Petrology — rock classification by origin Rocks formed by alteration of existing rocks due to heat and/or high pressure (without melting) are called:

Introduction / Context:
Engineering geology classifies rocks based on their origin: igneous, sedimentary, and metamorphic. This classification informs strength, anisotropy, weathering, and suitability for construction and aggregates.

Given Data / Assumptions:

• Metamorphism alters mineralogy and texture in the solid state.
• Agents: heat, pressure, chemically active fluids.
• No bulk melting; otherwise the product would be igneous.

Concept / Approach:
When pre-existing rocks (igneous or sedimentary) are subjected to elevated temperature and pressure, new minerals grow and original textures are reconstituted. The resulting rocks are called metamorphic (e.g., gneiss, schist, marble, quartzite).

Step-by-Step Solution:
Identify process: solid-state recrystallization under T–P conditions.Relate to products: foliation, banding, increased density and strength.Therefore, the correct class is metamorphic rocks.

Verification / Alternative check:
Field features like schistosity or gneissic banding confirm metamorphic processes; lab thin-sections show neoblastic minerals.

Why Other Options Are Wrong:
Sedimentary rocks form by deposition and lithification; igneous rocks form by crystallization from melt; “extrusive” is a subset of igneous, not a separate origin.

Common Pitfalls:
Confusing contact metamorphism near intrusions with partial melting; mislabeling metasomatic alterations as igneous.

Final Answer:
metamorphic rocks