. Which of the following statements is/are correct? 1. The major constituent mineral of granite rock is quartz. 2. The major constituent mineral of sandstone rock is feldspar. 3. The major constituent mineral of limestone rock is dolomite. Select the correct answer using the code given below :

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

Welcome to your first step in mastering the building blocks of our planet! Think of the Earth's crust as the thin, outermost "skin" of the Earth. While it feels massive to us, it actually makes up less than 1% of the Earth’s total mass and only about 0.5-1.0% of its volume Physical Geography by PMF IAS, Earths Interior, p.52. This crust isn't uniform; it is much thinner under the oceans (5-30 km) and significantly thicker under the continents (50-70 km), reaching its peak thickness of up to 100 km under massive mountain ranges like the Himalayas.

When we look at what the crust is actually made of, we distinguish between elements (the basic chemical ingredients) and minerals (the structures formed when those elements bond together). Interestingly, just eight elements account for over 98% of the crust's weight. Oxygen is the undisputed heavyweight champion, making up nearly half of the crust's weight, followed closely by Silicon. Together, they form the silicate minerals that dominate the rocks you see every day.

Element	% by Weight (Crust)	Key Note
Oxygen (O)	46.6%	Most abundant element in the crust.
Silicon (Si)	27.7%	Second most abundant; key to silicate minerals.
Aluminium (Al)	8.1%	The most abundant metal in the crust.
Iron (Fe)	5.0%	Major component of the whole Earth, but 4th in the crust.

Beyond these individual elements, we find minerals. The most common mineral group in the Earth's crust is Feldspar, which makes up about half of the crust's composition Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Quartz is another major player, consisting of silicon and oxygen. When these minerals contain a high enough concentration of a specific metal that we can extract it for profit, we call them ores—such as Bauxite for aluminium or Cinnabar for mercury Science class X (NCERT 2025), Metals and Non-metals, p.49.

Sources: Physical Geography by PMF IAS, Earths Interior, p.52-53; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175; Science class X (NCERT 2025), Metals and Non-metals, p.49

2. Major Rock-Forming Minerals (intermediate)

To understand the Earth's crust, we must look at the building blocks: minerals. While there are over 2,000 identified minerals, only a small group—often called rock-forming minerals—constitutes the vast majority of the crust. These are primarily silicates, as oxygen and silicon are the two most abundant elements in the crust. The chemistry of these minerals determines whether a rock is 'acidic' (light-colored and silica-rich) or 'basic' (dark-colored and magnesium/iron-rich) Physical Geography by PMF IAS, Chapter 13, p.170.

The 'Big Six' mineral groups you must remember are:

• Feldspar: The most abundant mineral group, making up nearly half of the Earth's crust. It contains silicon, oxygen, aluminum, and varying amounts of sodium, potassium, or calcium. It is a major component of granite.
• Quartz: Composed of pure silica (SiO₂). It is extremely hard and resistant to weathering, which is why it is the primary constituent of sandstone and quartzite Physical Geography by PMF IAS, Chapter 13, p.170.
• Pyroxene and Amphibole: These are darker minerals containing calcium, magnesium, and iron. Pyroxene is commonly found in meteorites and basaltic rocks, while Amphibole (like asbestos) is often found in metamorphic rocks Physical Geography by PMF IAS, Chapter 13, p.176.
• Mica: Known for its perfect 'cleavage' (it peels into thin, flat sheets). It is widely used in electrical instruments due to its insulating properties Physical Geography by PMF IAS, Chapter 13, p.176.
• Olivine: A greenish crystal typically found in basaltic (oceanic) rocks, composed of magnesium, iron, and silica Physical Geography by PMF IAS, Chapter 13, p.176.

Beyond silicates, Carbonate minerals play a massive role in sedimentary geology. The most important is Calcite (calcium carbonate, CaCO₃), which is the primary mineral in limestone and its metamorphic cousin, marble. While dolomite is similar, a rock must be dominated by calcite to be strictly classified as limestone Geography of India, Majid Husain, Resources, p.29.

