Earthquakes & Volcanoes – NDA Geography Notes

Introduction

The Earth’s surface looks permanent and stable to us. Mountains seem fixed. The ground feels solid. 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 heavily. Questions come from earthquake terminology, seismic wave properties, volcanic features, specific volcano-country pairs, and the Richter scale. This chapter requires both concept understanding and precise factual recall.

Understand the Topic

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):
The focus — also called the hypocentre — is the point inside the Earth where the earthquake actually begins. This is where the rocks first slip and energy is released. [NDA 2021-I]

Students frequently confuse focus and epicentre. The focus is always underground — it is the origin point inside the Earth. [NDA 2023-I]

Epicentre:
The epicentre is the point on the Earth’s surface directly above the focus. It is usually where shaking is most intense. The epicentre is always on the surface — directly above the underground focus. [NDA 2024-I | NDA tested via CDS]

The term hypocentre means exactly the same as focus — the origin point inside the Earth. Saying “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. [NDA 2023-I]

Depth of Focus:

• Shallow-focus earthquakes — focus less than 70 km deep — most common and cause 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]

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. Think of pushing and pulling a spring.

P waves can travel through solids, liquids, and gases. This is critical — because P waves travel through all states of matter, they pass through the entire Earth including the liquid outer core. [NDA tested via CDS]

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. [NDA tested via CDS]

S waves are transverse waves — the material moves perpendicular to the direction of wave travel. Think of 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. [NDA tested via CDS]

S waves are more destructive to buildings than P waves — because they shake the ground sideways, which is more damaging to structures. [NDA tested via CDS]

S waves create troughs and crests in the material they travel through. Their direction of vibration is perpendicular to the wave direction. [NDA tested via CDS]

L Waves (Surface Waves or 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 roughly constant speed. [NDA 2025-I]

Summary of Seismic Wave Properties:

Wave	Speed	Travels Through	Motion	Destruction
P (Primary)	Fastest	Solid + Liquid + Gas	Compressional	Moderate
S (Secondary)	Medium	Solid only	Transverse	High
L (Surface/Long)	Slowest	Surface only	Along surface	Highest

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Shadow Zones

When an earthquake occurs, seismic waves spread outward through the Earth. Scientists noticed that 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. This creates a ring-shaped shadow zone for P waves between about 105° and 145° from the epicentre.

Shadow zone for S waves:
S waves cannot pass through the liquid outer core at all. So there is a very large shadow zone for S waves starting at about 103° from the epicentre and 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 the shadow zone for P waves. [NDA 2025-II]

The statement that “the shadow zone of P waves is much larger than that of S waves” 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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Richter Scale

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

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

1. It was devised in 1935 by Charles F. Richter — TRUE
2. It describes the quantity of energy released by a single earthquake — TRUE
3. The Richter Scale has no upper limit — TRUE (theoretically there is no maximum, though the largest recorded earthquakes are around magnitude 9.5)

Diastrophism and Folds

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. [NDA tested via CDS]

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

Anticline — a fold that arches upward. [NDA 2015-II]

Syncline — a fold that bends downward.

Recumbent Fold — a fold so extreme that the axial plane (the plane dividing the fold) is nearly horizontal. [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 are not at plate boundaries but sit above stationary hot spots — plumes of hot mantle material.

The Himalayas have no volcanoes — they were formed by continental-continental collision with no subduction, so no magma is generated there. [NDA tested via CDS]

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.

Example: Mauna Loa, Hawaii.

Individual lava flows are only a few feet thick, but repeated flows over a long period of time build up the volcano. Such volcanoes are termed shield volcanoes. [NDA tested via CDS]

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.

Examples: Mount Fuji (Japan), Mayon (Philippines), Mount Vesuvius (Italy).

Composite volcanic cones are called strato-cones because of their alternating sheets of lava and pyroclastic materials. [NDA tested via CDS]

Arc Volcanoes:
Form above subducting plates at convergent boundaries. Mount Fuji and Mayon are classical examples of arc volcanoes. Some of the highest volcanoes — like Nevado Ojos del Salado in the Chilean Andes — are arc type. [NDA tested via CDS]

Hawaiian Islands — Hot Spot:
The 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]

Volcanic Features

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. The caldera is a product of vulcanicity — direct result of volcanic activity. [NDA 2008-II]

Intrusive Igneous Features:

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

Batholith:
The largest of all intrusive features. A massive dome of solidified magma deep in the Earth’s crust. The deepest and innermost intrusive igneous rock. [NDA 2021-I | NDA 2023-I]

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

Lopolith:
A saucer-shaped intrusion — concave upward.

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

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

Important Volcano-Country Pairs

This is one of the most reliably tested match-list topics in NDA.

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

NDA 2006-I matched:
A. Semeru — Indonesia
B. Cotopaxi — Ecuador
C. Etna — Italy
D. Kilimanjaro — Country given as Kenya in PYQ but correct answer is Tanzania

NDA 2009-I matched:
A. Mount Etna — Italy
B. Kilimanjaro — Tanzania
C. Katmai — USA
D. Barren Island — India
Answer: (d) A-4, B-3, C-2, D-1

Mount Merapi is on the island of Java in Indonesia. [NDA 2007-I | NDA 2007-II]

Cotopaxi is in Ecuador — not Chile. Students frequently confuse this.

The Deccan Traps

The Deccan Traps are a large volcanic province covering much of Maharashtra and neighbouring states. They were formed about 66 million years ago 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. This is a common trap in NDA. [NDA 2011-II]

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Seismic Gaps

A seismic gap is a section of a plate boundary that has NOT ruptured in recent times. Scientists watch these areas carefully because the built-up stress may be released as a major earthquake in the future. [NDA tested via CDS]

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Advantages of Volcanic Activity

Volcanoes are not only destructive. All three of these statements about the advantages of tectonic activity are correct: [NDA 2021-I]

1. Source of natural geothermal energy (Iceland uses volcanic heat)
2. Creation of new land (volcanic eruptions build up new land)
3. Attraction of tourists (volcanic landscapes attract tourism)

Volcanic soils are rich, deep and fertile — excellent for agriculture. [NDA tested via CDS]