Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth

1. How we know the interior: direct and indirect sources

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• The earth's radius is about 6,378 km at the equator (mean about 6,371 km). No one can reach the centre, so most of what we know is estimates and inferences (Class 11, Interior of the Earth).
• Direct sources of information about the interior (material we can actually see and test):
• surface rocks and mine rocks. South African gold mines go 3-4 km deep. Deeper than that it is too hot to work.
• deep drilling, such as the "Deep Ocean Drilling Project" and the "Integrated Ocean Drilling Project".

• magma thrown out by volcanoes. It can be tested in a laboratory, but we cannot tell how deep it came from.

• Kola Superdeep Borehole: the deepest hole ever drilled. It is on the Kola Peninsula in Arctic Russia and reached 12,262 m in 1989. It found about 180 °C at the bottom, far hotter than expected (NCERT: "Kola, in Arctic Ocean, 12 km"; actually on land, about 12.2 km).

• Indirect sources of information about the interior (clues we read from outside):
• Temperature, pressure and density all rise with depth.
• The geothermal gradient (how fast temperature rises as you go down) is about 25-30 °C per km in the upper crust and slows deeper down. The centre is about 5,000-6,000 °C.
• Meteors do not come from the interior, but they are made of similar material. Iron meteorites look like the core.
• Gravity anomaly: the gap between the gravity we measure and the gravity we expect. Gravity (g) is higher at the poles than at the equator because the equator is farther from the centre. Anomalies show how mass is spread unevenly in the crust.
• Magnetic surveys show where magnetic materials lie in the crust.

• Seismic waves are the most important clue (Sections 2-4).

• India: the Ministry of Earth Sciences runs a Scientific Deep Drilling project at Koyna (Maharashtra) to study reservoir-triggered quakes. Its pilot borehole is about 3 km deep (verify current depth).

2. Why the earth shakes: faults, focus and epicentre

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• An earthquake is the shaking of the ground. Energy is released suddenly, usually along faults, and sends waves out in all directions.
• How a quake happens (the steps):
• A fault is a sharp break in crustal rocks. The blocks on each side try to move in opposite directions.
• The weight of the rocks above and friction lock them together, so stress builds up.

• The stress beats the friction. The blocks bend and then slip suddenly, releasing energy. This is called elastic rebound (Reid, 1910, after the 1906 San Francisco quake; beyond NCERT).

• Focus (also called the hypocentre): the point inside the earth where the energy is first released.

• Epicentre: the point on the surface directly above the focus. It feels the waves first and is usually hit hardest.
• All natural earthquakes happen in the lithosphere, which NCERT puts at up to 200 km deep.
• Shallow-focus and deep-focus earthquakes (beyond NCERT): quakes are grouped by focus depth as shallow (0-70 km), intermediate (70-300 km) or deep (300-700 km).
• Under mid-ocean ridges, the foci are shallow.
• Along the Alpine-Himalayan belt and the rim of the Pacific, the foci are deep.
• This difference was an early clue to subduction (Class 11, Distribution of Oceans and Continents).

3. Earthquake waves: body waves, surface waves and the seismograph

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• A seismograph records the waves that reach the surface. Its record has three parts: P-waves arrive first, then S-waves, then surface waves.
• Body waves start at the focus and travel through the inside of the earth. There are two kinds, P and S.
• Reflection and refraction of seismic waves: denser material makes waves travel faster. Where the material changes, waves either bounce back (reflection) or bend (refraction). These changes show where the boundaries between layers are.

	P-waves (primary)	S-waves (secondary)	Surface waves
Arrival	First (fastest)	Second, after a time lag	Last
Pass through	Solids, liquids and gases (like sound)	Solids only	Along the surface
Vibration	Parallel to the direction of travel; squeeze and stretch the rock	At right angles to the direction of travel, in the vertical plane; make crests and troughs	At right angles to the direction of travel
Damage	Low	Moderate	Most destructive

• Surface waves form when body waves hit surface rocks. They shift rocks and bring buildings down. There are two kinds (beyond NCERT): Love waves, which shake the ground side to side, and Rayleigh waves, which roll like ocean waves.
• Locating a quake (beyond NCERT): the gap between the S-wave and P-wave arrival times at three stations fixes the epicentre. This is called triangulation.

4. Shadow zones and what they reveal

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• A shadow zone is an area where seismographs record no direct waves from a particular earthquake. Each earthquake has its own shadow zone.
• What the seismographs record (Class 11, Interior of the Earth):
• Within 105° of the epicentre: both P- and S-waves.
• Between 105° and 145°: neither wave. This band is the shadow zone for both.

