Which of the following rocks is sedimentary?

Step 1: Analyze the question You are asked to pick the one rock that is sedimentary. The other three must belong to the igneous or metamorphic groups.

Step 2: The concept Rocks fall into three main types:

• Igneous: formed when molten magma or lava cools and solidifies (e.g. granite, basalt).
• Sedimentary: formed when sediments such as sand, mud, shells, or plant remains are deposited in layers, then compacted and cemented over time (e.g. sandstone, limestone, shale, coal).
• Metamorphic: existing rocks changed by intense heat and pressure (e.g. gneiss, marble, slate, schist).

Step 3: Classify each option

• A. Coal: formed from the compaction of dead plant material buried in layers over millions of years. It is sedimentary (an organic sedimentary rock). Nigeria's Enugu coal deposits are a good local example.
• B. Gneiss: formed when granite or similar rocks are subjected to heat and pressure. It is metamorphic.
• C. Granite: formed from slowly cooled magma deep underground. It is igneous.
• D. Marble: formed when limestone is metamorphosed. It is metamorphic.

Step 4: Answer Only coal is sedimentary, so the correct option is A.

Exam tip: Remember that sedimentary rocks are often layered and may contain fossils or organic material. Coal, limestone, sandstone, and shale are the usual examples. For metamorphic rocks, think of changed versions: limestone → marble, granite → gneiss, shale → slate, and sandstone → quartzite.