UPSC Prelims 2024 Q10: Fold vs Block Mountains of the World — Explained

Consider the following information:

| Sl.No. | Region | Name of the mountain range | Type of mountain |

| 1 | Central Asia | Vosges | Fold mountain |

| 2 | Europe | Alps | Block mountain |

| 3 | North America | Appalachians | Fold mountain |

| 4 | South America | Andes | Fold mountain |

In how many of the above rows is the given information correctly matched?

  1. Only one
  2. Only two
  3. Only three
  4. All four

Answer: (b) Only two

Statement-wise explanation

  • 1: incorrect — The Vosges are in north-eastern France, so they are in Europe, not Central Asia. They are also not fold mountains. They are a classic block mountain (horst): a block of crust pushed up along faults on the western side of the Rhine Rift Valley, a down-dropped block (graben). Both the region and the mountain type are wrong.
  • 2: incorrect — The Alps are in Europe, so the region is right, but the type is wrong. The Alps are young fold mountains. They formed when the African and European plates collided during the Mesozoic and Cenozoic eras. That collision folded and thrust the rock layers; the Alps were not raised as one block along faults.
  • 3: correct — The Appalachians run along eastern North America. They are fold mountains, formed in the late Paleozoic when North America collided with Gondwana while Pangaea was assembling. That collision produced the compressional folding and faulting of the Ridge and Valley Province. They are old fold mountains, and long erosion has worn them down to rounded, low ridges.
  • 4: correct — The Andes run along the western edge of South America. They are young fold mountains, built where the oceanic Nazca Plate sinks (subducts) beneath the South American Plate. That convergence lifts the range and feeds its volcanoes. Aconcagua (6,959 m) is the highest peak in the Western Hemisphere.

How to eliminate

Start with the region column, which needs only a world map. The Vosges are in France, so row 1 is wrong without any geology. The Alps, Appalachians and Andes are clearly in the right continents. So the answer is at most three, and 'All four' is out. Next, use one basic fact: the big mountain chains at plate collision zones (the Alps–Himalaya belt and the Pacific-rim Rockies–Andes belt) are fold mountains. That makes rows 3 and 4 correct. It also makes the 'Block mountain' label for the Alps doubtful. The one fact to remember is that the Alps are young fold mountains, like the Himalayas. Typical examples of block mountains are the Vosges and Black Forest, the two horsts on either side of the Rhine graben. In this question, UPSC swapped a block-mountain example into the region and type columns of other rows.

Know more

Fold mountains vs block mountains

Both form through endogenic forces (diastrophism), but the rocks react differently.

  • Fold mountains form under compression. Where plates converge, rock layers buckle into up-folds (anticlines) and down-folds (synclines), often with thrust faults. Most of the world's long, high chains are fold mountains.
  • Block mountains form through faulting, usually under tension. The crust breaks along faults. A block raised between faults is a horst (block mountain). A block dropped between faults is a graben (rift valley).
Feature Fold mountains Block mountains
Main force Compression at convergent boundaries Tension (sometimes compression) along faults
Structure Anticlines, synclines, thrusts Horst raised between parallel faults
Shape Long, arc-like chains Flat-topped or tilted blocks with steep fault scarps
Examples Himalayas, Alps, Andes, Rockies (young); Appalachians, Urals, Aravallis (old) Vosges, Black Forest (Europe); Sierra Nevada (USA)

Young vs old fold mountains. Young fold mountains, such as the Himalayas, Alps, Andes and Rockies, rose in the Cenozoic during the Alpine orogeny. They are high, rugged and often seismically active. Old fold mountains, such as the Appalachians, Urals and Aravallis, are much older and have been eroded into low, rounded ranges.

The Rhine pair. The Rhine Rift Valley is a graben about 300 km long. It has a horst on each side: the Vosges in France (highest point 1,423 m) and the Black Forest in Germany (Feldberg, 1,493 m). This horst-graben-horst pattern is the standard textbook example of block mountains and a rift valley.

Trap to watch: UPSC often keeps the right name and region in a row but swaps the type, or the reverse. Check every column of each row separately.

NCERT Concepts to Revise

  • Fold mountain: Three rows label a range as a fold mountain. You need this definition to judge which of those labels hold.
  • Block mountain: Two rows use the block-mountain (horst) label. Knowing that block mountains form by faulting helps test those claims.
  • Rift valley: Some block mountains are paired with a graben. Knowing the rift-valley setting helps you recognise them.
  • Convergent boundary: Collisions at convergent boundaries build most major fold chains. This links mountain type to plate setting for each row.
  • Subduction zone: One South American range rises above a subduction zone. This explains how ocean-continent convergence builds fold mountains.

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