--- title: "Petrogenesis, igneous rock identification, rock significance" subject: "Engineering Geology I" chapter: 4 key: "semester_3/engineering_geology_i:petrogenesis-igneous-rock-identification-rock" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/petrogenesis-igneous-rock-identification-rock site: BCT Engineering --- # Petrogenesis, igneous rock identification, rock significance Engineering Geology I · Chapter 4: Petrology - Asked in: 2078 Bhadra, 2+2+3 marks; 2075 Chaitra, 2+3 marks (asked 2 times) - Video: [What Are Igneous Rocks?](https://www.youtube.com/watch?v=PrN7jygu4cQ) — ClickView - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/petrogenesis-igneous-rock-identification-rock - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4 ## Question Describe petrogenesis. How do you identify igneous rocks in the field? Describe the civil engineering significance of granite, phyllite and sandstone. ## Short answer Petrogenesis deals with rock origin (igneous from magma crystallization, sedimentary from lithification, metamorphic by heat/pressure). Igneous rocks are identified by crystalline texture, massive structure, no fossils, minerals like quartz/feldspar, and occurrence as dykes or lava flows. Granite is hard (100–250 MPa) for foundations; phyllite is weak and foliated, unstable in slopes; sandstone strength depends on cement, it is porous and used as building stone. ## Answer #### Petrogenesis **Petrogenesis** is the branch of petrology that deals with the origin and formation of rocks. - **Igneous rocks**: crystallisation of magma or lava, by cooling and differentiation. - **Sedimentary rocks**: weathering, transport, deposition and lithification of sediments. - **Metamorphic rocks**: transformation of pre-existing rocks in the solid state by heat, pressure and chemically active fluids. #### Identification of igneous rocks in the field - **Crystalline texture**: interlocking crystals (coarse in granite, fine in basalt), or glassy. - **Massive** structure; no layers or foliation; may show columnar jointing. - **Vesicles** or amygdales in volcanic rocks. - **No fossils**. - **Minerals**: quartz, feldspar, mica, pyroxene, hornblende, olivine. - **Occurrence**: dykes, sills, batholiths or lava flows, with baked contacts on neighbouring rock. - Colour from light (acidic) to dark (basic). #### Civil engineering significance - **Granite**: hard, strong and durable (100-250 MPa); used for foundations, bridge piers, aggregate, dressed stone and monuments. Weathered or jointed granite needs care. - **Phyllite**: weak, foliated and easily weathered; it makes slopes and tunnels unstable in the Lesser Himalaya, so it is generally unsuitable for aggregate or heavy foundations. - **Sandstone**: building stone, aggregate and flooring; strength depends on the cement (silica cement is strong, clay cement weak); bedding affects stability; it is porous, so it may leak. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Petrogenesis deals with A. the description of rocks only B. fossils in rocks C. the origin and formation of rocks D. strength testing of rocks Answer: C. It is the branch of petrology on origin. (Key point: Petrogenesis meaning) 2. Which feature identifies igneous rocks in the field? A. Bedding with fossils and ripple marks B. Foliation with folded bands C. Rounded pebbles in cement D. Interlocking crystals or glassy texture, massive, with no fossils Answer: D. Igneous rocks are crystalline or glassy, massive and unfossiliferous. (Key point: Field identification of igneous) 3. Why is phyllite generally unsuitable for aggregate or heavy foundations? A. It is weak, foliated and easily weathered B. It is too hard to crush C. It is too soluble D. It is too glassy Answer: A. Weak foliated phyllite makes unstable slopes and tunnels. (Key point: Phyllite significance) 4. What controls sandstone strength? A. Only its colour B. The type of cement; silica cement is strong and clay cement weak C. Its age only D. The presence of foliation Answer: B. Cement type controls strength; bedding affects stability. 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