In geology, igneous rocks are primarily classified on the basis of which two key properties?

Introduction / Context:
Igneous rocks form when molten rock material, known as magma or lava, cools and solidifies. Geologists classify igneous rocks to understand how and where they formed, which in turn helps interpret the history of Earth crust. Two main properties are used in this classification. This question asks you to recall that both mineral composition and texture are important in naming and understanding igneous rocks, rather than relying on a single property like colour alone.

Given Data / Assumptions:

• The rock type is igneous, not sedimentary or metamorphic.
• The options mention composition, texture, both together, and colour pattern only.
• We assume school level or introductory geology classification methods.

Concept / Approach:
Composition describes which minerals and chemical elements are present in the rock, such as quartz, feldspar, mica, or olivine, and in what proportions. This affects whether a rock is felsic, intermediate, mafic, or ultramafic. Texture describes the size, shape, and arrangement of the mineral grains, such as fine grained, coarse grained, glassy, or porphyritic. Texture reflects the cooling history; rapid cooling produces fine or glassy textures, while slow cooling produces large crystals. Geologists use both composition and texture to classify igneous rocks into common types like granite, basalt, rhyolite, and gabbro.

Step-by-Step Solution:
1. Recognize that igneous rocks originate from cooled magma or lava.2. Identify composition as the mineral and chemical makeup of the rock, which determines categories such as felsic or mafic.3. Identify texture as the physical appearance of the crystals or grains, such as coarse or fine, which indicates cooling rate and environment.4. Understand that granite and rhyolite, for example, have similar felsic compositions but different textures, so they are classified separately.5. Similarly, basalt and gabbro are both mafic in composition but differ in texture because of different cooling histories.6. Therefore, igneous rocks are classified using both composition and texture together, not just one of these properties or colour alone.

Verification / Alternative check:
Classification charts for igneous rocks in textbooks are typically two dimensional, with composition on one axis and texture on the other. For instance, a coarse grained felsic rock is named granite, while a fine grained felsic rock is named rhyolite. The same approach is used for intermediate and mafic compositions. Colour can provide a rough indication of composition, such as lighter colours for felsic rocks and darker colours for mafic rocks, but colour alone is not enough to determine the exact rock type. This confirms that both composition and texture are the primary classification criteria.

Why Other Options Are Wrong:
Option A, composition only, is incomplete because many rocks share similar compositions but differ in texture and therefore have different names. Option B, texture only, is also incomplete because rocks with similar textures but different compositions have different names and properties. Option D, colour pattern only, is incorrect because colour can be misleading and is influenced by surface weathering and lighting conditions, and it does not uniquely determine composition or texture.

Common Pitfalls:
Students sometimes rely on colour alone to identify rocks in field trips or pictures, which can lead to misclassification. Another pitfall is to memorize a few rock names without understanding the combined role of composition and texture. To avoid these errors, it helps to remember that classification charts always consider what minerals are present and how big the crystals are. Practising with rock samples or diagrams can make the combined use of composition and texture more intuitive.

Final Answer:
Igneous rocks are primarily classified on the basis of both composition and texture together, which reflect their mineral makeup and cooling history.