Volcanism and Earthquakes

UPSC tests this chapter almost entirely through specifics: which volcano is India's only active one, what a volcanic eruption actually releases, how P-waves and S-waves differ, and which parts of India carry the highest earthquake risk. General familiarity with "volcanoes are dangerous" or "earthquakes happen along faults" answers none of these. This note builds the mechanism from the ground up: how a volcano forms and which type it becomes, how an earthquake's energy travels and gets measured, and where India's own volcanic and seismic activity actually sits on that map.

Physical Geography Essentials covers the evidence for continental drift and the fold, block and residual mountain types. This note does not repeat that ground; the only plate-tectonic background it needs is that moving plates create most of the world's volcanoes and earthquakes, which is where each section below picks up.

Volcanoes: how they form and what shapes them

A volcano is a vent in the Earth's crust through which magma (molten rock beneath the surface) escapes as lava (the same material once it reaches the surface), along with gases and fragmented rock. What a given volcano actually looks like, a broad gentle dome or a steep explosive cone, depends almost entirely on the lava's viscosity (how easily it flows) and how much gas it carries, not on its size or fame.

Shield volcanoes are built from repeated flows of highly fluid, low-viscosity basaltic lava that spreads far before it cools, producing a broad, gently sloping profile that resembles a warrior's shield lying face up. Because the lava flows rather than explodes, shield volcanoes are typically low-explosivity; they turn dangerous mainly if water enters the vent and flashes to steam. Excluding flood basalt provinces, shield volcanoes are the largest volcanoes on Earth by volume. The Hawaiian volcanoes, Mauna Loa (the largest single volcano on Earth by volume) and Kilauea, are the standard textbook examples.

Composite volcanoes (also called stratovolcanoes) are the classic postcard-cone shape: steep-sided, symmetrical, built from alternating layers of lava, ash and pyroclastic debris. Their lava is cooler and more viscous than a shield volcano's, so gas cannot escape smoothly; eruptions are correspondingly more explosive, and large volumes of ash and rock fragments accumulate around the vent as distinct layers, which is exactly what gives composite volcanoes their steep, layered form. Mount Fuji (Japan), Cotopaxi (Ecuador) and Mount St Helens (United States) are named examples.

Cinder cones are the simplest type: built from a single vent that ejects gas-charged lava explosively into the air, where it breaks into small fragments (cinders) that fall back and pile up around the vent in a circular or oval cone. Cinder cones are small compared to shield or composite volcanoes and typically finish erupting within a short span, sometimes just months. Paricutin in Mexico is the standard example, growing from a farmer's field to over 400 metres within a year of its first eruption.

Calderas form when an eruption is so explosive that the emptied magma chamber beneath the volcano can no longer support the structure above it, and the volcano collapses in on itself rather than building a tall cone. Calderas mark the most explosive volcanoes on Earth.

Flood basalt provinces are a different category altogether: instead of building a cone, they release enormous volumes of highly fluid basaltic lava that spreads out over vast areas as successive horizontal flows, sometimes hundreds of kilometres long and, cumulatively, several kilometres thick. India's Deccan Traps are the textbook example, covered in detail below.

Active, dormant and extinct: a genuinely fuzzy line

Volcanoes are conventionally sorted into three categories by eruptive history. An active volcano is currently erupting, or has erupted recently enough to be considered capable of erupting again (a common working definition is any eruption within roughly the last 10,000 years, alongside ongoing seismic or gas activity as a live sign of an active system). A dormant volcano has not erupted recently but retains a connection to a magma source and could erupt again. An extinct volcano has shown no eruptive activity for a very long span and is not expected to erupt again, often because it has lost its magma supply entirely. Geologists themselves treat these as working risk categories rather than sharp, permanent labels, since volcanoes long assumed extinct have occasionally reawakened.

India's volcanism: Barren Island and the Deccan Traps

India has extremely limited active volcanism, and almost the entire chapter's India-specific content concentrates on two features.