Mineral Group	Key Characteristics	Common Rock Association
Feldspar	Most abundant; light-colored	Granite, Basalt
Quartz	Hard, resistant to chemical decay	Sandstone, Granite
Calcite	Reacts with acid; carbonate-based	Limestone, Marble
Olivine	Greenish; rich in Iron/Magnesium	Basalt

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170, 176; Geography of India, Majid Husain, Resources, p.29

3. Igneous Rocks: Formation and Chemistry (basic)

Igneous rocks, often called primary rocks, are the starting point of the rock cycle. They form through the cooling and solidification of molten rock material. When this molten material is below the Earth's surface, it is known as magma; once it erupts onto the surface, it is called lava. Because these rocks originate under conditions of extreme heat, they are unfossiliferous (they do not contain fossils) and are generally hard and crystalline Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169.

We classify igneous rocks primarily based on where they cool and their chemical makeup. From a physical perspective, Plutonic rocks (like Granite) cool slowly deep within the crust, allowing large, easily recognized crystals to form. In contrast, Volcanic rocks (like Basalt) cool rapidly at the surface, resulting in a much finer-grained texture Certificate Physical and Human Geography, The Earth's Crust, p.18. Chemically, the most important factor is the silica (silicon dioxide) content, which determines the rock's color, weight, and flow behavior.

Feature	Acidic Igneous Rocks	Basic Igneous Rocks
Silica Content	High (up to 80%)	Low (approx. 40-50%)
Key Minerals	Quartz and Feldspar	Iron and Magnesium (Ferromagnesian)
Physical Traits	Lighter color, less dense	Darker color, heavier/denser
Magma Nature	Viscous (thick), does not flow far	Fluid, spreads over large areas
Example	Granite	Basalt, Gabbro

Beyond their structure, igneous rocks are the Earth’s primary "treasure chests." Since magma is the chief source of metal ores, these rocks are often rich in valuable minerals like gold, iron, nickel, copper, and platinum. Sometimes, gas bubbles escaping from cooling basalt leave behind cavities called amygdales, which later fill with secondary minerals like quartz or gemstones Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-170; Certificate Physical and Human Geography, The Earth's Crust, p.18

4. Sedimentary Rocks: Origins and Classification (basic)

Welcome back! Now that we understand how molten magma cools into igneous rocks, let’s look at the second major family: Sedimentary Rocks. The word itself comes from the Latin sedimentum, meaning 'settling.' These rocks are formed through a process called lithification—which is the consolidation and compaction of sediments derived from the weathering and erosion of pre-existing rocks Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171. One of the most distinctive features of sedimentary rocks is their stratification; because they accumulate over time, they appear in layers of varying thickness.

It is fascinating to note that while sedimentary rocks cover nearly 75% of the Earth's surface, they only make up about 5% of the total volume of the crust. This is because they form a relatively thin 'veneer' or skin over the deeper igneous and metamorphic basement rocks Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171. Based on their mode of origin, we classify them into three primary categories:

• Mechanically Formed (Clastic): These are created from fragments of older rocks that have been cemented together. Sandstone is the most common example, typically composed of quartz grains. Other examples include Shale (from clay), Loess (deposited by wind), and Tillite (deposited by glaciers) Certificate Physical and Human Geography, Chapter 2: The Earth's Crust, p.19.
• Organically Formed: These result from the accumulation of organic debris. Calcareous rocks like limestone and chalk form from the shells and skeletons of marine organisms. Carbonaceous rocks, such as coal, form from highly compressed plant matter from ancient swamps Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.172.
• Chemically Formed: These precipitate directly from mineral-rich water. For instance, when salt lakes evaporate, they leave behind deposits of Rock Salt (Halite), Gypsum, or Potash Certificate Physical and Human Geography, Chapter 2: The Earth's Crust, p.19.

To help you distinguish between them effectively, keep this comparison in mind:

Type	Primary Process	Classic Examples
Mechanical	Physical accumulation of rock fragments	Sandstone, Shale, Conglomerate
Organic	Remains of plants/animals (shells, vegetation)	Coal, Limestone, Chalk
Chemical	Precipitation from solution (evaporation)	Rock Salt, Gypsum, Potash

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.172; Certificate Physical and Human Geography, Chapter 2: The Earth's Crust, p.19

5. Weathering and Mineral Stability (intermediate)

Weathering is the fundamental process of rock disintegration and decomposition in situ (on-site). While physical weathering involves the mechanical breaking of rocks into smaller fragments, chemical weathering is a far more transformative process that alters the internal mineral structure of the rock Certificate Physical and Human Geography, Weathering, Mass Movement and Groundwater, p.36. This decomposition is driven by agents like water, oxygen, and organic acids produced by plants and microbes, which loosen the chemical bonds between mineral grains Physical Geography by PMF IAS, Geomorphic Movements, p.90.

