Minerals, Rocks and the Rock Cycle

1. Elements of the crust and what a mineral is

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• Elements of the Earth's crust (by weight): oxygen 46.6%, silicon 27.7, aluminium 8.1, iron 5.0, calcium 3.6, sodium 2.8, potassium 2.6, magnesium 2.1. These eight make up about 98% of the crust.
• Aluminium is the most common metal in the crust. For the whole earth, iron comes first, because the core is nife (nickel + iron).
• Most of these elements are joined together, not free. Oxygen and silicon combine to form silicate minerals.
• Mineral (Class 10, Minerals and Energy Resources): "a homogenous, naturally occurring substance with a definable internal structure". Rocks are made of minerals. Limestone has a single mineral; most rocks have several.
• NCERT says over 2,000 minerals are known. The International Mineralogical Association now recognises over 6,000 (verify current). Only a few are common in rocks.
• Ore: a mineral mixed with other elements and rich enough to be worth mining. Workable deposits are only about 1% of the crust. Minerals are non-renewable.
• Geographers study minerals for landforms and for where resources and economic activity are found. Geologists study how minerals formed, their age and their composition.

2. Identifying minerals: physical properties and the Mohs scale

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• Physical properties of minerals (the features used to identify them):
• Crystal form: the outer shape, set by how the molecules are arranged inside (cubes, octahedrons, hexagonal prisms).
• Cleavage: breaking along flat planes, as in mica. Fracture: uneven breaking when there is no such plane, as in quartz.
• Lustre: how the surface shines (metallic, silky, glossy).
• Colour: can mislead, because impurities change it. Streak (the colour of the powder) is more reliable. Hematite always gives a red-brown streak.

• Transparency, structure, hardness and specific gravity (weight compared with an equal volume of water).

• Mohs scale of hardness (Friedrich Mohs, 1812) ranks minerals by how well they resist scratching. A harder mineral scratches any softer one.

1	2	3	4	5	6	7	8	9	10
Talc	Gypsum	Calcite	Fluorite	Apatite	Orthoclase feldspar	Quartz	Topaz	Corundum	Diamond

• The scale only gives the order of hardness. Diamond is far more than ten times as hard as talc.
• Everyday checks: fingernail about 2.5, copper coin about 3.5, glass or a steel knife about 5.5.
• Trap: graphite (about 1–2) and diamond (10) are both pure carbon. Their internal structure makes the difference.

3. Major mineral groups, metallic and non-metallic minerals, and how ores occur

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• Silicate minerals are built from silicon–oxygen units and form over 90% of the crust.

Group	Make-up and features	Use / note
Feldspar	Si, O with Na, K, Ca, Al; light-coloured; about half the crust (older NCERT figure)	Ceramics, glass
Quartz	Silica; hardness 7; does not dissolve in water; white or colourless	Main grain of sand; in granite; glass, electronics, watches
Pyroxene	Ca, Al, Mg, Fe + silica; about 10% of the crust; green or black	Common in meteorites
Amphibole	Al, Ca, Fe, Mg + silica; about 7%; green or black	Asbestos is one form
Mica	K, Al, Mg, Fe + silica; splits into thin sheets	Electrical goods; Koderma–Gaya–Hazaribagh, Ajmer, Nellore
Olivine	Mg, Fe + silica; green	In basalt and the mantle; peridot gem

• Metallic minerals are precious (gold, silver, platinum), ferrous (iron and manganese) or non-ferrous (copper, lead, zinc, tin, aluminium). Non-metallic minerals include mica, limestone, gypsum, sulphur and phosphates. Mineral fuels (coal, oil, gas) come from living matter. Cement is a manufactured product, not a mineral.
• How minerals occur (Class 10, Minerals and Energy Resources):
• Veins (small) and lodes (large) in cracks of igneous and metamorphic rocks: tin, copper, zinc, lead.
• Beds in sedimentary rocks: coal and some iron ore. Gypsum, potash and salt form by evaporation in dry regions.
• Residual deposits, left behind when weathering removes soluble parts: bauxite.
• Placer deposits in valley-floor sands: gold, silver, tin, platinum.
• Ocean water gives salt, magnesium and bromine; the ocean floor has manganese nodules.

4. Igneous rocks: from magma to granite and basalt

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• Petrology is the science of rocks. By origin, rocks fall into three families: igneous, sedimentary and metamorphic.
• Igneous rock forms when magma or lava cools and becomes solid. These are called primary rocks because the first crust was igneous. Class 11, Interior of the Earth: rock that cools at the surface is volcanic (extrusive); rock that cools inside the crust is plutonic (intrusive).
• Cooling rate sets crystal size:
• Slow cooling at depth gives coarse grains (granite, gabbro).
• Fast cooling at the surface gives fine grains (basalt).

