CHAPTER 5
MINERALS & ROCKS

Minerals and Rocks

The Earth’s crust is mainly made up of minerals and rocks. Minerals are the basic building blocks of rocks, while rocks form the outer solid layer of the Earth. Different geological processes such as cooling, weathering, deposition, heat, and pressure continuously change rocks from one type to another through the rock cycle.

Rocks

A rock is a naturally occurring solid mass made up of one or more minerals. Rocks form the Earth’s crust and vary greatly in color, texture, hardness, and composition. They may be hard like granite or soft like clay and sandstone.

Characteristics of Rocks

  • Made up of one or more minerals.
  • Form the Earth’s crust.
  • Can be hard or soft.
  • Classified into three major types:
    • Igneous Rocks
    • Sedimentary Rocks
    • Metamorphic Rocks

Minerals

A mineral is a naturally occurring, inorganic, homogeneous solid substance with a definite chemical composition and an orderly atomic structure. Minerals possess definite physical and chemical properties.

About 2,000 minerals are known, but only about 12 major minerals are responsible for forming most rocks.

Characteristics of Minerals

  • Naturally occurring.
  • Inorganic in origin.
  • Definite chemical composition.
  • Crystalline structure.
  • Basic building blocks of rocks.

Igneous Rocks (Primary Rocks)

The word Igneous comes from the Latin word “Ignis”, meaning Fire.

Igneous rocks are formed by the cooling and solidification of molten magma or lava. Since they are the first rocks formed on the Earth and all other rocks are ultimately derived from them, they are called Primary Rocks.

Characteristics of Igneous Rocks

  • Formed from magma or lava.
  • Hard and compact.
  • Non-layered (massive).
  • Generally contain no fossils.
  • Rich in mineral crystals.

Intrusive Forms of Igneous Rocks

When magma cools below the Earth’s surface, it forms different intrusive structures.

1. Batholith

  • Largest intrusive body.
  • Dome-shaped.
  • Forms deep inside the Earth.

2. Stock

  • Smaller than a batholith.
  • Irregular-shaped intrusive body.

3. Laccolith

  • Mushroom-shaped intrusion.
  • Flat base with a dome-like top.

4. Dyke (Dike)

  • Vertical wall-like sheet.
  • Cuts across rock layers.

5. Sill

  • Horizontal sheet of magma.
  • Forms between existing rock layers.

 

Question & Answers

 

Q1) What do you mean by Rocks? Write the definition of Rocks.
 
Ans: Definition of Rocks: A rock is an aggregate of one or more minerals. According to petrologists, rocks may simply be defined as any mineral matter that forms the crust of the Earth. They can be hard like granite or soft like clay, gravel, or sandstone. In other words, the Earth’s crust is entirely composed of rocks.
 
 
 
Q2) Define the term Minerals.
 
Ans: Minerals: A mineral is a naturally occurring, homogeneous, inorganic solid substance having an orderly atomic structure and a definite chemical composition and physical properties. There are about 2,000 different minerals known to science, out of which only 12 minerals are recognized as major rock-forming minerals.
 
 
 
 
Q3) Why are Igneous rocks called Primary Rocks?
 
Ans: Igneous Rocks: The word “igneous” is derived from the Latin word ‘ignis’, which means fire. Igneous rocks are formed through the cooling and solidification of hot, molten materials.
All volcanic rock-forming materials originate deep within the Earth’s interior as a hot, liquid, and sticky mass called magma or lava, which moves toward the surface through geological cracks and joints. Because they form directly out of magma and lava and serve as the foundational material from which all other rock types are eventually derived, they are called Primary Rocks.
 
 
 
 
Q4) What do you mean by the Rock Cycle?
 
Ans: Rock Cycle: The rock cycle is a continuous, dynamic process through which rocks are constantly transformed from one type into another within the Earth’s crust.
  1. First, Igneous rocks are formed from solidified magma.
  2. When exposed to the elements of nature, they undergo weathering and denudation, breaking down into particles that accumulate to form Sedimentary rocks.
  3. Under the intense influence of high temperature and pressure, both igneous and sedimentary rocks transform into Metamorphic rocks.
  4. If these rocks are buried deep enough down into the mantle, they melt down once again into molten magma, eventually cooling back into igneous rocks.

Through this continuous loop, the total material volume of the Earth’s crust remains balanced and is never lost.
 
 
 
Q5) What are igneous rocks? Write the major intrusive forms of igneous rocks.
 
