Volcanoes, earthquakes and tsunamis are the most dramatic expressions of the Earth's internal energy. They destroy lives and property within minutes, yet they also build new land, create fertile soils and reveal what lies deep inside the planet. Most of them occur along plate boundaries. This third part of the series covers volcanicity, earthquakes and tsunamis in detail, with short tricks and Indian examples.
Geomorphology Series (5 parts):
Part 1 – Basics, Forces, Earth's Crust, Interior and Geomagnetism
Part 2 – Geosynclines, Continental Drift, Isostasy, Plate Tectonics and Mountain Building
Part 3 – Volcanicity, Earthquakes and Tsunamis
Part 4 – Geomorphic Cycles, Denudation Chronology, Channel Morphology, Erosion Surfaces and Slopes
Part 5 – Applied Geomorphology, Economic Geology and Environment
Part A: Volcanicity
Volcanicity (volcanism) includes all processes by which molten rock (magma) and gases from inside the Earth move towards or onto the surface. Molten rock below the surface is called magma; once it comes out it is called lava.
Parts of a Volcano
- Magma chamber – a reservoir of molten rock below the surface.
- Vent / pipe – the passage through which magma rises.
- Crater – the bowl-shaped opening at the top.
- Caldera – a very large depression formed when the top of a volcano collapses after a huge eruption (e.g., Lake Toba in Sumatra, Krakatoa).
- Volcanic materials – lava, gases (mostly water vapour, along with CO2 and sulphur gases) and solid fragments called pyroclasts (ash, lapilli, volcanic bombs).
Types of Volcanoes by Activity
| Type | Meaning | Examples |
|---|---|---|
| Active | Erupts frequently or has erupted in recent times | Etna and Stromboli (Italy), Kilauea (Hawaii), Barren Island (India) |
| Dormant | Has not erupted for a long time but may erupt again | Mount Fuji (Japan), Kilimanjaro (Tanzania), Narcondam (India) |
| Extinct | No eruption in historical times and not expected to erupt | Mount Kenya, Arthur's Seat (Scotland) |
India: Barren Island in the Andaman Sea is India's only active volcano. Narcondam, also in the Andaman Sea, is considered dormant.
Types of Eruptions (Least to Most Explosive)
| Type | Nature | Example |
|---|---|---|
| Hawaiian | Quiet outpouring of runny basaltic lava; lava fountains | Kilauea, Mauna Loa |
| Strombolian | Regular, small explosions throwing out glowing fragments | Stromboli – called the "Lighthouse of the Mediterranean" |
| Vulcanian | Short, violent explosions of thick lava and ash clouds | Vulcano (Italy) |
| Pelean | Very violent; produces a glowing avalanche of hot gas and ash called nuée ardente (pyroclastic flow) | Mount Pelée, Martinique (1902) |
| Plinian | Most explosive; huge columns of gas and ash rise tens of kilometres | Vesuvius (79 CE, which buried Pompeii); Mount St. Helens (1980); Pinatubo (1991) |
Short trick: "Happy Students Very Proudly Pass" → Hawaiian → Strombolian → Vulcanian → Pelean → Plinian (explosiveness increases).
The Plinian type is named after Pliny the Younger, who described the eruption of Vesuvius in 79 CE.
A separate type is the fissure eruption, where lava flows quietly out of long cracks and spreads over large areas to form lava plateaus – the Deccan Traps of India, the Columbia Plateau (USA) and the Laki eruption in Iceland (1783).
The size of eruptions is measured on the Volcanic Explosivity Index (VEI), from 0 to 8.
Volcanic Landforms – Extrusive
| Landform | Features | Example |
|---|---|---|
| Shield volcano | Broad, gently sloping; formed by fluid basaltic lava; the largest volcanoes on Earth | Mauna Loa (largest active volcano), Mauna Kea (Hawaii) |
| Composite (strato) volcano | Steep, cone-shaped; layers of lava and ash; explosive | Fuji, Vesuvius, Etna, Mount St. Helens |
| Cinder cone | Small, steep cones of loose fragments | Parícutin (Mexico), which grew from a cornfield in 1943 |
| Caldera | Huge depression after collapse; may hold a lake | Toba, Krakatoa, Crater Lake (USA) |
| Lava plateau / flood basalt | Layer upon layer of fissure lava | Deccan Traps (India) |
Mauna Kea in Hawaii is the tallest mountain on Earth when measured from its base on the sea floor (over 10,000 m). Ojos del Salado (Chile–Argentina) is the highest active volcano above sea level.
