Which one among the following is not an igneous rock ?

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

To understand rocks, we must first look at the 'skin' of our planet: the Earth's Crust. The crust is the outermost solid shell of the Earth, and though it feels massive to us, it accounts for less than 1% of the Earth's total mass and volume Physical Geography by PMF IAS, Earths Interior, p.52. It isn't uniform; the oceanic crust is relatively thin (5–30 km), while the continental crust is much thicker, reaching up to 70–100 km beneath massive mountain ranges like the Himalayas Physical Geography by PMF IAS, Earths Interior, p.52. This crust is the storehouse of all the elements we use in our daily lives. At a chemical level, the crust is composed of various elements, but just eight of them make up over 98% of its weight. Oxygen is the most abundant element in the crust, followed by Silicon. This is why most minerals in the crust are 'silicates.' However, if we look at the Entire Earth (including the core), Iron becomes the most abundant element because the Earth's core is primarily made of iron and nickel Physical Geography by PMF IAS, Earths Interior, p.53.

Element	% in Earth's Crust	% in Whole Earth
Oxygen (O)	46.6% (1st)	29.5% (2nd)
Silicon (Si)	27.7% (2nd)	15.2% (3rd)
Iron (Fe)	5.0% (4th)	34.6% (1st)
Aluminium (Al)	8.1% (3rd)	Small amount

Naturally occurring elements or compounds found in the crust are called minerals. When a mineral contains a high enough concentration of a specific metal that it can be extracted profitably, we call it an ore Science Class X (NCERT 2025), Metals and Non-metals, p.49. While we often focus on metals like Iron or Gold, non-metallic minerals like Mica, Limestone, and Phosphate are equally vital for industries like construction and electronics India People and Economy (NCERT 2025), Mineral and Energy Resources, p.57.

Sources: Physical Geography by PMF IAS, Earths Interior, p.52-53; Science Class X (NCERT 2025), Metals and Non-metals, p.49; India People and Economy (NCERT 2025), Mineral and Energy Resources, p.57

2. Primary Rocks: Formation of Igneous Rocks (basic)

To understand the Earth's crust, we must start with its very first building blocks: Igneous Rocks. Often referred to as "Primary Rocks," they earn this title because they are the first rocks to form from the cooling of molten material. Every other rock you see on Earth—whether it’s the sand on a beach or the marble in a building—ultimately traces its ancestry back to an igneous origin Physical Geography by PMF IAS, Chapter 13, p.174.

The formation process begins deep within the Earth where intense heat melts rock into a thick, soup-like substance called magma. When this magma cools and solidifies, it crystallizes into rock. The location of this cooling process is crucial as it determines the rock's texture:

• Intrusive (Plutonic) Rocks: These form when magma cools slowly deep inside the Earth's crust. Because the cooling is leisurely, minerals have time to grow into large, visible crystals. Granite and Gabbro are classic examples of these coarse-grained rocks Physical Geography by PMF IAS, Chapter 13, p.169.
• Extrusive (Volcanic) Rocks: These form when magma reaches the surface as lava. Exposed to the air or ocean, the lava cools rapidly, leaving no time for large crystals to grow. This results in a fine-grained or even glassy texture, like Basalt Certificate Physical and Human Geography, GC Leong, Chapter 2, p.18.

One defining characteristic you must remember for the UPSC is that igneous rocks are unfossiliferous. Because they originate from scorching molten matter, any organic remains would be instantly incinerated, meaning you will never find a dinosaur bone or a leaf imprint inside a true igneous rock Physical Geography by PMF IAS, Chapter 13, p.169. Chemically, they are also classified based on their silica content: Acidic rocks (like Granite) are high in silica and lighter in color, while Basic rocks (like Gabbro or Basalt) are denser and darker.

Feature	Intrusive (Plutonic)	Extrusive (Volcanic)
Cooling Rate	Very Slow	Rapid
Crystal Size	Large / Coarse-grained	Small / Fine-grained
Example	Granite, Gabbro	Basalt

Sources: Physical Geography by PMF IAS, Chapter 13: Types of Rocks & Rock Cycle, p.169; Physical Geography by PMF IAS, Chapter 13: Types of Rocks & Rock Cycle, p.174; Certificate Physical and Human Geography, GC Leong, Chapter 2: The Earth's Crust, p.18

3. Intrusive (Plutonic) vs. Extrusive (Volcanic) Rocks (intermediate)

At its simplest level, all Igneous rocks (also called Primary rocks) are born from fire—or more accurately, from the cooling and solidification of molten rock. However, not all igneous rocks look the same. The fundamental difference between them lies in where the molten material cools and how fast it loses its heat. As molten rock below the surface is called magma and molten rock above the surface is called lava, the location of cooling dictates the rock's entire personality Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169.

