Rocks & Minerals – NDA Geography Notes

Everything solid on Earth’s surface, mountains, river beds, cliffs, plains, desert floors, is made of rock. Rock is the raw material of the Earth’s crust. Over millions of years, rocks form, break down, transform, and form again in a continuous cycle.

This chapter is one of the most reliably tested in NDA World Geography. Questions appear in almost every paper. They test three things: which rock belongs to which type, which rock transforms into which under heat and pressure, and specific mineral properties. A student who knows that shale becomes slate, sandstone becomes quartzite, and limestone becomes marble will score full marks from this chapter.

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What is a Rock?

A rock is a naturally occurring solid mass of one or more minerals, the building blocks of the Earth’s crust.

A mineral is a naturally occurring, inorganic, solid substance with a definite chemical composition and a regular crystal structure. Quartz, feldspar, and mica are minerals. Granite is a rock made of these minerals.

Rocks do not remain in their original form for long. They undergo transformation caused by weathering, erosion, and metamorphic action. [NDA 2022-I]

All rocks on Earth belong to one of three families, igneous, sedimentary, or metamorphic, based on how they were formed.

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Igneous Rocks

The word igneous comes from the Latin ignis, meaning fire. Igneous rocks are formed from cooled and solidified magma or lava. Magma is molten rock inside the Earth; when it reaches the surface through a volcano, it is called lava.

Intrusive Igneous Rocks (Plutonic Rocks)

Magma that cools and solidifies slowly deep inside the Earth, before reaching the surface. Because cooling is slow, there is plenty of time for large crystals to grow. The result is a coarse-grained rock with large visible crystals.

Example: Granite: the most common intrusive igneous rock, made of large crystals of quartz, feldspar, and mica. Gabbro is another intrusive igneous rock, coarse-grained and dark coloured.

Extrusive Igneous Rocks (Volcanic Rocks)

Lava that reaches the surface and cools very quickly, sometimes in minutes. There is no time for large crystals to grow. The result is a fine-grained rock with tiny or no crystals.

Example: Basalt: the most common extrusive igneous rock, dark coloured and fine-grained. The Deccan Plateau of India is covered by ancient basalt flows. Obsidian is volcanic glass. It cools so fast that no crystals form.

When magma cools slowly, large crystals form. When lava cools quickly at the surface, it forms very small crystals.

Igneous rocks are called primary rocks because they were the first rocks to form when the molten Earth cooled. All other rock types derive ultimately from igneous rocks. [NDA 2011-II]

Radioactive decay provides internal heat for the Earth, keeping rock molten in the mantle and eventually leading to the formation of igneous rocks. [NDA 2013-I]

What is NOT an igneous rock: Dolomite is NOT igneous: it is sedimentary. [NDA 2017-II] Slate is NOT igneous: it is metamorphic. [NDA 2023-I]

Gneiss is NOT igneous: it is metamorphic (formed from granite).

Granite can be considered both igneous and metamorphic: because it originated as an igneous rock but under extreme heat and pressure transforms into gneiss. [NDA 2024-II]

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Sedimentary Rocks

Sedimentary rocks are formed from sediments, fragments of other rocks, minerals, or organic material that have been broken down by weathering, transported by water or wind, deposited in layers, and then compacted and cemented over time.

Sedimentary rocks always form in layers called strata. This layering is their most distinctive feature.

Mechanically Formed (Clastic) Sedimentary Rocks

From broken fragments of other rocks. Grain size sequence from smallest to largest: Shale → Siltstone → Sandstone → Conglomerate. [NDA 2014-I]

Shale: made of clay particles; finest grains; the most common sedimentary rock. [NDA 2010-I]

Siltstone: slightly coarser than shale. Sandstone: made of sand grains cemented together. Conglomerate: made of large rounded pebbles and gravel cemented together; the coarsest.

Chemically Formed Sedimentary Rocks

From dissolved minerals that precipitate from water: Limestone (from calcium carbonate precipitating from seawater), Rock salt/Halite (from evaporation of salty water), Chert (from silica), Geyserite (silica deposited by hot springs: NOT formed mechanically).

Organically Formed Sedimentary Rocks

From remains of living organisms: Coal (from compressed plant material), Chalk (from shells and skeletons of microscopic marine organisms). [NDA 2019-II]

Coal is a sedimentary rock. Basalt is an igneous rock. Both statements are correct. [NDA 2008-I]

Chalk and marble are both made of calcium carbonate (CaCO₃). Chalk is the sedimentary form; marble is the metamorphic form. [NDA 2020-I]

Fold mountains like the Himalayas contain enormous thicknesses of sedimentary rocks, formed from the compressed sediments of the ancient Tethys Sea in a geosyncline over millions of years.

