Consider the following statements: 1. Many of the world's largest mountain chains exist beneath the sea. 2. Some mountain chains are revealed as island arcs. 3. The mid-oceanic ridges form the longest mountain chains. 4. The mid-Atlantic ridge rises thirty-three metres above the floor of the Atlantic. Which of the statements given above are correct?

Correct — D, 1, 2 and 3 only. The first three are sound. Most of the planet's mountain relief is under water, because the mid-ocean ridge system runs unbroken through every ocean basin for something like 65,000 km — far longer than the Andes, the Rockies and the Himalaya put together. Some submarine chains do break the surface as island arcs, the curved volcanic strings such as the Aleutians, Japan and the Indonesian arc that mark subduction zones. Statement 4 is where the item is decided: the Mid-Atlantic Ridge does not rise thirty-three metres above the Atlantic floor, it rises by the order of two to three thousand metres, and in Iceland it clears the sea surface altogether. Thirty-three metres would be a ten-storey building placed on the abyssal plain.

• (a)1 and 2 only — Correctly rejects statement 4 but also drops statement 3, though the mid-ocean ridge system is precisely the longest mountain chain on Earth — that is its single most quoted property.
• (b)3 and 4 only — Keeps the one false statement and throws away two true ones. Statement 4's thirty-three metres is out by roughly two orders of magnitude.
• (c)1, 2 and 4 only — Accepts the thirty-three metre figure and rejects the ridge's status as the longest chain — exactly the wrong way round on both counts.

Mid-ocean ridges are constructive plate margins. Magma rises where two plates pull apart, cools to form new oceanic crust, and the fresh, hot, low-density rock stands high above the older, colder, denser floor on either side. The ridge is therefore a mountain range built by sea-floor spreading, with a rift valley along its crest, transform faults cutting across it, and symmetrical magnetic stripes recorded in the rocks on both flanks.

Island arcs come from the opposite kind of margin and it is worth keeping the two apart. A ridge is where plates separate; an arc is where an oceanic plate is pushed under another, melting at depth and feeding a curved line of volcanoes that can rise above sea level — the Aleutians, the Kuriles, Japan, the Philippines, Indonesia and the Antilles. Both are submarine mountain chains, which is why statements 1 and 2 sit together comfortably. On the numbers, the useful ones to remember are that the ridge system is around 65,000 km long, that it stands roughly 2 to 3 km above the neighbouring abyssal plains, and that Iceland, the Azores, Ascension and Tristan da Cunha are its exposed summits. Any option putting the ridge's relief in tens of metres has confused a mountain range with a sandbank.

• The mid-ocean ridge system runs about 65,000 km through the world's oceans and is the longest mountain chain on Earth.
• Ridges stand roughly 2 to 3 km above the adjacent ocean floor; Iceland is the Mid-Atlantic Ridge above sea level.
• Ridges are constructive margins where sea-floor spreading creates new oceanic crust; a central rift valley runs along the crest.
• Island arcs are curved volcanic chains at subduction zones — the Aleutians, Japan, the Philippines and Indonesia among them.
• Symmetrical magnetic striping on either side of a ridge was the decisive evidence for sea-floor spreading.

• Accepting an oddly precise small number in a statement because precision reads as authority.
• Treating every submarine volcanic chain as a ridge feature — Hawaii is a hotspot track, not a ridge.
• Forgetting that the ridge system is continuous across ocean basins, which is what makes it the longest chain.

Asked as a four-statement item where three lines are textbook and the fourth carries a planted numerical error.

• practice — not a real PYQ

  The longest mountain chain on the Earth's surface is

  • (a)the Andes
  • (b)the Himalaya
  • (c)the mid-ocean ridge system
  • (d)the Rockies

  Answer(c) the mid-ocean ridge system — continuous through the world's oceans for roughly 65,000 km, almost all of it under water.

• practice — not a real PYQ

  Island arcs are typically associated with which one of the following plate boundaries?

  • (a)Divergent boundaries
  • (b)Convergent boundaries with subduction
  • (c)Transform boundaries
  • (d)Passive continental margins

  Answer(b) Convergent boundaries with subduction — the descending plate melts at depth and feeds a curved chain of volcanoes.