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

📌 Investigated Precursors — Not Reliable Prediction Radon and unusual animal behaviour have been investigated/reported, but neither reliably predicts earthquakes.

〜 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
Compressional push–pull motion

Secondary Waves (S-waves)

Transverse / Perpendicular waves
Average speed (4–6 km/s)
Travel only through solids
Slower than primary waves
Damage depends on wave amplitude, local ground conditions and structures; no universal S-wave damage ranking.
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 arrive first; Love/Rayleigh surface waves are usually most damaging near the surface. Speed alone does not set damage.

📏 Recording Waves; Magnitude vs Intensity

Richter Scale	Mercalli Scale	Rossi-Forel Scale
Magnitude, logarithmic and open-ended; +1 magnitude = 10× wave amplitude. Mw is preferred for large earthquakes.	Observed shaking/damage intensity at a place: I–XII	Historical intensity scale: I–X

🚫 Seismic Shadow Zone

Region where particular seismic-wave types are not recorded
Direct P-wave shadow zone: approximately 104°–140° from epicentre
Direct S-wave shadow zone: beyond approximately 104°

⚡ 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

🇮🇳 Indian Seismic Zones — IS 1893 (Part 1):2016 Used by OP

Zone-2 (41%)

Zone II: lower design hazard; zone factor 0.10 (OP’s IS 1893:2016 map)

Zone-3 (30%)

Moderate hazard in the IS 1893:2016 map used by OP; a whole state need not fall within one zone.

Zone-4 (18%)

Zone IV: high design hazard; zone factor 0.24; OP examples include Patna and Arrah.

Zone-5 (11%)

Zone V: highest design hazard; zone factor 0.36; OP examples include Darbhanga, Purnia and Port Blair.

⚠ 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
Ejecta: lava, gases and pyroclasts. Water vapour is usually the dominant volcanic gas; tuff is lithified ash, and lapilli are 2–64 mm pyroclasts.

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

🏗️ Volcanic Parts / Activity

Part	Description
Volcanic Vent	Opening through which volcanic material reaches the surface
Volcanic Tube	Thin tube connected to the interior
Crater	Bowl-shaped depression around a volcanic vent
Caldera	Large crater formation due to collapse • e.g. Aso (Japan), Crater Lake (USA)
Geyser	Intermittently erupting hot spring; e.g. Old Faithful, Yellowstone (USA)
Fumaroles	Release of gas and water vapour • Solfatara (Sulphur-rich Fumaroles)

Fumarole examples include the Katmai/Valley of Ten Thousand Smokes region (Alaska) and White Island (New Zealand); Koh-i-Sultan is in Pakistan.

📌 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

Erupting, geologically recently erupted, or showing continuing eruptive potential; not necessarily continuously erupting.

Kīlauea (Hawaii) — one of the world’s most active volcanoes
Mt. Etna (Italy)
Stromboli (Italy)
Mauna Loa (Hawaiian Islands)
Cotopaxi (Ecuador)
Barren Island (Andaman-Nicobar, India)
Krakatau (Indonesia) — active/recently active; not a safe dormant example

Dormant Volcano

No recent eruptions, but eruption is possible

Vesuvius (Italy)
Fujiyama (Japan)
Narcondam: last eruption unknown; no documented Holocene eruption, so a confident dormant label is not justified.

Extinct Volcano

Considered unlikely to erupt again; this is an assessment, not an absolute guarantee.

Use activity classifications cautiously; they may change as geological evidence improves.
Do not treat a long quiet interval as proof that future eruption is impossible.
A safer ancient-field example in OP: Koh-i-Sultan
Koh-i-Sultan, Pakistan: no known Holocene eruption

📌 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 means capable of future eruption despite quiescence; extinct means considered unlikely to erupt, not guaranteed impossible. Krakatau is active/recently active; Vesuvius/Fuji remain potentially eruptive.

🌐 Global Distribution of Earthquakes

Circum-Pacific Ring of Fire: about 90% of global earthquakes
Alpide belt: convergent collision/subduction from the Mediterranean toward Asia
Mid-Atlantic Ridge: divergent seismic-volcanic belt

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

🌋 Pacific Ocean — Ring of Fire

Most earthquakes and volcanoes occur in this region
Common approximate recall: ~90% of earthquakes and ~75% of active volcanoes

🗺️ Global Distribution of Volcanoes

Circum-Pacific Zone

About 75% of active volcanoes; mainly convergent/subduction settings, with other boundary segments

Mid-Continental Belt

Alpine mountain chains • Destructive plate boundaries

Mid-Atlantic Zone

Oceanic ridges • Constructive plate boundaries

Intra-plate Volcanoes

Hotspots / mantle plumes beneath moving plates (e.g. Hawaii)

⚡ Quick Recall

Common approximate Ring of Fire recall: ~90% of earthquakes, ~75% of active volcanoes; percentages depend on definitions/catalogue.

🧩 Plate Tectonic Theory

Study related to behaviour and movement of plates
Earth's rigid lithosphere is broken into Plates
Harry Hess (1962): seafloor spreading, a major precursor to modern plate tectonics
Quantitative plate-motion theory: W. Jason Morgan and independently McKenzie & Parker (late 1960s)
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: South America, Africa, Antarctica, Australia, India, Madagascar and Arabia

⚠ 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	Much of the Pacific Ocean basin, not the entire ocean
Indo-Australian Plate	India + Australia + Indian Ocean
African Plate	African continent + Eastern Atlantic ocean + Western Indian ocean
Eurasian Plate	Most of Europe and Asia, excluding separate major plates such as India and Arabia; not Antarctica
Antarctic Plate	Antarctica plus surrounding oceanic crust
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