• Beyond 145°: P-waves but no S-waves.

• S-wave shadow zone: everything beyond 105°, a little over 40 per cent of the earth's surface. S-waves cannot pass through the liquid outer core, so they stop there.

• P-wave shadow zone: a band around the earth between 105° and 145°. P-waves bend sharply when they enter the denser liquid core. This leaves a gap, and they come back to the surface beyond 145°.
• What it reveals:
• The missing S-waves show that the outer core is liquid.

• The bending of P-waves shows the size of the core.

• Inge Lehmann (1936) noticed faint P-waves inside the P-wave shadow zone. From this she worked out that there is a solid inner core that reflects them (Section 8; beyond NCERT).

• If you know where the epicentre is, you can draw its shadow zone.

5. Types, measurement and effects of earthquakes

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• Types of earthquakes (NCERT lists five):

Type	Cause	Indian example (beyond NCERT)
Tectonic earthquake (most common)	Rocks slide along a fault plane	Bhuj, 26 Jan 2001 (Mw 7.7); Latur-Killari, 30 Sep 1993 (M 6.2, in the stable peninsula)
Volcanic	A special kind of tectonic quake, found only near active volcanoes	—
Collapse	Roofs of underground mines cave in; minor tremors	Mining belts
Explosion	Chemical or nuclear blasts	Pokhran tests (1974, 1998)
Reservoir-induced	Near large reservoirs	Koyna, 11 Dec 1967 (M about 6.3)

• Magnitude vs intensity:
• Richter scale (Charles Richter, 1935) measures magnitude, which relates to the energy released. NCERT writes it as 0-10, but the scale has no fixed top. It is logarithmic: each whole number means 10 times the ground motion and about 32 times the energy.
• Mercalli scale (Giuseppe Mercalli, Italy) measures intensity, rated I-XII by the damage you can see. Intensity changes with distance from the epicentre.

• Moment magnitude scale (Mw; Hanks and Kanamori, 1979) uses the fault area, the amount of slip and the strength of the rock. It has replaced the Richter scale for large quakes because the Richter scale stops rising (saturates) at very large sizes. India's NCS reports magnitudes and locations.

• Effects (NCERT lists 12): ground shaking, differential ground settlement, land and mud slides, soil liquefaction (wet soil acting like a liquid), ground lurching, avalanches, ground displacement, floods from dam and levee failures, fires, structural collapse, falling objects and tsunami. The first six shape landforms.

• A tsunami follows only when the epicentre is under the sea and the magnitude is high enough. Serious damage starts above about magnitude 5.
• Quakes of magnitude 8 or more happen about once in 1-2 years. "Tiny" ones happen almost every minute.
• Varahamihira's Brihatsamhita has an early Indian account of earthquakes ("bhukampa") (Class 9).
• India's seismic zones and NDMA guidelines are covered in natural-hazards-disasters.

6. Where earthquakes strike: global belts, the Himalaya and recent great earthquakes

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• Global belts (Class 11, Distribution of Oceans and Continents; Class 9):
• a line down the mid-Atlantic that continues into the Indian Ocean. South of India it splits: one branch goes to East Africa and the other joins a Myanmar-New Guinea line.
• the Alpine-Himalayan belt.

• the rim of the Pacific, which releases most of the world's seismic energy (beyond NCERT).

• Alpine-Himalayan belt: runs from the Alps through Turkey and Iran to the Himalayas, Myanmar and Indonesia. It forms where the African, Arabian and Indian plates push into the Eurasian plate. Its quakes are often deep-seated.

• Great Himalayan quakes:
• Kangra (1905).
• Bihar-Nepal (1934).
• Assam-Tibet earthquake (1950): 15 August 1950, about M 8.6, the strongest ever recorded on land. Its landslides blocked rivers in Arunachal and Assam, which later flooded.

• Gorkha earthquake (2015): 25 April 2015, central Nepal, Mw 7.8, about 9,000 dead, avalanches on Everest. It was caused by thrust movement (one block pushed up over another) on the Main Himalayan Thrust.

• Seismic gap: a stretch of an active fault that has had no big quake for a long time, so one is thought likely. The standard example is the "central seismic gap" between the 1905 and 1934 ruptures (Uttarakhand and western Nepal).

