stands out as a ridge.

• It is discordant: it cuts across the structure of the country rock [4].

• India: western Maharashtra.
• Dykes are the most common intrusive form there.
• They are thought to be the feeders of the Deccan Trap eruptions. They were the cracks through which lava rose to build the flood basalts.
• Dyke swarms along India's western margin linked to the Deccan Traps [6]:
  • Narmada–Tapi swarm: runs ENE–WSW.
  • Nasik–Pune swarm: runs NNE–SSW.
  • Coastal swarm: runs N–S.
• Around Pune, the basalt lava flows of the Khandala, Bushe, Poladpur and Ambenali Formations are cut by a N–S swarm of five dykes. These dykes are 0.3 m to 8.9 m wide (study published 2025) [7].

Comparison: sill vs dyke

Feature	Sill	Dyke
Position	Near-horizontal	Near-vertical (upright)
Relation to rock layers	Concordant: parallel to the beds	Discordant: cuts across the beds
How magma enters	Squeezes between bedding planes	Fills vertical cracks and fissures
Shape after erosion	Flat bench or ledge	Wall or ridge
Indian example	—	Western Maharashtra, the feeders of the Deccan Traps

Comparison: the large intrusive bodies

Form	Shape	Base / roof	Concordant?	Example
Batholith	Huge dome, several km deep	No visible floor	Usually discordant	Exposed granite masses; Karnataka domes
Laccolith	Dome or mushroom	Flat base, arched roof	Concordant	Karnataka plateau granite domes
Lopolith	Saucer, concave to the sky	Sagging floor	Concordant	Bushveld, South Africa
Phacolith	Wavy lens	Follows the fold	Concordant	Folded igneous country

Link to other sections

• Extrusive forms are landforms made when lava flows out on the surface: lava plateaus (e.g. the Deccan Traps), cones, craters and calderas (large hollows formed when a volcano's top collapses). They are covered in Section 9.
• How these intrusive bodies change as erosion goes on is covered in landforms-evolution.

Prelims Hooks

• Concordant (parallel to layers): sill, sheet, laccolith, lopolith, phacolith. Discordant (cuts across layers): dyke, stock.
• Sill and sheet are near-horizontal. Thick = sill, thin = sheet. A dyke is near-vertical.
• A batholith is a cooled magma chamber. It is mostly granite, very large and several km deep.
• Laccolith: flat base, domed roof, fed by a pipe from below. NCERT calls it an "underground composite volcanic dome".
• Lopolith: saucer shape, concave to the sky. Example: Bushveld Complex, South Africa (about 66,000 km², platinum, chromium, vanadium).
• Phacolith: wavy, lens-shaped mass at the base of synclines or top of anticlines.
• Pairs: dykes → western Maharashtra (Deccan Trap feeders); exfoliated granite domes → Karnataka plateau (laccoliths or batholiths).
• Deccan dyke swarms and their direction: Narmada–Tapi = ENE–WSW; Nasik–Pune = NNE–SSW; Coastal = N–S.
• Trap: intrusive forms are made of plutonic rock and show at the surface only after denudation. They do not build up on the surface the way volcanic cones do.
• Trap: a stock is a small batholith-type body covering less than 100 km². It is not the same as a laccolith.

Mains Points

• Rocks under the Deccan Plateau:
• Dyke swarms (Narmada–Tapi, Nasik–Pune, Coastal) were the cracks that fed the Deccan flood basalts.
• The way these swarms line up follows the old rift zones of India's western margin.

• This helps explain the region's ridges, its drainage patterns and how groundwater moves through fractured basalt.

• Resources from intrusive bodies (GS-III):

• Layered intrusions like the Bushveld lopolith hold most of the world's chromite and platinum-group elements.
• These metals are critical minerals for catalytic converters, hydrogen fuel cells and alloy steel.

• So intrusive geology shapes the global supply of critical minerals and India's need to import them.

• Landscape and geo-heritage:

• Exposed batholiths and laccoliths, such as the exfoliated granite domes of the Karnataka plateau, show how denudation uncovers buried rock.

• Hard granite and dolerite (dyke) bodies resist erosion and stand above softer rock as hills, tors and ridges.

• Use in answers: the concordant–discordant classification and Indian examples (Maharashtra dykes, Karnataka domes) can enrich GS-I answers on volcanism, the rock cycle and landform evolution.