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Petrogenesis, igneous rock identification, rock significance

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.

Describe petrogenesis. How do you identify igneous rocks in the field? Describe the civil engineering significance of granite, phyllite and sandstone.

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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