---
title: "Petrography versus petrogenesis"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:petrography-versus-petrogenesis"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/petrography-versus-petrogenesis
site: BCT Engineering
---

# Petrography versus petrogenesis

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2079 Bhadra, 1 marks; 2073 Shrawan, 1 marks (asked 2 times)
- Video: [🔍 Petrology Explained | Igneous, Sedimentary & Metamorphic Rocks | Geology for B.Sc. Students](https://www.youtube.com/watch?v=w6C8eJ3an7k) — GeoParth
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/petrography-versus-petrogenesis
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Differentiate between petrography and petrogenesis (define petrography and petrogenesis).

## Short answer

Petrography is the systematic description of rocks (composition, texture, structure), while petrogenesis studies their origin and formation. Petrography is descriptive, petrogenesis interpretative.

## Answer

| Basis | Petrography | Petrogenesis |
|---|---|---|
| Meaning | Systematic **description** of rocks: mineral composition, texture, structure and classification | Study of the **origin** and formation of rocks |
| Concerned with | What the rock is like | How and why the rock formed |
| Method | Hand specimen and thin-section microscope | Field relations, experiments and chemistry |
| Example | Granite is coarse-grained, with quartz, feldspar and mica | Granite forms by slow cooling of acidic magma deep underground |
| Nature | Descriptive | Interpretative |

Both are parts of petrology.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Petrography is the
   A. systematic description and classification of rocks
   B. study of origin of rocks
   C. study of fossils
   D. study of earthquakes
   Answer: A. Petrography describes mineral composition, texture and structure. (Key point: Petrography)

2. Which question does petrogenesis answer?
   A. What the rock looks like in thin section
   B. How and why the rock formed
   C. How hard the rock is
   D. How many minerals are in it
   Answer: B. Petrogenesis is about origin; petrography about description. (Key point: Petrogenesis)

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