Which of the following statements about volcanoes are correct? 1. The strength of a volcano is measured by the Volcanic Explosive Index. 2. Iceland and Philippines were created by volcanic activity. 3. Volcanic soils are rich, deep and fertile and allow intensive agriculture to take place. Select the correct answer using the code given below.
- (a)1 and 2 only
- (b)2 and 3 only
- (c)1 and 3 only
- (d)1, 2 and 3
Correct — D, 1, 2 and 3. Every statement holds. The scale used to size an explosive eruption is the Volcanic Explosivity Index, devised by Christopher Newhall and Stephen Self in 1982; it runs from 0 for quiet effusive activity to 8 for the mega-colossal eruptions, and it grades an eruption by the volume of material thrown out, the height of the eruption column and the duration. Statement 2 holds for both places named, though for opposite plate-tectonic reasons: Iceland stands where the Mid-Atlantic Ridge, a spreading boundary, rises above sea level, while the Philippine islands belong to the arcs of the Pacific Ring of Fire, built where one plate is driven under another. Statement 3 is the agricultural half of the story — soils weathered from volcanic ash are deep and nutrient-rich, and they carry some of the densest farming populations on earth, from the rice terraces of Java to the coffee and tea country of the tropical highlands.
- (a)1 and 2 only — Rejects the soil claim, which is the best-supported of the three. Ash-derived soils are classed as andosols precisely because their weathering products hold nutrients well and support intensive cultivation.
- (b)2 and 3 only — Rejects the index. There is a standard scale for eruption size, and the printed name is a slightly loose rendering of it rather than an invented one.
- (c)1 and 3 only — Rejects the claim about Iceland and the Philippines, both of which are volcanic in origin — Iceland along a spreading ridge, the Philippines in a subduction arc.
Volcanoes cluster where plates meet or split. At spreading ridges magma rises to fill the gap, and where a ridge is high enough it breaks the surface as land, which is what Iceland is. At convergent margins one plate sinks beneath another, water drives melting in the wedge above, and a chain of volcanic islands or mountains forms — the pattern all round the Pacific Ring of Fire, the Philippines included. Explosive eruptions are graded on the Volcanic Explosivity Index, a logarithmic scale on which each step upward represents roughly a tenfold increase in the material ejected.
This is a three-true item, and the way to be confident of that is to test each claim separately instead of hunting for the odd one out. Two small points are worth carrying away. First, the paper prints 'Volcanic Explosive Index'; the scale's proper name is the Volcanic Explosivity Index, so this is the booklet's own wording and not a different instrument — do not let the wording talk you out of a correct statement. Second, the same volcanic process that destroys is what makes the soil: ash weathers into minerals that hold nutrients, which is why the flanks of active volcanoes in Java, the Philippines and the East African highlands are farmed as hard as any land in the world despite the risk.
- The Volcanic Explosivity Index was devised by Christopher Newhall and Stephen Self in 1982 and runs from 0 to 8.
- It grades eruptions by volume of ejecta, height of the eruption column and duration, and is logarithmic — each step is about a tenfold increase.
- Iceland sits astride the Mid-Atlantic Ridge, a divergent boundary that rises above sea level there.
- The Philippine islands are part of the Pacific Ring of Fire, where volcanism follows subduction at convergent margins.
- Soils weathered from volcanic ash are classed as andosols and are highly fertile; Java's ash soils support some of the densest rural populations in the world.
- Rejecting a statement because the paper renders a technical name loosely — 'Volcanic Explosive Index' is the same scale as the Volcanic Explosivity Index.
- Assuming Iceland and the Philippines must share a tectonic setting because both are volcanic; one is a ridge, the other an arc.
- Treating volcanic regions as barren — the fertility of ash soils is the reason people live on active flanks.
As a statements item combining the measuring scale, the tectonic origin of specific islands and the soil consequence, or as a single question on where the Ring of Fire runs.
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?
- (a) Only one
- (b) Only two
- (c) Only three
- (d) All four
Answer(d) All four
The output side of the same topic. That item asks what an eruption throws out; this one asks how the throwing-out is measured and what the ash eventually becomes. The ash and dust of the UPSC list are the parent material of the fertile soils in statement 3 here.
Which of the following statements about Ring of Fire is/are correct ? 1. It is a zone of convergence plate boundaries 2. It is an active seismic and volcanic zone 3. It is associated with deep trench Select the correct answer using the code given below :
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 only
- (d) 1, 2 and 3
Answer(d) 1, 2 and 3
Directly supports statement 2 here. The Ring of Fire is the belt of convergent boundaries that built the Philippine arcs, and the NDA item spells out the convergence, the volcanism and the seismicity together.
- practice — not a real PYQ
The Volcanic Explosivity Index grades an eruption chiefly on which one of the following?
- (a)The temperature of the lava
- (b)The volume of ejecta and the height of the eruption column
- (c)The depth of the magma chamber
- (d)The number of casualties caused
Answer(b) The volume of ejecta and the height of the eruption column — with duration also taken into account, on a logarithmic scale from 0 to 8.
- practice — not a real PYQ
Iceland owes its origin to volcanic activity along which one of the following?
- (a)A subduction zone
- (b)A transform fault
- (c)A mid-oceanic ridge
- (d)A continental rift valley
Answer(c) A mid-oceanic ridge — Iceland is the part of the Mid-Atlantic Ridge that rises above sea level.