Volcanic Landforms – Intrusive
When magma cools below the surface, it forms intrusive bodies, which are exposed later by erosion.
| Landform | Shape |
|---|---|
| Batholith | The largest intrusive body; huge mass of granite forming the core of mountains |
| Laccolith | Dome-shaped body that lifts the overlying layers; flat base (e.g., Karnataka plateau domes) |
| Lopolith | Saucer-shaped (sagging) body |
| Phacolith | Lens-shaped body found at the crest or trough of folds |
| Sill | Horizontal sheet of magma parallel to rock layers (thin ones are called sheets) |
| Dyke | Vertical, wall-like sheet cutting across rock layers (common in the Deccan region) |
Short trick: Lacco = Lifted dome; Lopo = Low saucer; Phaco = Folds; Sill = Sleeping (horizontal); Dyke = Divides vertically (standing).
Secondary Volcanic Features
- Geyser – hot spring that throws hot water and steam into the air at intervals; e.g., Old Faithful in Yellowstone National Park (USA). Geysers are also found in Iceland and New Zealand.
- Hot springs – e.g., Manikaran (Himachal Pradesh), Bakreshwar (West Bengal), Rajgir (Bihar).
- Fumaroles – vents giving out steam and gases; the "Valley of Ten Thousand Smokes" in Alaska is famous. Solfataras give out sulphur gases.
- Mud volcanoes – found in Baratang Island (Andaman).
World Distribution of Volcanoes
- Circum-Pacific Belt ("Ring of Fire") – has about two-thirds to three-fourths of the world's active volcanoes; along the subduction zones around the Pacific Ocean.
- Mid-continental (Alpine–Mediterranean) Belt – Mediterranean volcanoes like Vesuvius, Etna and Stromboli; the Himalaya have no active volcanoes because they formed by continent–continent collision.
- Mid-Atlantic Ridge – e.g., Iceland, Azores, St. Helena.
- Intra-plate hotspots – Hawaii; and rift volcanoes of the East African Rift (Kilimanjaro, Nyiragongo).
Effects of Volcanoes
- Harmful: loss of life, lava and pyroclastic flows, ash falls, poisonous gases, lahars (volcanic mudflows), disruption of air traffic. Large eruptions can cool global climate – after Tambora (1815), 1816 became the "Year Without a Summer".
- Useful: very fertile soils (black cotton soil of the Deccan), new land, minerals, geothermal energy (Iceland, New Zealand; Puga Valley in Ladakh is India's prominent geothermal site) and tourism.
Part B: Earthquakes
An earthquake is a sudden shaking of the Earth's surface caused by the release of energy, usually when rocks break and move along a fault. The study of earthquakes is called seismology, and the instrument that records them is the seismograph.
Key Terms
- Focus (hypocentre): the point inside the Earth where the earthquake starts.
- Epicentre: the point on the surface directly above the focus. Damage is usually greatest here.
- Isoseismal lines: lines joining places of equal intensity.
- Homoseismal / Coseismal lines: lines joining places where shocks arrive at the same time.
