Which statement best defines igneous rock?

Which statement best defines igneous rock?

• A. Rock formed by the solidification of molten magma
• B. Rock formed when sediments are compacted and cemented together
• C. Rock formed by precipitation from mineral-rich solutions
• D. Rock changed in structure by heat, moisture, and pressure

Answer: (A)

Rocks are classified by how they form. Igneous rocks form from molten material that cools and solidifies, whether that happens underground as magma or on the surface as lava. This is why the statement about solidification of molten magma is the best description. The other descriptions fit other rock types: sedimentary rocks form when sediments are compacted and cemented; chemical sedimentary rocks form by precipitation from mineral-rich solutions; metamorphic rocks are created when existing rocks are altered by heat, pressure, and chemical processes without melting. So the defining idea is the cooling and solidification of molten rock.

Rocks are classified by how they form. Igneous rocks form from molten material that cools and solidifies, whether that happens underground as magma or on the surface as lava. This is why the statement about solidification of molten magma is the best description. The other descriptions fit other rock types: sedimentary rocks form when sediments are compacted and cemented; chemical sedimentary rocks form by precipitation from mineral-rich solutions; metamorphic rocks are created when existing rocks are altered by heat, pressure, and chemical processes without melting. So the defining idea is the cooling and solidification of molten rock.