The Deccan Trap Formation was caused by

1. Basics of Magma: Viscosity and Chemical Composition (basic)

Welcome to our journey through Indian geology! To understand the massive rock formations of the Indian subcontinent, we must first understand the "DNA" of the rocks themselves: Magma. At its simplest, magma is molten rock found beneath the Earth's surface. However, not all magma is created equal. Its behavior—whether it flows like water or explodes like a cork from a champagne bottle—is dictated primarily by two factors: Chemical Composition and Viscosity.

Viscosity is a measure of a fluid's resistance to flow. Think of the difference between water and honey; honey has higher viscosity. In magma, the primary driver of viscosity is Silica (SiO₂). Silica molecules tend to link together into long chains (polymerization), which acts like a "glue" that makes the magma thick and sticky. Magma with high silica content (known as Acidic or Felsic magma) is highly viscous, while magma with low silica but high Iron (Fe) and Magnesium (Mg) content (known as Basic or Mafic magma) is much thinner and more fluid Physical Geography by PMF IAS, Volcanism, p.140.

These differences lead to very different geological landscapes. Because Basaltic lava is so fluid, it can travel for dozens of miles before solidifying, often creating flat, expansive plateaus or gently sloping "shield" volcanoes Physical Geography by PMF IAS, Divergent Boundary, p.131. Conversely, Andesitic lava is so viscous that it often plugs the volcanic vent. This traps gases underneath, leading to immense pressure buildup and violent, explosive eruptions like the Peléan or Vulcanian types, which eject ash and "volcanic bombs" high into the atmosphere Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.11.

Sources: Physical Geography by PMF IAS, Volcanism, p.139-140; Physical Geography by PMF IAS, Divergent Boundary, p.131; Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.11

2. Classification of Volcanic Landforms (basic)

To understand the geological map of India, we must first master how volcanoes shape the land. The form a volcano takes—whether it is a steep mountain or a flat plateau—is primarily decided by the viscosity (thickness) of the lava and how much gas is trapped inside it. Think of it like pouring honey versus pouring water; water spreads out thin and wide, while honey piles up.

Volcanic landforms are generally classified into four major types based on their structure and eruption style:

• Shield Volcanoes: These are the largest volcanoes on Earth (after flood basalts). They are made of basaltic lava, which is extremely fluid. Because the lava flows easily, it travels long distances before cooling, creating a broad, low-profile shape resembling a warrior’s shield. They are usually non-explosive unless water enters the vent. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 3: Interior of the Earth, p. 23
• Composite Volcanoes (Stratovolcanoes): These are the iconic "cone-shaped" mountains. They erupt andesitic lava, which is cooler and more viscous (sticky) than basalt. This stickiness causes the lava to pile up near the vent. They are built from alternating layers of lava, ash, and pyroclastic material, making them prone to violent, explosive eruptions. Physical Geography by PMF IAS, Chapter 11: Volcanism, p. 140
• Calderas: These are the most explosive volcanoes. The eruption is so intense that the magma chamber underneath empties quickly, causing the entire volcanic structure to collapse into itself. This leaves behind a massive, tub-shaped depression called a caldera, which often turns into a lake over time. Physical Geography by PMF IAS, Chapter 11: Volcanism, p. 150
• Flood Basalt Provinces: These occur when highly fluid lava erupts through long cracks or fissures rather than a single pipe. The lava can cover thousands of square kilometers in thick layers. The Deccan Traps in India are one of the world's most famous examples of this. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 3: Interior of the Earth, p. 24

Comparison of Primary Volcanic Types:

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 3: Interior of the Earth, p.23-24; Physical Geography by PMF IAS, Chapter 11: Volcanism, p.140, 141, 150

3. Flood Basalt Provinces and Fissure Eruptions (intermediate)

To understand Flood Basalt Provinces, we must first look at the plumbing of the Earth. Unlike the explosive, mountain-building volcanoes we often see in movies (like Mt. Vesuvius), flood basalts are born from fissure eruptions. These occur when the Earth’s crust cracks open in long, linear vents—sometimes kilometers long—allowing magma to pour out steadily without the build-up of massive gas pressure Physical Geography by PMF IAS, Volcanism, p.149.

The secret to why these eruptions cover entire subcontinents lies in the viscosity of the lava. Magma in these provinces is Basaltic (Mafic), meaning it is low in silica and extremely fluid. Instead of cooling quickly and piling up into a steep cone, it behaves more like thick oil, spreading out over vast distances to form relatively flat, horizontal layers called Lava Plateaus. When these layers stack up over millions of years, they create a step-like landscape known as 'Traps' (from the Swedish word 'Trappa' meaning stairs) Geography of India, Physiography, p.51.

