Granite

Topic: Minerals, Rocks and the Rock Cycle · NCERT: Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 5 "Geomorphic Processes"

Meaning

Granite is a light-coloured, coarse-grained igneous rock. It forms when silica-rich magma cools slowly deep inside the Earth's crust, and it is made mainly of quartz and feldspar. It matters because granite is the main rock of the continental crust and of the old shields (the stable cores of continents). Batholiths are made of it, and so are the bare, rounded domes of the Karnataka plateau.

Explanation

How granite forms: slow cooling at depth

• Magma stays trapped inside the crust
• Magma is molten rock below the surface. When it does not reach the surface, it cools underground.

• Rock that forms this way is called plutonic or intrusive. "Plutonic" comes from Pluto, the Roman god of the underworld, so it means "formed deep below".

• The rocks around it hold in the heat

• So the magma cools very slowly, over thousands of years.

• Mineral crystals have time to grow large. The longer magma takes to cool, the more time its crystals have to grow [3].

• Result: a coarse-grained texture

• Texture means the size, shape and arrangement of the grains in a rock.
• In granite, the crystals are big enough to see with the naked eye.

What granite is made of: a felsic rock

• Main minerals:
• Quartz: glassy grains.
• Feldspar: pink or white grains.

• A little mica or hornblende: dark specks.

• Felsic (acid) rock: felsic = feldspar + silica. It has more than about 65% silica (SiO₂, the compound in quartz and sand).

• Because it is rich in silica:
• it is light in colour (pink, white or grey)
• it is less dense than mafic rocks such as gabbro and basalt
• the same magma makes thick (viscous) lava if it reaches the surface. That lava traps gas and gives explosive eruptions.

Granite bodies and how they reach the surface

• Batholith: a very large, dome-shaped body of magma that cooled deep in the crust. Batholiths are granitic.
• Granite forms deep down, so we see it only after the rocks above it are worn away by erosion.
• Exfoliation then shapes the exposed granite:
• Exfoliation is onion-peel weathering.
• Daily heating and cooling makes the outer layers expand and shrink. The release of pressure, as the rock above wears away, does the same.
• So the outer layers peel off in curved sheets → smooth, rounded domes and tors (piles of rounded boulders) are left behind.

Granite in the rock cycle

• Like all igneous rocks, granite is a primary rock. The Earth's first crust was igneous.
• Weathering breaks granite into pieces, which can later become sedimentary rock.
• Heat and pressure turn granite into gneiss, a banded metamorphic rock.
• Melting turns it back into magma, and new igneous rock forms.

Where it occurs

• World:
• Granite is the main rock of the continental crust.
• It forms the old shields (stable cores of continents).

• It makes up the batholiths under mountain belts, which are exposed after long erosion.

• India: Karnataka plateau

• Exfoliated domal hills (bare granite domes) and tors around Bengaluru, Ramanagara and Savandurga.

• India: Archaean basement of Peninsular India

• The Archaean is the oldest rock system of Peninsular India. Here, old granitic rock was changed by heat and pressure into gneiss.
• Peninsular Gneiss, Lalbagh, Bengaluru (Karnataka) is a GSI National Geological Monument.

• GSI has declared 26 sites across India as National Geological Monuments (PIB, 2016) [2].

• Southern Peninsula: hard granite and gneiss lie under much of this region. They shape the landforms, the soils and the groundwater supply (see Mains Points).

Don't confuse with

• Gabbro: also coarse-grained and plutonic, but it is mafic (rich in iron and magnesium). So it is dark and dense. Granite is felsic and light-coloured.
• Rhyolite: has the same felsic, silica-rich make-up as granite, but it is extrusive. It cools fast at the surface, so it is fine-grained. Granite cools slowly at depth, so it is coarse-grained.
• Basalt: dark, fine-grained, mafic and extrusive. It makes up the Deccan Traps and the ocean floor. Granite makes up batholiths and the continental crust.
• Gneiss: a banded metamorphic rock made when granitic rock is changed by heat and pressure. The Lalbagh monument is gneiss, not igneous granite.

Prelims Hooks

• Granite = intrusive (plutonic) + slow cooling + coarse-grained + felsic. Its main minerals are quartz and feldspar.
• Batholiths are made of granite. The Deccan Traps and the ocean floor are made of basalt.
• Granite and gabbro are both coarse-grained plutonic rocks. Granite is felsic (more than about 65% silica, light). Gabbro is mafic (dark, dense).
• Process–landform pair: exfoliation → granite domes and tors, e.g. Bengaluru, Ramanagara and Savandurga on the Karnataka plateau.
• Trap: the GSI National Geological Monument at Lalbagh, Bengaluru is Peninsular Gneiss. This is a metamorphic rock formed from granitic rock, not igneous granite. GSI has 26 such monuments (PIB, 2016) [2].
• "Felsic" = feldspar + silica. Felsic rocks are light in colour, low in density, and give viscous lava and explosive eruptions.

Mains Points

• Granite shapes the land and how people use it (GS-I):
• The Archaean granite and gneiss of the Karnataka plateau → bare domes, tors and thin red soils → farming that depends on tanks (small reservoirs).

• Hard, crystalline granite holds groundwater only in its cracks → groundwater is scarce across the southern Peninsula. This makes tank and watershed management very important there.

• Granite and its mineral wealth (GS-III resources):

• Granites and pegmatites (very coarse-grained granitic rocks) host mica, feldspar, tin and rare metals. Old granite shields are therefore key targets for mineral exploration by GSI.

• Protecting geoheritage (GS-I/GS-III environment):

• The Peninsular Gneiss at Lalbagh shows India's ancient granitic basement. It sits inside a growing city and faces pressure from urban growth.
• GSI has notified 26 National Geological Monuments (PIB, 2016) [2]. This supports the case for a national geoheritage law to protect such sites.

Related concepts

• Igneous rock
• Felsic and mafic rocks
• Basalt
• Kimberlite

Read more

• Igneous rocks: from magma to granite and basalt