Barren Island, in the Andaman Sea about 135 km north-east of Port Blair, is India's only confirmed active volcano. It sits on the same north-south volcanic arc running between Sumatra and Myanmar, produced by the subduction of the Indian Plate beneath the Burma microplate. Barren Island's first recorded eruption was in 1787, with further activity through the eighteenth and nineteenth centuries, before it fell quiet for over a century and erupted again in 1991, the eruption a real 2018 PYQ was built around. Since 1991 it has erupted repeatedly: in the mid-1990s, the mid-2000s, 2017 to 2019, 2022 and again in 2025. So the trap in that PYQ, the claim that Barren Island "remained completely inactive since 1991", is false on the facts: it has been intermittently active, not dormant, across the three and a half decades since. A short distance away on the same arc, Narcondam is classified as dormant to extinct, with no confirmed historical eruption, a useful statement-matching contrast to Barren Island's genuinely active status.

The Deccan Traps are a flood basalt province covering roughly 500,000 sq km, centred on the Maharashtra plateau and extending into Madhya Pradesh, Gujarat and Karnataka, with individual basalt layers stacking to a cumulative thickness of more than 2 km in places. The bulk of the eruptions occurred around 66 million years ago, in high-volume pulses spanning the Cretaceous-Palaeogene (K-Pg) boundary, the same boundary marked by the mass extinction that ended the age of dinosaurs. The Traps are attributed to a mantle plume, a hotspot, that sat beneath peninsular India at the time of eruption; as the Indian Plate has since drifted north-east, that same hotspot now sits beneath Reunion Island in the western Indian Ocean. The precise causal weight of Deccan volcanism in the K-Pg extinction is still debated alongside the Chicxulub asteroid impact in Mexico, the other major candidate cause, but current geochronological evidence places peak Deccan eruptive activity in the same narrow window as the extinction, so most current research treats sustained volcanic gas release (sulphur dioxide and carbon dioxide, among others) as a major contributing driver, alongside the impact rather than in place of it.

Intrusive volcanic landforms, briefly

Not all magma reaches the surface. Magma that cools while still within the crust forms intrusive (plutonic) landforms, as opposed to the extrusive (volcanic) rock formed by lava cooling at the surface. A large body of magma cooling slowly at great depth forms a batholith, a granite dome that becomes visible only once erosion strips away the overlying rock; much of the domal granite on the Karnataka plateau is an eroded batholith or the smaller, shallower lacolith, a dome-shaped intrusion fed by a narrow, pipe-like conduit. Magma spreading along a near-horizontal weak plane cools into a sill (the thicker deposit) or a sheet (the thinner one); where it bends upward into a saucer shape it is a lapolith; and a wavy intrusive mass at the base of a syncline or the crest of an anticline in folded rock is a phacolith. Where magma instead forces its way up through a vertical crack or fissure, it cools into a wall-like dyke, the most common intrusive form in western Maharashtra and the feeder structure behind many of the eruptions that built the Deccan Traps.

Earthquake mechanics: focus, epicentre and seismic waves

An earthquake is the shaking of the Earth caused by a sudden release of energy along a fault, a sharp break in crustal rock. Rocks on either side of a fault are pushed together or dragged past each other by ongoing stress; friction holds them locked in place until accumulated stress overcomes that friction, the rocks slip abruptly, and the stored energy releases as waves travelling outward in all directions. The point inside the Earth where this energy is released is the focus (or hypocentre); the point on the surface directly above the focus, the first to feel the waves, is the epicentre.

Earthquake waves come in two families. Body waves travel through the Earth's interior and are generated directly at the focus; they split into two types. P-waves (primary waves) are the faster of the two and always the first to arrive at a seismograph; they are longitudinal, meaning particles vibrate parallel to the direction the wave travels (the same mechanism as sound waves), and they can travel through solid, liquid and gaseous material alike. S-waves (secondary waves) arrive after a time lag; they are transverse, with particle vibration perpendicular to the wave's direction, and, critically, S-waves can travel only through solids. Surface waves are generated once body waves reach the surface and interact with surface rock; they travel along the surface rather than through the Earth's body, arrive last on a seismograph, and are the most destructive of the three, since most structural damage comes from surface shaking rather than the deeper body waves passing beneath a building.