The primary driver behind such massive outpourings is often a Mantle Plume—a column of hot rock rising from deep within the Earth. When the head of this plume hits the lithosphere, it undergoes widespread decompression melting, generating enormous volumes of magma Physical Geography by PMF IAS, Hotspot Volcanism, p.162. The Indian Deccan Trap is a world-class example of this, formed roughly 66 million years ago during the Late Cretaceous. It covers about 5 lakh sq km, with some layers in the Western Ghats reaching a staggering thickness of over 2,000 meters Geography of India, Geological Structure and formation of India, p.19.

Sources: Physical Geography by PMF IAS, Volcanism, p.149; Geography of India, Physiography, p.51; Physical Geography by PMF IAS, Hotspot Volcanism, p.162; Geography of India, Geological Structure and formation of India, p.19

4. Geological History of the Indian Peninsular Block (intermediate)

The Indian Peninsular Block is one of the oldest and most stable landmasses on Earth. To understand its geological history, we must view it as a massive "shield" that has remained relatively rigid while the rest of the world shifted around it. Its story begins in the Archaean Era with the formation of the Basement Complex. These are the "fundamental gneisses"—rocks so old they form the literal foundation of the subcontinent. These rocks, including granite and gabbro, represent the crust's first solidification Geography of India, Chapter 1, p.4. Over time, these were topped by the Purana Group (Cuddapah and Vindhyan systems), which consist of ancient sedimentary rocks deposited in massive basins, marking the first major cycle of erosion and deposition on the shield.

As we move into the Permo-Carboniferous period, the Peninsular Block underwent a dramatic transformation known as the Gondwana System. This wasn't about volcanic fire, but rather about tectonic sinking. Huge sections of the crust subsided along linear tracks, creating rift valleys (like the Damodar and Mahanadi valleys). Freshwater sediments and massive amounts of vegetation were buried in these troughs, eventually forming over 95% of India’s coal deposits Geography of India, Chapter 1, p.18. This era turned the Peninsula into a literal powerhouse of mineral wealth, hosting high-quality bituminous and anthracite coal.

The most spectacular event, however, occurred during the Late Cretaceous period (about 66 million years ago): the birth of the Deccan Traps. As the Indian plate migrated northward, it passed over the Reunion Hotspot. This triggered flood basalt eruptions through long fissures and cracks in the crust Fundamentals of Physical Geography, Chapter 3, p.24. Unlike the explosive volcanoes we see in movies, this was a relatively quiet but relentless pouring of highly fluid, low-viscosity tholeiitic basalt. Layer upon layer of lava spread across 5 lakh sq km, creating a stepped (or "trap") landscape that reaches thicknesses of over 3,000 meters in the Western Ghats Geography of India, Chapter 2, p.51.

Sources: Geography of India (Majid Husain), Chapter 1: Geological Structure and formation of India, p.4, 16, 18, 19; Geography of India (Majid Husain), Chapter 2: Physiography, p.51, 52; Fundamentals of Physical Geography (NCERT), Chapter 3: Interior of the Earth, p.24

5. The Reunion Hotspot and Plate Tectonics (exam-level)

To understand the geological face of modern India, we must look at the Reunion Hotspot — a stationary "blowtorch" of molten magma rising from deep within the Earth's mantle. Unlike most volcanoes that form at plate boundaries (like the Ring of Fire), hotspot volcanism occurs intra-plate, meaning it punches through the middle of a moving tectonic plate. Think of it like holding a moving piece of paper over a fixed candle flame; the flame will burn a trail of holes as the paper passes by Physical Geography by PMF IAS, Chapter 11, p.161.

During the Late Cretaceous period (roughly 66 million years ago), the Indian plate, drifting rapidly northward from the southern hemisphere, passed directly over this Reunion plume. The result was a geological event of cataclysmic proportions. The intense heat caused the crust to rift, separating India from the Seychelles Plateau, and triggered the outpouring of massive volumes of highly fluid, low-viscosity basaltic lava through long fissures in the Earth's crust Physical Geography by PMF IAS, Chapter 11, p.165. This wasn't a single explosive eruption but a series of flood basalt events that layered the landscape, creating the Deccan Traps (from the Swedish word 'Trappa' meaning steps). These lava flows eventually covered nearly 5 lakh sq km, with thicknesses reaching over 2,000 meters in the Western Ghats Geography of India, Majid Husain, Chapter 2, p.51.

