Understanding Rock Formation: Igneous Rocks

Rocks are naturally occurring solid aggregates of minerals. They are classified based on how they are formed. There are three main types of rocks:

• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks

The question asks about rocks formed by the cooling and crystallisation of magma. This specific process describes the formation of igneous rocks.

Formation of Igneous Rocks

Igneous rocks are formed when molten rock material, known as magma (when beneath the Earth's surface) or lava (when erupted onto the Earth's surface), cools and solidifies. The cooling process causes minerals within the molten rock to crystallise. The size of the crystals depends on how fast the cooling occurs:

• Slow cooling beneath the surface allows for larger crystals to form (intrusive or plutonic igneous rocks).
• Rapid cooling on the surface results in smaller crystals or even a glassy texture (extrusive or volcanic igneous rocks).

Analyzing the Options

Let's examine each option based on how it is formed:

• Granite: Granite is a common type of felsic intrusive igneous rock. It is formed from the slow cooling and solidification of magma deep beneath the Earth's surface. This process results in its characteristic coarse-grained texture, where individual mineral crystals are visible to the naked eye. Therefore, Granite fits the description of a rock formed by the cooling and crystallisation of magma.
• Chalk: Chalk is a soft, porous, sedimentary rock. It is primarily composed of calcium carbonate (CaCO₃), largely derived from the fossilised remains of microscopic marine organisms called coccolithophores. Sedimentary rocks are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Chalk is not formed from magma.
• Coal: Coal is a combustible black or brownish-black sedimentary rock, classified as a fossil fuel. It is formed from the accumulation and preservation of plant remains, usually in a swamp setting. Over millions of years, these plant remains are subjected to heat and pressure, transforming them into coal. Coal is an organic sedimentary rock and is not formed from magma.
• Shell: A shell is the hard, protective outer layer of certain invertebrates, such as molluscs. Shells are made of calcium carbonate. While shells can be deposited and compacted to form sedimentary rocks (like some types of limestone), a shell itself is an organic structure, not a rock type formed by the cooling of magma.

Conclusion

Based on the formation processes, Granite is the only rock among the options that is formed by the cooling and crystallisation of magma.

The final answer is Granite.

Revision Table: Rock Types & Formation

Additional Information on Igneous Rock Formation

Igneous rocks are often called "primary rocks" because they are formed from the original molten material of the Earth's crust and mantle. The process of crystallisation involves the arrangement of atoms and molecules into an ordered, repetitive structure as the molten rock cools. Different minerals crystallise at different temperatures.

Igneous rocks make up a significant portion of the Earth's crust. Examples include basalt (common extrusive rock, forms oceanic crust) and granite (common intrusive rock, forms continental crust). The composition of the magma or lava also influences the type of igneous rock formed.