Maarg Manthan · Topic 2.5

Rocks as Aggregates of Minerals; Petrology

Geography › Geology – Minerals and Rocks · Topic 2.5

A rock is a natural mass of one or more minerals, held together in a solid, such as granite, which is made of quartz, feldspar and mica. Rocks are therefore aggregates of minerals, and some, like limestone, are made almost wholly of one mineral. Petrology is the branch of geology that studies the origin, composition, texture, structure and classification of rocks. It has three main branches, igneous, sedimentary and metamorphic petrology, matching the three families of rocks.

5 min read · UPSC Prelims · Geography · Geology – Minerals and Rocks

At a Glance

Rock Natural aggregate of minerals
Petrology Study of rocks
Monomineralic One mineral, such as limestone
Polymineralic Several minerals, such as granite
Main features Texture, structure, composition
Exam link UPSC Prelims, Geography: Geology – Minerals and Rocks

Where Does It Fit?

This topic moves from single minerals to the rocks they make. It prepares the way for the topics on how rocks affect landforms and soils, and for the detailed study of the three rock families. Previous topic: Non-Metallic Minerals (Sulphur, Phosphates, Cement Minerals). Next topic: Relationship Between Rocks and Landforms.

What Is a Rock?

In short: a rock is a natural solid made of minerals, and it can be hard or soft.

In geology, the word rock covers every solid part of the Earth’s crust, whether it is hard like granite or loose like clay and sand. Rocks are made of minerals, but they differ from minerals in not having a fixed chemical formula or a regular crystal shape. Some rocks consist of one mineral, such as quartzite, made of quartz, or limestone, made of calcite. Most rocks, like granite and basalt, contain several minerals.

Rock	Minerals in it	Type
Granite	Quartz, feldspar, mica	Polymineralic igneous
Basalt	Pyroxene, plagioclase feldspar, olivine	Polymineralic igneous
Limestone	Calcite	Monomineralic sedimentary
Sandstone	Quartz grains cemented together	Mostly one mineral with cement
Quartzite	Quartz	Monomineralic metamorphic
Marble	Calcite or dolomite	Monomineralic metamorphic

The mineral make-up of rocks follows from the abundance of elements, given in Minerals: Major Elements of the Earth’s Crust.

What Is Petrology and What Are Its Branches?

Petrology: the study of rocks, their origin, history, composition and relationships. The word comes from the Greek petra, meaning rock.
Petrography: the description and classification of rocks, usually under the microscope.
Petrogenesis: the study of how rocks originate.
Igneous petrology: rocks formed by the cooling of magma and lava.
Sedimentary petrology: rocks formed by deposition and compaction of sediments.
Metamorphic petrology: rocks changed by heat and pressure.

Petrology helps to find mineral deposits, plan dams and tunnels, locate building stone, and read the history of mountains and plates. The three families are taken up in Three Families of Rocks: Igneous, Sedimentary, Metamorphic.

Rocks are also grouped by origin. Igneous rocks are made when molten rock cools, sedimentary rocks when loose material is laid down and hardened, and metamorphic rocks when existing rocks are changed by heat and pressure without melting. A rock can pass from one family to another over geological time, which is the idea behind the rock cycle in The Rock Cycle.

What Are Texture and Structure?

Term	Meaning	Example
Texture	The size, shape and arrangement of the grains or crystals in a rock	Coarse crystals in granite, fine crystals in basalt
Phaneritic	Crystals large enough to see; cooled slowly inside the Earth	Granite, gabbro
Aphanitic	Crystals too small to see; cooled quickly on the surface	Basalt, rhyolite
Glassy	No crystals; cooled so fast that no crystals grew	Obsidian
Porphyritic	Large crystals set in a fine-grained mass	Some granites and andesites
Clastic	Made of broken fragments of other rocks	Conglomerate, sandstone
Foliated	Minerals aligned in layers	Schist, gneiss
Structure	Large-scale features, such as bedding, joints and folds	Layers in sandstone

Texture shows the conditions of formation. Slow cooling gives large crystals, and rapid cooling gives small crystals or glass. In sedimentary rocks the texture records how far the grains travelled: rounded, well-sorted grains point to long transport by water or wind, while angular, mixed grains point to short transport. In metamorphic rocks, the alignment of minerals shows the direction of pressure.

