Igneous rocks: from magma to granite and basalt

Detail

1. Petrology and the three rock families

• Petrology is the science of rocks. A rock is a natural mass made of one or more minerals.
• Rocks are grouped by how they were formed (their origin) into three families:
• Igneous: formed from molten rock.
• Sedimentary: formed from layers of broken bits that settle and harden.

• Metamorphic: formed when old rocks are changed by heat and pressure.

• Igneous rock forms when magma or lava cools and becomes solid.

• Magma is molten rock below the surface, inside the crust or upper mantle.
• Lava is the same molten rock after it comes out on the surface.

• "Igneous" comes from the Latin ignis, meaning fire.

• Igneous rocks are called primary rocks. The Earth's first crust formed when molten material cooled, so it was igneous. All other rocks later formed from igneous rocks, directly or indirectly.

• In the rock cycle, igneous rocks are the starting point:
• Weathering breaks them down → the pieces become sedimentary rock.
• Heat and pressure change them → they become metamorphic rock.
• Melting turns any rock back into magma → new igneous rock forms.

2. Where the magma 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 (too small to see easily) 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. Batholiths are made of granite. They are seen at the surface only after the rocks above them are worn away.

3. Cooling rate controls crystal size (texture)

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

• Slow cooling at depth → coarse grains
• Magma cools over thousands of years, because the rocks around it hold in the heat.
• Mineral crystals have time to grow large → 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 → fine grains

• Lava meets air or water and loses heat quickly.
• Crystals stay small → fine-grained basalt.

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

• Very fast cooling → glass (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 → pumice

• Gas bubbles get trapped in lava as it cools quickly.
• The result is a very light rock full of holes (high porosity). It is glassy, not crystalline [5].
• Pumice floats on water. Rafts of pumice can drift across oceans after an undersea eruption.

4. Felsic (acid) and mafic (basic) rocks

• Silica (SiO₂, the compound in quartz and sand) is the most important part of magma. The amount of silica decides colour, weight and how the volcano behaves.

Feature	Felsic (acid)	Mafic (basic)
Silica	More than about 65%	Less silica
Other elements	Rich in feldspar and quartz	More iron (Fe) and magnesium (Mg)
Colour	Light (pink, white, grey)	Dark (black, dark green)
Density	Lower	Higher
Lava	Thick (viscous), traps gas	Thin (fluid), gas escapes easily
Eruption	Explosive	Quiet, flowing
Intrusive example	Granite	Gabbro
Extrusive example	Rhyolite, obsidian, pumice	Basalt

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

5. Granite: the old basement rock

• Granite is light-coloured and coarse-grained. It is felsic and plutonic.
• Its main minerals are quartz (glassy grains), feldspar (pink or white) and a little mica or hornblende (dark specks).
• Batholiths are granitic. Granite is the main rock of the continental crust and of old shields (stable cores of continents).
• In India:
• The exfoliated domal hills of the Karnataka plateau (e.g. around Bengaluru, Ramanagara and Savandurga) are bare granite domes.
  • Exfoliation is onion-peel weathering. 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.

  • Gneiss is a banded rock. It formed when old granitic rock was changed by heat and pressure.
  • It is part of the Archaean basement, the oldest rock system of Peninsular India.

• National Geological Monuments: GSI has declared 26 sites across India as National Geological Monuments. GSI or the state governments take steps to protect them (PIB, 2016) [2].

6. Basalt: the rock of ocean floors and the Deccan Traps

• Basalt is dark and fine-grained. It is mafic and extrusive. It forms from very fluid, low-silica lava.
• Basalts are extrusive igneous rocks that cooled quickly, so they have a fine-grained structure [5].
• Where basalt is found:
• 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 of India, the Columbia River Plateau (USA) and the Siberian Traps (Russia).

• Deccan Traps:

• Fissure eruption: lava poured out of long cracks in the crust, not from one crater → it spread in thin sheets, flow over flow → it formed a thick plateau with step-like edges.
• "Trap" comes from the Swedish trappa, meaning stairs. It refers to this step-like look.
• They cover large parts of Maharashtra, plus parts of Gujarat, Madhya Pradesh, Karnataka and Telangana. They formed near the end of the Cretaceous period.

• Basalt weathers into black soil (regur), which is good for cotton.

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

• Columnar jointing: a thick lava flow cools and shrinks → cracks spread in a regular pattern → this leaves tall columns that are usually six-sided (hexagonal).
• The same feature is seen at the Giant's Causeway (Northern Ireland, UK).

7. Kimberlite: the diamond carrier

• Kimberlite is an ultramafic rock. It is very rich in iron and magnesium and very low in silica. It is named after Kimberley, South Africa.
• How it forms:
• Magma starts very deep in the mantle, below about 150 km, where the pressure is high enough for carbon to form diamond.
• The magma, full of gas, shoots up very fast through narrow, carrot-shaped volcanic pipes.