Ans: Igneous rocks are formed directly through the cooling and solidification of molten materials. The molten magma that cools inside the Earth’s crustal portion assumes various structures and geometric orientations known as Intrusive Forms.
The primary types of intrusive igneous forms include:
  1. Batholith: Large, deep-seated dome-shaped bodies of magma.
  2. Stock: A smaller, discordant intrusive feature similar to a batholith.
  3. Laccolith: Large mushroom-shaped intrusive bodies with a flat floor.
  4. Dyke (Dike): Vertical, wall-like discordant sheets of rock cutting across older layers.
  5. Sill: Horizontal, sheet-like concordant bodies injected between older rock layers.

 

Q6) What is meant by Sedimentary rocks? Explain the classification of Sedimentary rocks in brief.
 
Ans: Sedimentary Rocks are formed from pre-existing rock materials that have been broken down, transported, and deposited at favorable sites by agents of denudation such as rivers, wind, glaciers, and ocean waves. This accumulated material is called sediment. Because these sediments accumulate layer by layer over long periods, sedimentary rocks typically feature distinct structural layers or strata, which is why they are also called Stratified Rocks.
Classification of Sedimentary Rocks
Sedimentary rocks are broadly classified based on their mode of formation into two primary sets—Clastic and Non-Clastic rocks:
  1. Clastic Sedimentary Rocks: The term “clastic” comes from the Greek word ‘klastos’, which means “broken”. These rocks are composed of solid mineral fragments and individual grains derived mechanically from pre-existing parent rocks. Their formal naming depends heavily on the grain size of these components.
    • Examples: Sandstone, Shale, Limestone, and Loess.
  2. Non-Clastic Sedimentary Rocks: These are formed without direct mechanical rock fragmentation and are split into two groups:
    • Chemical Precipitates: Formed when dissolved mineral matter solidifies and settles directly out of an aqueous (water) solution. Examples: Rock Salt (Halite), Chert, and Gypsum.
    • Organically Derived Sediments: Formed from the accumulated skeletal debris, shells, or organic remains of ancient plants and animals. Examples: Coal, Chalk, Geyserite, and organic Limestone.

 

Q7) What are Metamorphic rocks? Discuss their classification and features.
 
Ans: The word “metamorphic” is derived from the Greek root words Meta (meaning change) and Morphic (meaning form). Metamorphic rocks are formed when pre-existing igneous or sedimentary rocks undergo structural changes in their color, hardness, texture, and mineral composition due to extreme conditions of heat and pressure deep within the Earth’s crust.
Classification of Metamorphic Processes
  1. Mechanical Disruption (Dynamic Metamorphism): Rocks are broken and ground down mechanically due to immense structural pressures without significant chemical alterations. For instance, under high pressure, granite changes into gneiss, while clay and shale convert into schist.
  2. Recrystallized Rocks: Formed when the internal crystals of the original minerals organize into new structures. This is divided into two subclasses:
    • Contact Metamorphism: Changes occur locally on a small scale when cool surrounding rocks come into direct contact with hot, intruding magma vents. New mineral components emanating from the magma are added directly into the host rock.
    • Regional Metamorphism: Occurs on a vast, widespread scale when tectonic plate movements, massive deep-burial mountain building, and heat combine to alter rocks across an entire region.

 

MULTIPLE CHOICE QUESTIONS 
 
 
Q1. Which one of the following is the hardest mineral?
  • a) Gypsum
  • b) Topaz
  • c) Calcite
  • d) Diamond (Correct Answer)

Q2. Which one of the following is a non-metallic mineral?
  • a) Copper
  • b) Lead
  • c) Sulphur (Correct Answer)
  • d) Aluminum

Q3. Which one of the following is a sedimentary rock?
  • a) Sandstone (Correct Answer)
  • b) Mica
  • c) Granite
  • d) Gneiss

Q4. Which one of the following are the two main constituents of granite?
  • a) Iron & Nickel
  • b) Iron & Silver
  • c) Silica & Aluminum (Correct Answer)
  • d) Iron oxide & Potassium

Q5. Which one of the following is a salient feature of metamorphic rocks?
  • a) Changeable
  • b) Quite crystalline
  • c) Crystalline
  • d) Foliation (Correct Answer)

Q6. Which one of the following is not a single element mineral?
  • a) Gold
  • b) Silver
  • c) Mica (Correct Answer – It is a complex silicate compound)
  • d) Graphite

Q7. Which of the following is not a sedimentary rock?
  • a) Tillite
  • b) Borax
  • c) Breccia
  • d) Marble (Correct Answer – Marble is a metamorphic rock)
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