Igneous rock

Also called: Primary rock · Topic: Minerals, Rocks and the Rock Cycle · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

Igneous rock is rock that forms when hot, molten rock cools and becomes solid. The molten rock is called magma when it is below the surface and lava after it comes out on the surface. Igneous rocks are called primary rocks because the Earth's first crust was igneous. All other rocks later formed from them, directly or indirectly. They shape major landforms such as the Deccan Traps, the ocean floor and the granite domes of Karnataka. They also hold key minerals, including diamonds in kimberlite.

Explanation

How igneous rocks form and fit into the rock cycle

• Petrology is the science of rocks. Rocks are grouped by how they formed into three families: igneous, sedimentary and metamorphic.
• "Igneous" comes from the Latin ignis, meaning fire.
• Formation:
• Rock melts deep inside the crust or upper mantle and becomes magma.
• The magma either stays below the surface or comes out as lava.

• It cools, mineral crystals grow, and it hardens into igneous rock.

• Igneous rock is the starting point of the rock cycle:

• Weathering (rock breaking down in the open air) turns it into pieces. These pieces settle and harden into sedimentary rock.
• Heat and pressure change it into metamorphic rock.
• Melting turns any rock back into magma, and new igneous rock forms.

Where it cools: extrusive and intrusive

Feature	Volcanic (extrusive)	Plutonic (intrusive)
Where it cools	On the surface, as lava	Inside the crust, as magma
Cooling speed	Fast	Slow
Crystal size	Fine or glassy	Coarse (crystals can be seen by eye)
Examples	Basalt, obsidian, pumice	Granite, gabbro
Landforms	Lava flows, shield volcanoes, flood basalts	Batholiths, laccoliths, sills, dykes

• Plutonic comes from Pluto, the Roman god of the underworld. It means "formed deep below".
• Batholith: a very large, dome-shaped body of magma that cooled deep in the crust. It is made of granite. We see it at the surface only after the rocks above it wear away.

Cooling rate controls texture

Texture means the size, shape and arrangement of the grains in a rock.

• Slow cooling at depth gives coarse grains.
• The rocks around the magma hold in the heat, so it cools over thousands of years.
• Crystals have time to grow large, which gives coarse-grained granite and gabbro.

• The longer magma takes to cool, the more time its crystals have to grow [5].

• Fast cooling at the surface gives fine grains.

• Lava meets air or water and loses heat quickly, so the crystals stay small. This gives basalt.

• In rocks that cool fast, glass may form between the tiny crystals [5].

• Very fast cooling gives glass with no crystals.

• Obsidian is natural volcanic glass. It is shiny and usually black.

• It forms when thick (viscous), silica-rich lava cools almost instantly [5].

• Gas-filled froth gives pumice.

• Gas bubbles get trapped in lava as it cools quickly. The result is a very light, glassy rock full of holes (high porosity) [5].
• Pumice floats on water. Rafts of pumice can drift across oceans after an undersea eruption.

Silica content: felsic, mafic and ultramafic

Silica (SiO₂, the compound found in quartz and sand) is the most important part of magma. The amount of silica decides a rock's colour, its weight and how its volcano behaves.

Feature	Felsic (acid)	Mafic (basic)
Silica	More than about 65%	Less
Rich in	Feldspar, quartz	Iron (Fe), magnesium (Mg)
Colour	Light (pink, white, grey)	Dark (black, dark green)
Density	Lower	Higher
Lava	Thick, traps gas	Runny, lets gas escape
Eruption	Explosive	Quiet, flowing
Intrusive / extrusive	Granite / rhyolite, obsidian, pumice	Gabbro / basalt

• The names: felsic = feldspar + silica. Mafic = magnesium + ferrum (Latin for iron).
• Ultramafic rock has even less silica and even more Fe and Mg than mafic rock. It comes from the mantle. Examples are kimberlite and peridotite.
• How silica decides the type of volcano:
• High silica: silica chains link up, so the lava becomes thick and sticky (high viscosity, meaning it resists flowing).
  • The thick lava traps gas, so pressure builds and the eruption is explosive.
  • This builds steep composite volcanoes (layered cones of lava and ash), e.g. Fuji in Japan.
• Low silica: basalt lava is runny and spreads far.
  • This builds gentle shield volcanoes (e.g. Hawaii) and wide flood basalts (e.g. the Deccan Traps).

Where it occurs

• Granite (felsic, plutonic):
• Granite is the main rock of the continental crust and of old shields (stable cores of continents). Batholiths are made of granite.
• Karnataka plateau: bare granite domes stand around Bengaluru, Ramanagara and Savandurga.
  • Exfoliation (onion-peel weathering) shapes them. Daily heating and cooling, and the release of pressure as the rock above wears away, make the outer layers peel off in curved sheets.
  • This leaves smooth, rounded domes and tors (piles of rounded boulders).

• Peninsular Gneiss, Lalbagh, Bengaluru (Karnataka) is a GSI National Geological Monument. It is part of the Archaean basement, the oldest rock system of Peninsular India. Gneiss is a banded rock that formed when old granitic rock was changed by heat and pressure.

