How are rocks classified into igneous, sedimentary, and metamorphic rocks?

• A. By how they form: igneous from cooling lava/m magma, sedimentary from sediments, metamorphic from existing rocks under heat/pressure.
• B. By color and grain size.
• C. By how hard they are.
• D. By mineral content only.

Answer: (A)

Rocks are grouped by how they form. Igneous rocks come from molten material that cools and hardens, whether lava erupting at the surface or magma cooling underground. Sedimentary rocks form from sediments—bits of other rocks, minerals, or organic material—that are deposited, buried, compacted, and cemented over time. Metamorphic rocks start as existing rocks and are transformed by heat and pressure deep inside Earth, which changes their minerals and structure without melting them.

This formation-based classification is why igneous rocks like granite and basalt are linked to cooling molten material, sedimentary rocks like sandstone and limestone are linked to deposition and cementation of sediments, and metamorphic rocks like marble and slate are linked to alteration under heat and pressure.

Color and grain size can vary within any rock type and aren’t reliable alone for classification. Hardness also isn’t a consistent basis for grouping rocks, since hardness varies widely within each category. Mineral content helps describe rocks, but rocks with similar minerals can belong to different groups depending on how they formed.