Earthquakes & Volcanoes – NDA Geography Notes

The Earth’s surface looks permanent and stable. But the Earth’s interior is anything but still. Tectonic plates are always moving. Stress builds up along faults and plate boundaries. Then: in seconds: that stress releases as an earthquake, or magma forces its way to the surface as a volcanic eruption.

These are not random events. They follow a clear pattern: earthquakes and volcanoes cluster along plate boundaries. Understanding why they occur where they do, how they are measured, and what they produce is the heart of this chapter.

NDA tests this chapter very heavily, from earthquake terminology and seismic wave properties to volcano-country pairs, to Richter scale facts and intrusive igneous structures. This chapter requires both concept understanding and precise factual recall.

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What is an Earthquake?

An earthquake is the sudden shaking of the ground caused by the rapid release of energy stored in rocks along faults or plate boundaries. The Earth’s tectonic plates are constantly being pushed and pulled. Stress builds up in rocks along faults. For a long time, friction keeps the rocks locked in place. Then suddenly the stress exceeds the strength of the rock. The rocks slip, releasing enormous energy as seismic waves that travel through the Earth and shake the ground.

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Earthquake Terminology

Focus (Hypocentre) is the point inside the Earth where the earthquake actually begins, where rocks first slip and energy is released. [NDA 2021-I]

The focus is always underground. It is the origin point inside the Earth. The hypocentre and focus are exactly the same thing. They are synonyms. [NDA 2023-I]

Epicentre is the point on the Earth’s surface directly above the focus. It is usually where shaking is most intense, because the energy reaches there with the shortest path through the Earth. [NDA 2024-I]

The statement “the hypocentre is the point on the surface nearest to the focus” is incorrect. The surface point above the focus is the epicentre, not the hypocentre. Hypocentre = focus = inside the Earth. [NDA 2023-I]

Depth of Focus

Shallow-focus earthquakes: focus less than 70 km deep: are the most common and cause the most damage.

Intermediate-focus: 70 to 300 km deep.

Deep-focus: more than 300 km deep. Deep-focus earthquakes cause less damage than shallow-focus, because the energy has farther to travel and dissipates more before reaching the surface. [NDA 2024-I]

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Seismic Waves

When an earthquake occurs, it sends waves in all directions through the Earth. These are called seismic waves. There are three main types.

P Waves (Primary Waves)

P waves are the fastest seismic waves: they are the first to arrive at a seismograph after an earthquake, which is why they are called Primary waves. [NDA 2023-I]

P waves are longitudinal waves: they compress and expand the material in the same direction as the wave travels (push-pull, like a spring). P waves can travel through solids, liquids, and gases, all states of matter. This is why P waves pass through the entire Earth including the liquid outer core.

The velocity of earthquake waves is higher in denser materials. [NDA 2023-I]

S Waves (Secondary Waves)

S waves are slower than P waves. They arrive at the seismograph after P waves.

S waves are transverse waves: the material moves perpendicular to the direction of wave travel (like shaking a rope up and down while it moves forward). S waves can travel only through solids: they cannot pass through liquids or gases. This is why scientists concluded that the outer core is liquid: S waves disappear when they reach it.

S waves are more destructive to buildings than P waves, because they shake the ground sideways (transverse motion), which is more damaging to structures. S waves create troughs and crests in the material they travel through; their direction of vibration is perpendicular to the wave direction.

L Waves (Surface Waves / Long Waves)

L waves travel along the Earth’s surface rather than through the interior. They are the slowest of all seismic waves but the most destructive: they cause most of the ground shaking and building damage during an earthquake. [NDA 2025-I]

L waves follow the Earth’s circumference and travel at a roughly constant rate. [NDA 2025-I]

Wave	Speed	Travels Through	Motion	Destruction	NDA Tag
P (Primary)	Fastest	Solid + Liquid + Gas	Compressional (longitudinal)	Moderate	[NDA 2023-I]
S (Secondary)	Medium	Solid only	Transverse (⊥ to direction)	High	—
L (Surface)	Slowest	Surface only	Along surface	Highest	[NDA 2025-I]

★ IMPORTANT Two key facts from seismic waves that NDA tests together: P waves arrive first (fastest) and travel through everything. S waves arrive second and travel through solids only, proving the outer core is liquid. L waves arrive last but are the most destructive. [NDA 2023-I | NDA 2025-I]

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Shadow Zones

When an earthquake occurs, seismic waves spread outward through the Earth. In certain areas, no waves arrive. These are called shadow zones.

Shadow zone for P waves: P waves are refracted (bent) as they enter the liquid outer core, creating a ring-shaped shadow zone between about 105° and 145° from the epicentre.

Shadow zone for S waves: S waves cannot pass through the liquid outer core at all, creating a very large shadow zone starting at about 103° from the epicentre covering a large area on the opposite side of the Earth.

The statement that the zone between 105° and 145° is the shadow zone for both P and S waves is incorrect: this range is specifically for P waves only. [NDA 2025-II]

The statement that the P wave shadow zone is much larger than the S wave shadow zone is also incorrect: the S wave shadow zone is actually larger because S waves cannot pass through the entire outer core. [NDA 2025-II]

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The Richter Scale

The Richter Scale measures the magnitude of an earthquake, the amount of energy released.