The rate at which a rock weathers is largely determined by its mineral stability. Not all minerals react to the environment the same way. For instance, Quartz is one of the most chemically stable minerals at the Earth's surface, making it highly resistant to decomposition. In contrast, Feldspar—the most abundant mineral in the Earth's crust—is susceptible to hydrolysis, a process where it reacts with water to turn into soft clay. This explains why an exposed piece of Granite (which contains both quartz and feldspar) often looks pitted and rough; the feldspar has "rotted" away into clay and washed off, leaving the resilient quartz grains standing out in relief Certificate Physical and Human Geography, Weathering, Mass Movement and Groundwater, p.36.

Different chemical environments trigger different reactions based on the mineral content:

• Oxidation: When oxygen reacts with iron-rich minerals (like olivine or pyroxene), it forms iron oxide, or rust. This brownish crust is much weaker than the original mineral, causing the rock to crumble easily Certificate Physical and Human Geography, Weathering, Mass Movement and Groundwater, p.37.
• Carbonation: Rainwater absorbs atmospheric CO₂ to form a weak carbonic acid (H₂CO₃). This acid is particularly effective at dissolving Calcite, the primary mineral in Limestone.
• Solution: Some minerals are so soluble that they dissolve completely in water, which is why limestone landscapes often feature caves and underground drainage.

Rock Type	Primary Mineral	Weathering Behavior
Granite	Quartz & Feldspar	Feldspar turns to clay; Quartz remains as sand.
Sandstone	Quartz	Very resistant to chemical change; breaks down physically.
Limestone	Calcite	Highly susceptible to dissolution by acidic rainwater.

Sources: Certificate Physical and Human Geography, Weathering, Mass Movement and Groundwater, p.36-37; Physical Geography by PMF IAS, Geomorphic Movements, p.90

6. Metamorphic Rocks: The Transformation Path (intermediate)

The term metamorphism literally translates to a 'change of form'. Unlike igneous rocks (which form from cooling magma) or sedimentary rocks (which form from lithified deposits), metamorphic rocks are the 'survivors' of the geological world. They are formed when existing rocks are subjected to intense Pressure, Volume, and Temperature (PVT) changes. Crucially, this transformation occurs primarily in a solid state; if the rock melts completely, it becomes magma and returns to the igneous cycle. This process involves the recrystallization and reorganization of minerals, often creating entirely new textures and mineral compositions Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173.

Metamorphism generally follows two primary paths: Thermal (Contact) Metamorphism and Dynamic (Regional) Metamorphism. Thermal metamorphism occurs when rocks come into contact with hot, rising magma. The intense heat 'bakes' the surrounding rock—for instance, changing limestone into marble or sandstone into quartzite. A fascinating example of this is the peak of Mount Everest, which consists of metamorphosed limestone Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173. Regional metamorphism, however, occurs during massive tectonic movements (like mountain building), where high pressure and heat transform rocks over vast areas, often resulting in complex structural changes.

A hallmark of these rocks is their unique texture. Under intense pressure, minerals often align themselves in parallel layers or lines, a process called foliation or lineation. In some cases, minerals of different densities segregate into alternating light and dark thick layers, known as banding, which gives rocks like Gneiss their signature appearance Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.174.

