Earthquakes, Tsunamis and Volcanoes

🌍 Earthquake

Vibrations in the Earth's crust • Seismic or elastic waves generated
Study of seismic waves → Seismology
Focus: Origin point of earthquake
Epicentre: Point on the surface directly above the focus
Isoseismal line: Connects areas of equal seismic intensity

📌 Earthquake Warning Signs Increased Radon gas in atmosphere • Animal behaviour changes (e.g. dogs)

〜 Types of Earthquake Waves

Primary Waves (P-waves)

Longitudinal / Sound / Compressional waves
Reach the surface first
Highest speed (8–13 km/s)
Travel through solid, liquid and gas
Capable of passing through the central part of Earth
Fill and Push

Secondary Waves (S-waves)

Transverse / Perpendicular waves
Average speed (4–6 km/s)
Travel only through solids
Slower than primary waves
Less destructive
Reach the seismograph after primary waves

Surface Waves (L-waves)

Love / Rayleigh (L / R-waves)
Restricted to the surface
Slowest speed (1–3 km/s)
Most destructive
Final waves to reach the surface and seismograph
Impact on water and land

⚠ Exam Trap

P-waves are the fastest but least destructive; Surface (L) waves are the slowest but most destructive — speed and destructiveness are inversely related here.

📏 Measurement of Seismic Wave Intensity — Seismograph

Richter Scale	Mercalli Scale	Rossi-Forel Scale
Intensity / Magnitude – 0–10 • Mathematical scale (Logarithmic) • Each point is 10 times more intense than the previous	Intensity / Magnitude – 1–12 • Scale of 12 • Reflects the intensity of shaking	Scale – 1–11

🚫 Seismic Shadow Zone

Region where no seismic waves are recorded
P-wave shadow zone – Between 105° to 145°
S-wave shadow zone – Beyond 105°

⚡ Quick Recall

Richter Scale = Logarithmic, measures Magnitude · Mercalli Scale = measures perceived Intensity/shaking

🏗️ Types of Earthquake

1. Tectonic Earthquake – Due to collision and slipping of plates
2. Collapse Earthquake – Underground collapse of mine or cave
3. Volcanic Earthquake – Due to volcanoes
4. Reservoir Induced Earthquake – Due to dam construction
5. Explosion Earthquake – Due to nuclear tests or bomb explosion

💥 Effects of Earthquake

Tsunami occurrence
Landslides / Avalanches
Floods / Forest fires
Soil liquefaction

🇮🇳 Earthquake Zones in India

Zone-2 (41%)

Low Intensity (6) • Peninsular Plateau

Zone-3 (30%)

Medium Intensity (7) • Bihar, Jharkhand, UP, Odisha, etc.

Zone-4 (18%)

High Intensity (8) • Delhi, Punjab, Jammu & Kashmir, etc.

Zone-5 (11%)

Very High Intensity / Most active • Himalayan region, North-east India, Gujarat

⚠ Exam Trap

Zone-5 covers the smallest area (11%) but is the most seismically active; Zone-2 covers the largest area (41%) but has the lowest intensity — area share and risk level are inversely related.

🌊 Tsunami (Japanese word)

Tsu: Harbour • Nami: Wave — "Harbour Wave"
Waves generated due to undersea earthquakes or volcanic eruptions
Devastating Tsunami of Indian Ocean: 26 December, 2004

⚡ Quick Recall

Tsunami = "Harbour Wave" (Japanese) → triggered mainly by undersea tectonic earthquakes.

🌋 Volcano — "Safety Valve of Nature"

Natural vent or fissure through which molten substances of Earth like lava, ash, steam, etc. emerge
Materials emitted: Gases and water vapour (60–90%), Tuff (rock fragments), Lapilli (pea-sized particles), Breccia, Lava, Pyroclasts (solid rock fragments), etc.

📌 Magma vs Lava Magma (inside crust) → becomes Lava on surface

🏗️ Volcanic Parts / Activity

Part	Description
Volcanic Vent	Funnel-shaped opening, lava emerges
Volcanic Tube	Thin tube connected to the interior
Crater	Expanded form of volcanic vent (mouth of volcano)
Caldera	Large crater formation due to collapse • e.g. Aso (Japan), Crater Lake (USA)
Geyser	Water and steam emerge from fissures or vents • e.g. Old Faithful Geyser (Yellowstone Park, USA)
Fumaroles	Release of gas and water vapour • Solfatara (Sulphur-rich Fumaroles)

Examples of Fumaroles: Mt. Katmai – Valley of a Thousand Smokes (Alaska, USA), Koh Sultan Fumarole (Iran), White Island Fumaroles (New Zealand)

📌 Pele's Tears Formed during volcanic eruption

📌 Solidified lava deposits Cinder

⚡ Quick Recall

Crater → mouth of the volcano · Caldera → enlarged crater formed by collapse (much bigger than a crater)

🔥 Types of Volcano based on Eruption Activity

Active Volcano

Constant release of dust, smoke, vapour, etc.