• Recent great quakes elsewhere:
• Sumatra-Andaman, 26 Dec 2004, about Mw 9.1, which caused the Indian Ocean tsunami.
• Tohoku, Japan, 11 Mar 2011, Mw 9.0-9.1, which caused a tsunami and the Fukushima nuclear accident. It was a thrust quake.
• Turkey-Syria earthquake (2023): 6 Feb 2023, Mw 7.8 on the East Anatolian Fault, followed by Mw 7.5 hours later. Over 50,000 died. India sent relief under Operation Dost.
• Myanmar earthquake (2025): 28 Mar 2025, Mw 7.7 on the Sagaing Fault near Mandalay. The Sagaing Fault is a strike-slip fault (the blocks slide past each other sideways). India sent relief under Operation Brahma.

7. Structure of the earth I: crust, mantle, lithosphere and asthenosphere

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• The crust is the outermost layer. It is solid and brittle.
• Oceanic crust: mean about 5 km thick (5-10 km). It is made of dense basalt (sima: silica + magnesium). It is nowhere older than about 200 million years, because it is always being made at ridges and destroyed at trenches.

• Continental crust: about 30 km thick on average and up to 70 km under the Himalayas. It is lighter and mostly granite (sial: silica + aluminium). Its oldest rocks are about 4 billion years old (NCERT: over 3,200 million years).

• Conrad discontinuity: a boundary inside the continental crust between the granitic upper crust and the basaltic lower crust. It is not found everywhere.

• Mohorovicic discontinuity (Moho): the boundary between the crust and the mantle, where seismic wave speeds jump sharply. Andrija Mohorovicic (Croatia) found it in 1909. It lies 5-10 km below the ocean floor and 30-70 km below the continents.
• Mantle: runs from the Moho down to 2,900 km. It is denser than the crust and makes up about 84 per cent of the earth's volume. It is silicate rock rich in iron and magnesium.
• Asthenosphere ("astheno" means weak): a weak, plastic, partly molten zone in the upper mantle, down to about 400 km (NCERT). It is the main source of magma, and the plates move over it. The lower mantle below it is solid.
• Lithosphere: the crust plus the uppermost mantle, 10-200 km thick. It is thinner under oceans (5-100 km) and about 200 km under continents. It is broken into plates.
• Trap: the lithosphere is "crust and upper mantle", NOT "crust and core".

8. Structure of the earth II: the core and the magnetic field

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• Core: from 2,900 km down to the centre at about 6,371 km. It is made mostly of nickel and iron, so it is called nife. Seismic wave speeds revealed it.
• Outer core: liquid, from 2,900 to 5,150 km. S-waves cannot pass through it.
• Inner core: solid, from 5,150 to 6,371 km. It is about 5,000-6,000 °C, but the huge pressure keeps it solid.
• Gutenberg discontinuity: the boundary between the mantle and the core at about 2,900 km. Beno Gutenberg found it in 1913. Here S-waves stop, and P-waves slow down and bend.
• Lehmann discontinuity: the boundary between the outer and inner core at about 5,150 km. Inge Lehmann (Denmark) found it in 1936.

Layer	Depth (km)	State	Main material	Boundary at its base
Crust	0 to 5 (ocean) / 30 (continent) / 70 (Himalaya)	Solid, brittle	Basalt; granite	Moho
Mantle	Moho to 2,900	Solid; asthenosphere is plastic	Fe-Mg silicates	Gutenberg
Outer core	2,900-5,150	Liquid	Fe-Ni	Lehmann
Inner core	5,150-6,371	Solid	Fe-Ni	—

• How the magnetic field is made (the geodynamo):
• Heat and the earth's spin stir the liquid iron in the outer core.
• The moving metal acts like a dynamo and produces the geomagnetic field.

• This field forms the magnetosphere around the earth.

• Geomagnetic reversal: the north and south magnetic poles swap places. This has happened hundreds of times. The last full reversal (Brunhes-Matuyama) was about 780,000 years ago.

• Lava cooling at ridges records each flip as magnetic stripes, which is evidence for sea-floor spreading.

• The magnetic north pole is drifting from the Canadian Arctic towards Siberia (verify current). The field is weak over the South Atlantic Anomaly.

9. Volcanoes: materials and types

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• A volcano is an opening where gases, ash and/or molten rock escape to the surface. Volcanoes are classed by how they erupt and the shape they build.
• Magma is molten rock in the upper mantle and crust, coming mainly from the asthenosphere. Once it moves towards or reaches the surface, it is called lava:
• fluid basaltic lava flows far and builds gentle slopes.