How Are Rocks Studied?

Field study: geologists map outcrops, record the layers, folds and faults, and collect samples.
Thin sections: a slice of rock ground to about 0.03 mm is examined under a polarising microscope, where each mineral shows distinctive colours.
Chemical and isotopic analysis: gives the composition and age of rocks, as with the methods in Geological Time Scale.
Remote sensing and geophysics: satellites, gravity and magnetic surveys map rocks under cover. The Geological Survey of India carries out such work in India.

As an analytical point, rocks are the record of the Earth’s history, and each type shows the process that formed it. The links with landforms and soils are taken up in Relationship Between Rocks and Landforms and Black soil From basalt (Deccan Traps) Red soil From granite and gneiss…">Relationship Between Rocks and Soils.

Why Is This Topic Important?

Concept Distinguishes mineral and rock
Method Basis for classification
Use Resources and engineering

What Came Next?

The next topic explains how the kind of rock decides the landforms that develop on it. Next topic: Relationship Between Rocks and Landforms.

Key People and Terms

Petrology Study of rocks
Petrography Description of rocks
Texture Size and arrangement of grains
Phaneritic Coarse-grained
Aphanitic Fine-grained
Thin section Slice of rock for the microscope

Exam Corner

Points to Remember

A rock is an aggregate of minerals.
Granite contains quartz, feldspar and mica.
Slow cooling gives coarse crystals.
Obsidian is volcanic glass.
Limestone and marble are made of calcite.

Do Not Confuse With

Petrology and petrography Whole study of rocks versus description of them
Texture and structure Grain-level versus large-scale features
Rock and mineral Aggregate versus single substance

Memory Hook

Mineral is the brick, rock is the wall; texture tells how fast it fell.

Mains Angle

Questions use rocks as the base for landforms and resources. These points add depth.

Concept Rocks record Earth processes
Use Resource search and engineering
Link Rocks, landforms and soils

A Question You May Face

An original practice question, not a past paper question.

“What is petrology? Explain how texture reveals the conditions under which a rock was formed.”

How to Answer

Define Rock and petrology.
Branches Three families.
Texture Cooling rate and grain size.
Uses Mining and engineering.

GS Relevance

Prelims: definition of rock, petrology, texture, monomineralic versus polymineralic rocks. GS Paper 1: geomorphology and rocks.

Frequently Asked Questions

What is a rock?

A rock is a naturally formed solid mass made of one or more minerals. In geology it includes hard rocks like granite and also loose materials like sand and clay. Rocks differ from minerals because they have no fixed chemical formula and are usually mixtures of different minerals.

What is petrology?

Petrology is the branch of geology that studies the origin, composition, texture, structure and classification of rocks. It has three main branches, the petrology of igneous, sedimentary and metamorphic rocks, and it helps in finding minerals, building dams and understanding the geological history of a region.

What is the difference between a mineral and a rock?

A mineral is a single natural inorganic substance with a definite chemical composition and crystal structure, such as quartz. A rock is an aggregate of one or more minerals, for example granite, which contains quartz, feldspar and mica. Rocks do not have a fixed chemical formula.

What is the texture of a rock?

Texture is the size, shape and arrangement of the grains or crystals in a rock. In igneous rocks, slow cooling inside the Earth produces coarse crystals, as in granite, and rapid cooling at the surface produces fine crystals or glass, as in basalt and obsidian.

How do geologists study rocks?

Geologists study rocks by mapping them in the field, examining thin slices under a polarising microscope, analysing their chemistry and isotopes, and using satellite and geophysical surveys to map hidden rocks. In India, the Geological Survey of India carries out much of this work.

PYQ Practice — Statement Analysis

1 A rock is always made of a single mineral.

False

Most rocks contain several minerals.

2 Petrology is the study of rocks.

True

It covers origin, texture and classification.

3 Slow cooling of magma produces coarse crystals.