• Very fast cooling gives glass (obsidian). Gas-filled froth gives pumice, which floats.

• Felsic and mafic rocks: felsic (acid) rocks have more than about 65% silica and are light-coloured and less dense (granite). Mafic (basic) rocks have less silica and more iron and magnesium, and are dark and dense (basalt, gabbro). More silica makes lava thicker and eruptions more explosive.

• Granite is light-coloured and coarse-grained, rich in quartz and feldspar. Batholiths are granitic. Examples are the exfoliated dome hills of Karnataka and the Peninsular Gneiss at Lalbagh, Bengaluru (a GSI National Geological Monument).
• Basalt is dark and fine-grained and forms from fluid, low-silica lava. It makes up the ocean floor, shield volcanoes and the Deccan Traps. Columnar lava at St Mary's Island, Udupi is a GSI monument.
• Kimberlite is an ultramafic rock (very rich in iron and magnesium) that shoots up quickly from deep in the mantle through pipes and often carries diamonds. Examples are the Majhgawan pipe near Panna, Madhya Pradesh (worked by NMDC) and Vajrakarur, Andhra Pradesh.

5. Sedimentary rocks: layers, fossils and fuels

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• Sedimentary rock (Latin sedere, to settle) forms from rock bits, shells or chemical deposits laid down in layers, mostly in water.
• Lithification turns loose sediment into rock in two steps:
• Compaction: the weight of upper layers presses the sediment.

• Cementation: silica or calcite from groundwater glues the grains.

• These rocks cover about three-quarters of the land surface but form only a thin skin of the crust. They are layered (stratified), hold fossils, and contain coal, oil and gas. Petroleum sits in porous limestone or sandstone.

• Clastic, chemical and organic sedimentary rocks:
• Clastic (mechanically formed) rocks are made of rock fragments: sandstone, conglomerate, shale, loess.
• Chemical rocks settle out of water, often by evaporation: rock salt, gypsum, potash, some limestone, chert.

• Organic rocks come from remains of living things: coal (plants), chalk and much limestone (shells).

• Sandstone is made of sand-sized grains, mostly quartz, cemented together. Red Vindhyan sandstone was used in the Red Fort, Fatehpur Sikri and Sanchi.

• Limestone (calcium carbonate) is the raw material for cement and a flux in blast furnaces. It dissolves in water to form karst landforms.
• Banded iron formation (BIF) has thin, alternating bands of iron oxide and silica. Most BIF formed 3.5–1.8 billion years ago, when oxygen from early photosynthesis reacted with iron dissolved in seawater. BIFs hold most of the world's iron ore, including India's Dharwar ores (Bailadila, Kudremukh, Odisha–Jharkhand belt).

6. Metamorphic rocks: change of form under heat and pressure

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• Metamorphism (meta = change, morph = form) changes a rock's minerals and texture without melting it. It happens under changes in pressure, volume and temperature (PVT). Class 11, Geomorphic Processes: mountain building, earthquakes and plate movements cause these PVT changes.
• Metamorphic rock can form from igneous, sedimentary or older metamorphic rock. It is harder and more compact than the parent rock, so it often stands as ridges.
• Dynamic metamorphism: pressure crushes and recrystallises rock with little chemical change. Thermal metamorphism: heat recrystallises the minerals.
• Contact and regional metamorphism:
• Contact metamorphism is the baking of rock next to hot magma over a small area, such as around a dyke.

• Regional metamorphism changes large areas under heat and pressure during mountain building.

• Foliation means minerals line up in layers under pressure. Alternating light and dark bands are called banding (seen in gneiss).

• Parent rock → metamorphic rock: shale → slate (→ phyllite → schist); limestone or dolomite → marble; sandstone → quartzite; granite → gneiss; coal → graphite.
• Schist and gneiss: schist is medium-grained, with flaky mica that makes it shiny. Gneiss is coarse-grained, with light and dark bands. Both are common in India's Archaean rocks.
• Marble forms from limestone or dolomite. Makrana (Nagaur, Rajasthan) marble was used for the Taj Mahal, and the Narmada flows through marble gorges at Bhedaghat, near Jabalpur.

7. The rock cycle

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• The rock cycle is the never-ending process by which rocks change from one type into another.

        MAGMA ──cooling──► IGNEOUS
          ▲                   │ weathering, erosion
       melting                ▼
          │              SEDIMENTS ──compaction + cementation──► SEDIMENTARY
          │                   ▲                                       │
          │           weathering (shortcut)                    heat + pressure
          │                   │                                       ▼
          └──────────── METAMORPHIC ◄──────heat + pressure──── (igneous too)
                 (uplift and erosion bring any rock back to the surface)

• Steps:
• Igneous rock at the surface is weathered and eroded into fragments.
• The fragments are deposited and lithified into sedimentary rock.
• Heat and pressure turn igneous or sedimentary rock into metamorphic rock.

