NDA (I) 2025 General Ability Test · Question 104 of 150

Correct — D, Tuff. Tuff is the rock formed when volcanic ash (fine pyroclastic material thrown out in an eruption) settles and becomes compacted and cemented into a solid layer. When that ash is first picked up and moved by running water before being deposited in beds, it forms what is called water-laid (reworked) tuff — a rock that is volcanic in origin yet layered like a sediment. That exactly matches the description in the question.

• (a)Basalt — Basalt is a fine-grained igneous rock formed when fluid lava cools and solidifies (as in the Deccan Traps). It comes from molten lava, not from wind- or water-borne ash, so it is not the answer.
• (b)Lapilli — Lapilli are small pyroclastic fragments (roughly 2-64 mm) blown out during an eruption. They are loose ejected particles, not a consolidated, water-deposited sedimentary layer.
• (c)Slate — Slate is a metamorphic rock formed when shale (a clay-rich sedimentary rock) is compressed and heated. It has nothing to do with volcanic ash deposited by water.

Material thrown out by an explosive volcanic eruption is called pyroclastic (or tephra), and it ranges from fine ash to small lapilli to large blocks and bombs. When the fine ash accumulates and is compacted and cemented, it hardens into a rock called tuff. If water sorts and re-deposits that ash in layers before it lithifies, the result is a water-laid tuff.

Sort the options by origin. Basalt = solidified lava (igneous, from melt, not ash). Lapilli = loose eruption fragments (not a rock layer). Slate = metamorphosed shale (no volcanic ash). Only tuff is the consolidated, layered rock made specifically from volcanic ash — and it is the one that can be deposited by running water, so it fits the description.

• Tuff = rock formed from consolidated volcanic ash (a pyroclastic/volcaniclastic rock).
• Water-laid (reworked) tuff forms when ash is transported and deposited by water in layers.
• Basalt is a fine-grained igneous rock formed from cooled fluid lava.
• Lapilli are pyroclastic fragments about 2-64 mm across; slate is metamorphosed shale.

• Picking basalt because it is 'volcanic' — basalt is from lava, tuff is from ash.
• Confusing loose lapilli fragments with a consolidated ash rock like tuff.

Rock-origin questions give a formation description — match the process (lava, ash, sediment or metamorphism) to the right rock name.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following is a fine-grained igneous rock formed from solidified lava?

  • (a)Tuff
  • (b)Basalt
  • (c)Slate
  • (d)Marble

  Answer(b) Basalt — it forms when fluid lava cools quickly at or near the surface.

• practice — not a real PYQ

  Slate is a metamorphic rock formed from

  • (a)Limestone
  • (b)Sandstone
  • (c)Shale
  • (d)Granite

  Answer(c) Shale — heat and pressure convert clay-rich shale into slate.