Earthquakes and Volcanoes
NCERT Class 11 - Fundamentals of Physical Geography, Chapters 3 and 6
Earthquakes and Volcanoes
Earthquakes and volcanoes are sudden endogenic processes - they release energy from within the earth in minutes rather than over millions of years. Both are concentrated along plate boundaries, which is why their world map matches the plate map.
Earthquakes
- An earthquake is the shaking of the earth caused by the release of energy along a fault, which travels outward as seismic waves.
- The point of origin inside the earth is the focus or hypocentre; the point on the surface directly above it is the epicentre, where damage is usually greatest.
- Types - tectonic, caused by movement along faults and by far the commonest; volcanic, linked to eruptions; collapse, in mining areas; explosion, from nuclear or chemical blasts; and reservoir induced, from the weight of water behind a large dam.
- Measurement - the Richter scale measures magnitude, the energy released, on a logarithmic scale in which each whole number is about thirty-two times more energy; the Mercalli scale measures intensity, the observed damage, from I to XII.
- Effects include ground shaking, liquefaction of loose saturated soil, landslides, fires, damage to dams, and tsunamis where the sea floor is displaced.
Seismic zones of India
- India is divided into four seismic zones, from Zone II with the lowest risk to Zone V with the highest.
- Zone V covers the whole of the north-east, parts of Jammu and Kashmir, Himachal Pradesh and Uttarakhand, the Rann of Kutch, and the Andaman and Nicobar Islands.
- The Himalayan belt is highly active because the Indian plate is still pushing into the Eurasian plate.
- Bihar lies largely in high risk zones; the Bihar-Nepal earthquake of 1934 is one of the most destructive in Indian records, and a further major earthquake struck the same belt in 1988.
Volcanoes
| Type | Character |
|---|---|
| Shield volcano | Broad and gently sloping, from fluid basaltic lava; the Hawaiian volcanoes |
| Composite or strato volcano | Steep cone of alternating lava and ash from more viscous lava; Fujiyama |
| Caldera | A large depression formed when the volcano collapses into its emptied chamber |
| Flood basalt province | Vast sheets of lava covering large areas; the Deccan Traps of India |
| Mid-ocean ridge volcano | Continuous eruption along the ridge, forming new ocean floor |
- Volcanoes are classed as active, dormant or extinct. India's only active volcano is on Barren Island in the Andaman Sea.
- The Deccan Traps were formed by flood basalt eruptions and are the source of the black soil of western and central India.
- Most of the world's volcanoes lie in the Pacific Ring of Fire along subduction boundaries, along mid-ocean ridges, and along rift valleys such as the East African rift.
Intrusive volcanic landforms
- Batholith - a very large mass of cooled magma deep in the crust, forming the core of many mountain ranges.
- Laccolith - a dome shaped intrusion with a flat base connected by a pipe below.
- Lopolith - a saucer shaped intrusion that sags in the middle.
- Phacolith - magma solidified in the crest or trough of a fold.
- Sill - a sheet of magma lying parallel to the rock layers; a dyke is a sheet cutting across them.
- Extrusive forms include lava plateaus, cinder cones and volcanic necks left when the surrounding cone erodes away.
हिंदी संकेत: सिल चट्टानी परतों के समानांतर होता है और डाइक उन्हें काटते हुए आर-पार। यह जोड़ी सबसे अधिक पूछी जाती है।
Exam me kaise aata hai
Factual questions ask the difference between focus and epicentre, what the Richter and Mercalli scales measure, and where India's only active volcano is. Match-the-following links intrusive landform with its shape, and volcano type with example.
UPSC / State PSC ke liye
Distinguish magnitude from intensity carefully - magnitude is a single value for the event, intensity varies from place to place with distance and ground conditions. On volcanoes, note that lava viscosity decides the shape - fluid basaltic lava spreads into shields, viscous lava piles into steep cones.
Yahan confuse hote hain
✗ The Richter scale measures the damage caused | ✓ It measures magnitude; damage is measured by the Mercalli scale
✗ The focus is the point on the surface above the origin | ✓ That is the epicentre; the focus is inside the earth
✗ A dyke lies parallel to rock layers | ✓ A sill lies parallel; a dyke cuts across them
Ek nazar me
- Focus is inside the earth; epicentre is the point on the surface above it.
- Types - tectonic, volcanic, collapse, explosion and reservoir induced.
- Richter scale measures magnitude on a logarithmic scale; Mercalli measures intensity of damage.
- India has four seismic zones, from Zone II to the highest risk Zone V.
- Volcano types - shield, composite, caldera, flood basalt and mid-ocean ridge.
- Barren Island has India's only active volcano; the Deccan Traps are flood basalt.
- Sill lies parallel to rock layers and a dyke cuts across them.