• Basalt (mafic, extrusive):

• Ocean floor: new ocean crust forms as basalt at mid-ocean ridges, e.g. the Mid-Atlantic Ridge.
• Shield volcanoes: Mauna Loa and Kilauea in Hawaii.
• Flood basalts: the Deccan Traps (India), the Columbia River Plateau (USA) and the Siberian Traps (Russia).
• Deccan Traps:
  • They formed near the end of the Cretaceous period.
  • They cover large parts of Maharashtra, plus parts of Gujarat, Madhya Pradesh, Karnataka and Telangana.
  • They formed by fissure eruption: lava poured out of long cracks, not from one crater. It spread in thin sheets, flow over flow, and built a thick plateau with step-like edges.
  • "Trap" comes from the Swedish trappa, meaning stairs.

• Columnar basalt at St Mary's Island, Udupi district (Karnataka coast, Arabian Sea) is a GSI National Geological Monument [2].

  • Here a thick lava flow cooled and shrank, so cracks spread in a regular pattern. This left tall columns that are usually six-sided.
  • The Giant's Causeway in Northern Ireland (UK) has the same feature.

• Kimberlite (ultramafic, carries diamonds):

• It is named after Kimberley, South Africa.
• Majhgawan pipe, near Panna (Madhya Pradesh), is run by NMDC. It is the only diamond mine working on a commercial scale in India (as on 1.4.2015) [3].

• Vajrakarur (Wajrakarur), Anantapur district (Andhra Pradesh) is a kimberlite field that GSI has explored [3].

• Active volcano in India: Barren Island (Andaman Sea).

Don't confuse with

• Extrusive vs intrusive: extrusive (volcanic) rock cools fast on the surface and is fine-grained, e.g. basalt. Intrusive (plutonic) rock cools slowly inside the crust and is coarse-grained, e.g. granite and gabbro.
• Granite vs gabbro: both are coarse-grained and plutonic. Granite is felsic (light-coloured, high silica). Gabbro is mafic (dark, rich in Fe and Mg). Gabbro is the slow-cooled version of basalt.
• Gneiss (metamorphic) vs granite (igneous): the Peninsular Gneiss at Lalbagh is metamorphic. It formed when granitic rock was changed by heat and pressure, so it is not an igneous rock itself.
• Obsidian and pumice vs crystalline igneous rock: both are glassy and have no proper crystal structure. Obsidian is dense volcanic glass. Pumice is full of gas holes and floats on water.

Prelims Hooks

• Extrusive = volcanic = fast cooling = fine-grained (basalt). Intrusive = plutonic = slow cooling = coarse-grained (granite, gabbro).
• Batholiths are made of granite. The Deccan Traps and the ocean floor are made of basalt.
• Felsic rock has more than about 65% silica: light, less dense, viscous lava, explosive eruptions. Mafic rock is dark and dense, with fluid lava that builds shield volcanoes and flood basalts.
• GSI National Geological Monuments, both in Karnataka: Peninsular Gneiss – Lalbagh, Bengaluru and Columnar basalt – St Mary's Island, Udupi [2]. Trap: gneiss is metamorphic, not igneous.
• Kimberlite is ultramafic and carries diamonds. Match: Majhgawan – Panna, MP – NMDC; Vajrakarur – Anantapur, AP. Diamond resources by state (1.4.2015): MP (90.17%) > AP (5.73%) > Chhattisgarh (4.10%) [3].
• Igneous rocks are called primary rocks because the Earth's first crust was igneous.

Mains Points

• Rock type shapes land, soil and water use (GS-I):
• Deccan basalt weathers into black soil (regur), which is good for cotton. It also gives flat-topped plateaus and step-like slopes, e.g. the Western Ghats in Maharashtra.
• The Archaean granite and gneiss of the Karnataka plateau give bare domes, tors and thin red soils. Farming there depends on tanks (small reservoirs).

• Hard crystalline rock holds groundwater only in its cracks, so groundwater is scarce in the southern Peninsula.

• Silica controls volcanic hazard (GS-I / GS-III disasters):

• Felsic, explosive volcanoes on the Pacific Ring of Fire (Japan, Indonesia) are far more dangerous than mafic shield or fissure eruptions (Hawaii, Iceland, the ancient Deccan).

• India's only active volcano, Barren Island, needs monitoring for its effects on the islands and nearby seas.

• Igneous rocks as a mineral base (GS-III resources):

• Kimberlite hosts diamonds. Mafic and ultramafic rocks host chromite, nickel and platinum-group elements. Granites and pegmatites host mica, feldspar, tin and rare metals.
• India has diamond resources of 31,836,091 carats (1.4.2015) [3]. Yet it produced only 28,816 carats in 2019-20, down 25% from 38,437 carats in 2018-19 [4].
  • So the cutting hub at Surat (Gujarat) depends on imported rough diamonds [4].
  • This argues for more GSI exploration in the Dharwar and Bastar cratons (old, stable blocks of crust) and for auctioning diamond blocks.
• GSI has notified 26 National Geological Monuments (PIB, 2016) [2]. Igneous sites such as St Mary's Island face pressure from tourism, which supports the need for a national geoheritage law.

Related concepts

• Felsic and mafic rocks
• Granite
• Basalt
• Kimberlite

Read more

• Igneous rocks: from magma to granite and basalt