To master this topic, you must remember the specific "transformation paths" that common rocks follow:

Original (Parent) Rock	Metamorphic Result
Granite (Igneous)	Gneiss
Clay / Shale (Sedimentary)	Slate / Schist
Sandstone (Sedimentary)	Quartzite
Limestone (Sedimentary)	Marble
Coal (Sedimentary)	Graphite / Diamond

Certificate Physical and Human Geography, The Earth's Crust, p.19

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173-174; Certificate Physical and Human Geography, The Earth's Crust, p.19; Geography of India, Resources, p.29

7. Detailed Mineralogy: Granite, Sandstone, and Limestone (exam-level)

To master mineralogy, we must look beyond just "rocks" and understand the specific chemical building blocks that define them. Granite, Sandstone, and Limestone represent the three pillars of the Earth's crust, each with a distinct mineral signature. Granite is the quintessential "acidic" igneous rock, characterized by a very high silica content (up to 80%). While many students associate granite solely with Quartz, it is actually a composite where Feldspar—the most abundant mineral group in the Earth's crust—often forms the bulk of the volume Physical Geography by PMF IAS, Chapter 13, p. 170. This combination of Quartz (hard and resistant) and Feldspar (light-colored and silica-rich) gives Granite its characteristic light color and immense structural strength.

In contrast, Sandstone is a mechanically formed sedimentary rock. It is essentially a collection of "recycled" mineral grains, primarily Quartz fragments derived from the weathering of older rocks like granite Certificate Physical and Human Geography, GC Leong, Chapter 2, p. 19. Quartz is favored in sandstone because it is chemically stable and survives the journey of erosion better than other minerals. However, if a sandstone contains a significant amount of Feldspar (usually over 25%), it is specifically termed Arkose. Finally, we have Limestone, which shifts the chemistry from silica to carbonates. Its primary mineral is Calcite (Calcium Carbonate - CaCO₃), often formed from the skeletal remains of marine organisms. While Dolomite (Magnesium Carbonate) is frequently found alongside it, a rock must be dominated by Calcite to be strictly termed a limestone.

Understanding these distinctions is crucial for identifying how these rocks behave under environmental stress. For instance, because Limestone is composed of Calcite, it is highly susceptible to chemical weathering through solution, especially in humid climates, whereas the Quartz in Sandstone and Granite makes them far more resistant to dissolving Certificate Physical and Human Geography, GC Leong, Chapter 4, p. 37.

Rock Type	Primary Mineral(s)	Key Mineral Property
Granite (Igneous)	Quartz & Feldspar	Acidic, high silica, light-colored.
Sandstone (Sedimentary)	Quartz	Mechanically formed; resistant grains.
Limestone (Sedimentary)	Calcite (CaCO₃)	Soluble in rainwater; organic origin.

Sources: Physical Geography by PMF IAS, Manjunath Thamminidi, Types of Rocks & Rock Cycle, p.170; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.19; Certificate Physical and Human Geography, GC Leong, Weathering, Mass Movement and Groundwater, p.37

8. Solving the Original PYQ (exam-level)

Now that you have mastered the classification of rocks, this question tests your ability to drill down into their mineralogical composition. The building blocks you learned—specifically that Granite is an intrusive igneous rock and Sandstone/Limestone are sedimentary—come together here to define the physical character of the Earth's crust. The key to solving this lies in identifying the dominant mineral versus a secondary one. For instance, while Quartz is a significant part of granite, Feldspar is actually the most abundant mineral group in the crust and often the primary constituent of granite by volume (Physical Geography by PMF IAS).

To arrive at the correct answer, (B) 3 only, we must apply elimination logic based on mineral resistance. Statement 2 is a classic swap: Sandstone is primarily composed of Quartz because it is highly resistant to chemical weathering; if it contains high amounts of feldspar, it is specifically called Arkose. Statement 1 is a "partial truth" trap—quartz is present, but feldspar is the majority. This leaves Statement 3. While technical geology distinguishes between Calcite (Limestone) and Dolomite (Dolostone), general geography resources like Certificate Physical and Human Geography, GC Leong often group them together under the category of calcareous rocks. In the UPSC framework, when two statements are definitively reversed (as in 1 and 2), the remaining statement is often the intended answer, even if it uses a broader classification.

UPSC frequently uses this "Constituent Swap" trap to confuse students. By assigning Feldspar to Sandstone and Quartz to Granite, the examiner tests if you know which mineral is truly the defining major component. Always remember: Quartz defines the durability of Sandstone, while Feldspar defines the bulk of Granite. Mastering these primary mineral associations allows you to navigate even the most ambiguously phrased questions with confidence.