S-waves' inability to cross liquid is exactly what let seismologists map the Earth's interior without ever drilling to it. Seismographs at increasing distance from an earthquake's epicentre record a shadow zone, an angular band where a given wave type simply never arrives. Between 105 degrees and 145 degrees from the epicentre, no direct P-waves are recorded, a band caused by P-waves refracting sharply as they cross into the core; beyond 105 degrees from the epicentre, no S-waves are recorded at all, a shadow zone covering a little over 40% of the Earth's surface. Because S-waves cannot pass through liquid, the total absence of S-waves beyond 105 degrees is the direct evidence that the Earth's outer core is liquid, while the narrower P-wave shadow zone reflects only the sharp refraction (bending) P-waves undergo at the liquid outer core's boundary, not an inability to cross it.

Magnitude versus intensity: how an earthquake gets measured

UPSC regularly tests the difference between these two, and they measure genuinely different things. Magnitude measures the energy released at the earthquake's source; it is a single number for a given earthquake, the same wherever it is reported. The original magnitude scale, developed in 1935 by Charles Richter, is the Richter scale, based on the amplitude of seismic waves recorded on a seismograph; it works well for smaller, more local earthquakes but loses accuracy (a phenomenon called saturation) for very large earthquakes, since amplitude alone stops scaling reliably with the enormous energy such quakes actually release. For that reason, seismological agencies today report large earthquakes on the Moment Magnitude Scale (Mw), calculated from the physical scale of the rupture itself, the rigidity of the rock that broke, the area of the fault that ruptured, and the average distance the fault slipped, combined into a single measure of total energy released. Both scales are logarithmic and open-ended in principle, but only the Moment Magnitude Scale is considered reliable at the very largest magnitudes, which is why it is now the standard for major earthquakes even though "Richter scale" remains the term most people use loosely for any earthquake magnitude.

Intensity, in contrast, measures the felt effects and visible damage at a specific place, and so varies from location to location for the very same earthquake, generally decreasing with distance from the epicentre. The standard intensity scale is named after the Italian seismologist Giuseppe Mercalli (used today in its Modified Mercalli Intensity form), and it runs from 1 to 12, based on observed damage and human perception rather than any instrument reading.

Plate boundaries and their characteristic seismicity

Earthquakes concentrate overwhelmingly along plate boundaries, and the three boundary types produce systematically different patterns. At divergent boundaries, where plates pull apart (as along the mid-ocean ridges), earthquakes are frequent but confined to a narrow zone and remain shallow-focus. At transform boundaries, where plates slide past each other horizontally (California's San Andreas Fault is the standard example), earthquakes are also shallow but can be severe, since built-up stress along a locked fault releases in a single abrupt slip. At convergent boundaries, particularly subduction zones where one plate is forced beneath another, earthquakes are the most abundant and cover the broadest range of depths, from shallow to deep-focus events as deep as 700 km within the descending slab, since the cold, rigid subducting plate stays brittle enough to fracture even where the surrounding mantle is too hot and ductile to do so. This is also why the world's most powerful earthquakes, magnitude 9 and above, occur almost exclusively at subduction zones, unlike transform boundaries, which rarely exceed magnitude 8.

India's seismic zonation: the four-zone map and the Bhuj exception

India's seismic risk is mapped by the Bureau of Indian Standards under IS 1893 into four zones, Zone II (lowest hazard) through Zone V (highest). Zone V, the most seismically active, covers roughly 11% of India's area; Zone IV covers around 18%; Zone III around 30%; and the remainder falls in Zone II. Combined, close to three-fifths of India's land area sits in Zones III, IV or V, facing at least a moderate earthquake hazard.

The Himalayan belt and India's north-eastern states dominate Zone IV and V precisely because that is where the Indian Plate is actively colliding with and being thrust beneath the Eurasian Plate, a convergent boundary generating continuous stress build-up (the mechanics of that collision are covered in Physical Geography Essentials, so this note does not re-derive them). The scale of that stress is not theoretical: within a roughly fifty-year span, the Himalayan belt produced four earthquakes exceeding magnitude 8, the 1897 Shillong, 1905 Kangra, 1934 Bihar-Nepal and 1950 Assam-Tibet earthquakes, a concentration of great earthquakes rarely matched anywhere else on Earth in so short a span. Zone V also extends beyond the Himalayan arc to cover Kutch in Gujarat and the Andaman and Nicobar Islands.

