In geology, marble is a metamorphic rock formed by the recrystallisation of which sedimentary rock?

Introduction / Context:
Rocks are broadly classified into igneous, sedimentary and metamorphic types. Metamorphic rocks are formed when existing rocks are subjected to high temperature and pressure, causing physical and chemical changes. Marble is a classic example often used in school geology to illustrate metamorphism. This question asks you to recall which sedimentary rock, when subjected to metamorphic conditions, transforms into marble.

Given Data / Assumptions:

• We know that marble is a metamorphic rock.
• We must identify the original sedimentary rock from which marble forms.
• Options are sandstone, limestone, granite and clay.

Concept / Approach:
Marble is formed when limestone, a sedimentary rock composed mainly of calcium carbonate, undergoes metamorphism. Under heat and pressure, the calcite grains recrystallise to form a denser rock with interlocking crystals, giving marble its characteristic texture and appearance. Granite, in contrast, is an igneous rock, not sedimentary, and sandstone and clay form different types of metamorphic rocks such as quartzite and slate, not marble. Therefore, the correct original rock for marble is limestone.

Step-by-Step Solution:
Step 1: Recall that limestone is a sedimentary rock made largely of calcium carbonate from shells and marine organisms. Step 2: Understand that when limestone experiences high temperature and pressure deep in the crust, its structure changes. Step 3: This recrystallisation produces marble, a metamorphic rock valued for sculpture and building. Step 4: Compare with sandstone, which under metamorphism forms quartzite, not marble. Step 5: Recognise that granite is igneous, so it cannot be the sedimentary parent rock of marble. Step 6: Clay rich sediments usually form slate or schist under metamorphism, not marble. Step 7: Therefore, choose limestone as the correct answer.

Verification / Alternative check:
Standard geology tables show common metamorphic transformations: limestone to marble, sandstone to quartzite, shale or clay to slate and schist. If you remember this table from school, it becomes easy to pair marble with limestone. Additionally, many references explicitly describe marble as metamorphosed limestone, confirming the link.

Why Other Options Are Wrong:
Sandstone: Under metamorphism, sandstone typically turns into quartzite, which is hard and composed mainly of quartz, not marble.
Granite: This is an intrusive igneous rock formed from cooling magma and is not a sedimentary parent rock; it gives rise to different metamorphic forms, not marble.
Clay: Clay rich rocks metamorphose into slate and later schist, not into marble, which requires a carbonate precursor like limestone.

Common Pitfalls:
Students sometimes mix up pairs of rock transformations because they learn them all at once. A helpful memory trick is to group by main mineral: carbonate rich limestone leads to marble, quartz rich sandstone leads to quartzite and clay rich shale or mudstone leads to slate. Keeping this pattern in mind reduces confusion when faced with such matching questions in exams.

Final Answer:
Marble is a metamorphic rock formed by the recrystallisation of the sedimentary rock Limestone.