1. What is a Volcano?
A volcano is an opening (vent) in the Earth’s crust through which magma, gases and ash come out from the interior.
When magma reaches the surface, it is called lava.
हिंदी: ज्वालामुखी = पृथ्वी की सतह पर ऐसा मुहाना जहाँ से गरम लावा (पिघला हुआ पत्थर), गैसें, राख आदि बाहर निकलती हैं।
मैल (अंदर) = magma, बाहर आने पर = lava।
2. Main Parts of a Volcano
- Magma Chamber – underground store of molten rock.
- Vent – passage through which magma rises.
- Crater – bowl-shaped opening at the top.
- Lava – molten rock flowing on the surface.
3. Types of Volcanoes (Simple NDA View)
(A) Active Volcano
• Erupts frequently, shows regular activity. • Example: Stromboli, Etna, Kilauea etc.
(B) Dormant Volcano
• Has not erupted for a long time but may erupt again. • Example: Vesuvius (Italy).
(C) Extinct Volcano
• No signs of future eruption; considered “dead”. • Example: Many eroded cones in different parts of the world.
• Erupts frequently, shows regular activity. • Example: Stromboli, Etna, Kilauea etc.
(B) Dormant Volcano
• Has not erupted for a long time but may erupt again. • Example: Vesuvius (Italy).
(C) Extinct Volcano
• No signs of future eruption; considered “dead”. • Example: Many eroded cones in different parts of the world.
हिंदी Short-cut:
Active = अभी भी जीवित, आग उगलता रहता है
Dormant = सोया हुआ (फिर से फट सकता है)
Extinct = मर चुका, दोबारा फटने की संभावना बहुत कम
4. Pacific Ring of Fire
Most of the world’s active volcanoes and earthquakes are found around the edges of the Pacific Ocean.
This zone is called the “Ring of Fire” – due to frequent volcanic and seismic activity.
हिंदी: प्रशांत महासागर के चारों ओर का इलाका, जहाँ सबसे ज़्यादा भूकम्प और ज्वालामुखी मिलते हैं, उसे Ring of Fire कहते हैं।
5. What is an Earthquake?
An earthquake is a sudden shaking or vibration of the Earth’s surface caused by the release of energy from within the Earth’s crust.
हिंदी: भूकम्प = पृथ्वी की सतह का अचानक हिलना, जो अंदर से ऊर्जा निकलने के कारण होता है।
6. Focus, Epicentre & Seismic Waves
- Focus (Hypocentre) – point inside the Earth where the earthquake starts.
- Epicentre – point on the Earth’s surface directly above the focus.
- Seismic Waves – waves of energy that travel through the Earth.
हिंदी:
Focus = भूकम्प का असली (source) बिंदु अंदर।
Epicentre = उसी के ठीक ऊपर सतह पर स्थित बिंदु – यहाँ झटका सबसे ज़्यादा होता है।
7. Measurement of Earthquakes
- Richter Scale – measures magnitude (energy released).
- Seismograph – instrument that records seismic waves.
8. Causes of Earthquakes (Very Short)
- Movement of tectonic plates.
- Volcanic activity.
- Human activities – dam construction, mining, nuclear tests (local quakes).
9. Effects of Volcanoes & Earthquakes
Positive:
- Volcanic lava forms fertile soils (e.g., Deccan plateau).
- Mineral deposits and geothermal energy.
- Loss of life and property.
- Tsunamis, landslides, fires, building collapse.
हिंदी: ज्वालामुखी और भूकम्प खतरनाक होते हैं, लेकिन लंबे समय में उपजाऊ मिट्टी, खनिज और ऊर्जा के स्रोत भी देते हैं।
📘 Practice Questions (NDA Style)
1. Magma that reaches the Earth’s surface is called:
a) Lava b) Ash c) Sediment d) Crust
2. The point inside the Earth where an earthquake originates is called:
a) Epicentre b) Focus c) Crater d) Vent
3. The instrument used to record earthquakes is:
a) Thermometer b) Barometer c) Seismograph d) Hygrometer
4. Most of the world’s active volcanoes lie in:
a) Atlantic belt b) Ring of Fire (Pacific) c) Arctic Circle d) Equatorial belt
5. Which rock type is commonly associated with volcanic activity?
a) Sandstone b) Basalt c) Limestone d) Shale
a) Lava b) Ash c) Sediment d) Crust
2. The point inside the Earth where an earthquake originates is called:
a) Epicentre b) Focus c) Crater d) Vent
3. The instrument used to record earthquakes is:
a) Thermometer b) Barometer c) Seismograph d) Hygrometer
4. Most of the world’s active volcanoes lie in:
a) Atlantic belt b) Ring of Fire (Pacific) c) Arctic Circle d) Equatorial belt
5. Which rock type is commonly associated with volcanic activity?
a) Sandstone b) Basalt c) Limestone d) Shale
Answers:
1 – a) Lava
2 – b) Focus
3 – c) Seismograph
4 – b) Ring of Fire (Pacific)
5 – b) Basalt
1 – a) Lava
2 – b) Focus
3 – c) Seismograph
4 – b) Ring of Fire (Pacific)
5 – b) Basalt
Quick Recap | सार
Now you should be able to:
- Recall the most important ideas of this topic.
- Solve typical NDA-style questions from this chapter.
- Spot and avoid common traps used in competitive exams.
Self-check: 3–4 random questions उठा कर देखिए – अगर आसानी से हो रहे हैं, तो chapter strong है।
Common Exam Mistakes | आम गलतियाँ
- Concept ठीक से clear न करना और सीधे रटने की कोशिश करना।
- Question पूरी तरह पढ़े बिना answer mark कर देना।
- Figures, dates, या formulas को last step में verify न करना।
NDA में smart strategy यह है कि आसान वाले सवाल पहले, doubtful बाद में – और silly mistakes से बचना सबसे ज़रूरी है।