Engineering Geology – Rock Class Formed from Molten Magma Rocks that originate by crystallization or solidification of molten magma are classified as which rock type?

Introduction:
Engineering geology divides rocks into three major genetic classes: igneous, sedimentary, and metamorphic. Correctly identifying the origin class helps predict mechanical properties, weathering behavior, and suitability for use as building stones and aggregates.

Given Data / Assumptions:

• Molten magma cools either intrusively (slowly underground) or extrusively (rapidly at the surface).
• Crystallization produces interlocking mineral textures.
• Examples include granite (intrusive) and basalt (extrusive).

Concept / Approach:

Rocks formed directly from magma are classed as igneous. Their coarse or fine textures reflect cooling rate, which in turn affects properties such as strength and durability—key considerations in civil works.

Step-by-Step Solution:

Verification / Alternative check:

Standard geology texts define igneous rocks as those formed by magma solidification and contrast them with sedimentary and metamorphic categories.

Why Other Options Are Wrong:

Sedimentary: formed from sediments by deposition/lithification. Metamorphic: formed by heat/pressure acting on pre-existing rocks. Evaporitic: a subset of sedimentary formed by chemical precipitation.

Common Pitfalls:

Confusing metamorphosed igneous rocks with original igneous class; using textural terms (e.g., crystalline) without considering genesis.

Final Answer:

Igneous rocks