Type	How Formed	Examples
Mechanical (clastic)	Fragments of other rocks deposited and cemented	Shale, Siltstone, Sandstone, Conglomerate, Loess
Chemical	Dissolved minerals precipitated from water	Limestone, Halite, Chert, Geyserite
Organic	Remains of living organisms compressed	Coal, Chalk

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Metamorphic Rocks

The word metamorphic comes from Greek words meaning change of form. Metamorphic rocks form when existing rocks (igneous or sedimentary) are subjected to extreme heat and pressure, changing their mineral structure without melting them completely.

Agents of metamorphism: Heat (from magma or deep burial), Pressure (from overlying rocks or tectonic compression), Chemical fluids (hot fluids that cause chemical changes).

Decomposition is NOT an agent of metamorphism. It is a process of weathering at the surface. [NDA 2018-II]

Among shale, limestone, slate, and sandstone: slate is the one that does NOT belong to the sedimentary group. Shale, limestone, and sandstone are all sedimentary. Slate is metamorphic. [NDA 2012-II]

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Parent Rock to Metamorphic Rock: The Key Pairs

★ IMPORTANT These parent-metamorphic rock pairs are the single most tested set of facts in this chapter. [NDA 2010-II | NDA 2014-II | NDA 2020-I]

Original Rock (Parent)	Metamorphic Rock Formed	NDA Tag
Shale	Slate	[NDA 2010-II]
Sandstone	Quartzite	[NDA 2014-II]
Limestone / Chalk	Marble	[NDA 2020-I]
Granite	Gneiss	—
Coal	Graphite	—

Shale → Slate: Subjected to tremendous pressure and high temperature. [NDA 2010-II] Sandstone → Quartzite: Hard, dense, highly resistant to erosion. [NDA 2014-II] Limestone/Chalk → Marble: Both are calcium carbonate (CaCO₃). [NDA 2020-I] Granite → Gneiss: Gneiss is metamorphic: NOT igneous, despite resembling granite. [NDA tested via CDS]

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Minerals

A mineral is a naturally occurring, solid, inorganic substance with a definite chemical composition.

Quartz: made almost entirely of silica (SiO₂). The most common mineral in sedimentary rocks. Very resistant to weathering, which is why beaches are made of quartz sand.

Feldspar: the most abundant mineral group in the Earth’s crust, making up about 50% of Earth’s crust. Light cream to salmon pink colour. Magnesium is NOT a common element in feldspar. Feldspar contains potassium, sodium, or calcium, not magnesium. [NDA 2025-II]

Pyroxene: found commonly in basaltic rocks and also commonly in meteorites. [NDA 2024-II]

Olivine: a green mineral found in basaltic and ultra-mafic rocks. Major elements: magnesium, iron, and silica.

Mica: distinctive layered, sheet-like structure. Can be split into thin transparent sheets. Excellent electrical insulator.

Ascending order of abundance in Earth’s crust among these three: Mica → Pyroxene → Amphibolite. [NDA 2025-I]

Mineral	Key Property	NDA Tag
Quartz	Silica (SiO₂); most common in sedimentary rocks; resistant to weathering	—
Feldspar	~50% of Earth’s crust; cream/salmon pink; NO magnesium	[NDA 2025-II]
Pyroxene	Found in basalt and meteorites	[NDA 2024-II]
Olivine	Green; Mg + Fe + Si; in basalt and mantle	—
Mica	Sheet-like; electrical insulator	—

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Volcanic Deposits: Tuff and Lapilli

Lava flows and solidifies to form basalt or other volcanic rocks.

Lapilli are small rock fragments (pea-sized) thrown out directly by a volcanic explosion.

Tuff: when volcanic ash is carried by running water and deposited as a sedimentary layer, it forms a rock called tuff. [NDA 2025-I]

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The Rock Cycle

Rocks change from one type to another in a continuous process:

Magma cools → Igneous rock. Igneous rock breaks down through weathering and erosion → sediment → compacts → Sedimentary rock. Any rock under extreme heat and pressure → Metamorphic rock. Any rock that melts completely → Magma again.

This cycle has been running for billions of years. The rocks you see today may have been through it many times over geological history.

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