Kilauea (Hawaiian Islands) — Most active
Mt. Etna (Italy)
Stromboli (Italy)
Mauna Loa (Hawaiian Islands)
Cotopaxi (Ecuador)
Barren Island (Andaman-Nicobar, India)

Dormant Volcano

No recent eruptions, but eruption is possible

Vesuvius (Italy)
Fujiyama (Japan)
Krakatoa (Indonesia)
Narcondam Island (Andaman-Nicobar)

Extinct Volcano

No possibility of future eruptions

Chimborazo (Ecuador)
Kilimanjaro (Tanzania)
Mount Popa (Myanmar)
Demavend and Koh Sultan (Iran)

📌 Flood Basalt Large area covered by basaltic lava flow

🏔️ Volcanic Landforms

Internal Landforms

Dyke, Sill, Laccolith, Phacolith, Lapolith, Batholith

External Landforms

Cinder or Ash cone
Lava cone (Shield – less viscous)
Composite Volcanic cone (Viscous lava)
Parasitic or dependent cone

⚠ Exam Trap

Dormant ≠ Extinct: Dormant volcanoes (e.g. Vesuvius, Fujiyama) can still erupt again; Extinct volcanoes (e.g. Kilimanjaro) have no eruption potential — Krakatoa is famously "dormant," not extinct, despite long quiet periods.

🌐 Global Distribution of Earthquakes

Pacific Ocean Coastal Belt (13+ earthquakes)
Mid-Continental Belt (21+ earthquakes)
Mid-Atlantic Belt (most earthquakes near equator)

📌 'Ring of Fire' ~90% of world's earthquakes occur here

🌋 Pacific Ocean — Ring of Fire

Most earthquakes and volcanoes occur in this region
Circum-Pacific (Pacific Belt) → ~15% of volcanoes and ~63% of earthquakes

🗺️ Global Distribution of Volcanoes

Circum-Pacific Zone

2/3rd of world's volcanoes • Destructive plate boundaries

Mid-Continental Belt

Alpine mountain chains • Destructive plate boundaries

Mid-Atlantic Zone

Oceanic ridges • Constructive plate boundaries

Intra-plate Volcanoes

Micro-plate activities

⚡ Quick Recall

Circum-Pacific Zone alone hosts about 2/3rd of all the world's volcanoes.

🧩 Plate Tectonic Theory

Study related to behaviour and movement of plates
Earth's rigid lithosphere is broken into Plates
Theory proposed by: Harry Hess (1962)
Scientific/mathematical explanation: W.J. Morgan
Other key contributors: McKenzie, Parker, Holmes
Driven by heat generated from radioactive substances inside the Earth

📌 Mechanism Movement of the lithosphere over the asthenosphere

🔀 Plate Boundaries

Destructive Plate Boundary – Formation of Rockies, Andes, Himalayas
Constructive Plate Boundary – Formation of new crust
Conservative Plate Boundary – Formation of transform faults (e.g. San Andreas Fault, California)

📜 Other Important Theories

Precursor Hypothesis: F.B. Taylor (early continental displacement idea)
Continental Drift Theory: Alfred Wegener (German Scientist) – 1912, based on gravitational force, Guy-Sam-Fit fit of coastlines, buoyancy, and tidal force
Pangea – the original supercontinent
Angara Land – North America, Europe, Asia
Gondwana Land – South America, Africa, India, Madagascar, Australia

⚠ Exam Trap

The widely credited Continental Drift Theory belongs to Alfred Wegener (1912) — F.B. Taylor's idea was an earlier, separate precursor concept, not the main "Continental Drift Theory" itself.

🗺️ Major Plates of the World

Plate	Included Areas
Pacific Plate	Entire Pacific Ocean
Indo-Australian Plate	India + Australia + Indian Ocean
African Plate	African continent + Eastern Atlantic ocean + Western Indian ocean
Eurasian Plate	Europe + Asia + Eastern Atlantic ocean + Antarctic continent
Antarctic Plate	Oceanic plate encircling the Antarctic continent from all sides
North American Plate	North-west Atlantic ocean + North American continent
South American Plate	South-west Atlantic ocean + South American continent

⚡ Quick Recall

The Pacific Plate is the largest tectonic plate, made up almost entirely of oceanic crust.

🌋 EARTHQUAKES, TSUNAMIS & VOLCANOES World Geography · Ch 09

Primary Waves (P-waves)

Secondary Waves (S-waves)

Surface Waves (L-waves)

Zone-2 (41%)

Zone-3 (30%)

Zone-4 (18%)

Zone-5 (11%)

Active Volcano

Dormant Volcano

Extinct Volcano

Internal Landforms

External Landforms

Circum-Pacific Zone

Mid-Continental Belt

Mid-Atlantic Zone

Intra-plate Volcanoes