Earthquakes & Volcanoes
How earthquakes are measured (magnitude vs intensity), volcano types and landforms, and their distribution along the Pacific Ring of Fire.
Key Takeaways
- An earthquake's magnitude is measured on the Richter scale; its intensity on the Mercalli scale.
- Most earthquakes and volcanoes occur along plate boundaries — notably the Pacific Ring of Fire.
- Volcanic landforms are intrusive (below surface) or extrusive (on the surface).
Core concept
An earthquake is the shaking of the ground caused by the sudden release of energy along a fault. A volcano is a vent through which molten rock (magma) erupts. Both are concentrated along plate boundaries — the Earth's most tectonically active zones.
Static foundation — measuring an earthquake
- Focus (hypocentre): the point inside the Earth where the rupture begins.
- Epicentre: the point on the surface directly above the focus — where damage is usually greatest.
Magnitude vs Intensity
| Aspect | Magnitude (Richter) | Intensity (Mercalli) |
|---|---|---|
| Measures | Energy released at the source | Effects/damage felt at a place |
| Scale | Logarithmic (each step = ~32× energy) | Descriptive (I to XII) |
| Value per quake | A single value | Varies with distance from the epicentre |
| Instrument | Seismograph | Observation of damage |
Types of Volcanoes
Active — erupts frequently (e.g., Barren Island, India's only active volcano). Dormant — 'sleeping', may erupt again. Extinct/Dead — no signs of future activity.
The Pacific Ring of Fire
About two-thirds of the world's volcanoes and most large earthquakes occur along the Pacific Ring of Fire — a horseshoe of subduction zones around the Pacific Ocean. This confirms the link between tectonic activity and plate boundaries. Hotspots (like Hawaii) are the main exception, occurring away from boundaries.
Relevance & current linkage
Much of the Himalayan region and the north-east lie in high seismic zones (IV–V). Disaster preparedness — building codes, early-warning systems, the NDMA's guidelines — is a recurring theme. (Add a recent earthquake or tsunami event if relevant.)
Prelims trap zones
- Magnitude (Richter) ≠ Intensity (Mercalli) — one measures energy, the other measures felt damage.
- Focus is inside the Earth; the epicentre is on the surface.
- Barren Island (Andaman) is India's only active volcano.
Prelims Pointers
- The point of origin inside the Earth is the focus/hypocentre; the point directly above on the surface is the epicentre.
- The Richter scale (magnitude) is logarithmic; the Mercalli scale measures observed intensity.
- India's seismic zoning ranges from Zone II (least) to Zone V (highest risk).
- Intrusive volcanic landforms include the batholith, laccolith, sill and dyke.
Mains Angle
- 'India is increasingly vulnerable to earthquakes.' Discuss the reasons and mitigation measures.
- Explain the global distribution of volcanoes with reference to plate tectonics.
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