Consider the following statements about rock structure: 1. Conglomerates are clastic sedimentary rocks. 2. Slate is a non-clastic sedimentary rock. 3. Granite is a plutonic igneous rock. 4. Basalt is an extrusive igneous rock. Which of the statements given above are correct ?
- A.1,2 and 3 only
- B.3 and 4 only
- C.1 and 2 only
- D.1,3 and 4 only
Correct answer
Explanation
The correct answer is D, 1, 3 and 4 only. Statement 2 is wrong because slate is a metamorphic rock, formed when shale is squeezed and heated, not a sedimentary rock.
Rocks fall into three groups. Sedimentary rocks form from layers of sediment; clastic ones are made of broken fragments, such as conglomerate (rounded pebbles cemented together), sandstone and shale. Igneous rocks form from cooled magma: plutonic or intrusive rocks like granite cool slowly deep underground and have large crystals, while extrusive or volcanic rocks like basalt cool quickly at the surface and are fine-grained. The Deccan Traps are made of basalt. Metamorphic rocks form when existing rocks are changed by heat and pressure: shale becomes slate, limestone becomes marble and sandstone becomes quartzite.
Why the others are wrong: A and C both include statement 2, which is false. B leaves out statement 1, which correctly describes conglomerate as a clastic sedimentary rock.
Exam tip: shale to slate, limestone to marble, sandstone to quartzite, granite to gneiss; granite intrusive, basalt extrusive.
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