Intrusive (Plutonic) Rocks form when magma never reaches the surface. Instead, it remains trapped deep within the Earth's crust. Because the surrounding crust acts like a thick thermal blanket, this magma cools very slowly—often over thousands or millions of years. This slow pace gives mineral molecules enough time to migrate and organize into large, well-defined crystals that are easily visible to the naked eye. Granite, Gabbro, and Diorite are classic examples of these coarse-grained rocks Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.18. They are eventually exposed at the surface only after millions of years of erosion and uplift.

In contrast, Extrusive (Volcanic) Rocks are formed when magma erupts onto the surface as lava. Exposed to the air or ocean water, the lava cools rapidly. This sudden drop in temperature prevents large crystals from forming, resulting in a fine-grained or even glassy texture where individual minerals cannot be seen without a microscope. Basalt is the most famous example, forming vast landscapes like the Deccan Traps in India Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170. These rocks are often denser and darker because they are rich in iron and magnesium (mafic) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT, Interior of the Earth, p.24.

Feature	Intrusive (Plutonic)	Extrusive (Volcanic)
Cooling Location	Deep within the crust	At or near the surface
Cooling Rate	Slow	Rapid
Grain/Crystal Size	Large (Coarse-grained)	Small (Fine-grained)
Common Examples	Granite, Gabbro	Basalt, Pumice

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-170; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.18; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT, Interior of the Earth, p.24

4. The Rock Cycle: Transformation Processes (intermediate)

The Rock Cycle is nature’s ultimate recycling program—a continuous, self-sustaining process where rocks are transformed from one type to another over geological time. Think of it not as a circle, but as a web of possibilities. At the heart of this cycle are Igneous rocks, often called Primary Rocks because they are the first to form from the cooling of molten magma or lava Physical Geography by PMF IAS, Chapter 13, p.169. Whether it is Granite cooling slowly deep within the crust or Basalt hardening quickly on the surface, these rocks provide the raw material for everything else that follows. Once formed, no rock is safe from the elements. Through denudation (the combined forces of weathering and erosion), igneous and metamorphic rocks break down into fragments. These fragments undergo lithification—a process of compaction and cementation—to become Sedimentary rocks like sandstone or shale Physical Geography by PMF IAS, Chapter 13, p.171. If these sedimentary or igneous rocks are later subjected to intense heat and pressure (without melting), they recrystallize into Metamorphic rocks. For instance, the igneous rock Granite can be squeezed and heated until its minerals align into the distinct bands characteristic of Gneiss Physical Geography by PMF IAS, Chapter 13, p.174.

Process	Input Rock Type	Resulting Rock Type
Cooling & Solidification	Magma/Lava	Igneous (Primary)
Lithification	Any Rock (as sediments)	Sedimentary
Metamorphism	Igneous or Sedimentary	Metamorphic
Melting (Subduction)	Any Crustal Rock	Magma

The cycle closes when crustal rocks are carried down into the Earth’s mantle through subduction. Here, they melt back into magma, ready to begin the journey again as new igneous formations. This ensures that the Earth's crust is in a constant state of renewal, moving minerals from the deep interior to the surface and back again.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.174

5. Sedimentary Rocks and Stratification (intermediate)

If igneous rocks are the 'primary' rocks born of fire, sedimentary rocks are the Earth's great storytellers. The word 'sedimentary' is derived from the Latin sedimentum, meaning 'settling.' These rocks form when the debris of older rocks, plants, and animals is transported by agents like water, wind, or ice and settles in layers. Over vast periods, the weight of overlying materials squeezes these deposits together in a process called lithification (literally, 'turning into stone') Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171.

The most distinctive characteristic of these rocks is stratification. Because sediments are deposited in distinct intervals, they form clear horizontal layers or 'strata.' This is why you will often see beautiful bands of different colors and textures in a canyon wall. While these rocks are incredibly widespread—covering nearly 75% of the Earth's land surface—they are actually quite thin in the grand scheme of things, making up only about 5% of the total volume of the Earth's crust Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171.

Geographers classify sedimentary rocks into three main categories based on their origin:

Category	Formation Process	Examples
Mechanically Formed	Accumulation of sand, clay, or pebbles cemented together.	Sandstone, Shale, Loess (wind-deposited), Tillite (ice-deposited) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171.
Organically Formed	Derived from the remains of living organisms like corals, shells, or vegetation.	Limestone (from shells), Coal (from compressed forests/swamps) Certificate Physical and Human Geography, The Earth's Crust, p.19.
Chemically Formed	Precipitated out of solution when water evaporates.	Rock Salt (Halite), Gypsum, Potash Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171.

