The upper crust of the earth consists mostly of silicates. The acidic rocks containing a larger proportion of silica do not include I.Dolerite II. Granite III.Basalt. IV.Quartz,

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

To understand the ground we walk on, we must look at the Earth's crust—the ultra-thin, solid 'skin' of our planet. While it feels massive to us, the crust actually makes up less than 1% of the Earth’s total volume and mass Physical Geography by PMF IAS, Earths Interior, p.52. It isn't uniform, though; it varies significantly in thickness. Under the vast oceans, the oceanic crust is quite thin (5–30 km), whereas the continental crust is much thicker, averaging 50–70 km and reaching up to 100 km beneath massive mountain ranges like the Himalayas Physical Geography by PMF IAS, Earths Interior, p.52.

When we break the crust down into its chemical building blocks, we find that it is dominated by just a few elements. Unlike the Earth as a whole (where Iron is the most abundant element), the crust is dominated by Oxygen and Silicon. These elements combine with others to form silicate minerals, which are the primary components of most rocks. Elements or compounds occurring naturally in the crust are called minerals, and when a mineral contains a high enough concentration of a metal to be extracted profitably, we call it an ore Science, Class X, Metals and Non-metals, p.49.

The chemistry of these rocks allows us to classify them into two broad categories based on their Silica (SiO₂) content. Acidic (or Felsic) rocks, like Granite, are rich in silica (above 65%) and lighter elements like Aluminum. In contrast, Basic (or Mafic) rocks, like Basalt and Dolerite, have lower silica content (45–55%) and are heavier because they are enriched with Magnesium and Iron Physical Geography by PMF IAS, Earths Interior, p.53.

Top Elements in the Earth's Crust (by weight):

Rank	Element	Percentage (%)
1	Oxygen (O)	46.6%
2	Silicon (Si)	27.7%
3	Aluminium (Al)	8.1%
4	Iron (Fe)	5.0%

Sources: Physical Geography by PMF IAS, Earths Interior, p.52-53; Science, Class X, Metals and Non-metals, p.49

2. The Rock Cycle and Primary Rock Families (basic)

To understand the Earth's crust, we must first look at rocks, which are simply aggregates of one or more minerals held together by chemical bonds Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169. While there are thousands of variations, geologists classify them into three primary families based on their mode of origin: Igneous (born of fire/magma), Sedimentary (born of fragments), and Metamorphic (born of change). Igneous rocks are often called primary rocks because they form directly from the cooling of molten magma or lava; all other rock types are eventually derived from them Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.174.

A crucial way to distinguish these primary rocks is through their chemical composition, specifically their silica (SiO₂) content. This chemistry determines the rock's color, weight, and even how it weathers. We generally divide them into two categories:

Feature	Acidic (Felsic) Rocks	Basic (Mafic) Rocks
Silica Content	High (Above 65%)	Low (45-55%)
Key Minerals	Quartz and Feldspar	Iron and Magnesium
Physical Traits	Lighter color, less dense	Darker color, denser
Examples	Granite, Rhyolite	Basalt, Dolerite, Gabbro

The Rock Cycle is the continuous process that links these families in an eternal loop. It is driven by Earth's internal heat and external forces like weathering. For instance, when igneous rocks are exposed to the atmosphere, they break down into fragments to form sedimentary rocks. If any rock is subjected to intense heat and pressure—often during mountain-building orogenic movements—it undergoes recrystallization to become metamorphic Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173. Over millions of years, even these can be subducted back into the mantle, melt into magma, and start the cycle over as new igneous rocks.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169, 173, 174; Certificate Physical and Human Geography (GC Leong), The Earth's Crust, p.19

3. Igneous Rocks: Mode of Occurrence (intermediate)

Igneous rocks, often called the 'primary rocks,' form when molten material—either magma (below the surface) or lava (on the surface)—cools and solidifies. The defining characteristic of their mode of occurrence is the location and speed of this cooling process. If magma reaches the surface, it is termed Extrusive or Volcanic rock; if it cools within the Earth's crust, it is termed Intrusive or Plutonic rock NCERT Geography Class XI, Interior of the Earth, p.24.

The fundamental rule of thumb in igneous petrology is that cooling rate determines texture. When lava erupts onto the surface, it is exposed to air or water, causing it to cool rapidly. This sudden cooling prevents large crystals from forming, resulting in fine-grained or even glassy rocks like Basalt, which forms the vast Deccan Traps in India Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170. Conversely, magma trapped deep underground is insulated by the surrounding rocks, cooling very slowly. This allows mineral crystals to grow large, resulting in coarse-grained rocks like Granite.

Intrusive rocks often take on specific shapes, known as plutons, depending on how the magma forced its way into the crustal layers. These are categorized into several fascinating forms:

• Batholiths: Massive, deep-seated bodies of magma that form the core of many mountain ranges.
• Laccoliths: Dome-shaped intrusions that push the overlying strata upward.
• Phacoliths: Lens-shaped masses found at the crest of an anticline or the base of a syncline in folded terrain Physical Geography by PMF IAS, Volcanism, p.154.
• Sills and Dykes: Sills are horizontal sheets of rock that solidify between layers of older rock, while Dykes are vertical or near-vertical 'walls' that cut across rock layers Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170.