The 2001 Bhuj earthquake (26 January, Kutch district, Gujarat, itself in Zone V) is the chapter's other essential named event, worth knowing specifically because it breaks the plate-boundary pattern the rest of this section describes. At a moment magnitude of roughly 7.6 to 7.7, it killed approximately 20,000 people and remains one of the most destructive earthquakes India has recorded, yet it was not generated at a plate boundary at all; Kutch sits well within the interior of the Indian Plate. Earthquakes like this are called intraplate earthquakes, rarer and less understood than boundary earthquakes; the leading explanation for Kutch's own seismicity is that the region sits above old, buried rift structures from an earlier phase of continental breakup, structures that can still reactivate far from an active plate edge. Bhuj is the standard example UPSC reaches for whenever a question tests whether "earthquakes only happen at plate boundaries" holds up, and the honest answer it demonstrates is that this does not always hold.

Quick revision points

• Volcano types by lava and eruption style: shield (fluid basaltic lava, gentle slope, low explosivity, e.g. Mauna Loa), composite/stratovolcano (viscous lava, alternating ash-lava layers, explosive, e.g. Mount Fuji), cinder cone (single vent, ejected fragments, small and short-lived, e.g. Paricutin), caldera (explosive collapse of the magma chamber roof), flood basalt province (vast fluid basalt outpourings, e.g. the Deccan Traps).
• Active/dormant/extinct is a working classification, not a strict rule; some "extinct" volcanoes have reawakened.
• Barren Island (Andaman Sea) is India's only active volcano; it has erupted repeatedly since 1991, so a statement claiming it went dormant after 1991 is false. Narcondam, nearby, is dormant to extinct.
• Deccan Traps: flood basalt province, roughly 500,000 sq km, erupted mainly around 66 million years ago across the Cretaceous-Palaeogene boundary, formed over the hotspot now under Reunion Island, and a major contributor (alongside the Chicxulub impact) to the K-Pg mass extinction.
• Intrusive landforms: batholith (large, deep, granitic dome), lacolith (smaller dome, pipe conduit), sill/sheet (horizontal, thick/thin), lapolith (saucer-shaped), phacolith (wavy, at folds), dyke (vertical, through fractures; the main Deccan Trap feeder form).
• Focus (hypocentre) is where energy releases inside the Earth; epicentre is the surface point directly above it.
• P-waves: fastest, longitudinal, pass through solid, liquid and gas. S-waves: slower, transverse, solid only. Surface waves: slowest, most destructive.
• Shadow zones: no P-waves between 105 and 145 degrees from the epicentre; no S-waves at all beyond 105 degrees, proof the outer core is liquid.
• Magnitude (Richter for smaller quakes, Moment Magnitude for large ones) measures energy at the source and is one number per earthquake; intensity (Modified Mercalli scale, 1 to 12) measures felt damage and varies by location.
• Divergent and transform boundaries produce shallow earthquakes; convergent (subduction) boundaries produce the broadest depth range, up to 700 km, and the world's largest earthquakes.
• India's BIS seismic zonation has four zones (II to V); Zone V is roughly 11% of India's area and includes the Himalayan belt, the north-east, Kutch and the Andaman and Nicobar Islands.
• The Himalayan belt produced four earthquakes over magnitude 8 in about fifty years (1897, 1905, 1934, 1950).
• The 2001 Bhuj earthquake (moment magnitude roughly 7.6 to 7.7, Kutch, Zone V) was a rare major intraplate earthquake, not generated at a plate boundary, the standard exception UPSC uses to test that "boundary" reasoning.

Nearly every fact in this chapter shows up as a statement-based or matching-pair question rather than a direct "define this" prompt, so the habit that pays off most is pairing each named example, Barren Island, the Deccan Traps, Bhuj, with the specific mechanism or number attached to it, rather than just the general category it belongs to.