Beyond their geological interest, these rocks are economically vital. For instance, carbonaceous rocks like coal, formed from compressed organic matter, provide a significant portion of the world's energy Certificate Physical and Human Geography, The Earth's Crust, p.19. Furthermore, because they are formed at the surface without the melting heat of volcanoes, sedimentary rocks are the only ones that typically preserve fossils, providing us with a record of ancient life.

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

6. Metamorphism: Heat, Pressure, and Foliation (exam-level)

The word metamorphism literally translates to a "change of form." In the geological world, this refers to the process where pre-existing rocks—whether igneous or sedimentary—undergo a complete transformation in their mineralogy, texture, and chemical composition without ever fully melting into magma. This transformation occurs primarily due to changes in Pressure, Volume, and Temperature (PVT) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173. When rocks are pushed deep into the Earth's crust by tectonic movements or come into contact with searing hot magma, they are "cooked" and "squeezed," causing their minerals to recrystallize into more stable forms for their new environment.

Metamorphism is generally categorized into two main types based on the dominant agent of change. Thermal (or Contact) Metamorphism occurs when rocks come into contact with hot magma; the intense heat causes minerals to recrystallize, such as when limestone turns into marble or sandstone into quartzite Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173. On the other hand, Regional Metamorphism occurs over vast areas during mountain-building processes where intense pressure and heat work together to deform the rock structure. This massive pressure often leads to foliation—a characteristic arrangement where mineral grains align in parallel layers or bands, giving the rock a striped appearance, as seen in Gneiss or Schist Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173.

Understanding the "parent rock" (protolith) is essential for identifying metamorphic rocks. Use the table below to see how common rocks change under these extreme conditions:

Original Rock (Protolith)	Metamorphic Result	Primary Agent
Clay / Shale	Slate	Pressure/Heat
Limestone	Marble	Heat
Sandstone	Quartzite	Heat
Granite	Gneiss	Pressure/Heat
Coal	Graphite	Heat/Pressure

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

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.19

7. Identifying Key Rock Examples and Origins (exam-level)

To master the rock cycle, we must distinguish between rocks based on their genetic origin—how they were born. Igneous rocks (or primary rocks) are formed from the cooling and solidification of molten matter. If this cooling happens deep underground, we get intrusive (plutonic) rocks like Granite and Gabbro. Because they cool slowly, they develop large, visible crystals. Conversely, if magma reaches the surface as lava, it cools rapidly to form extrusive (volcanic) rocks like Basalt. Since these rocks originate from extreme heat, they are unfossiliferous—they contain no organic remains Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169.

Metamorphic rocks, on the other hand, are 'changed' rocks. They are formed when pre-existing igneous or sedimentary rocks are subjected to intense heat and pressure, causing their minerals to recrystallize without melting. A classic example is Gneiss, which often forms from the metamorphism of Granite. In India, the Bundelkhand Gneiss is a famous ancient example of such massive, coarse-grained rocks Geography of India by Majid Husain, Geological Structure and formation of India, p.5-6. Understanding these transitions is key to identifying rock types in the field.

Below is a quick reference for common rock transformations:

Original Rock	Metamorphic Equivalent	Primary Factor
Granite (Igneous)	Gneiss	Pressure
Limestone (Sedimentary)	Marble	Heat
Sandstone (Sedimentary)	Quartzite	Heat
Shale/Clay (Sedimentary)	Schist / Slate	Pressure/Heat

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

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169, 174; Certificate Physical and Human Geography by GC Leong, The Earth's Crust, p.19; Geography of India by Majid Husain, Geological Structure and formation of India, p.5-6

8. Solving the Original PYQ (exam-level)

This question brings together your understanding of the Rock Cycle and the classification of Primary Rocks. You have learned that Igneous rocks are formed through the cooling and solidification of molten material (magma or lava). To solve this, you must apply the building blocks of lithology: identifying rocks based on their mode of origin. When you encounter such a list, your first step should be to categorize each rock type according to the physical processes that created it.

Walking through the reasoning, Granite and Gabbro are classic examples of intrusive (plutonic) igneous rocks that crystallized slowly deep within the Earth's crust, as explained in Certificate Physical and Human Geography, GC Leong. Conversely, Basalt is the most common extrusive (volcanic) igneous rock formed at the surface. The trap UPSC often sets is providing rocks with vastly different textures—coarse-grained versus fine-grained—to see if you can still group them by their shared origin. However, Gneiss stands out because it is a metamorphic rock characterized by distinct mineral banding or foliation.

Therefore, the correct answer is (D) Gneiss. As highlighted in Physical Geography by PMF IAS, Gneiss forms when pre-existing rocks like granite undergo intense heat and pressure, a process known as metamorphism. Mastery Tip: UPSC frequently tests the relationship between "parent" rocks and their metamorphic derivatives; since Gneiss is often the result of Granite being transformed, it fundamentally cannot be an igneous rock itself.