Intrusive volcanic landforms

Also called: Intrusive forms · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

Intrusive volcanic landforms are bodies of rock that form when magma (molten rock below the ground) cools and hardens inside the Earth's crust instead of flowing out on the surface. The main types are batholiths, laccoliths, lopoliths, phacoliths, sills, sheets and dykes. They show at the surface only after denudation (the wearing away of land by weathering and erosion) removes the rock above them.

They matter for two reasons. They explain many hills, domes and ridges in India, such as the granite domes of Karnataka and the dyke ridges of Maharashtra. Some of them also hold large stores of critical minerals, like the platinum and chromium of the Bushveld Complex.

Explanation

How they form: magma → cooling underground → exposure

• Magma moves into older rock
• Magma rises and pushes into the rock above it. This older rock that surrounds the magma is called the country rock.

• The magma may enter a crack, a gap between rock layers, or a bend in folded rock.

• Magma cools slowly underground

• Deep inside the crust the magma loses heat slowly, so it forms large crystals.
• This gives plutonic rocks such as granite. Lava that cools fast at the surface forms volcanic rocks with small crystals, such as basalt.
• A pluton is a deep-seated igneous intrusion, meaning a body of magma that cooled deep underground [5].

• At this stage nothing shows at the surface.

• Erosion uncovers the hard body

• Over a long time, denudation removes the softer cover rock.

• The hard intrusive body is left standing as a hill, dome or ridge.

• These shapes are shown in NCERT Figure 3.4 (Class 11, Ch 3).

Main classification: concordant vs discordant

• Concordant bodies lie parallel to the rock layers, like sills and laccoliths [4]. Lopoliths, phacoliths and sheets are also concordant.
• Discordant bodies cut across the rock layers, like dykes and stocks [4]. Batholiths are usually discordant too.

Types of intrusive bodies

• Batholith
• 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 the volcanoes above it.
• Far below the surface, a large magma chamber cools slowly and forms small plutons and large batholiths [5].
• It is exposed only after a lot of erosion, as large masses of granite.

• A stock is the small form of the same thing. It is a plutonic body, or what is left of a volcano's vent, covering less than 100 km² (40 square miles) [5].

• Laccolith

• A large dome-shaped body with a flat base. A pipe-like conduit (feeding channel) joins it to the magma source below.
• NCERT calls it a composite volcanic dome that formed underground.
• Like a sill, it is concordant. Unlike a sill, it has a flat floor and a domed, mushroom-shaped roof [4].

• How it forms: the magma is thick, so it cannot spread far sideways. It pushes the upper layers up into a blister-shaped dome [4].

• Lopolith (NCERT spells it "lapolith")

• A saucer-shaped body that is concave to the sky: its middle sags down like a dish.

• How it forms:

  • Rising magma reaches a weak plane between rock layers.
  • It spreads widely along that plane.
  • Its weight makes the floor sink, so the body sags into a saucer.

• Phacolith

• A wavy, lens-shaped mass found at the base of synclines (downfolds, U-shaped bends in rock layers) or the top of anticlines (upfolds, arch-shaped bends).
• Magma moves into the weak spaces at the bends of folds and hardens in the curved shape of the fold.

• A conduit joins it to a magma chamber below. NCERT notes that this link still needs to be checked in the field.

• Sill and sheet

• A near-horizontal body lying between rock beds. A thick body is a sill and a thin one is a sheet.

• Sills form when molten material squeezes between horizontal layers [5], so they are concordant.

• Dyke

• A wall-like body that stands almost upright (perpendicular to the ground).
• Magma fills cracks and fissures and cools in that vertical crack [5].
• It is discordant because it cuts across the structure of the country rock [4].
• When the softer rock around it wears away, the dyke stands out as a ridge.

Form	Shape	Base / roof	Concordant?
Batholith	Huge dome, several km deep	No visible floor	Usually discordant
Laccolith	Dome or mushroom	Flat base, arched roof	Concordant
Lopolith	Saucer, concave to the sky	Sagging floor	Concordant
Phacolith	Wavy lens	Follows the fold	Concordant
Sill / sheet	Near-horizontal layer	Parallel to beds	Concordant
Dyke	Near-vertical wall	Cuts across beds	Discordant

Where it occurs

• Western Maharashtra (dykes):
• Dykes are the most common intrusive form here.
• They are thought to be the feeders of the Deccan Trap eruptions, meaning 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, 0.3 m to 8.9 m wide [7].

• Karnataka plateau (laccoliths or batholiths):

• Its domal granite hills are laccoliths or batholiths that are now exfoliated.

• Exfoliation means the rock peels off in curved sheets, like onion skins, because it heats and cools again and again.

• Bushveld Complex, South Africa (lopolith):

• 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, about 1 m thick, is almost pure chromite (the ore of chromium) and runs for tens of kilometres [4].

Don't confuse with

• Extrusive volcanic landforms: these form when lava cools on the surface, for example lava plateaus like the Deccan Traps, cones, craters and calderas. Intrusive forms cool underground and show only after denudation.
• Sill vs dyke: a sill is near-horizontal and concordant, squeezed between rock beds. A dyke is near-vertical and discordant, filling cracks that cut across the beds.
• Laccolith vs lopolith: a laccolith has a flat base and a domed roof, like a mushroom. A lopolith is a saucer, concave to the sky, with a sagging floor.
• Stock vs laccolith: a stock is a small batholith-type body (less than 100 km²) and is discordant [5]. A laccolith is a concordant dome fed by a pipe.

Prelims Hooks

• Concordant: sill, sheet, laccolith, lopolith, phacolith. Discordant: dyke, stock (and usually batholith).
• Sill and sheet are near-horizontal (thick = sill, thin = sheet). A dyke is near-vertical.
• Lopolith = saucer, concave to the sky. Example: Bushveld Complex, South Africa, rich in chromite, platinum-group elements and vanadium [2][3].
• 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). Swarm directions: Narmada–Tapi ENE–WSW, Nasik–Pune NNE–SSW, Coastal N–S [6].
• Trap: intrusive bodies are made of plutonic rock (slow cooling, large crystals, e.g. granite). They do not build up on the surface the way volcanic cones do. A batholith is a cooled magma chamber.

Mains Points

• Rocks under the Deccan Plateau (GS-I):
• Dyke swarms (Narmada–Tapi, Nasik–Pune, Coastal) were the cracks that fed the Deccan flood basalts [6].
• The way the 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.

• Critical minerals from intrusive bodies (GS-III):

• Layered intrusions such as the Bushveld lopolith hold most of the world's chromite and platinum-group elements [3].
• These metals are needed for catalytic converters, hydrogen fuel cells and alloy steel.

• So intrusive geology shapes the global supply of critical minerals and how much India must import.

• Landscape and geo-heritage (GS-I):

• 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. This makes them useful examples in answers on the rock cycle and how landforms change over time.

Related concepts

• Batholith
• Laccolith
• Lopolith
• Phacolith
• Sill
• Dyke

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• Intrusive volcanic landforms