All three of the following statements about the Richter Scale are correct: [NDA 2019-I]

It was devised in 1935 by Charles F. Richter: TRUE. It describes the quantity of energy released by a single earthquake: TRUE. The Richter Scale has no upper limit: TRUE (theoretically unlimited; largest recorded earthquakes are around magnitude 9.5).

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Diastrophism: Folds and Faults

Diastrophism refers to all large-scale deformation of the Earth’s crust through folding and faulting. These forces are so slow and gradual that they pass unnoticed for a long period of time.

When rocks are compressed, they can either fold or fracture.

Anticline: a fold that arches upward (like an inverted U). [NDA 2015-II]

Syncline: a fold that bends downward (like a bowl).

Recumbent Fold: a fold so extreme that the axial plane (the plane dividing the fold into two halves) is nearly horizontal: the fold has been pushed over on its side. [NDA 2019-II]

Graben: a block of land that has sunk between two parallel faults: forming a rift valley.

Horst: a block of land that has been uplifted between two parallel faults: forming a block mountain.

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Volcanoes

A volcano is an opening or vent in the Earth’s crust through which magma, gas, and ash escape to the surface. Most volcanoes occur along plate boundaries: at convergent boundaries (where subducting oceanic plates melt) and at divergent boundaries (where plates pull apart). Some volcanoes sit above stationary hot spots: plumes of hot mantle material, like the Hawaiian Islands.

The Himalayas have no volcanoes, as they formed by continental-continental collision with no subduction, so no magma is generated.

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Types of Volcanoes

Shield Volcano: low, broad, gently sloping (like a shield lying on the ground). Built up by many thin lava flows. Lava is runny (low viscosity) and flows easily. Not very explosive. Individual lava flows are only a few feet thick, but repeated flows over a long period build up the volcano, hence the name shield volcano. Example: Mauna Loa, Hawaii.

Composite Volcano (Strato-Volcano): tall, steep-sided, cone-shaped. Built from alternating layers of lava AND pyroclastic material (ash, cinders, bombs). Lava is thick (high viscosity). Can be very explosive. Called strato-cones because of their alternating sheets of lava and pyroclastic materials. Examples: Mount Fuji (Japan), Mayon (Philippines), Mount Vesuvius (Italy).

Arc Volcanoes: form above subducting plates at convergent boundaries. Mount Fuji and Mayon are classical examples. Some of the highest volcanoes, like Nevado Ojos del Salado in the Chilean Andes, are arc type.

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Volcanic Features and Intrusive Structures

Caldera: a large, circular depression that forms when a volcano’s summit collapses into itself after a massive eruption empties the magma chamber below. Much larger than a regular volcanic crater. A caldera is a product of vulcanicity, a direct result of volcanic activity. [NDA 2008-II]

Hawaiian Islands were NOT formed at a mid-oceanic ridge. They are a hot-spot chain: the Pacific Plate moved over a stationary hot spot in the mantle, creating a chain of volcanic islands. The Azores, Ascension Islands, and Tristan da Cunha are associated with mid-oceanic ridges. [NDA 2025-II]

Intrusive Igneous Structures

When magma cools underground without reaching the surface, it forms intrusive features, visible only after millions of years of erosion expose them.

Batholith: the largest intrusive feature; a massive dome of solidified magma at great depth. The deepest and innermost intrusive igneous rock. [NDA 2021-I | NDA 2023-I]

Laccolith: dome-shaped intrusion where magma pushes up the overlying rock layers.

Lopolith: saucer-shaped intrusion, concave upward.

Phacolith: lens-shaped intrusion that forms in the folds of rocks.

From deepest to shallowest: Batholith → Lopolith → Laccolith → Phacolith → Sill → Dyke

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Important Volcano-Country Pairs

Volcano	Country	Key Notes	NDA Tag
Mount Merapi	Indonesia	Active; on island of Java	[NDA 2007-I
Semeru	Indonesia	Highest volcano in Java	—
Mount Etna	Italy	Active; on Sicily	[NDA 2009-I]
Kilimanjaro	Tanzania	Africa’s highest peak; NOT Kenya	[NDA 2009-I]
Katmai	USA (Alaska)	Active volcano	[NDA 2009-I]
Barren Island	India	India’s only active volcano; Andaman Islands	[NDA 2009-I]
Cotopaxi	Ecuador	High active volcano; NOT Chile	—
Mauna Loa	USA (Hawaii)	Shield volcano; hot spot	—
Mount Vesuvius	Italy	Destroyed Pompeii in 79 CE	—
Guallatiri	Chile	Active Andean arc volcano	—

Mount Merapi is on the island of Java in Indonesia: NOT Sumatra, Borneo, or Sulawesi. [NDA 2007-I | 2007-II]

Cotopaxi is in Ecuador, NOT Chile. This is a common confusion.

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The Deccan Traps

The Deccan Traps are a large volcanic province covering much of Maharashtra and neighbouring states. They formed about 66 million years ago (end of Cretaceous Period) through massive fissure eruptions, where lava poured out through long cracks in the crust rather than through a single volcanic cone. [NDA 2011-II]

The lava spread out in horizontal sheets. [NDA 2011-II]

The regur soil (black cotton soil) formed from the Deccan Traps is poor in nitrogen: not rich. [NDA 2011-II]

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