NDA (I) 2021 General Ability Test · Question 122 of 149

Correct — D, Himalayan Mountains. The lithosphere (the rigid crust plus the uppermost solid mantle) is thinnest under the oceans and thickest under high fold mountains. Where the Indian and Eurasian plates collided, the crust was buckled and doubled in thickness to form deep mountain 'roots' beneath the Himalayas (crust reaching roughly 70 km). Of the given options, the lithosphere therefore reaches its maximum depth under the Himalayan Mountains.

• (a)Pacific Ocean. — Oceanic lithosphere is thin — a few kilometres of crust over young mantle — so depth here is least, not most.
• (b)Siberian Plain. — A stable continental plain has moderate crustal thickness but lacks the deep collision-thickened root that lifts the Himalayas.
• (c)Patagonian Desert. — A low continental desert surface has no great mountain root, so the lithosphere is far shallower than under the Himalayas.

The lithosphere is the Earth's rigid outer shell — the crust and the topmost part of the mantle. Its thickness is not uniform: oceanic lithosphere is thin, continental lithosphere is thicker, and it reaches its greatest depth beneath high mountain belts, where crust piled up by plate collision extends downward as a root to keep the range isostatically balanced.

The idea to apply is isostasy: tall mountains 'float' on the denser mantle by having deep roots, like an iceberg. So the tallest range in the list — the Himalayas — has the deepest lithosphere. Oceans and plains lack such roots.

• The lithosphere = crust + rigid uppermost mantle.
• It is thinnest under oceans and thickest under high fold mountains.
• The India–Eurasia collision thickened the crust to about 70 km under the Himalayas.
• Deep mountain roots keep ranges balanced by isostasy.

• Assuming the lithosphere is uniform in thickness everywhere.
• Picking the ocean because it is 'deep' — that refers to water depth, not lithosphere thickness.

Asked on where lithosphere/crust is thickest or thinnest, or through isostasy and mountain-root reasoning.

• practice — not a real PYQ

  The Earth's lithosphere is thinnest beneath the

  • (a)high fold mountains
  • (b)ocean basins
  • (c)continental shields
  • (d)plateaus

  Answer(b) ocean basins — oceanic lithosphere is the thinnest.

• practice — not a real PYQ

  The principle by which high mountains have deep 'roots' to stay balanced is called

  • (a)isostasy
  • (b)eustasy
  • (c)subduction
  • (d)denudation

  Answer(a) isostasy — mountains float on the mantle with proportionate roots.