Which of the following are the two main constituents of granite?

1. Composition of the Earth's Crust (basic)

Welcome to the start of our journey into the building blocks of our planet! To understand rocks and minerals, we must first look at the Earth's Crust — the thin, brittle, outermost shell that we call home. Think of it like the skin of an apple; it is incredibly thin compared to the rest of the planet, making up less than 1% of the Earth's total mass and volume Physical Geography by PMF IAS, Earths Interior, p.52.

The crust isn't uniform; it has two distinct personalities based on where it sits. The oceanic crust is thinner (averaging 5 km) and denser, while the continental crust is much thicker (averaging 30 km). In massive mountain ranges like the Himalayas, the crust can even reach depths of 70 to 100 km FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT 2025 ed.), Interior of the Earth, p.22. Despite these differences, the average density of the crust is about 2.7 g/cm³, which is significantly lighter than the Earth's overall average density of 5.51 g/cm³.

When we look at the chemistry of the crust, a few "superstar" elements dominate. While the whole Earth is dominated by Iron, the Crust is overwhelmingly made of Oxygen and Silicon. Together, they make up about 75% of the crust's weight!

Rank	Element (Crust)	% by Weight	Notes
1	Oxygen (O)	46.6%	The most abundant element.
2	Silicon (Si)	27.7%	The primary building block of silicate minerals.
3	Aluminium (Al)	8.1%	The most abundant metal in the crust.
4	Iron (Fe)	5.0%	Common, but much more concentrated in the Earth's core.

Physical Geography by PMF IAS, Earths Interior, p.53

These elements don't usually exist in isolation; they combine to form minerals. For instance, Feldspar alone accounts for half of the Earth's crust! It is a complex mineral containing silicon, oxygen, aluminium, and other elements like sodium or potassium. Another major player is Quartz, which is simply a combination of silicon and oxygen (SiO₂) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175.

Sources: Physical Geography by PMF IAS, Earths Interior, p.52-53; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Interior of the Earth, p.22; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175

2. Introduction to Igneous Rocks (basic)

At the very heart of geology, Igneous rocks are known as 'Primary Rocks' because they are the first to form from the cooling and solidification of molten matter. This molten material is called magma when it is trapped below the Earth's surface and lava once it erupts above. Because these rocks originate under conditions of intense heat, they are unfossiliferous—any organic remains would simply be incinerated. Physical Geography by PMF IAS, Chapter 13, p.169

The texture and appearance of an igneous rock are determined by where it cools. When magma stays deep within the crust, it cools very slowly, allowing large crystals to grow; these are called Plutonic (or Intrusive) rocks, such as Granite. Conversely, if the molten material reaches the surface, it cools rapidly, resulting in fine-grained or even glassy textures because there isn't enough time for large crystals to form. These are known as Volcanic (or Extrusive) rocks, with Basalt being the most common example. Fundamentals of Physical Geography (NCERT 2025 ed.), Interior of the Earth, p.24

Chemically, we classify these rocks based on their silica (SiO₂) content. This distinction is vital because it determines the rock's color, density, and even how the volcano behaves. We generally group them into two main categories:

Feature	Acidic Igneous Rocks	Basic Igneous Rocks
Silica Content	High (approx. 70-77% SiO₂)	Low (approx. 45-55% SiO₂)
Dominant Elements	Silica and Aluminium	Iron, Magnesium, and Calcium
Appearance	Lighter in color and less dense	Darker in color and much denser
Examples	Granite	Basalt, Gabbro

Physical Geography by PMF IAS, Chapter 13, p.170

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-170; Fundamentals of Physical Geography (NCERT 2025 ed.), Interior of the Earth, p.24; Certificate Physical and Human Geography (GC Leong), The Earth's Crust, p.18

3. Chemical Classification: Acidic vs. Basic Rocks (intermediate)

When we classify igneous rocks chemically, we look primarily at the amount of silica (SiO₂) they contain. Silica isn't just a component; it acts as the primary "acid-forming" radical in geology. Depending on the concentration of silica versus metallic oxides (like iron and magnesium), we categorize rocks into two main groups: Acidic and Basic. This chemical makeup determines everything from the rock's color and density to how the original magma flowed across the landscape Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169.

