Building Stones MCQ with answer and solution

Concept

Some sedimentary rocks form from remains of living things (organic sedimentary rocks), so biological activity is part of their formation.

Step-by-step solution

1. Igneous rocks form by cooling of magma or lava. No biological activity. 2. Metamorphic rocks form when existing rocks change under heat and pressure. No biological activity. 3. Sedimentary rocks form by weathering, transport, deposition and cementation (mechanical and chemical processes). 4. Some sedimentary rocks are organic. Coal forms from plant matter, and some limestone and chalk form from shells and skeletons of sea organisms. 5. So biological activity is involved only in sedimentary rocks. Answer is (C).

Explanation

Sedimentary rocks form by mechanical and chemical processes such as erosion, deposition and cementation. Some of them, like coal, chalk and fossil limestone, also form from the remains of plants and animals. So their formation can involve biological activity.

Why the other options are wrong

A: Metamorphic rocks form by heat and pressure, not by biological activity. B: Metamorphism is a solid-state change of existing rock, with no living organisms involved. D: Igneous rocks form from molten magma, and life plays no role.

Shortcut

Think 'fossils and coal': both are found in sedimentary rocks, so biology means sedimentary.

Common mistake

Choosing (A) because marble comes from limestone. The biological activity happens only in forming the sedimentary parent rock, not in metamorphism.