Which of the following statements regarding Arc Volcanoes is/are correct? 1. Arc volcanoes are found near divergent plate boundaries. 2. Some of the world's classical examples of cone-shaped stratovolcanoes, such as Mount Fuji in Japan, Mayon in the Philippines are Arc volcanoes. 3. Some of the highest volcanoes on Earth are of the Arc type, notably Nevado Ojos del Salado in the Chilean Andes. Select the answer using the code given below :
- (a)2 only
- (b)2 and 3 only
- (c)1 and 3 only
- (d)1, 2 and 3
Correct — A, 2 only. That is the option marked for this paper. Statement 1 is certainly false, and it is the reversal the item is built on: arc volcanoes are the product of convergent boundaries, where one plate is forced beneath another, water driven out of the descending slab lowers the melting point of the mantle above it, and the magma that results rises to build a curved chain of volcanoes parallel to the trench. Divergent boundaries make volcanoes too, but they make mid-ocean ridges and rift valleys, not arcs. Statement 2 is right: Fuji in Japan and Mayon in the Philippines are the textbook cone-shaped stratovolcanoes, both standing on island arcs above subduction zones, and their steep symmetrical profile comes from alternating layers of lava and ash. Statement 3 is where this card parts company with the marked answer. Nevado Ojos del Salado, at 6,893 metres on the Chile–Argentina border, is the highest volcano on Earth, and it belongs to the Central Volcanic Zone of the Andes — a continental volcanic arc raised by the subduction of the Nazca plate beneath South America. On that reading statement 3 is true and the answer would be (b). The case for excluding it is that ‘arc volcano’ is sometimes reserved for island arcs, and that the summit sits on the border rather than wholly in Chile. Carry away the mechanism, which no reading disputes: arcs sit above subduction zones.
- (c)1 and 3 only — Keeps the one statement that is definitely false. Arc volcanism is a convergent-margin process; nothing about it belongs at a divergent boundary.
- (d)1, 2 and 3 — Fails on statement 1 for the same reason. The plate boundary named there is the wrong one.
Volcanoes cluster along plate boundaries, and which boundary decides what kind of volcano you get. At divergent margins, magma wells up to fill the gap and builds the mid-ocean ridges, with Iceland as the place the ridge rises above sea level. At convergent margins, the descending slab releases water into the overlying mantle wedge, melting starts, and the magma is more viscous and gas-rich, so eruptions are explosive and the cones are steep. Where ocean meets ocean the result is an island arc — Japan, the Philippines, the Aleutians, the Andamans with Barren Island. Where ocean meets continent it is a continental arc, as along the western edge of South America. The Pacific Ring of Fire is the sum of these arcs.
Read the plate boundary first. An item that says arc volcanoes belong to divergent boundaries has already made its error, and no amount of knowledge about Mount Fuji will change that, so statement 1 falls at once and every option containing it goes with it. That leaves a choice between statement 2 alone and statements 2 and 3, and since the paper has no published key the choice cannot be settled cleanly. Two facts are worth taking away regardless: Ojos del Salado is the highest volcano on Earth, and the Andean chain is a continental volcanic arc. Both are standard, and both are the kind of thing a later question is likely to ask directly.
- Arc volcanoes form at convergent boundaries, above a subducting slab, in a chain curved parallel to the ocean trench.
- Mount Fuji in Japan and Mayon in the Philippines are cone-shaped stratovolcanoes built of alternating lava and ash on island arcs.
- Nevado Ojos del Salado, on the Argentina–Chile border, rises to 6,893 metres and is the highest volcano on Earth.
- Ojos del Salado belongs to the Central Volcanic Zone of the Andes and marks its southern boundary.
- Divergent boundaries produce mid-ocean ridges and rift volcanism rather than arcs; Iceland is the ridge above sea level.
- Attaching arc volcanoes to divergent boundaries because both settings produce volcanoes.
- Assuming an arc must be a chain of islands; the Andes are a continental arc.
- Confusing a shield volcano, built by repeated thin lava flows, with the steep composite cones of an arc.
As a statements item on where arc volcanoes form, or as a naming question giving a cone-shaped volcano and asking which setting it belongs to.
Individual lava flows are normally only a few feet thick, but over a long period of time, repeated flows may build up a volcano. Such volcanoes are termed as:
- (a) Shield volcano.
- (b) Composite volcano.
- (c) Strato-volcano.
- (d) Cinder-cone volcano.
Answer(a) Shield volcano.
The contrasting form. A shield is built by repeated thin flows of fluid lava at a hot spot or a ridge; the steep composite cones named in this item are what subduction magma builds instead.
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
Answer(d) 1, 2 and 3
Puts the two settings side by side. Iceland sits on a spreading ridge and the Philippines on an island arc, which is the divergent-against-convergent contrast that statement 1 of this item gets the wrong way round.
- practice — not a real PYQ
Volcanic arcs are most closely associated with which type of plate boundary?
- (a)Divergent
- (b)Convergent
- (c)Transform
- (d)Passive continental margin
Answer(b) Convergent — the chain forms above a subducting slab, parallel to the ocean trench.
- practice — not a real PYQ
Which of the following is the highest volcano on Earth?
- (a)Mount Kilimanjaro
- (b)Mauna Loa
- (c)Nevado Ojos del Salado
- (d)Mount Fuji
Answer(c) Nevado Ojos del Salado — 6,893 metres, on the Argentina–Chile border in the Andes.