• viscous (thick, sticky) lava causes explosions and builds steep cones.

• Pyroclastic material: rock pieces, ash, dust and volcanic bombs thrown out in explosive eruptions. It can form deadly pyroclastic flows.

• The gases include nitrogen and sulphur compounds, with small amounts of chlorine, hydrogen and argon.
• Active volcano: one that is erupting or has erupted recently (Barren Island). A dormant volcano is quiet but may erupt again (Narcondam). An extinct volcano is not expected to erupt again.
• Types:
• Shield volcano: the largest volcanoes apart from flood basalts. Broad, gentle domes of fluid basalt. Low explosivity unless water gets into the vent. Examples: Mauna Loa and Kilauea (Hawaii).
• Cinder cone: a small, steep cone built when lava fountains out and falls back as cinders around the vent.
• Composite volcano: cooler, viscous lava and explosive eruptions. Steep cones made of alternating layers of lava and pyroclastic material. Examples: Fuji, Vesuvius, St Helens.
• Caldera: the most explosive type. The volcano collapses into its own emptied magma chamber, which shows a huge chamber lies close below. Examples: Yellowstone, Toba, Krakatoa.
• Supervolcano: can throw out more than 1,000 cubic km of material (VEI 8). Examples: Yellowstone and Toba (about 74,000 years ago; its ash reached India).
• Flood basalt province: very fluid lava pours from fissures (long cracks) over thousands of sq km. Flows are over 50 m thick and can be hundreds of km long. Examples: the Deccan Traps (about 66 million years ago, covering most of the Maharashtra plateau and once much larger), the Columbia Plateau and the Siberian Traps.
• Mid-ocean ridge volcanoes: along a ridge system over 70,000 km long, with frequent eruptions at its centre.

10. Intrusive volcanic landforms

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• Magma that cools at the surface makes volcanic rocks. Magma that cools inside the crust makes plutonic rocks.
• Intrusive volcanic landforms are the shapes magma takes when it cools inside the crust. They show at the surface only after erosion strips away the rock above.

Form	Shape and setting	Example
Batholith	Very large, deep granite dome, several km deep; a cooled magma chamber	Exposed granite masses
Laccolith	Dome with a flat base, fed by a pipe from below, like an underground composite dome	Exfoliated granite domal hills of the Karnataka plateau (laccoliths or batholiths)
Lopolith (NCERT: "lapolith")	Saucer shape, concave to the sky; magma spreads sideways along a weak plane and sags	Bushveld complex, South Africa (platinum, chromium)
Phacolith	Wavy, lens-shaped mass at the base of synclines (downfolds) or the top of anticlines (upfolds), with a pipe down to a magma chamber	Folded igneous country
Sill / sheet	Near-horizontal layer between rock beds; thick = sill, thin = sheet	—
Dyke	Wall-like body; magma fills cracks and sets almost upright	Western Maharashtra; the feeders of the Deccan Traps

• Sill vs dyke: a sill is concordant (lies parallel to the rock layers). A dyke is discordant (cuts across them).
• Extrusive forms (lava plateaus, cones, craters, calderas) are covered in Section 9. How these landforms change under erosion is covered in landforms-evolution.

11. Where volcanoes are: the Ring of Fire, hotspots, India's volcanoes and eruptions that change climate

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• Volcanoes follow the same belts as earthquakes: the mid-ocean ridges, the Alpine-Himalayan zone (Etna, Vesuvius) and, most of all, the Pacific rim.
• Ring of Fire: a horseshoe-shaped belt of volcanoes and frequent quakes around the Pacific, along plate edges that are pushing together. It runs from the Andes through Alaska, Japan and the Philippines to Indonesia and New Zealand. Most active volcanoes and most tsunamis are here.
• Hotspot volcanism: volcanoes away from plate edges, fed by a plume of very hot mantle rock rising under a moving plate.
• The hotspot stays fixed while the plate moves over it.
• This leaves a chain of volcanoes that get older with distance from the hotspot.

• Examples: Hawaii-Emperor chain, Iceland (a hotspot on a ridge), Yellowstone, Reunion (linked to the Deccan Traps).

• Barren Island volcano: India's only confirmed active volcano. It is in the Andaman Sea, about 140 km north-east of Port Blair (Class 7, Geographical Diversity of India). It sits where the Indian plate subducts (slides down) beneath the Burma microplate. Eruptions are recorded from 1787 and repeatedly since 1991 (verify latest activity).