The evidence of this journey is still visible today as a "breadcrumb trail" of volcanic features. As the Indian plate continued its northward journey toward the Eurasian plate, the hotspot continued to erupt, leaving behind a volcanic track. This track includes the Chagos-Laccadive Ridge (of which Lakshadweep is a part) and the Mascarene Plateau Physical Geography by PMF IAS, Chapter 11, p.165. Today, the hotspot sits dormant under the Island of Reunion in the Indian Ocean, but its legacy remains the fertile black cotton soil (Regur) of the Deccan, born from the weathering of those ancient basaltic rocks.

Sources: Physical Geography by PMF IAS, Chapter 11: Volcanism, p.161, 165; Geography of India, Majid Husain, Chapter 2: Physiography, p.51; Fundamentals of Physical Geography, NCERT, Chapter 3: Interior of the Earth, p.24, 34; Physical Geography by PMF IAS, Chapter 9: Convergent Boundary, p.121

6. The Deccan Traps: Structure and Soil (exam-level)

The Deccan Traps represent one of the most significant geological features of the Indian subcontinent, classified as a Large Igneous Province (LIP). The term 'Trap' is derived from the Swedish word 'trappa', meaning 'stairs,' which describes the characteristic step-like landscape of the region. This unique morphology was formed during the Late Cretaceous period (approximately 66 million years ago) when India, during its northward journey, passed over the Réunion hotspot. Unlike the explosive volcanoes we see in movies, this was flood basalt volcanism, where highly fluid, low-viscosity basaltic lava poured out through massive fissures in the Earth's crust, spreading over an area of about 5 lakh sq km Majid Husain, Geography of India, Chapter 1, p.19.

The structure of the Deccan Traps is essentially a massive pile of solidified lava flows. The thickness of these layers is greatest in the West, reaching over 3,000 meters near the Mumbai coast, and gradually thins out as you move towards the South and East Majid Husain, Geography of India, Chapter 1, p.20. A fascinating feature of this structure is the presence of Inter-trappean beds—sedimentary layers sandwiched between successive lava flows, which often contain fossils of the plants and animals that lived during the brief quiet periods between eruptions.

From a resource perspective, the weathering of these basaltic rocks over millions of years has produced the fertile Black Soil (Regur), which is rich in minerals like iron, magnesium, and aluminum but poor in nitrogen and phosphorus. While this soil is excellent for moisture retention (ideal for cotton), the underlying basaltic rock is quite dense. Consequently, the Deccan Trap region is often deficient in underground water, as water can only seep through cracks and fissures rather than through the rock itself Majid Husain, Geography of India, Chapter 3, p.44.

Sources: Geography of India ,Majid Husain, (McGrawHill 9th ed.), Geological Structure and formation of India, p.19; Geography of India ,Majid Husain, (McGrawHill 9th ed.), Geological Structure and formation of India, p.20; Geography of India ,Majid Husain, (McGrawHill 9th ed.), The Drainage System of India, p.44

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental types of volcanoes and India's geological timeline, this question brings those concepts together. To solve this, you must connect the viscosity of magma with the resulting landform scale. You learned that the Deccan Traps formed during the Late Cretaceous period as the Indian plate migrated over a hotspot. This event wasn't a localized explosion but a massive discharge of highly fluid, low-silica lava. As explained in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT Class XI, these types of eruptions cover thousands of square kilometers in thick, successive layers, which is the defining characteristic of a Large Igneous Province (LIP).

To reach the correct answer, think about the morphology of the Deccan region—it is a vast, step-like plateau (from the Swedish 'Trappa'), not a single peak. This tells you that the lava had to be extremely fluid to travel hundreds of kilometers from its source fissures. While Shield eruptions also involve fluid basalt, they typically build broad, sloped mountains. In contrast, (D) flood basalt eruption specifically describes the mechanism where lava flows out of fissures to drown the landscape in flat sheets. This perfectly matches the stratigraphic sequence of the Western Ghats described in Physical Geography by PMF IAS, where lava reached thicknesses of over 2,000 meters.

UPSC often uses similar-sounding volcanic terms to create traps. You can eliminate Composite eruption immediately because it involves high-viscosity, 'sticky' lava that creates steep, explosive cones—the exact opposite of the flat Deccan layers. A caldera eruption is a trap for those who confuse 'large scale' with 'explosive collapse'; a caldera occurs when a volcano cave in on itself, which does not produce a sub-continental plateau. By focusing on the fluidity and the immense area covered, you can see why the flood basalt eruption is the only logical choice for a formation that reshaped nearly half of the Indian subcontinent.