Acidic rocks are defined by a high silica content, often ranging between 70% and 80%. Because silica is relatively light, these rocks are characterized by a lower density and a lighter color. They are dominated by minerals like quartz and feldspar, which are rich in silicon and aluminium — often referred to as the 'Sial' layer of the crust Physical Geography by PMF IAS, Earths Interior, p.53. A critical point for geography students is the viscosity of the parent magma: because acidic magma has so much silica, it is thick and "sticky." It doesn't flow very far before cooling, which is why acidic rocks often form massive, compact structures like Granite Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170.

In contrast, Basic rocks (also called Mafic rocks) have a lower silica content (typically 45% to 55%) but are much richer in heavier elements like Iron (Fe) and Magnesium (Mg). This makes them significantly denser and darker in appearance (think of the dark rocks of the Deccan Plateau). Common minerals in these rocks include olivine and pyroxene Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176. Basic magma is very fluid, allowing it to flow over vast distances before solidifying into rocks like Basalt.

Feature	Acidic Rocks	Basic Rocks
Silica Content	High (70% - 80%)	Low (45% - 55%)
Primary Elements	Silicon + Aluminium (Sial)	Iron + Magnesium (Mafic)
Physical Properties	Lighter, less dense, hard	Darker, heavy/dense
Magma Flow	Viscous (thick); cools fast	Fluid; spreads far
Examples	Granite, Quartz	Basalt, Gabbro, Olivine

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-170, 176; Physical Geography by PMF IAS, Earths Interior, p.53

4. The Sial and Sima Layers (intermediate)

In our journey through the Earth's composition, we encounter the two primary chemical layers that make up the crust: Sial and Sima. These terms are shorthand for the chemical elements that dominate them. The Sial layer (Silica + Aluminium) forms the bulk of the continental crust. It is primarily composed of granite, an acidic rock characterized by high silica content (often 70% to 77%) and alumina (Al₂O₃) derived from feldspar Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170. Because Sial is relatively light (average density of ~2.7 g/cm³), it is highly buoyant and essentially 'floats' atop the denser layers below. This buoyancy is the reason why continental masses do not easily subduct into the mantle during tectonic collisions, instead folding and compressing to form massive mountain ranges like the Himalayas Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.27. Beneath the Sial, and forming the oceanic crust, lies the Sima layer (Silica + Magnesium). This layer is dominated by basaltic rocks, which are basic in nature and significantly denser than granite (~3.0 g/cm³). While Sial is thick and varied—reaching up to 70 km under major mountain systems—Sima is much thinner, with the oceanic crust averaging only about 5 km in thickness FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Interior of the Earth, p.22. Geologically, Sima is often viewed as a continuous layer that forms the ocean floor and extends beneath the continental Sial blocks.

Feature	Sial (Continental Crust)	Sima (Oceanic Crust)
Main Minerals	Silica & Aluminium	Silica & Magnesium
Primary Rock	Granite (Acidic)	Basalt (Basic)
Density	Lower (~2.7 g/cm³)	Higher (~3.0 g/cm³)
Thickness	Thick (Avg 30km; up to 70km)	Thin (Avg 5km)

Understanding these layers is crucial for Plate Tectonics. For instance, when an oceanic plate (Sima) meets a continental plate (Sial), the denser Sima layer is forced downward (subducted) while the lighter Sial remains on top. This interaction is the fundamental driver of seafloor spreading and mountain building Physical Geography by PMF IAS, Divergent Boundary, p.129.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170; Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.27; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Interior of the Earth, p.22; Physical Geography by PMF IAS, Divergent Boundary, p.129

5. Rock-Forming Minerals: Quartz and Feldspar (intermediate)

To understand the earth's crust, we must look at its building blocks. While the crust contains many elements, they rarely exist in isolation; instead, they combine to form minerals. Two minerals stand out as the 'heavyweights' of the continental crust: Feldspar and Quartz. These are known as silicate minerals because their fundamental building block is silicon and oxygen. Together, they dominate the composition of granite, the most common rock of the continental landmasses Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Feldspar is arguably the most important mineral group to remember, as it makes up roughly half of the Earth's crust. It is a complex mineral that includes silicon and oxygen combined with various metals like aluminium, sodium, potassium, or calcium. Because it is rich in aluminium and silica, it is a primary reason why the upper crust is often called Sial (Silica + Alumina) Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.17. Feldspar is typically light-colored and is the main ingredient in the manufacturing of ceramics and glass. Quartz, on the other hand, is chemically simpler but physically tougher. It consists purely of Silicon and Oxygen (SiO₂) arranged in a hexagonal crystalline structure. Unlike many other minerals, quartz does not have 'cleavage' (it doesn't break along smooth planes), making it highly resistant to chemical weathering. This is why, as rocks erode, quartz remains behind as sand on our beaches and in our deserts Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175.

