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.
Correct — A, Shield volcano. The stem is a description of basalt. Basaltic lava is very fluid when it erupts, so it spreads far and thin instead of piling up steeply, and a volcano built by hundreds of such flows one on top of another grows into a broad, gently sloping dome — a shield. Barring the flood basalts, shield volcanoes are the largest volcanoes on earth, and the Hawaiian volcanoes are the classic examples. Because the lava is fluid these eruptions are usually of low explosivity, turning violent only if water reaches the vent. Note also the structure of the option set: composite volcano and strato-volcano are two names for the same thing, so neither can be the intended single answer.
- (b)Composite volcano. — A composite volcano is built by eruptions of cooler, more viscous lava than basalt, and typically explosively. Its cone is layered from lava together with large quantities of pyroclastic material and ash accumulated near the vent, which is what gives it the steep classic profile.
- (c)Strato-volcano. — Strato-volcano is simply the other name for a composite volcano — the strata being the alternating layers of lava and ash. Two options meaning the same thing is a structural signal that neither is the answer.
- (d)Cinder-cone volcano. — A cinder cone forms when upcoming lava moves as a fountain and throws out fragments that fall around the vent to build a cone at the top. It is a small, steep pile of ejected material rather than a broad structure built by repeated thin flows.
Volcanoes are grouped by the material they erupt and the shape that material builds. Shield volcanoes are made mostly of basalt, fluid enough to travel far, so they are broad and low. Composite or strato-volcanoes alternate viscous lava with pyroclastics and ash, and are steep and explosive. Cinder cones are small piles of ejecta around a vent. Calderas are the most explosive of all, collapsing into a depression rather than building a structure. Mid-ocean ridge volcanoes erupt along the seventy-thousand-kilometre ridge system that runs through every ocean basin. Flood basalt provinces such as the Deccan Traps are larger still — individual flows there may run for hundreds of kilometres and reach more than fifty metres in thickness.
Two clues in the stem do all the work. Repeated flows building up a volcano over a long period points to a structure made of stacked lava rather than of ash; individual flows being only a few feet thick points to lava fluid enough to spread out rather than pile up. Both describe basalt, and basalt builds shields. The second and stronger clue is in the options themselves — composite and strato name the same volcano, and a single-answer question cannot have two identical answers, so both fall together. The Indian angle worth carrying is the Deccan Traps, which cover most of the Maharashtra plateau and are a flood basalt province rather than a single volcano, formed by the same kind of fluid lava on an enormously larger scale.
- Shield volcanoes are mostly basalt, a lava that is very fluid when erupted, so the volcanoes are broad and not steep.
- Barring the basalt flood flows, shield volcanoes are the largest volcanoes on earth; the Hawaiian volcanoes are the standard examples.
- Shield eruptions are characterised by low explosivity and become explosive mainly if water reaches the vent.
- Composite volcanoes, also called strato-volcanoes, erupt cooler and more viscous lava with large quantities of pyroclastic material and ash.
- The Deccan Traps, covering most of the Maharashtra plateau, are a flood basalt province with individual flows more than 50 m thick that may extend for hundreds of kilometres.
- Choosing composite or strato-volcano without noticing that they are the same thing.
- Associating any large volcano with explosive eruptions; shields are the largest and the least explosive.
- Confusing a flood basalt province such as the Deccan Traps with a single shield volcano.
As a description-to-type item, as a match of volcano to location, or through the intrusive and extrusive landform vocabulary.
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
Widens the same topic from form to consequence — how eruptive strength is graded, which landmasses volcanic activity has built, and why volcanic soils support dense farming. Iceland in particular is built of the fluid basalt that makes shields.
- practice — not a real PYQ
Which type of volcano is built chiefly of basalt and is characterised by low explosivity and gentle slopes?
- (a)Shield volcano
- (b)Composite volcano
- (c)Cinder cone
- (d)Caldera
Answer(a) Shield volcano — fluid basaltic lava spreads far and thin, producing a broad low dome rather than a steep cone.
- practice — not a real PYQ
The Deccan Traps of peninsular India are an example of
- (a)a composite volcano
- (b)a flood basalt province
- (c)a caldera complex
- (d)a mid-ocean ridge volcano
Answer(b) A flood basalt province — vast sheets of fluid basalt erupted in succession, now covering most of the Maharashtra plateau.