• It rises so fast that the diamonds do not change into graphite on the way up → kimberlite often carries diamonds.

• India's kimberlites:

• Majhgawan pipe, near Panna (Madhya Pradesh), run by NMDC (National Mineral Development Corporation).
  • Majhgawan is a fully mechanised mine run by NMDC. It is worked by the opencast method (an open pit) in tuff rock (2020) [4].
  • It can produce about 30,000 carats a year (2020) [4].
  • The mined kimberlite is first left in piles to weather. It is then crushed and processed by Heavy Media Separation. An X-ray diamond sorter has raised diamond recovery to 98% (2020) [4].
  • Majhgawan is the only diamond mine working on a commercial scale in India (as on 1.4.2015) [3].
• Vajrakarur (Wajrakarur), Anantapur district (Andhra Pradesh): a kimberlite field that GSI has explored [3].

• The Majhgawan and Wajrakarur pipes were mined for diamonds long before they were recognised as kimberlite. This was only understood in 1930 [3].

• Diamond resources (National Mineral Inventory, 1.4.2015):

• India's total diamond resources are 31,836,091 carats [3].
• Shares by state: Madhya Pradesh 90.17% > Andhra Pradesh 5.73% > Chhattisgarh 4.10% [3].

• Anantapur district alone holds about 91.5% of Andhra Pradesh's resources [3].

• India's four diamond regions (2020) [4]: 1. South Indian tract (Andhra Pradesh): Anantapur, Kadapa, Guntur, Krishna, Mahabubnagar and Kurnool. 2. Central Indian tract (Madhya Pradesh): the Panna belt. 3. Chhattisgarh: Behradin–Kodawali (Raipur district) and Tokapal–Dugapal (Bastar district). 4. Eastern Indian tract: mostly in Odisha, between the Mahanadi and Godavari valleys.

• Production: India produced 28,816 carats in 2019-20, down 25% from 38,437 carats in 2018-19. Both reporting mines are in Panna district. About 36% of the output was gem quality and 64% industrial [4].

• Historic and global context:
• Before the 19th century, diamonds came from the river gravels of the Krishna. Golconda was the main trading centre in the 16th–17th centuries [3].
• Today India mines very few rough diamonds but cuts most of the world's diamonds, mainly in Surat (Gujarat). India imports most of its rough diamonds (2020) [4].
• Main world producers in 2019: Russia (33%), Botswana (17%), Canada (14%) [4].

Prelims Hooks

• Extrusive = volcanic = fast cooling = fine-grained (basalt). Intrusive = plutonic = slow cooling = coarse-grained (granite, gabbro).
• Obsidian = volcanic glass (very fast cooling). Pumice = frothy, glassy rock that floats. Neither has a proper crystal structure.
• Granite and gabbro are both coarse-grained. Granite is felsic; gabbro is mafic. Gabbro is the slow-cooled, plutonic version of basalt.
• Felsic (more than about 65% silica) → light, less dense, viscous lava, explosive eruptions. Mafic → dark, dense, fluid lava, shield volcanoes and flood basalts.
• Batholiths are made of granite. The Deccan Traps and the ocean floor are made of basalt.
• GSI National Geological Monuments: Peninsular Gneiss – Lalbagh, Bengaluru; Columnar basalt – St Mary's Island, Udupi. Both are in Karnataka. Trap: Peninsular Gneiss is a metamorphic rock formed from granite, not an igneous rock itself.
• Kimberlite is ultramafic, comes from the mantle, rises in pipes and carries diamonds. Match these: Majhgawan – Panna, MP – NMDC; Vajrakarur – Anantapur, AP.
• Diamond resources by state (1.4.2015): MP > AP > Chhattisgarh [3].
• Primary rocks = igneous, because the first crust was igneous.

Mains Points

• Rock type shapes the land and its use (GS-I):
• Deccan basalt → black cotton soil, flat-topped plateaus and step-like slopes (e.g. the Western Ghats in Maharashtra).
• Archaean granite and gneiss of the Karnataka plateau → bare domes, tors and thin red soils → farming that depends on tanks (small reservoirs).

• Hard, crystalline rocks hold groundwater only in cracks → groundwater is scarce in the southern Peninsula.

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

• Felsic, explosive volcanoes (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 (Andaman Sea), needs monitoring for its effects on the islands and nearby seas.

• Igneous rocks host many minerals (GS-III resources):

• Kimberlite carries 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 31.8 million carats (2015), but only one commercial kimberlite mine and a production of 28,816 carats (2019-20). So India's cutting hub at Surat depends on imported rough diamonds [3][4]. This argues for more GSI exploration in the Dharwar and Bastar cratons (old, stable blocks of crust) and for auctioning diamond blocks.

• Geoheritage protection: GSI has notified 26 National Geological Monuments (PIB, 2016) [2]. Lalbagh (in a city) and St Mary's Island (a tourist spot) face pressure from urban growth and tourism. This supports the need for a national geoheritage law.