UPSC Prelims 2024 Q3: Products of Volcanic Eruptions — Explained
Consider the following:
1. Pyroclastic debris
2. Ash and dust
3. Nitrogen compounds
4. Sulphur compounds
How many of the above are products of volcanic eruptions?
- Only one
- Only two
- Only three
- All four
Answer: (d) All four
Statement-wise explanation
- 1: correct — Pyroclastic debris is the broken rock thrown out of a volcano in an explosive eruption. USGS defines 'pyroclast' as material that is explosively ejected and fragmented; the word means 'fire-broken'. It includes volcanic bombs, lapilli and ash, together called tephra. NCERT's Class XI text lists pyroclastic debris among the materials that reach the ground during an eruption.
- 2: correct — Ash is the smallest kind of tephra. USGS describes it as pieces of crushed rock and volcanic glass smaller than about 2 mm, roughly the size of sand or silt. It forms when gases dissolved in magma expand violently and shatter it. NCERT lists 'ash and dust' as an eruption product.
- 3: correct — Nitrogen is a minor part of volcanic gas. Britannica lists it, along with hydrogen, helium, hydrogen chloride and hydrogen fluoride, as volatiles that come out in small amounts beside the main gases. NCERT names 'nitrogen compounds' among the gases released in an eruption.
- 4: correct — Sulphur gases are among the main volcanic emissions. USGS says water vapour, carbon dioxide and sulphur dioxide make up about 99% of the gas molecules from an eruption, and that hydrogen sulphide is another principal gas. Mount Pinatubo (1991) put about 20 million tonnes of SO2 into the stratosphere.
How to eliminate
Elimination does not get you far here. All four items are real eruption products, so you need to know that. Basic knowledge settles two of them: everyone has seen pictures of ash clouds, and 'pyroclastic' contains 'pyro' (fire) and 'clastic' (broken pieces). Most aspirants also know that SO2 from volcanoes cools the climate. That leaves nitrogen compounds as the only real doubt, and the item comes almost word for word from NCERT Class XI (Fundamentals of Physical Geography, 'Interior of the Earth'): the material reaching the ground includes 'lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon'. When a 'how many' list repeats NCERT wording this closely, every item is often from the same sentence. Remember that NCERT list.
Know more
What comes out of a volcano
Eruption products are usually grouped by physical state.
| State | Product | Key facts |
|---|---|---|
| Liquid | Lava | Molten rock (magma) that reaches the surface. Basaltic lava is runny; silica-rich lava is thick and sticky. |
| Solid (tephra) | Volcanic bombs (> 64 mm), lapilli (2–64 mm), ash (< 2 mm) | All are pyroclasts: rock broken up by explosive gas expansion. Ash is hard, abrasive, does not dissolve in water, and is a danger to jet engines. |
| Gas | H2O, CO2, SO2, H2S, HCl, HF; minor N2, H2, He, Ar, CO | Water vapour, CO2 and SO2 make up about 99% of the gas molecules (USGS). |
Pyroclastic flow vs pyroclastic debris. Debris is the fragments themselves. A pyroclastic flow is a fast, ground-hugging avalanche of hot gas and debris. Flows of this kind buried Pompeii in 79 CE.
Why the gases matter for climate
- SO2 that reaches the stratosphere turns into sulphate aerosols. These reflect sunlight, so the Earth cools. After Pinatubo in 1991, the surface cooled for about three years, by up to about 1.3 °F (USGS).
- CO2 is a greenhouse gas, but volcanoes emit far less of it each year than human activity does.
- Over geological time, outgassing of water vapour, nitrogen and CO2 from the interior helped build the early atmosphere and the oceans.
Link to volcano types. Eruption style controls the products. Shield volcanoes (Hawaii) mostly pour out fluid basalt lava. Composite volcanoes (Fuji, Barren Island) erupt explosively and produce much more tephra. Flood basalt provinces such as the Deccan Traps come from huge outpourings of fluid lava through fissures.
NCERT Concepts to Revise
- Volcanism and climate: Climate cooling after big eruptions comes from one of the gas groups listed in this question, which links the item to a frequently tested effect.