Which of the following statements with reference to stratovolcanoes is/are correct? 1. Stratovolcanoes are tall, steep cones built of layers of felsic lava and volcanic ash. 2. They can erupt explosively as felsic lavas from stratovolcanoes hold large amounts of gas under high pressure. Select the answer using the code given below.

Correct — C, Both 1 and 2. A stratovolcano, also called a composite volcano, is built of alternating layers of lava and pyroclastic ash, which is what the name records — strata. Its lava is silica-rich, running from andesite through to rhyolite, and silica-rich melt is stiff, so the cone it builds is tall and steeply sided rather than broad and low. That same stiffness explains statement 2. Dissolved gas cannot escape easily through viscous magma, so pressure builds until the plug fails and the eruption is explosive, throwing out the ash that forms the alternate layers. The two statements are two halves of one mechanism, which is why both stand. Krakatoa, Vesuvius, Fuji and Mount St Helens are the standard examples.

• (a)1 only — Accepts the shape and the layering but rejects the explosivity, which is the very consequence of the silica-rich lava named in statement 1. Stiff, gas-charged magma is what makes these the dangerous volcanoes.
• (b)2 only — Accepts the explosions but rejects the description of the cone. The alternating lava and ash layers are what the word stratovolcano means.
• (d)Neither 1 nor 2 — Both statements are standard. The contrast the examiner has in mind is with the shield volcano, which is built of runny basaltic lava, is broad and gently sloped, and erupts quietly.

Lava chemistry decides volcano shape. Basaltic lava is low in silica, runny and poor at trapping gas, so it spreads far and builds the broad, gently sloping shield volcanoes of Hawaii and Iceland. Andesitic and rhyolitic lava is high in silica, viscous, and holds gas under pressure, so it piles up near the vent and erupts violently — the tall, steep, layered cones of the subduction zones. Stratovolcanoes are therefore concentrated along the Pacific Ring of Fire.

One statement in this pair states a cause and the other its effect, and a candidate who understands the viscosity link cannot accept one and reject the other. It is worth carrying the shield volcano alongside as the contrast, because papers often ask the two together: gentle slope, fluid basalt, quiet effusive eruption for the shield; steep slope, viscous silica-rich lava, explosive eruption for the composite cone. The word felsic simply means rich in feldspar and silica.

• A stratovolcano or composite volcano is built of alternating layers of lava and pyroclastic ash.
• Its lava is silica-rich — andesitic to rhyolitic — and therefore viscous.
• Viscous magma traps gas, so pressure builds and eruptions are explosive.
• Shield volcanoes are the contrast: fluid basaltic lava, broad gentle slopes, quiet effusive eruptions.
• Vesuvius, Fuji, Krakatoa and Mount St Helens are standard stratovolcanoes, mostly on the Ring of Fire.

• Accepting one statement and rejecting the other when they are cause and effect.
• Associating all volcanoes with runny lava and quiet eruptions.
• Reading 'felsic' as an unfamiliar term rather than as silica-rich.

A two-statement item where both sentences describe one mechanism, so the pair stands or falls together.

• practice — not a real PYQ

  Broad, gently sloping volcanoes built by repeated flows of fluid basaltic lava are called

  • (a)shield volcanoes
  • (b)composite volcanoes
  • (c)cinder cones
  • (d)calderas

  Answer(a) shield volcanoes — Mauna Loa in Hawaii is the standard example.

• practice — not a real PYQ

  Explosive volcanic eruptions are chiefly associated with magma that is

  • (a)low in silica and low in gas
  • (b)high in silica and gas-rich
  • (c)high in iron and magnesium
  • (d)at very high temperature and very fluid

  Answer(b) high in silica and gas-rich — viscosity traps gas until pressure forces a violent release.