Feature	Extrusive (Volcanic)	Intrusive (Plutonic)
Cooling Speed	Rapid (on the surface)	Slow (deep underground)
Grain Size	Fine-grained / Small crystals	Coarse-grained / Large crystals
Example	Basalt	Granite

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Interior of the Earth, p.24; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Types of Rocks & Rock Cycle, p.170; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Volcanism, p.154

4. Sial and Sima: The Chemical Layers (intermediate)

When we look at the Earth's crust, it isn't just one uniform layer of rock. Instead, it is chemically divided into two distinct zones based on the minerals they contain. These are traditionally known as Sial and Sima. Think of these as the 'light' and 'heavy' layers of our planet's outer shell. Understanding this chemical arrangement is fundamental to understanding why continents float higher than the ocean floor.

Sial derives its name from Silica and Aluminium. This layer primarily makes up the continental crust. Because it is enriched with lighter elements like silicon, oxygen, aluminium, sodium, and potassium, it has a relatively low density (about 2.7 g/cm³). The rocks here are predominantly acidic or felsic, with granite being the most famous example Physical Geography by PMF IAS, Earths Interior, p.53. Because these rocks are less dense, they essentially 'float' on the denser layers beneath them, forming the high-standing landmasses we live on.

Directly beneath the Sial, and forming the foundation of the ocean floors, is the Sima layer. The name comes from Silica and Magnesium. This layer is much denser (about 3.0 g/cm³) and is composed of basic or mafic rocks, such as basalt Certificate Physical and Human Geography, The Structure of the Earth, p.17. While Sial is often thick and buoyant, Sima is thinner but significantly heavier, which is why the ocean basins are deep depressions in the Earth's surface.

Feature	Sial (Continental)	Sima (Oceanic)
Composition	Silica + Aluminium	Silica + Magnesium
Rock Type	Acidic / Felsic (e.g., Granite)	Basic / Mafic (e.g., Basalt)
Density	Lower (~2.7)	Higher (~3.0)
Location	Upper crust / Continents	Lower crust / Ocean floor

One fascinating aspect of these layers is their behavior during volcanic activity. Acidic rocks in the Sial portion contain up to 80% silica, which makes their parent magma very viscous (thick). This means the magma cools quickly and doesn't spread far Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170. In contrast, the basic rocks of the Sima layer have less silica and more iron and magnesium, resulting in magma that flows more easily over long distances.

Sources: Physical Geography by PMF IAS, Earths Interior, p.53; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170; Certificate Physical and Human Geography, The Structure of the Earth, p.17

5. Weathering and Soil Correlation (intermediate)

To understand the relationship between rocks and the soil they produce, we must first look at weathering. Weathering is the fundamental 'first step' in soil formation; it is the process that breaks down massive rocks into smaller particles, enriching the soil with minerals and making natural resources accessible (PMF IAS, Geomorphic Movements, p.83). The specific characteristics of the soil—its color, texture, and mineral content—are a direct legacy of its parent rock material and the chemical environment during its formation. Rocks are fundamentally categorized by their silica content, which determines their acidity. Acidic (or Felsic) rocks, such as Granite and Quartzite, are high in silica (often over 65%) and rich in minerals like quartz and feldspar. When these rocks weather in areas of low rainfall, they typically form Red Soils. The distinct reddish tint is caused by the diffusion of iron oxides (specifically haematite and limonite) within the crystalline structure (Majid Husain, Geography of India, p.10). If these soils become hydrated, the iron oxides change form, and the soil may appear yellow instead (NCERT Class X, Geography, p.11). In contrast, Basic (or Mafic) rocks like Basalt and Dolerite have a lower silica content (45-55%) but are much richer in iron and magnesium. In India, the weathering of basaltic lava flows in the Deccan Trap region has led to the formation of Black Soil, also known as Regur or 'Black Cotton Soil' (NCERT Class X, Geography, p.9). These soils are chemically distinct from those derived from acidic rocks, being generally more argillaceous (clayey) and moisture-retentive.

Feature	Acidic (Felsic) Origin	Basic (Mafic) Origin
Key Rocks	Granite, Quartzite, Gneiss	Basalt, Dolerite
Silica Content	High (>65%)	Low (45-55%)
Resulting Soil	Red and Yellow Soils	Black (Regur) Soils
Dominant Minerals	Quartz, Feldspar, Iron Oxides	Iron, Magnesium, Aluminum

Sources: Physical Geography by PMF IAS, Geomorphic Movements, p.83; Geography of India (Majid Husain), Soils, p.10-12; NCERT Class X Geography, Resources and Development, p.9-11

6. Chemical Classification: Acidic (Felsic) vs. Basic (Mafic) (exam-level)

When we classify igneous rocks chemically, we primarily look at their silica (SiO₂) content. This chemical makeup determines not just the rock's appearance, but also how the parent magma behaved before it solidified. Generally, we divide these into two broad categories: Acidic (Felsic) and Basic (Mafic).

