Understanding Marble and Metamorphism

The question asks about the original rock type that transforms into marble through a process called metamorphism. Metamorphism is a geological process where rocks are changed by heat, pressure, and chemical activity.

When an existing rock is subjected to high temperatures and pressures, its mineral composition, texture, or structure can change significantly, forming a new type of rock called a metamorphic rock. Marble is a classic example of a metamorphic rock.

The Formation of Marble

Marble is specifically formed from the metamorphism of carbonate rocks. The most common carbonate rock is limestone. Limestone is primarily composed of the mineral calcium carbonate ($\text{CaCO}_3$).

When limestone is buried deep within the Earth's crust or is subjected to intense heat and pressure due to tectonic activity, the calcium carbonate crystals within the limestone recrystallize. This recrystallization process results in the formation of a dense, interlocking mosaic of larger calcite crystals, which is what we know as marble.

Different types of original limestone and varying metamorphic conditions can lead to different colors and patterns in the resulting marble. Impurities present in the original limestone, such as clay, sand, iron oxides, or chert, can result in marble with swirls, veins, and a wide range of colors (grey, pink, green, black, etc.) in addition to the pure white marble formed from very pure limestone.

Analyzing the Options

Let's look at the given options and see what metamorphic rocks they typically form:

• Shale: Shale is a fine-grained sedimentary rock composed mainly of clay minerals. When shale undergoes metamorphism, it typically transforms into slate, then phyllite, schist, and eventually gneiss, depending on the intensity of the metamorphism. It does not form marble.
• Basalt: Basalt is an extrusive igneous rock. The metamorphism of basalt can produce various metamorphic rocks, such as greenschist or amphibolite, depending on the conditions. It does not form marble.
• Sandstone: Sandstone is a sedimentary rock composed mainly of sand grains, often quartz. When sandstone undergoes metamorphism, the quartz grains recrystallize and fuse together, forming quartzite. It does not form marble.
• Limestone: Limestone is a sedimentary rock composed primarily of calcium carbonate. As discussed, the metamorphism of limestone results in the formation of marble.

Parent Rock of Marble - Summary

Based on the process of metamorphism and the composition of the rocks, limestone is the parent rock that metamorphoses into marble.

Revision Table: Rock Types and Metamorphism

Additional Information on Metamorphism and Rocks

Metamorphism can occur in several settings:

• Contact Metamorphism: Occurs when rocks are heated by direct contact with magma or lava. This typically results in high temperature, low-pressure metamorphism.
• Regional Metamorphism: Occurs over large areas, often associated with mountain building (orogeny). This involves high pressure and high temperature metamorphism.
• Dynamic Metamorphism: Occurs along fault zones where rocks are subjected to intense pressure and shearing forces.
• Hydrothermal Metamorphism: Occurs when hot, chemically active fluids pass through rocks, altering their composition.

Marble is a non-foliated metamorphic rock, meaning it does not have layers or bands formed by the alignment of minerals, unlike foliated rocks like slate or schist. This is because its primary mineral, calcite, tends to recrystallize into equant (equally-sized) crystals rather than platy or elongated ones under metamorphic conditions.