• Rock carried deep, for example down a subduction zone, melts into magma, which cools into new igneous rock.

• Shortcuts: metamorphic rock can weather straight into sediment, and igneous rock can melt again.

• Energy behind the cycle:
• The earth's internal heat (from radioactivity and heat left over from its formation) drives melting, metamorphism and uplift. These are endogenic forces (forces from inside the earth).

• The sun's energy and gravity drive weathering, erosion and deposition. These are exogenic forces (forces acting on the surface).

• Crust is recycled at ocean trenches, so ocean-floor rocks are younger than 200 million years. Continental rocks can be up to 3,200 million years old (Class 11, Distribution of Oceans and Continents).

8. India's rock systems I: Archaean to Vindhyan (the Precambrian shield)

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• Indian rock systems are classified by age, from oldest to youngest: Archaean → Dharwar → Cuddapah → Vindhyan (together called Precambrian or Purana) → Gondwana → Deccan Traps → Tertiary → Quaternary. The Peninsula is an old, stable shield (a large, ancient block of hard rock); the north is young, folded and alluvial.
• Archaean system (older than about 2.5 billion years): India's oldest rocks.
• Mainly granites and gneisses, with schists, formed as the early crust cooled. They have no fossils (azoic).
• Found in Karnataka, Tamil Nadu, Andhra Pradesh, Odisha, Chota Nagpur, Bundelkhand (Bundelkhand gneiss) and Rajasthan.

• Charnockite is named after Job Charnock, whose Kolkata tombstone is cut from it. It forms the Chennai hills and the Nilgiris.

• Dharwar system (about 2.5–1.8 billion years): India's first sedimentary rocks, now highly metamorphosed (schists, quartzites, BIFs).

• First studied around Dharwar and Shimoga in Karnataka; also found in the Aravallis and Chota Nagpur.
• The economically most important system: iron ore (Bababudan, Kudremukh, Bailadila, Noamundi), manganese, copper, uranium and gold (Kolar, now closed; Hutti, working).

• The Aravallis, about 2.5 billion years old, belong to this Precambrian group.

• Cuddapah system (Proterozoic): sandstone, shale, limestone and quartzite with little metamorphism. They lie in the crescent-shaped basin of Kadapa, Andhra Pradesh, and in the Chhattisgarh and Kaladgi basins. They yield limestone, building stone, barytes, asbestos and uranium (Tummalapalle).

• Vindhyan system (Proterozoic, after the Cuddapah): thick, mostly unmetamorphosed sandstone, shale and limestone stretching from Sasaram (Bihar) to Chittorgarh (Rajasthan). It gives red sandstone and limestone and holds the Panna diamond belt. Fossils are rare; stromatolites are found at Bhojunda, Chittorgarh (a GSI monument).

9. India's rock systems II: Gondwana, Deccan Traps, Tertiary and Quaternary

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System	Age	Rocks and where	Resources and meaning
Gondwana system	about 300–150 million years (NCERT: "a little over 200 million years")	River and lake sediments in rift basins; named after the Gond region	About 98% of India's coal (verify current): Damodar (Jharia, Raniganj, Bokaro), Son, Mahanadi (Talcher, Ib), Godavari (Singareni), Wardha
Deccan Traps	about 66 million years (66–60)	Flood basalt over most of the Maharashtra plateau and parts of Gujarat, MP, Karnataka, Telangana	Black (regur) soil, building stone, few metals
Tertiary system	about 66–2.6 million years	Formed as the Himalaya rose; Siwalik sediments	Petroleum (Digboi, Naharkatiya, Ankleshwar, Mumbai High); lignite (Neyveli, Barmer, Palana, Kutch); north-east coal (about 55 million years)
Quaternary system	last 2.6 million years	Indo-Gangetic–Brahmaputra alluvium (bhangar older, khadar newer), deltas, Thar sands, Kashmir karewas	Almost no economic minerals; groundwater, fertile soil, saffron on the karewas

• Gondwana: the Talchir tillite (rock made from old glacier debris) and Glossopteris plant fossils match the southern continents. This is key evidence for continental drift.
• Deccan Traps: they formed as the Indian plate passed over the Reunion hotspot. The name comes from Swedish trappa ("stairs"), because flat lava flows weather into step-like terraces. Dykes fed these lava flows (Class 11, Interior of the Earth).
• Tertiary oil: oil is trapped in anticlines (upward folds) and in fault traps (Class 10, Minerals and Energy Resources).
• Ophiolite is a slice of old ocean crust and upper mantle pushed onto land where plates collided. In India it marks the Indus–Tsangpo suture (Ladakh), the Naga–Manipur hills (chromite, nickel) and the Andaman Islands.

