Rocks & Minerals – NDA Geography Notes

Introduction

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. The questions are very direct but require precise knowledge. A student who knows that shale becomes slate, sandstone becomes quartzite, and limestone becomes marble will score full marks from this chapter.

Understand the Topic

What is a Rock?

A rock is a naturally occurring solid mass of one or more minerals. Rocks are 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]

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The Three Types of Rocks

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

1. Igneous Rocks

The word igneous comes from the Latin word 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.

Two types based on where they cool:

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.

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.

When magma cools slowly, large crystals form. When lava cools quickly on the surface, very small crystals form. [NDA tested via CDS]

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 — and this heat helps keep rock molten in the mantle, which eventually leads to the formation of igneous rocks. [NDA 2013-I]

What is NOT an igneous rock:

Dolomite is not an igneous rock — it is sedimentary. [NDA 2017-II]

Slate is not an igneous rock — it is metamorphic. [NDA 2023-I]

Other igneous rocks students should know: Gabbro (coarse-grained, dark, intrusive) and Obsidian (volcanic glass, no crystals).

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

2. 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.

Think of a river carrying sand and mud to the sea. Over millions of years, layer upon layer builds up. The pressure of upper layers compresses the lower layers. Minerals in the water act as cement. Eventually the loose sediment becomes solid rock.

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

Types based on how they form:

Mechanically Formed (Clastic) — from broken fragments of other rocks:

The grain size sequence from smallest to largest is: 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 — from dissolved minerals that precipitate from water:

• Limestone — from calcium carbonate precipitating from seawater
• Rock salt (Halite) — from evaporation of salty water
• Chert — formed from silica; chemically formed
• Geyserite — silica deposited by hot springs; NOT formed mechanically [NDA tested via CDS]

Organically Formed — from remains of living organisms:

• Coal — from compressed plant material
• Chalk — from shells and skeletons of microscopic marine organisms [NDA 2019-II]
• Limestone — can also be organic in origin

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 thickness of sedimentary rocks — because they were formed from the compressed sediments of the ancient Tethys Sea that accumulated in a geosyncline over millions of years. [NDA tested via CDS]

3. Metamorphic Rocks

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

This happens deep within the Earth — where tectonic plates collide, where magma pushes against existing rock, or where rocks are buried under great depths.

Agents of metamorphism:

• Heat — from nearby 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]

Parent rock to metamorphic rock — the most tested pairs in this chapter:

Original Rock (Parent)	Metamorphic Rock Formed
Shale	Slate
Sandstone	Quartzite
Limestone / Chalk	Marble
Granite	Gneiss
Coal	Graphite

Shale becomes slate through metamorphism — when subjected to tremendous pressure and high temperature. [NDA 2010-II]

Quartzite is metamorphosed from sandstone. [NDA 2014-II]

Gneiss is a metamorphic rock — formed from granite under extreme heat and pressure. It is NOT an igneous rock. [NDA tested via CDS]

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]

Minerals

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

Quartz:
Made almost entirely of silica (silicon dioxide, SiO₂). It is the most common mineral found in sedimentary rocks and is very resistant to weathering — which is why beaches are made of quartz sand. [NDA tested via CDS]

Feldspar:
The most abundant mineral group in the Earth’s crust — makes up about 50% of Earth’s crust. It has 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. Its major elements are magnesium, iron, and silica. [NDA tested via CDS]

Mica:
Has a distinctive layered, sheet-like structure. Can be split into thin transparent sheets. It is an excellent electrical insulator.

Ascending order of minerals by presence in Earth’s crust:
Among Mica, Pyroxene, and Amphibolite — the order from least to most abundant is: Mica → Pyroxene → Amphibolite. [NDA 2025-I]

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Volcanic Deposits — Tuff and Lapilli

When a volcano erupts, it throws out different types of material.

Lava flows from the volcano 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]

The Rock Cycle

Rocks change from one type to another in a continuous process called the rock cycle:

• 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 → becomes Magma again

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

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