UPSC Prelims 2024 Q3: Geography – Physical Geography & Geomorphology
- (a) Only one
- (b) Only two
- (c) Only three
- (d) All four
Answer: (d) All four
Explanation
Products of Volcanic Eruptions
| Product | Present in Volcanic Eruptions? | Details |
|---|---|---|
| 1. Pyroclastic debris | YES | Fragments of solidified lava, rocks, and ash ejected explosively. Includes bombs, lapilli, and tephra. Pyroclastic flows are among the most deadly volcanic hazards. |
| 2. Ash and dust | YES | Fine particulate matter erupted into the atmosphere. Volcanic ash can travel thousands of kilometres, affecting aviation and climate (volcanic winter). |
| 3. Nitrogen compounds | YES | Volcanoes emit N2 (nitrogen gas), nitrogen oxides (NOx), and ammonia compounds. Volcanic outgassing was a major source of Earth's early nitrogen-rich atmosphere. |
| 4. Sulphur compounds | YES | SO2 (sulphur dioxide) and H2S (hydrogen sulphide) are major volcanic gases. SO2 reacts with water to form sulphuric acid aerosols, causing acid rain and global cooling (e.g., 1991 Mt. Pinatubo eruption cooled Earth by ~0.5°C). |
All four are genuine products of volcanic eruptions → Answer: All four
Major Volcanic Gas Emissions (by volume):
| Gas | Approximate % | Effect |
|---|---|---|
| H2O (steam) | ~60-90% | Major volcanic gas |
| CO2 | ~10-15% | Greenhouse gas |
| SO2 | 1-5% | Acid rain, global cooling |
| H2S, HCl, N compounds | smaller % | Various atmospheric effects |
Answer: (d) All four.
Why each option is right or wrong
- (a) Incorrect. '1 and 2 only' excludes nitrogen and sulphur compounds. Volcanoes are significant emitters of SO2 (causing acid rain) and nitrogen compounds (contributed to Earth's early atmosphere).
- (b) Incorrect. '1, 2 and 3 only' excludes sulphur compounds — but SO2 is one of the most well-known and consequential volcanic emissions, responsible for acid rain and post-eruption cooling.
- (c) Incorrect. '1, 3 and 4 only' would exclude ash and dust — ash fall is one of the most visually dramatic and well-documented volcanic products.
- (d) CORRECT. All four — pyroclastic debris, ash/dust, nitrogen compounds, and sulphur compounds — are genuine products of volcanic eruptions.
Key point
All four are volcanic products: pyroclastic debris, ash/dust, nitrogen compounds (N2, NOx), sulphur compounds (SO2, H2S). SO2 causes acid rain and volcanic cooling. Pyroclastic flows are most deadly.
Remember
Volcanic eruptions produce pyroclastic debris, ash/dust, nitrogen compounds, and sulphur compounds — all four. SO2 is the most climatically significant volcanic gas.
Common mistakes
- Forgetting that volcanoes emit nitrogen compounds — N2 and NOx are volcanic gases; primordial volcanic outgassing built Earth's early nitrogen-rich atmosphere.
- Underestimating sulphur compounds — SO2 from major eruptions (like Pinatubo 1991) can cool global temperatures by injecting aerosols into the stratosphere.
- Thinking volcanic products are only lava and ash — gases (sulphur, nitrogen, CO2, water vapour) make up the majority of volcanic emissions by volume.
Exam tip
Volcanic products = 3 categories: Gases (SO2, H2S, CO2, N2, H2O), Pyroclastic material (ash, bombs, lapilli), Lava flows. Major hazards: pyroclastic flows (fastest, deadliest), lahars (volcanic mudflows), ash fall, volcanic gases.
Sources
- NCERT Class 11 Geography — Chapter 3: Interior of the Earth (Volcanoes section)
- USGS (United States Geological Survey) — Volcanic Gases and Their Effects