Causes of Earthquakes
| Type | Cause |
|---|---|
| Tectonic (most common) | Movement along faults and plate boundaries – explained by the elastic rebound theory (H.F. Reid, after the 1906 San Francisco earthquake) |
| Volcanic | Movement of magma and explosions |
| Collapse | Collapse of underground mines or caves |
| Induced (man-made) | Filling of large reservoirs (reservoir-induced seismicity, e.g., Koyna, 1967), nuclear tests, mining, fluid injection |
Earthquakes by Depth of Focus
- Shallow: 0–70 km (most common and most damaging)
- Intermediate: 70–300 km
- Deep: 300–700 km (along subduction zones)
Measuring Earthquakes
| Scale | Measures | Details |
|---|---|---|
| Richter scale | Magnitude (energy released) | Developed by Charles F. Richter in 1935; logarithmic – each whole number rise means about 10 times more ground motion and about 32 times more energy |
| Moment Magnitude scale (Mw) | Magnitude | Introduced in 1979; now the standard scale for large earthquakes |
| Modified Mercalli scale | Intensity (damage and effects felt) | Grades I to XII; based on observed damage |
Short trick: Richter = Reading of energy (magnitude); Mercalli = Mess / damage (intensity). Magnitude has one value; intensity changes from place to place.
World Distribution of Earthquakes
- Circum-Pacific Belt – about 80–90% of the world's earthquakes, including the largest ones.
- Mid-continental (Alpine–Himalayan) Belt – from the Mediterranean through Turkey, Iran and the Himalaya to Myanmar; most of the remaining earthquakes.
- Mid-Atlantic Ridge and other mid-ocean ridges – mostly mild, shallow earthquakes.
Seismic Zones of India
Under the Bureau of Indian Standards code IS 1893 (Part 1): 2016, India is divided into four seismic zones – II, III, IV and V (Zone I was merged with Zone II in 2002).
| Zone | Risk | Areas (examples) |
|---|---|---|
| Zone V | Very high | Entire North-East, Jammu & Kashmir and Ladakh (parts), Himachal Pradesh and Uttarakhand (parts), Rann of Kutch, north Bihar, Andaman & Nicobar Islands |
| Zone IV | High | Delhi, parts of J&K, Himachal, Uttarakhand, Sikkim, northern UP, Bihar, West Bengal (north) |
| Zone III | Moderate | Kerala, Goa, parts of Maharashtra, Gujarat, MP, Odisha, Kolkata |
| Zone II | Low | Most of the peninsular plateau (Karnataka, Telangana, Chhattisgarh, etc.) |
Current affairs note: In late 2025, a draft revision of the code (IS 1893: 2025) proposed a new Zone VI for the most hazardous Himalayan areas. After objections from several agencies, the Centre withdrew the revised zoning in early 2026, so the four-zone system remains the reference for now.
Major Earthquakes to Remember
| Earthquake | Year | Magnitude | Note |
|---|---|---|---|
| Valdivia, Chile | 1960 | 9.5 | Strongest earthquake ever recorded |
| Sumatra–Andaman (Indian Ocean) | 2004 | 9.1 | Caused the deadly Indian Ocean tsunami |
| Tohoku, Japan | 2011 | 9.0–9.1 | Tsunami; Fukushima nuclear disaster |
| Shillong (Assam) | 1897 | About 8.1 | One of India's great earthquakes |
| Assam–Tibet | 1950 | 8.6 | Strongest earthquake recorded in India |
| Latur (Maharashtra) | 1993 | 6.2 | In a "stable" low-risk zone; very high casualties |
| Bhuj (Gujarat) | 2001 | 7.7 | On Republic Day, 26 January |
| Gorkha (Nepal) | 2015 | 7.8 | Severe damage in Kathmandu |
| Turkey–Syria | 2023 | 7.8 | Among the deadliest of the century |
Effects and Safety
- Effects: collapse of buildings, landslides, liquefaction of soil, fires, floods from dam failure, tsunamis, changes in river courses.
- Safety: earthquake-resistant building codes, base isolation, retrofitting old buildings, public drills ("Drop, Cover and Hold On"), early warning systems.
Part C: Tsunamis
Tsunami is a Japanese word meaning "harbour wave". It is a series of very long, powerful sea waves caused by a sudden vertical displacement of a large volume of water.
Causes
- Undersea earthquakes (most common) – especially at subduction zones, when the sea floor moves up or down suddenly. Usually only earthquakes above about magnitude 7 produce major tsunamis.
- Submarine or coastal landslides
- Volcanic eruptions – Krakatoa (1883) produced deadly tsunamis.