• Narcondam, to the north of Barren Island, is generally classed as dormant.
• Mud volcano: a vent that pushes out mud, water and gases (mainly methane) under underground pressure, not molten lava. India's best known is on Baratang Island, Andamans (Class 9). It has been active since 2003, with renewed activity after the 2004 earthquake (verify latest).
• Mount Pinatubo eruption (1991): Philippines, June 1991. It put about 15-20 million tonnes of SO₂ into the stratosphere. The sulphate haze cooled the global average temperature by about 0.5 °C for 1-2 years.
• Hunga Tonga-Hunga Ha'apai eruption (2022): Tonga, 15 Jan 2022. It was the largest explosion recorded by modern instruments. It sent a tsunami across the Pacific and pressure waves round the globe several times. It also put huge amounts of water vapour into the stratosphere, which warms the earth, unlike Pinatubo's cooling sulphur.
• Tambora (1815) caused "the year without a summer" (1816).

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Exam angles

Prelims: high-yield facts and traps

• Direct sources: mines (3-4 km), drilling (Kola, about 12.2 km, 1989), volcanic material. "Volcanoes are an indirect source": FALSE (NCERT exercise).
• Indirect sources: temperature, pressure, density, meteors, gravity anomaly, magnetic surveys, seismic waves. Gravity is higher at the poles than at the equator.
• P-waves pass through solids, liquids and gases and vibrate parallel to travel. S-waves pass through solids only and vibrate at right angles. Surface waves arrive last and do the most damage.
• Shadow zone: no P- or S-waves at 105°-145°. No S-waves anywhere beyond 105° (a little over 40 per cent of the surface).
• Boundaries, top to bottom: Conrad (inside continental crust) → Moho (crust/mantle, 1909) → Gutenberg (mantle/core, 2,900 km, 1913) → Lehmann (outer/inner core, about 5,150 km, 1936).
• Lithosphere = crust + uppermost mantle. "Crust and core": FALSE. Asthenosphere goes to about 400 km and is the main magma source. Core = nife.
• Richter measures magnitude (energy). Mercalli measures intensity (damage, I-XII). "Richter is capped at 10": FALSE, it has no fixed top. Each unit is about 32 times the energy. Mw is now used for large quakes.
• Five earthquake types. Koyna (1967) = reservoir-induced. Latur (1993) = stable peninsula. Bhuj (2001) = Mw 7.7.
• Thrust quakes: Gorkha, Tohoku. Strike-slip quakes: Turkey (East Anatolian Fault), Myanmar (Sagaing Fault).
• Volcano type and example: shield (Hawaii), composite (Fuji, Vesuvius), caldera (Toba, Yellowstone), flood basalt (Deccan Traps; "flood" eruptions per NCERT exercise).
• Intrusive forms: batholith (granite dome), laccolith (flat base; Karnataka domes), lopolith (saucer), phacolith (wavy, in folds), sill (horizontal, concordant), dyke (upright, discordant; Deccan feeders).
• Barren Island = India's only active volcano. Narcondam = dormant. Baratang = mud volcano. "Kola is in the Arctic Ocean": FALSE, it is on the Kola Peninsula.

Mains: GS-I themes

1. How have seismic waves helped us understand the earth's interior? Explain using shadow zones.
2. Why is the Himalaya so prone to earthquakes? Discuss seismic gaps and the risk to dense settlements and hydropower.
3. Magnitude vs intensity: why can two quakes of the same magnitude cause very different damage?
4. The distribution of earthquakes and volcanoes as evidence for plate tectonics.
5. Volcanic eruptions as climate forcers: compare Pinatubo (1991) and Hunga Tonga (2022).
6. Intrusive volcanic landforms and how they show up in Indian landscapes (Karnataka domes, Deccan dykes).

Current-affairs hooks

• Major earthquakes and India's disaster diplomacy: Operation Dost (2023), Operation Brahma (2025). Verify any newer events.
• Himalayan tremors reported by NCS, and the debate over building in seismic zones (verify current).
• Revisions to India's seismic zoning map and building code, BIS IS 1893 (verify current edition and zones).
• Activity reports on Barren Island and Baratang (verify current).
• Large eruptions and their climate effects, including Iceland's Reykjanes eruptions since 2021 (verify current).
• Koyna deep drilling and ocean drilling expeditions (verify current).