10. Rocks as the base of resources, soils and landforms

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• Rock type → minerals:
• The Archaean–Dharwar shield (Chota Nagpur, the "storehouse of minerals"; Karnataka; Odisha) holds metallic ores.
• Gondwana rocks hold coal; Tertiary rocks hold oil, gas and lignite.

• Quaternary alluvium is almost barren of minerals.

• Recent hooks: GSI reported inferred lithium resources in Reasi, J&K (February 2023). India listed 30 critical minerals (2023) and launched the National Critical Mineral Mission (2025) (verify current).

• Rock type → soil: basalt gives black soil; old granite and gneiss give red soil; heavily leached rocks in wet tropics give laterite and bauxite; river deposits give alluvial soil.
• Rock type → landform:
• Granite forms domes and tors (Karnataka, Hampi). Basalt forms step-like plateaus and mesas.
• Limestone forms karst caves (Borra, Andhra Pradesh; Belum, Andhra Pradesh).

• Quartzite forms ridges (Aravalli, Delhi Ridge), while soft shale wears into valleys.

• Heritage and building stone: Makrana marble; Vindhyan red sandstone; basalt caves at Ellora and Elephanta; granite and charnockite at Mahabalipuram.

• Geo-heritage: GSI's National Geological Monuments protect key sites. The draft Geo-heritage Sites and Geo-relics (Preservation and Maintenance) Bill, 2022 is pending (verify current status).

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Exam angles

Prelims — high-yield facts and traps

• Crust order: O > Si > Al > Fe > Ca > Na > K > Mg; the top eight make up about 98%. Aluminium is the most abundant metal in the crust. "Iron is the most abundant element of the crust" is FALSE (true for the whole earth).
• Mohs order: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz (7), topaz, corundum, diamond. "Diamond is ten times harder than talc" is FALSE.
• Streak is more reliable than colour. Mica shows cleavage; quartz shows fracture.
• Feldspar is the most abundant mineral group. Quartz resists weathering and forms sand. Mica is used in electrical industries.
• Parent → product: limestone → marble; sandstone → quartzite; shale → slate; granite → gneiss; coal → graphite.
• Granite is intrusive and felsic; basalt is extrusive and mafic. Pumice floats; obsidian is volcanic glass.
• How minerals occur: veins and lodes (tin, copper, zinc, lead); beds (coal, gypsum, salts); residual (bauxite); placer (gold, tin, platinum); ocean (salt, magnesium, bromine).
• Rock system → resource → place: Dharwar → iron ore and gold (Karnataka); Cuddapah → uranium (Tummalapalle) and barytes (AP); Vindhyan → red sandstone and Panna diamonds; Gondwana → coal (Damodar); Tertiary → oil and lignite (Assam, Gujarat, Neyveli).
• Deccan Traps: flood basalt, about 66 million years old, Reunion hotspot, black soil. "Trap" means stairs, not a trap for oil.
• Kimberlite → Majhgawan (Panna, MP). Ophiolites → Indus–Tsangpo suture and Naga–Manipur hills. Charnockite → Chennai and Nilgiris.
• GSI National Geological Monuments: Lalbagh Peninsular Gneiss, St Mary's Island columnar lava, Bhojunda stromatolites.

Mains — GS-I themes

1. Explain the rock cycle and the endogenic and exogenic energy sources that drive it.
2. "The geological structure of India explains the uneven distribution of its mineral resources." Discuss with reference to the rock systems.
3. Why does Peninsular India hold most of the country's metallic minerals and coal, while the northern plains hold almost none?
4. How do rock type and climate together decide soil types and landforms in India? Give examples (basalt → regur; granite → red soil and tors; limestone → karst).
5. Significance of the Gondwana system for India's energy security and as evidence for continental drift.
6. Need for geo-heritage protection in India: National Geological Monuments and the draft 2022 Bill.

Current-affairs hooks

• Critical minerals: India's list of 30 (2023), mineral block auctions and the National Critical Mineral Mission (2025); lithium finds in J&K and elsewhere (verify current).
• GSI exploration reports and new mineral discoveries (verify current).
• Diamond mining disputes in Madhya Pradesh (Buxwaha forest) and NMDC's Panna mine (verify current).
• Geo-heritage legislation and new National Geological Monuments (verify current).
• Coal reserves and lignite use as India's energy transition moves forward (verify current).