- Meteorite impacts (rare)
Characteristics
- In the deep ocean, tsunamis travel at 700–800 km/h (as fast as a jet plane), but their height is often less than a metre, so ships hardly notice them.
- The wavelength is very long – often over 100 km.
- Near the coast, as water becomes shallow, the waves slow down and grow much taller (shoaling), sometimes over 30 m.
- A common warning sign is the sudden withdrawal of the sea, exposing the sea bed, just before the wave arrives.
Tsunami vs ordinary waves: Ordinary waves are caused by wind and move only the surface water; a tsunami moves the entire column of water from the sea bed to the surface.
Indian Ocean Tsunami, 2004
On 26 December 2004, a magnitude 9.1 earthquake off Sumatra (Indonesia) triggered a tsunami that hit 14 countries and killed about 2,30,000 people. In India, Tamil Nadu, Andhra Pradesh, Kerala, Puducherry and the Andaman & Nicobar Islands were badly affected.
Tsunami Warning Systems
- Indian Tsunami Early Warning Centre (ITEWC) at INCOIS, Hyderabad, became operational in 2007. It can issue warnings within about 10 minutes of an earthquake.
- Deep-ocean DART buoys and bottom pressure recorders detect changes in sea level.
- World Tsunami Awareness Day is observed on 5 November (declared by the UN General Assembly in 2015).
- Natural barriers like mangroves and coral reefs reduce tsunami damage.
Quick Revision Points
- India's only active volcano: Barren Island; dormant: Narcondam.
- Explosiveness: Hawaiian < Strombolian < Vulcanian < Pelean < Plinian.
- Stromboli: Lighthouse of the Mediterranean.
- Deccan Traps: fissure eruption, flood basalt.
- Largest intrusive body: batholith; vertical: dyke; horizontal: sill.
- Ring of Fire: most volcanoes and about 80–90% of earthquakes.
- Richter: magnitude; Mercalli: intensity (I–XII).
- Strongest recorded earthquake: Chile 1960 (9.5).
- Tsunami = harbour wave; ITEWC at INCOIS Hyderabad (2007); World Tsunami Awareness Day: 5 November.
Practice MCQs
1. India's only active volcano is located at:
(a) Narcondam
(b) Barren Island
(c) Baratang
(d) Lakshadweep
2. Which volcano is called the "Lighthouse of the Mediterranean"?
(a) Etna
(b) Vesuvius
(c) Stromboli
(d) Vulcano
3. The Deccan Traps were formed by:
(a) Central eruption
(b) Fissure eruption
(c) Plinian eruption
(d) Glacial deposition
4. A vertical intrusive body cutting across rock layers is called:
(a) Sill
(b) Dyke
(c) Laccolith
(d) Lopolith
5. The point on the surface directly above the focus of an earthquake is:
(a) Hypocentre
(b) Epicentre
(c) Seismic centre
(d) Isoseism
6. The Mercalli scale measures:
(a) Magnitude
(b) Intensity
(c) Depth
(d) Wave speed
7. Which seismic zone of India has the highest risk under IS 1893: 2016?
(a) Zone II
(b) Zone III
(c) Zone IV
(d) Zone V
8. The word "tsunami" means:
(a) Storm surge
(b) Harbour wave
(c) Tidal wave
(d) Sea quake
9. India's Tsunami Early Warning Centre is located at:
(a) Chennai
(b) Kolkata
(c) Hyderabad
(d) Goa
10. The strongest earthquake ever recorded occurred in:
(a) Japan, 2011
(b) Chile, 1960
(c) Indonesia, 2004
(d) Alaska, 1964
Answers: 1 – (b) | 2 – (c) | 3 – (b) | 4 – (b) | 5 – (b) | 6 – (b) | 7 – (d) | 8 – (b) | 9 – (c) | 10 – (b)
Reference: NCERT Fundamentals of Physical Geography (Class XI) – Interior of the Earth; NCERT India: Physical Environment; Bureau of Indian Standards IS 1893; INCOIS.