When these two minerals are the primary components of a rock, as seen in granite, the rock is classified as acidic. These rocks are characterized by their light color and lower density compared to darker, 'basic' rocks like basalt which are rich in iron and magnesium.

Feature	Feldspar	Quartz
Chemical Makeup	Si + O + (Al, Na, K, Ca)	Pure Silicon + Oxygen (SiO₂)
Abundance	~50% of the Earth's crust	Major component of sand/granite
Common Use	Ceramics and Glassmaking	Radio, Radar, and Timepieces

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.17; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170

6. Defining Granite: Composition and Properties (exam-level)

At its heart, Granite is a plutonic (intrusive) igneous rock that serves as the foundation of our continental crust. To understand granite, we must look at its birth: it forms when magma cools very slowly at great depths. This slow-motion cooling allows mineral grains to grow large and visible to the naked eye Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169. Because it is rich in Silicon (Si) and Aluminium (Al), it is the primary component of the Sial layer of the Earth's crust Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170. Chemically, granite is categorized as an acidic rock. This classification isn't about pH levels in a lab but rather its silica (SiO₂) content, which typically ranges from 70% to 77%. The two dominant minerals that define granite are Quartz and Feldspar. Quartz provides the hard, crystalline structure, while Feldspar (the most abundant mineral in the crust) provides the aluminium and potassium that give many granites their pinkish or grey hues Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Due to this chemical makeup, granite possesses distinct physical properties that differentiate it from "basic" rocks like basalt:

Property	Granite (Acidic Rock)	Basalt (Basic Rock)
Density	Lower (less heavy minerals)	Higher (rich in Iron/Magnesium)
Color	Lighter (Whites, Pinks, Greys)	Darker (Blacks, Dark Greens)
Resistance	High; massive and resistant to weathering	Lower; prone to faster weathering

Beyond its raw form, granite is a parent material for both soil and other rocks. Under intense geological pressure, granite undergoes a transformation (metamorphism) to become Gneiss Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.174. Despite its strength, it eventually breaks down into soil particles, though it is considered a much more "resistant" parent material compared to volcanic ash or shale Geography of India by Majid Husain, Soils, p.1.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-170, 174-175; Geography of India by Majid Husain, Soils, p.1

7. Solving the Original PYQ (exam-level)

Now that you have mastered the classification of rocks, let's see how those building blocks solve this PYQ. You have learned that granite is a plutonic igneous rock characterized as acidic. This classification is primarily based on its high concentration of silica (SiO2), which typically accounts for over 70% of its weight. Furthermore, your study of rock-forming minerals like feldspar—the most abundant mineral group in the Earth's crust and a major component of granite—reveals that these are essentially alumino-silicates. By connecting the dots between the mineral composition (quartz and feldspar) and the chemical elements they contain, we find that the rock's mass is dominated by silica and aluminium, as noted in Physical Geography by PMF IAS.

To arrive at the correct answer, (C) Silica and aluminium, you must think like a geologist identifying bulk chemistry. While granite contains trace amounts of many elements, the term "main constituents" refers to the oxides that define its physical properties. Since the mineral quartz is pure silica and feldspar provides the alumina (Al2O3), these two form the chemical backbone of the rock. This explains why granite is lighter in color and less dense than basic rocks; the presence of aluminium and silica (the "Sial" layer) is what distinguishes continental crustal rocks from the iron-heavy oceanic crust.

UPSC often uses specific "distractors" to test your precision. Options (A), (B), and (D) all mention iron or iron oxide; however, iron is a hallmark of mafic or basic rocks (like basalt), not acidic rocks like granite. Option (B) includes silver, which is a rare precious metal and never a primary constituent of common rock types—this is a classic trap to see if you are guessing. Finally, while potassium is indeed found in certain types of feldspar (orthoclase), aluminium is more universally representative of the entire feldspar mineral family present in granite, making (C) the most chemically accurate choice for a main constituent.