Acidic rocks, also known as Felsic (a term derived from Feldspar and Silica), contain a high proportion of silica—often exceeding 65% and reaching up to 80% Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p. 170. Because they lack heavy metallic elements like iron and magnesium, these rocks are typically lighter in color and less dense. They are the primary building blocks of the Sial (Silica-Aluminium) layer of the Earth's crust Certificate Physical and Human Geography, The Earth's Crust, p. 17. A classic example is Granite. Interestingly, the high silica content makes the parent magma very viscous (thick), meaning it doesn't flow easily and tends to cool quickly near the source.

In contrast, Basic rocks, or Mafic (from Magnesium and Ferric/Iron), have a lower silica content (roughly 45-55%) but are rich in heavier minerals like Olivine and Pyroxene Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p. 176. These rocks are darker, denser, and much heavier. Because their magma is low in silica, it is highly fluid, allowing it to spread over vast distances—this is why Basalt often forms expansive plateaus. Dolerite is another key example, serving as the medium-grained equivalent to basalt.

To help you visualize the differences, here is a quick comparison:

Feature	Acidic (Felsic) Rocks	Basic (Mafic) Rocks
Silica Content	High (>65%)	Low (45-55%)
Key Minerals	Quartz, Feldspar	Magnesium, Iron, Olivine
Color & Density	Light color; Less dense	Dark color; More dense
Magma Flow	Viscous (Thick); Slower flow	Fluid; Faster flow
Examples	Granite, Quartz	Basalt, Dolerite

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176; Certificate Physical and Human Geography, The Earth's Crust, p.17

7. Identifying Common Igneous Examples (exam-level)

To master the classification of igneous rocks, we must look beyond their origin and focus on their chemical composition—specifically the presence of silica (silicon dioxide). Geologists categorize these primary rocks into two main groups based on how much silica they contain: Acidic (Felsic) and Basic (Mafic). Understanding this distinction is crucial because it determines the rock's color, density, and even the type of landforms it creates Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169.

Acidic rocks, also known as felsic rocks, are characterized by a high silica content (often reaching up to 80%). Because they lack heavy minerals like iron and magnesium, they are generally lighter in color and less dense. A defining characteristic of acidic magma is its high viscosity; it is thick and "sticky," meaning it doesn't flow very far before solidifying. Granite is the quintessential example of an acidic rock, forming the bulk of the continental crust (Sial). Other typical examples include Quartz and Feldspar, which are the primary minerals constituting these rocks Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170.

On the opposite end of the spectrum are Basic rocks (or mafic rocks). These are poor in silica (around 40-55%) but rich in heavier elements like iron, magnesium, and lime. This heavy mineral load makes them darker in color and much denser than their acidic counterparts. Basic magma is very fluid, allowing it to spread over vast distances and form extensive landforms like the Deccan Plateau in India Certificate Physical and Human Geography, The Earth's Crust, p.18. Common examples of basic rocks include Basalt, Gabbro, and Dolerite (a medium-grained equivalent of basalt) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170.

Feature	Acidic (Felsic) Rocks	Basic (Mafic) Rocks
Silica Content	High (Up to 80%)	Low (~40-55%)
Color & Density	Light color, Low density	Dark color, High density
Key Examples	Granite, Quartz, Feldspar	Basalt, Gabbro, Dolerite

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

8. Solving the Original PYQ (exam-level)

To solve this question, you must synthesize your knowledge of the chemical composition of the Earth's crust and the classification of igneous rocks. As you learned in Certificate Physical and Human Geography, GC Leong, the upper crust (Sial) is dominated by silica and aluminium, making it predominantly "acidic." Your primary task here is to differentiate between acidic (felsic) rocks, which have a high silica content (above 65%), and basic (mafic) rocks, which are lower in silica (45-55%) but rich in iron and magnesium.

Let’s walk through the reasoning step-by-step. The question asks which of the listed items are not acidic rocks. From your conceptual building blocks in Physical Geography by PMF IAS, you know that Granite (II) is the classic example of an acidic rock forming the continental crust, and Quartz (IV) is a mineral that defines high-silica environments. On the flip side, Basalt (III) is the most common basic/mafic rock. Crucially, you must remember that Dolerite (I) is simply the medium-grained, intrusive version of basalt; therefore, it also falls into the basic category. By identifying that I and III are basic, you arrive at (B) I and III as the correct answer.

UPSC often uses the "do not include" phrasing to test your focus under pressure. A common trap here is to get confused by Quartz (IV); while it is a mineral and not a rock type like Granite, it is the primary constituent of acidic rocks, and including it in the list tests whether you understand the mineralogical basis of acidity. Another trap is failing to link the relationship between Basalt and Dolerite. If you recognize Granite as acidic, you can instantly eliminate options (A) and (C), showing how a firm grasp of "anchor concepts" allows for the method of elimination even if you are unsure about one specific term.